Prof. Dr. Michael Farle

Experimental Physics
University of Duisburg-Essen

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  • Dipolar-stabilized first and second-order antiskyrmions in ferrimagnetic multilayers
    Heigl, M. and Koraltan, S. and Vaňatka, M. and Kraft, R. and Abert, C. and Vogler, C. and Semisalova, A. and Che, P. and Ullrich, A. and Schmidt, T. and Hintermayr, J. and Grundler, D. and Farle, M. and Urbánek, M. and Suess, D. and Albrecht, M.
    Nature Communications 12 (2021)
    Skyrmions and antiskyrmions are topologically protected spin structures with opposite vorticities. Particularly in coexisting phases, these two types of magnetic quasi-particles may show fascinating physics and potential for spintronic devices. While skyrmions are observed in a wide range of materials, until now antiskyrmions were exclusive to materials with D2d symmetry. In this work, we show first and second-order antiskyrmions stabilized by magnetic dipole–dipole interaction in Fe/Gd-based multilayers. We modify the magnetic properties of the multilayers by Ir insertion layers. Using Lorentz transmission electron microscopy imaging, we observe coexisting antiskyrmions, Bloch skyrmions, and type-2 bubbles and determine the range of material properties and magnetic fields where the different spin objects form and dissipate. We perform micromagnetic simulations to obtain more insight into the studied system and conclude that the reduction of saturation magnetization and uniaxial magnetic anisotropy leads to the existence of this zoo of different spin objects and that they are primarily stabilized by dipolar interaction. © 2021, The Author(s).
    view abstract10.1038/s41467-021-22600-7
  • Dispersion relation of nutation surface spin waves in ferromagnets
    Cherkasskii, M. and Farle, M. and Semisalova, A.
    Physical Review B 103 (2021)
    Inertia effects in magnetization dynamics are theoretically shown to result in a different type of spin waves, i.e., nutation surface spin waves, which propagate at terahertz frequencies in in-plane magnetized ferromagnetic thin films. Considering the magnetostatic limit, i.e., neglecting exchange coupling, we calculate dispersion relation and group velocity, which we find to be slower than the velocity of conventional (precession) spin waves. In addition, we find that the nutation surface spin waves are backward spin waves. Furthermore, we show that inertia causes a decrease of the frequency of the precession spin waves, namely magnetostatic surface spin waves and backward volume magnetostatic spin waves. The magnitude of the decrease depends on the magnetic properties of the film and its geometry. © 2021 authors.
    view abstract10.1103/PhysRevB.103.174435
  • Long-Range Ordering Effects in Magnetic Nanoparticles
    Myrovali, E. and Papadopoulos, K. and Iglesias, I. and Spasova, M. and Farle, M. and Wiedwald, U. and Angelakeris, M.
    ACS Applied Materials and Interfaces 13 (2021)
    The challenge for synthesizing magnetic nanoparticle chains may be achieved under the application of fixation fields, which are the externally applied fields, enhancing collective magnetic features due to adequate control of dipolar interactions among magnetic nanoparticles. However, relatively little attention has been devoted to how size, concentration of magnetic nanoparticles, and intensity of an external magnetic field affect the evolution of chain structures and collective magnetic features. Here, iron oxide nanoparticles are developed by the coprecipitation method at diameters below (10 and 20 nm) and above (50 and 80 nm) their superparamagnetic limit (at about 25 nm) and then are subjected to a tunable fixation field (40-400 mT). Eventually, the fixation field dictates smaller particles to form chain structures in two steps, first forming clusters and then guiding chain formation via "cluster-cluster"interactions, whereas larger particles readily form chains via "particle-particle"interactions. In both cases, dipolar interactions between the neighboring nanoparticles augment, leading to a substantial increase in their collective magnetic features which in turn results in magnetic particle hyperthermia efficiency enhancement of up to one order of magnitude. This study provides new perspectives for magnetic nanoparticles by arranging them in chain formulations as enhanced performance magnetic actors in magnetically driven magnetic applications. ©
    view abstract10.1021/acsami.1c01820
  • Magnetic Nanoparticles as a Tool for Remote DNA Manipulations at a Single-Molecule Level
    Nikitin, A.A. and Yurenya, A.Y. and Zatsepin, T.S. and Aparin, I.O. and Chekhonin, V.P. and Majouga, A.G. and Farle, M. and Wiedwald, U. and Abakumov, M.A.
    ACS Applied Materials and Interfaces 13 (2021)
    Remote control of cells and single molecules by magnetic nanoparticles in nonheating external magnetic fields is a perspective approach for many applications such as cancer treatment and enzyme activity regulation. However, the possibility and mechanisms of direct effects of small individual magnetic nanoparticles on such processes in magneto-mechanical experiments still remain unclear. In this work, we have shown remote-controlled mechanical dissociation of short DNA duplexes (18-60 bp) under the influence of nonheating low-frequency alternating magnetic fields using individual 11 nm magnetic nanoparticles. The developed technique allows (1) simultaneous manipulation of millions of individual DNA molecules and (2) evaluation of energies of intermolecular interactions in short DNA duplexes or in other molecules. Finally, we have shown that DNA duplexes dissociation is mediated by mechanical stress and produced by the movement of magnetic nanoparticles in magnetic fields, but not by local overheating. The presented technique opens a new avenue for high-precision manipulation of DNA and generation of biosensors for quantification of energies of intermolecular interaction. ©
    view abstract10.1021/acsami.0c21002
  • Numerical Ferromagnetic Resonance Experiments in Nanosized Elements
    Wagner, K. and Körber, L. and Stienen, S. and Lindner, J. and Farle, M. and Kákay, A.
    IEEE Magnetics Letters 12 (2021)
    In this letter, we present a numerical approach to obtain the ferromagnetic resonance spectra of micrometer- and nanometer-sized magnetic elements by micromagnetic simulations. By mimicking common experimental conditions, we applied a static magnetic field and used a linearly polarized oscillating magnetic field to excite magnetization dynamics. A continuous single-frequency excitation is utilized, which permits the study of the steady-state dynamics in the space and time domain. This gives direct access to resonance fields, linewidths, and relative amplitudes as observed in the experiments, which is not easily accessible in pulsed schemes and allows for a one-to-one identification between simulation and experiment. Similar to numerical approaches using pulsed excitations, the phases, ellipticity, and spatial mode profiles of the spin-wave excitations may also be accessed. Using large excitation powers, we then showcase that one can additionally study nonlinear responses by this method, such as the nonlinear shift of the resonance fields and the foldover of the absorption lines. Since the dynamic susceptibility is directly determined from standard outputs of common micromagnetic codes, the presented method is robust, efficient, and easy-to-use, adding to its practical importance. © 2010-2012 IEEE.
    view abstract10.1109/LMAG.2021.3055447
  • Optical and magneto-optical properties of epitaxial Mn2GaC MAX phase thin film
    Lyaschenko, S. and Maximova, O. and Shevtsov, D. and Varnakov, S. and Tarasov, I. and Wiedwald, U. and Rosen, J. and Ovchinnikov, S. and Farle, M.
    Journal of Magnetism and Magnetic Materials 528 (2021)
    We report measurements of the dielectric permittivity, optical conductivity and magnetic circular dichroism (MCD) of the epitaxial Mn2GaC MAX-phase thin film in an external magnetic field of up to 200 mT, at temperatures of 296 and 140 K and 1.4 to 3.5 eV. The optical conductivity and MCD spectra show absorption peaks which are consistent with the interband electronic transitions for different positions of Mn, Ga, and C ions as confirmed by theoretical calculations of the spin-dependent density of electronic states. The well-known structural phase transition at 214 K is also seen in the changes of optical, magneto-optical and surface magnetic properties of Mn2GaC in our experiment. © 2021 Elsevier B.V.
    view abstract10.1016/j.jmmm.2021.167803
  • Pulsed laser deposition of epitaxial Cr2AlC MAX phase thin films on MgO(111) and Al2O3(0001)
    Stevens, M. and Pazniak, H. and Jemiola, A. and Felek, M. and Farle, M. and Wiedwald, U.
    Materials Research Letters 9 (2021)
    Epitaxial Cr2AlC MAX phase thin films were grown on MgO(111) and Al2O3(0001) by pulsed laser deposition (PLD) at 600°C. X-ray diffraction and morphology studies of Cr2AlC thin films on MgO (111) reveal phase purity, columnar growth, the epitaxial relation Cr2AlC(0001) || MgO(111) and Cr2AlC [11-20] || MgO[10-1] and similar growth behaviour on Al2O3(0001). Resistivity measurements show semiconductor-like behaviour for 10 and 20 nm thick films, and metallic-like behaviour for thicker films, suggesting a percolation thickness slightly above 20 nm. Our results demonstrate the potential of PLD as a novel method for the growth of epitaxial MAX phase thin films. © 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/21663831.2021.1920510
  • Room temperature synthesized solid solution AuFe nanoparticles and their transformation into Au/Fe Janus nanocrystals
    Efremova, M.V. and Spasova, M. and Heidelmann, M. and Grebennikov, I.S. and Li, Z.-A. and Garanina, A.S. and Tcareva, I.O. and Savchenko, A.G. and Farle, M. and Klyachko, N.L. and Majouga, A.G. and Wiedwald, U.
    Nanoscale 13 (2021)
    Solid solution AuFe nanoparticles were synthesized for the first time under ambient conditions by an adapted method previously established for the Fe3O4-Au core-shell morphology. These AuFe particles preserved the fcc structure of Au incorporated with paramagnetic Fe atoms. The metastable AuFe can be segregated by transformation into Janus Au/Fe particles with bcc Fe and fcc Au upon annealing. The ferromagnetic Fe was epitaxially grown on low index fcc Au planes. This preparation route delivers new perspective materials for magnetoplasmonics and biomedical applications and suggests the reconsideration of existing protocols for magnetite-gold core-shell synthesis. © The Royal Society of Chemistry.
    view abstract10.1039/d1nr00383f
  • Towards laser printing of magnetocaloric structures by inducing a magnetic phase transition in iron-rhodium nanoparticles
    Nadarajah, R. and Landers, J. and Salamon, S. and Koch, D. and Tahir, S. and Doñate-Buendía, C. and Zingsem, B. and Dunin-Borkowski, R.E. and Donner, W. and Farle, M. and Wende, H. and Gökce, B.
    Scientific Reports 11 (2021)
    The development of magnetocaloric materials represents an approach to enable efficient and environmentally friendly refrigeration. It is envisioned as a key technology to reduce CO2 emissions of air conditioning and cooling systems. Fe-Rh has been shown to be one of the best-suited materials in terms of heat exchange per material volume. However, the Fe-Rh magnetocaloric response depends on its composition. Hence, the adaptation of material processing routes that preserve the Fe-Rh magnetocaloric response in the generated structures is a fundamental step towards the industrial development of this cooling technology. To address this challenge, the temperature-dependent properties of laser synthesized Fe-Rh nanoparticles and the laser printing of Fe-Rh nanoparticle inks are studied to generate 2D magnetocaloric structures that are potentially interesting for applications such as waste heat management of compact electrical appliances or thermal diodes, switches, and printable magnetocaloric media. The magnetization and temperature dependence of the ink’s γ-FeRh to B2-FeRh magnetic transition is analyzed throughout the complete process, finding a linear increase of the magnetization M (0.8 T, 300 K) up to 96 Am2/kg with ca. 90% of the γ-FeRh being transformed permanently into the B2-phase. In 2D structures, magnetization values of M (0.8 T, 300 K) ≈ 11 Am2/kg could be reached by laser sintering, yielding partial conversion to the B2-phase equivalent to long-time heating temperature of app. 600 K, via this treatment. Thus, the proposed procedure constitutes a robust route to achieve the generation of magnetocaloric structures. © 2021, The Author(s).
    view abstract10.1038/s41598-021-92760-5
  • Beyond Solid Solution High-Entropy Alloys: Tailoring Magnetic Properties via Spinodal Decomposition
    Rao, Z. and Dutta, B. and Körmann, F. and Lu, W. and Zhou, X. and Liu, C. and da Silva, A.K. and Wiedwald, U. and Spasova, M. and Farle, M. and Ponge, D. and Gault, B. and Neugebauer, J. and Raabe, D. and Li, Z.
    Advanced Functional Materials (2020)
    Since its first emergence in 2004, the high-entropy alloy (HEA) concept has aimed at stabilizing single- or dual-phase multi-element solid solutions through high mixing entropy. Here, this strategy is changed and renders such massive solid solutions metastable, to trigger spinodal decomposition for improving the alloys’ magnetic properties. The motivation for starting from a HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behavior using multiple components. The key idea is to form Fe-Co enriched regions which have an expanded volume (relative to unconstrained Fe-Co), due to coherency constraints imposed by the surrounding HEA matrix. As demonstrated by theory and experiments, this leads to improved magnetic properties of the decomposed alloy relative to the original solid solution matrix. In a prototype magnetic FeCoNiMnCu HEA, it is shown that the modulated structures, achieved by spinodal decomposition, lead to an increase of the Curie temperature by 48% and a simultaneous increase of magnetization by 70% at ambient temperature as compared to the homogenized single-phase reference alloy. The findings thus open a pathway for the development of advanced functional HEAs. © 2020 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
    view abstract10.1002/adfm.202007668
  • Controlling the oxidation of magnetic and electrically conductive solid-solution iron-rhodium nanoparticles synthesized by laser ablation in liquids
    Nadarajah, R. and Tahir, S. and Landers, J. and Koch, D. and Semisalova, A.S. and Wiemeler, J. and El-Zoka, A. and Kim, S.-H. and Utzat, D. and Möller, R. and Gault, B. and Wende, H. and Farle, M. and Gökce, B.
    Nanomaterials 10 (2020)
    This study focuses on the synthesis of FeRh nanoparticles via pulsed laser ablation in liquid and on controlling the oxidation of the synthesized nanoparticles. Formation of monomodal γ-FeRh nanoparticles was confirmed by transmission electron microscopy (TEM) and their composition confirmed by atom probe tomography (APT). For these particles, three major contributors to oxidation were analysed: (1) dissolved oxygen in the organic solvents, (2) the bound oxygen in the solvent and (3) oxygen in the atmosphere above the solvent. The decrease of oxidation for optimized ablation conditions was confirmed through energy-dispersive X-ray (EDX) and Mössbauer spectroscopy. Furthermore, the time dependence of oxidation was monitored for dried FeRh nanoparticles powders using ferromagnetic resonance spectroscopy (FMR). By magnetophoretic separation, B2-FeRh nanoparticles could be extracted from the solution and characteristic differences of nanostrand formation between γ-FeRh and B2-FeRh nanoparticles were observed. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10122362
  • Decomposition in Ni-Co-Mn-In functional Heusler alloys and its effect on shell-ferromagnetic and magnetocaloric effects
    Şaşmaz, M. and Dreist, F. and Iglesias, I. and Çaklr, A. and Farle, M. and Acet, M.
    Physical Review B 102 (2020)
    Ni-Co-Mn-In Heusler-based compounds are interesting for their magnetocaloric properties and have been widely investigated for this purpose. For Co compositions more than 5 at% in (Ni100-xCox)50Mn25+yIn25-y the material is no longer single phase, and for y<25, shell-ferromagnetic precipitation occurs. Our study is twofold: First we study here the shell-ferromagnetic properties of these systems and show that their ferromagnetic exchange can be strengthened by introducing Co into the precipitate. Second, we further show that both the multiphase character and shell-ferromagnetic precipitation have strong implications on the magnetocaloric properties. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.102.064401
  • Dynamic unidirectional anisotropy in cubic FeGe with antisymmetric spin-spin-coupling
    Josten, N. and Feggeler, T. and Meckenstock, R. and Spoddig, D. and Spasova, M. and Chai, K. and Radulov, I. and Li, Z.-A. and Gutfleisch, O. and Farle, M. and Zingsem, B.
    Scientific Reports 10 (2020)
    Strong unidirectional anisotropy in bulk polycrystalline B20 FeGe has been measured by ferromagnetic resonance spectroscopy. Such anisotropy is not present in static magnetometry measurements. B20 FeGe exhibits inherent Dzyaloshinskii-Moriya interaction, resulting in a nonreciprocal spin-wave dispersion. Bulk and micron sized samples were produced and characterized. By X-band ferromagnetic resonance spectroscopy at 276 K ± 1 K, near the Curie temperature, a distribution of resonance modes was observed in accordance with the cubic anisotropy of FeGe. This distribution exhibits a unidirectional anisotropy, i.e. shift of the resonance field under field inversion, of KUD = 960 J/m3 ± 10 J/m3, previously unknown in bulk ferromagnets. Additionally, more than 25 small amplitude standing spin wave modes were observed inside a micron sized FeGe wedge, measured at 293 K ± 2 K. These modes also exhibit unidirectional anisotropy. This effect, only dynamically measurable and not detectable in static magnetometry measurements, may open new possibilities for directed spin transport in chiral magnetic systems. © 2020, The Author(s).
    view abstract10.1038/s41598-020-59208-8
  • Dynamics of chiral state transitions and relaxations in an FeGe thin plate: Via in situ Lorentz microscopy
    Chai, K. and Li, Z.-A. and Liu, R. and Zou, B. and Farle, M. and Li, J.
    Nanoscale 12 (2020)
    Studying the magnetic transition between different topological spin textures in noncentrosymmetric magnets under external stimuli is an important topic in chiral magnetism. Here, using in situ Lorentz transmission electron microscopy (LTEM) we directly visualize the thermal-driven magnetic transitions and dynamic characteristics in FeGe thin plates. A novel protocol-dependent phase diagram of FeGe thin plates was obtained via pulsed laser excitation. Moreover, by setting the appropriate specimen temperature, the relaxation of chiral magnetic states in FeGe specimens was recorded and analyzed with an Arrhenius-type relaxation mechanism. We present the field-dependent activation energy barriers for chiral state transitions and the magnetic transition pathways of these spin textures for FeGe thin plates. Our results unveil the effects of thermal excitation on the topological spin texture transitions and provide useful information about magnetic dynamics of chiral magnetic state relaxation. © 2020 The Royal Society of Chemistry.
    view abstract10.1039/d0nr03278f
  • Heterogeneous magnetism and kinetic arrest in antiperovskite Mn3-xNixGaC compounds with Ni2MnGa Heusler insertions
    Çaklr, Ö. and Dias, E. and Priolkar, K.R. and Hoser, A. and Farle, M. and Acet, M.
    Physical Review B 102 (2020)
    Mn-based antiperovskite compounds in the form Mn3AX, where A is a main group element and X is C or N, undergo magnetostructural transitions with which these materials acquire magnetocaloric, giant magnetoresistance, and spin-transport properties, which can be modified or tailored by manipulating the compositions of numerous compounds. This enables closer investigations and better understandings of the underlying principles governing these properties. Mn3-xNixGaC, which is a derivative of the prototype Mn3GaC antiperovskite, would normally be expected to form a cubic structure with a homogeneous composition. Contrary to this, we find that the addition of Ni leads to a heterogenous compound consisting of an antiperovskite part and a Ni2MnGa Heusler insertions. The system shows kinetic arrest features, which we study as a function of Ni composition using the techniques of x-ray diffraction, magnetization, and neutron diffraction under a magnetic field. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.102.024431
  • Non-standing spin-waves in confined micrometer-sized ferromagnetic structures under uniform excitation
    Pile, S. and Feggeler, T. and Schaffers, T. and Meckenstock, R. and Buchner, M. and Spoddig, D. and Zingsem, B. and Ney, V. and Farle, M. and Wende, H. and Ohldag, H. and Ney, A. and Ollefs, K.
    Applied Physics Letters 116 (2020)
    A non-standing characteristic of directly imaged spin-waves in confined micrometer-sized ultrathin Permalloy (Ni 80 Fe 20) structures is reported along with evidence of the possibility to alter the observed state by modifications to the sample geometry. Using micromagnetic simulations, the presence of the spin-wave modes excited in the Permalloy stripes along with the quasi-uniform modes was observed. The predicted spin-waves were imaged in direct space using time resolved scanning transmission X-ray microscopy, combined with a ferromagnetic resonance excitation scheme (STXM-FMR). STXM-FMR measurements revealed a non-standing characteristic of the spin-waves. Also, it was shown by micromagnetic simulations and confirmed using STXM-FMR results that the observed characteristic of the spin-waves can be influenced by the local magnetic fields in different sample geometries. © 2020 Author(s).
    view abstract10.1063/1.5139881
  • Nutation resonance in ferromagnets
    Cherkasskii, M. and Farle, M. and Semisalova, A.
    Physical Review B 102 (2020)
    The inertial dynamics of magnetization in a ferromagnet is investigated theoretically. The analytically derived dynamic response upon microwave excitation shows two peaks: ferromagnetic and nutation resonances. The exact analytical expressions of frequency and linewidth of the magnetic nutation resonance are deduced from the frequency-dependent susceptibility determined by the inertial Landau-Lifshitz-Gilbert equation. The study shows that the dependence of nutation linewidth on the Gilbert precession damping has a minimum, which becomes more expressive with increase of the applied magnetic field. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevB.102.184432
  • Reversal of uniaxial magnetic anisotropy in Fe/GaAs (110) films driven by surface relaxation: An in situ ferromagnetic resonance study
    Bhagat, B. and Semisalova, A. and Meckenstock, R. and Farle, M.
    AIP Advances 10 (2020)
    We report an in situ study of the time evolution of magnetic anisotropy constants of an uncapped 4 nm [∼27 monolayers (ML)] Fe film epitaxially grown on a GaAs (110) substrate at room temperature under ultra-high vacuum (UHV) conditions. The structural and chemical properties are monitored by low energy electron diffraction and Auger spectroscopy with a sensitivity of 0.01 ML. The in situ UHV ferromagnetic resonance (FMR) study over a period of 6 days in <10-9 Pa reveals that there is a slow magneto-morphological transition of the Fe film surface at room temperature. The resonance field measured in situ in the [11-0] direction initially changes at a rate of 0.3 mT/h within 30 h after deposition and later at 0.1 mT/h over 80 h. We determine the time-dependent changes in the in-plane and out-of-plane anisotropy constants and find a sign change in the uniaxial in-plane anisotropy in the first 24 h due to morphological changes at the surface. The in situ FMR measurements and the Auger analysis allow us to exclude changes in the magnetization and anisotropy due to the contamination and oxidation of the Fe film. © 2020 Author(s).
    view abstract10.1063/5.0004261
  • Structural evolution and magnetic properties of high-entropy CuCrFeTiNi alloys prepared by high-energy ball milling and spark plasma sintering
    Shkodich, N.F. and Spasova, M. and Farle, M. and Kovalev, D.Y. and Nepapushev, A.A. and Kuskov, K.V. and Vergunova, Y.S. and Scheck, Y.B. and Rogachev, A.S.
    Journal of Alloys and Compounds 816 (2020)
    A powder of equiatomic CuCrFeTiNi high-entropy alloy (HEA) was prepared by short-term (30 min) high-energy ball milling (HEBM). Our structural and chemical analysis showed that micron sized particles of bcc CuCrFeTiNi consisting of nanosized crystalline grains (∼6 nm) could be obtained after 30 min of HEBM. The influence of milling time (30 ÷ 240 min) on structural and magnetic transformations of CuCrFeTiNi powder mixture was investigated. The HEA powders were thermally stable up to 500°С based on DSC results. The HEA powder was subsequently consolidated by spark plasma sintering at 700 °C resulting in a consolidated bulk HEA with co-existing bcc and fcc phases. The as-milled CuCrFeTiNi powder blend contained a solid solution with bcc (Im3m) structure. Annealing at 600°С (t = 180 min) increased the crystallinity of the α-phase (bcc) and gave rise to formation of the γ-phase (fcc, Fm3m) whose amount grew with increasing dwell time. Between 800 and 1000°С, a tetragonal intermetallic σ-phase – most likely FeCr - appeared and subsequently vanished. At 1000°С, the final product was found to contain two solid solutions based on the γ-phase (fcc). The Vickers hardness HvHEBM = 7.7 GPa of the SPS consolidated CuCrFeTiNi alloy (milled for t = 180 min) was markedly higher than the one of SPS-produced ones without HEBM (Hv = 2.1 GPa). Paramagnetic behavior at room temperature with a small ferromagnetic contribution at low fields was observed for as-milled powder after 180 min of HEBM. A small magnetic hysteresis was observed at 5 K and 300 K with a coercive field of around 16 kA/m. Above 100 K, the inverse susceptibility of a HEA powder ball-milled for t = 240 min showed a clear paramagnetic response. The Curie temperature TC ∼50 K was found. © 2019 Elsevier B.V.
    view abstract10.1016/j.jallcom.2019.152611
  • Transport properties of shell-ferromagnetic Heusler precipitates in decomposed Ni49.8Mn45.1Sn5.1 and decomposition limit for Ni50Mn50−xSnx alloys
    Çakır, A. and Koyun, H.N. and Acet, M. and Farle, M.
    Journal of Magnetism and Magnetic Materials 499 (2020)
    Shell-ferromagnetism is observed in Mn-rich NiMn-based Heusler alloys as a result of phase-separation. Off-stoichiometric NiMn-based Heusler alloys decompose into a dual-phase composite when annealed under a magnetic field. As a result of this process, an initially anti-ferromagnetic Heusler alloy gains hard-ferromagnetic properties with a nearly 10 Tesla coercive field of a core/shell structured precipitate. In the present study, Ni50.1Mn42.1Sn7.8 and Ni50.4Mn36.6Sn13.0 alloys are investigated for magnetic and structural instabilities and shell-ferromagnetic decomposition to obtain information on the compositional limits of the decomposition. Furthermore, we investigate the magneto-transport properties of shell-ferromagnets obtained by annealing Ni49.8Mn45.1Sn5.1 in a magnetic-field. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmmm.2019.166265
  • Unravelling the nucleation, growth, and faceting of magnetite-gold nanohybrids
    Nalench, Y.A. and Shchetinin, I.V. and Skorikov, A.S. and Mogilnikov, P.S. and Farle, M. and Savchenko, A.G. and Majouga, A.G. and Abakumov, M.A. and Wiedwald, U.
    Journal of Materials Chemistry B 8 (2020)
    The chemical synthesis of nanoparticles with a preassigned size and shape is important for an optimized performance in any application. Therefore, systematic monitoring of the synthesis is required for the control and detailed understanding of the nucleation and growth of the nanoparticles. Here, we study Fe3O4-Au hybrid nanoparticles in detail using probes of the reaction mixture during synthesis and their thorough characterization. The proposed approach eliminates the problem of repeatability and reproducibility of the chemical synthesis and was carried out using laboratory equipment (standard transmission electron microscopy, X-ray diffraction, and magnetometry) for typically 10 μL samples instead of, for example, a dedicated synthesis and inspection at a synchrotron radiation facility. From the three independent experimental techniques we extract the nanoparticle size at 12 stages of the synthesis. These diameters show identical trends and good quantitative agreement. Two consecutive processes occur during the synthesis of Fe3O4-Au nanoparticles, the nucleation and the growth of spherical Fe3O4nanoparticles on the surface of Au seeds during the heating stage and their faceting towards octahedral shape during reflux. The final nanoparticles with sizes of 15 nm Fe3O4and 4 nm Au exhibit superparamagnetic behavior at ambient temperature. These are high-quality, close to stoichiometric Fe3O4nanocrystals with nearly volumetric magnetic behavior as confirmed by the presence of the Verwey transition. Understanding the processes occurring during the synthesis allows the nanoparticle size and shape to be adjusted, improving their capabilities in biomedical applications. © The Royal Society of Chemistry 2020.
    view abstract10.1039/c9tb02721a
  • Additive manufacturing of soft magnetic permalloy from Fe and Ni powders: Control of magnetic anisotropy
    Schönrath, H. and Spasova, M. and Kilian, S.O. and Meckenstock, R. and Witt, G. and Sehrt, J.T. and Farle, M.
    Journal of Magnetism and Magnetic Materials 478 (2019)
    The influence of the process parameters in Laser Beam Melting (LBM) on the element distribution and magnetic properties of permalloy (Ni 78.5 Fe 21.5 ) is studied. Iron and nickel powders are mixed in the respective proportions to build twenty-five permalloy samples. The process parameters for each sample are varied to achieve different volume energy densities. An increase of the saturation magnetization M S up to 14% of the samples with respect to the initial powder blend is found. For a volume energy density of 428 [Formula presented] we detect a stripe-like segregation of iron and nickel in the uppermost layer. In the volume a homogeneous element distribution is found. The segregation at the surface leads to a sizable uniaxial magnetic anisotropy. When using parameter combinations resulting in similar volume energy densities, we observe different surface morphologies depending on scan speed and laser power. The implications for creating tailored magnetic anisotropy directions in Fe-Ni soft magnets are discussed. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmmm.2018.11.084
  • Atomically Layered and Ordered Rare-Earth i-MAX Phases: A New Class of Magnetic Quaternary Compounds
    Tao, Q. and Lu, J. and Dahlqvist, M. and Mockute, A. and Calder, S. and Petruhins, A. and Meshkian, R. and Rivin, O. and Potashnikov, D. and Caspi, E.N. and Shaked, H. and Hoser, A. and Opagiste, C. and Galera, R.-M. and Salikhov, R. and Wiedwald, U. and Ritter, C. and Wildes, A.R. and Johansson, B. and Hultman, L. and Farle, M. and Barsoum, M.W. and Rosen, J.
    Chemistry of Materials 31 (2019)
    In 2017, we discovered quaternary i-MAX phases - atomically layered solids, where M is an early transition metal, A is an A group element, and X is C - with a (M12/3M21/3)2AC chemistry, where the M1 and M2 atoms are in-plane ordered. Herein, we report the discovery of a class of magnetic i-MAX phases in which bilayers of a quasi-2D magnetic frustrated triangular lattice overlay a Mo honeycomb arrangement and an Al Kagomé lattice. The chemistry of this family is (Mo2/3RE1/3)2AlC, and the rare-earth, RE, elements are Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The magnetic properties were characterized and found to display a plethora of ground states, resulting from an interplay of competing magnetic interactions in the presence of magnetocrystalline anisotropy. © 2019 American Chemical Society.
    view abstract10.1021/acs.chemmater.8b05298
  • Biologically encoded magnonics
    Zingsem, B.W. and Feggeler, T. and Terwey, A. and Ghaisari, S. and Spoddig, D. and Faivre, D. and Meckenstock, R. and Farle, M. and Winklhofer, M.
    Nature Communications 10 (2019)
    Spin wave logic circuits using quantum oscillations of spins (magnons) as carriers of information have been proposed for next generation computing with reduced energy demands and the benefit of easy parallelization. Current realizations of magnonic devices have micrometer sized patterns. Here we demonstrate the feasibility of biogenic nanoparticle chains as the first step to truly nanoscale magnonics at room temperature. Our measurements on magnetosome chains (ca 12 magnetite crystals with 35 nm particle size each), combined with micromagnetic simulations, show that the topology of the magnon bands, namely anisotropy, band deformation, and band gaps are determined by local arrangement and orientation of particles, which in turn depends on the genotype of the bacteria. Our biomagnonic approach offers the exciting prospect of genetically engineering magnonic quantum states in nanoconfined geometries. By connecting mutants of magnetotactic bacteria with different arrangements of magnetite crystals, novel architectures for magnonic computing may be (self-) assembled. © 2019, The Author(s).
    view abstract10.1038/s41467-019-12219-0
  • Direct measurement of anisotropic conductivity in a nanolaminated (Mn0.5Cr0.5)2GaC thin film
    Flatten, T. and Matthes, F. and Petruhins, A. and Salikhov, R. and Wiedwald, U. and Farle, M. and Rosen, J. and Bürgler, D.E. and Schneider, C.M.
    Applied Physics Letters 115 (2019)
    The direct and parameter-free measurement of anisotropic electrical resistivity of a magnetic Mn+1AXn (MAX) phase film is presented. A multitip scanning tunneling microscope is used to carry out 4-probe transport measurements with variable probe spacing s. The observation of the crossover from the 3D regime for small s to the 2D regime for large s enables the determination of both in-plane and perpendicular-to-plane resistivities ρab and ρc. A (Cr0.5Mn0.5)2GaC MAX phase film shows a large anisotropy ratio ρ c / ρ ab = 525 ± 49. This is a consequence of the complex bonding scheme of MAX phases with covalent M-X and metallic M-M bonds in the MX planes and predominately covalent, but weaker bonds between the MX and A planes. © 2019 Author(s).
    view abstract10.1063/1.5115347
  • Dynamics of the magnetoelastic phase transition and adiabatic temperature change in Mn1.3Fe0.7P0.5Si0.55
    Fries, M. and Gottschall, T. and Scheibel, F. and Pfeuffer, L. and Skokov, K.P. and Skourski, I. and Acet, M. and Farle, M. and Wosnitza, J. and Gutfleisch, O.
    Journal of Magnetism and Magnetic Materials 477 (2019)
    The adiabatic temperature change ΔTad of a Mn1.3Fe0.7P0.5Si0.55 Fe2P-type alloy was measured under different magnetic field-sweep rates from 0.93 Ts−1 to 2870 Ts−1. We find a field-sweep-rate independent magnetocaloric effect due to a partial alignment of magnetic moments in the paramagnetic region overlapping with the magnetocaloric effect of the first-order phase transition. Additionally, the first-order phase transition is not completed even in fields up to 20 T leading to a non-saturating behavior of ΔTad. Measurements in different pulsed fields reveal that the first-order phase transition cannot follow the fast field changes as previously assumed, resulting in a distinct field-dependent hysteresis in ΔTad. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmmm.2018.12.086
  • Extracting the dynamic magnetic contrast in time-resolved X-ray transmission microscopy
    Schaffers, T. and Feggeler, T. and Pile, S. and Meckenstock, R. and Buchner, M. and Spoddig, D. and Ney, V. and Farle, M. and Wende, H. and Wintz, S. and Weigand, M. and Ohldag, H. and Ollefs, K. and Ney, A.
    Nanomaterials 9 (2019)
    Using a time-resolved detection scheme in scanning transmission X-ray microscopy (STXM), we measured element resolved ferromagnetic resonance (FMR) at microwave frequencies up to 10 GHz and a spatial resolution down to 20 nm at two different synchrotrons. We present different methods to separate the contribution of the background from the dynamic magnetic contrast based on the X-ray magnetic circular dichroism (XMCD) effect. The relative phase between the GHz microwave excitation and the X-ray pulses generated by the synchrotron, as well as the opening angle of the precession at FMR can be quantified. A detailed analysis for homogeneous and inhomogeneous magnetic excitations demonstrates that the dynamic contrast indeed behaves as the usual XMCD effect. The dynamic magnetic contrast in time-resolved STXM has the potential be a powerful tool to study the linear and nonlinear, magnetic excitations in magnetic micro-and nano-structures with unique spatial-temporal resolution in combination with element selectivity. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano9070940
  • Fexni100-x thin film systems with slight deviations from zero magnetostriction compositions: Focus on pressure sensor applications
    Chlenova, A.A. and Novoselova, I.P. and Salikhov, R. and Farle, M. and Lepalovskij, V.N. and Totoricaguena, G.J. and Kurlyandskaya, G.V.
    Key Engineering Materials 826 KEM (2019)
    The structure, magnetic properties, ferromagnetic resonance and giant magnetoimpedance effect (GMI) were studied in FexNi100-x thin films and multilayered systems having compositions with small deviation from zero magnetostriction in order to find the best conditions for possible applications in the area of small pressure sensors. A comparative analysis of the effective magnetization and g-factor was carried out for the thin films of FexNi100-x (x = 19.8, 17.5, 15.0, 11.9) alloys. Comparison of the concentration dependences for static 4πMs and dynamic 4πMeff magnetization values allows to select a narrow interval of concentrations around Fe15Ni85 for the development of a microfluidic small pressure sensitive elements based on GMI effect. The maximum value of GMI ratio (ΔZ/Z) ratio shows linear dependence on the iron content in the FexNi100-x alloy for the concentration range under consideration. © 2019 Trans Tech Publications Ltd, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.826.11
  • Gilbert damping in NiFeGd compounds: Ferromagnetic resonance versus time-resolved spectroscopy
    Salikhov, R. and Alekhin, A. and Parpiiev, T. and Pezeril, T. and Makarov, D. and Abrudan, R. and Meckenstock, R. and Radu, F. and Farle, M. and Zabel, H. and Temnov, V.V.
    Physical Review B 99 (2019)
    Engineering the magnetic properties (Gilbert damping, saturation magnetization, exchange stiffness, and magnetic anisotropy) of multicomponent magnetic compounds plays a key role in fundamental magnetism and its applications. Here, we perform a systematic study of (Ni81Fe19)100-xGdx films with x=0%, 5%, 9%, and 13% using ferromagnetic resonance (FMR), element-specific time-resolved x-ray magnetic resonance, and femtosecond time-resolved magneto-optical pump-probe techniques. The comparative analysis of field and time domain FMR methods, with complimentary information extracted from the dynamics of high-frequency exchange magnons in ferromagnetic thin films, is used to investigate the dependence of Gilbert damping on the Gd concentration. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.104412
  • Long-term stability and thickness dependence of magnetism in thin (Cr 0.5 Mn 0.5 ) 2 GaC MAX phase films
    Novoselova, I.P. and Petruhins, A. and Wiedwald, U. and Weller, D. and Rosen, J. and Farle, M. and Salikhov, R.
    Materials Research Letters 7 (2019)
    The thickness dependence and long-term stability of the magnetic properties of epitaxial (Cr 0.5 Mn 0.5 ) 2 GaC MAX phase films on MgO (111) were investigated. For 12.5- to 156-nm-thick films, which corresponds to 10–125 c-axis unit cells, samples were found to be phase pure with negligible c-axis lattice strain of less than 10 −4 nm even for the thinnest films. No influence of the interface layers on the magnetic anisotropy, the magnetization or the para- to ferromagnetic phase transition was observed. All samples remained stable for more than one year in ambient conditions. © 2019, © 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/21663831.2019.1570980
  • Magnetic anisotropy of single-crystal antiperovskite Mn3GaC studied by ferromagnetic resonance and dynamic magnetic-response simulations
    Scheibel, F. and Zingsem, B. and Feggeler, T. and Meckenstock, R. and Spoddig, D. and Farle, M. and Acet, M.
    Physical Review Materials 3 (2019)
    The Mn-based antiperovskite Mn3GaC is an interesting material for magnetocaloric cooling applications below room temperature. To optimize its use, knowledge of the anisotropic magnetic properties is required. Here, we use a single crystal to study the magnetic anisotropy by the ferromagnetic resonance technique and magnetic-response simulations. The anisotropy constant K4=-5.49kJ/m3 obtained in combination from experiment and simulations is about an order of magnitude smaller than that of bcc Fe. The magnetically soft nature of this material practically in all crystallographic directions is favorable for using it in polycrystalline form for magnetic refrigeration. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.054403
  • Narrow transitional hysteresis in (Mn-Cr-Co)2Sb pnictides for room-temperature magnetic refrigeration
    Tekgül, A. and Acet, M. and Scheibel, F. and Farle, M.
    Journal of Physics D: Applied Physics 52 (2019)
    Mn1.96-xCrxCo0.04Sb with x = 0.05 and 0.09 has been investigated for its magnetocaloric properties using structural, magnetization and calorimetric methods. These compounds exhibit an antiferromagnetic-ferrimagnetic transition of which the temperature can be adjusted by the Cr concentration. The transition temperature is at room temperature for x = 0.09. Magnetocaloric properties are presented as entropy-change and direct temperature-change. We show that these compounds have a narrow transitional hysteresis and exhibit the inverse magnetocaloric effect. We find a nearly hysteresis-free transition with a 2.6 K temperature-change around 287 K making it attractive for magnetic-cooling. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/ab026e
  • Shell-ferromagnetism and decomposition in off-stoichiometric Ni 50 Mn 50-x Sb x Heuslers
    Wanjiku, Z. and Çaklr, A. and Scheibel, F. and Wiedwald, U. and Farle, M. and Acet, M.
    Journal of Applied Physics 125 (2019)
    Off stoichiometric Heuslers in the form Ni 50Mn 50 - x Z x, where Z can be a group 13-15 element of the periodic system, decompose at about 650 K into a ferromagnetic full Heusler Ni 50Mn 25 Z 25 and an antiferromagnetic Ni 50Mn 50 component. We study here the case for Z as Sb and report on shell-ferromagnetic properties as well as thermal instabilities. Unlike the case for other Z-elements, in Ni 50Mn 50 - xSb x, the minimum decomposition temperature corresponds to a temperature lying within the austenite state so that it is possible to observe the change in the martensitic transition temperature while annealing, thus providing further information on the change of composition during annealing. Scherrer analysis performed on emerging peaks related to the cubic full-Heusler shows that the precipitate size for shell-FM properties to become observable is around 5-10 nm. Other than vertical shifts in the field-dependence of the magnetization, which are also observed in compounds with Z other than Sb, concurrent exchange-bias effects are observed in the case with Z as Sb. © 2019 Author(s).
    view abstract10.1063/1.5057763
  • Shell-ferromagnetism and decomposition in off-stoichiometric Ni50Mn50-xSbx Heuslers
    Wanjiku, Z. and Çaklr, A. and Scheibel, F. and Wiedwald, U. and Farle, M. and Acet, M.
    Journal of Applied Physics 125 (2019)
    Off stoichiometric Heuslers in the form Ni 50Mn 50 - x Z x, where Z can be a group 13-15 element of the periodic system, decompose at about 650 K into a ferromagnetic full Heusler Ni 50Mn 25 Z 25 and an antiferromagnetic Ni 50Mn 50 component. We study here the case for Z as Sb and report on shell-ferromagnetic properties as well as thermal instabilities. Unlike the case for other Z-elements, in Ni 50Mn 50 - xSb x, the minimum decomposition temperature corresponds to a temperature lying within the austenite state so that it is possible to observe the change in the martensitic transition temperature while annealing, thus providing further information on the change of composition during annealing. Scherrer analysis performed on emerging peaks related to the cubic full-Heusler shows that the precipitate size for shell-FM properties to become observable is around 5-10 nm. Other than vertical shifts in the field-dependence of the magnetization, which are also observed in compounds with Z other than Sb, concurrent exchange-bias effects are observed in the case with Z as Sb. © 2019 Author(s).
    view abstract10.1063/1.5057763
  • Unusual nature of confined modes in a chiral system: Directional transport in standing waves
    Zingsem, B.W. and Farle, M. and Stamps, R.L. and Camley, R.E.
    Physical Review B 99 (2019)
    Nonreciprocity of spin-wave propagation is a well-known consequence of antisymmetric exchange contributions in magnetic spin systems that lack inversion symmetry. In this case, the energy of a state depends on the sign of its momentum as Latin small letter h with strokeω(k)≠Latin small letter h with strokeω(-k). We discuss here the consequences of this nonreciprocity on counterpropagating traveling spin-wave states. In a confined geometry we find states with well-defined nodes which are inherently phase modulated such that space-inversion symmetry of the mode profile is lost. This entails that additional spectral features become visible in ferromagnetic resonance studies of microelements with Dzyaloshinskii-Moriya interaction (DMI), allowing a quantification of the amplitude and direction of the DMI. Moreover, this interference between nonreciprocal modes forms the basis for a generalized concept of mode confinement. ©2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.214429
  • A variable pole magnet
    Çakir, A. and Krenke, T. and Farle, M. and Acet, M.
    Journal of Physics Condensed Matter 30 (2018)
    The off-stoichiometric antiferromagnetic Heusler alloy Fe50Mn45Ga5 decomposes and forms ferromagnetic Fe50Mn25Ga25 precipitates embedded in an antiferromagnetic Fe50Mn50 matrix when temper-annealed at temperatures T &gt; 550 K. The ferromagnetism of the precipitates is soft so that the magnetization direction of the non-interacting precipitates in a macroscopic material can be manipulated by locally applied fields so that even two similar poles can form at the ends of a centimeter-long bar. The cause for the soft magnetic behavior is due to the weak AF exchange anisotropy of the cubic Fe50Mn50 matrix and the precipitate. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aaa5f5
  • An effective non-enzymatic biosensor platform based on copper nanoparticles decorated by sputtering on CVD graphene
    Soganci, T. and Ayranci, R. and Harputlu, E. and Ocakoglu, K. and Acet, M. and Farle, M. and Unlu, C.G. and Ak, M.
    Sensors and Actuators, B: Chemical 273 (2018)
    It has become inevitable to design non-enzymatic biosensors to eliminate the drawbacks of enzymatic biosensors prepared using enzymes which are expensive and without long-term stability. For this purpose, a single layer graphene film was prepared by chemical vapor deposition method on Cu foil and transferred to the FTO glass slide. After that copper nanoparticles (CuNP) were decorated by the inert-gas condensation method based on DC magnetron sputtering on it. The prepared CuNP decorated graphene film was characterized and used as a non-enzymatic sensor platform for the detection of glucose. The sensor platform exhibited a fast response time of less than 4 s and the sensitivity of 430.52 μA mM−1 cm-2 with linear concentration range (0.01–1.0 mM) having detection limit 7.2 μM. Electrochemical investigations indicate that the sensor platform which is decorated CuNP graphene film possess an excellent performance toward glucose. Prepared biosensors platform could be used and applied in the field of new drug discovery, biomedical, clinical diagnosis and forensic science to miniaturize of detection instrument and reduce detection sample and period. © 2018 Elsevier B.V.
    view abstract10.1016/j.snb.2018.07.064
  • Annealing-time and annealing-temperature dependencies of the size of Ni-Mn-In shell-ferromagnetic nano-precipitates by Scherrer analysis
    Dincklage, L. and Scheibel, F. and Çaklr, A. and Farle, M. and Acet, M.
    AIP Advances 8 (2018)
    Shell-ferromagnetic effects are observed in Ni-Mn-based off-stoichiometric Heuslers decomposed into ferromagnetic precipitates embedded in an antiferromagnetic matrix when the surface-to-volume ratio of the precipitates are sufficiently large. However, since the size of the precipitates have until now not been determined, it is not known which ratios are involved. Here we carry out a Scherrer analysis on decomposed specimens to determine the precipitate-size as a function of decomposition temperature and time. © 2018 Author(s).
    view abstract10.1063/1.5018851
  • Coherently strained [Fe-Co(C)/Au-Cu]n multilayers: A path to induce magnetic anisotropy in Fe-Co films over large thicknesses
    Giannopoulos, G. and Salikhov, R. and Varvaro, G. and Psycharis, V. and Testa, A.M. and Farle, M. and Niarchos, D.
    Journal of Physics D: Applied Physics 51 (2018)
    Among novel critical-element-free materials for permanent magnets, the nearly equiatomic Fe-Co alloy has recently attracted a great deal of attention as a large magneto-crystalline anisotropy can be induced by straining the Fe-Co unit cell. In thin film systems, the use of a suitable underlayer allows a tetragonal reconstruction of the Fe-Co to be triggered up to a critical thickness of few nanometers, above which the crystal structure relaxes to the magnetically soft cubic phase. Scaling-up the thickness of the metastable tetragonal Fe-Co phase is of crucial significance for different nanoscale applications, such as magnetic micro- and nano-electromechanical systems. To suppress the strain relaxation occurring at high thicknesses, we explored a novel approach based on Fe-Co(C)/Au-Cu multilayer films, where both Au-Cu interlayers and carbon (C) doping were used to stabilize the strained Fe-Co tetragonal phase over large thicknesses. Both doped and un-doped multilayer structures show a coherently strained regime, persisting up to a thickness of 60 nm, which leads, possibly in combination with the surface anisotropy induced at the Au-Cu interfaces, to the appearance of a large out-of-plane anisotropy (up to 0.4 MJ m-3), thus suggesting the potential of such an approach to develop critical-element-free thin film permanent magnets for a variety of nanoscale applications. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aaa41c
  • Formation of nickel nanoparticles and magnetic matrix in nickel phthalocyanine by doping with potassium
    Manukyan, A.S. and Avakyan, L.A. and Elsukova, A.E. and Zubavichus, Y.V. and Sulyanov, S.N. and Mirzakhanyan, A.A. and Kolpacheva, N.A. and Spasova, M. and Kocharian, A.N. and Farle, M. and Bugaev, L.A. and Sharoyan, E.G.
    Materials Chemistry and Physics 214 (2018)
    A method for synthesis of nickel nanoparticles in a magnetic nickel phthalocyanine anions matrix has been developed. The method is based on intercalation of potassium atoms to the nickel phthalocyanine (NiPc) polycrystalline powder at 300 °C. The structure of (K2NiPc) was investigated by using high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopes. Magnetic properties were studied by SQUID magnetometry and magnetic resonances methods. It is revealed that the resultant compound contains of 1 wt% Ni nanoparticles with the average size of 15 nm. The measured values of the magnetization and absorption of the ferromagnetic resonance considerably exceed the magnetism which can be attributed to metallic Ni nanoparticles. The obtained results indicate the presence of room temperature molecular ferromagnetism caused by anionic molecules of NiPc. © 2018 Elsevier B.V.
    view abstract10.1016/j.matchemphys.2018.04.068
  • Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications
    Scheibel, F. and Gottschall, T. and Taubel, A. and Fries, M. and Skokov, K.P. and Terwey, A. and Keune, W. and Ollefs, K. and Wende, H. and Farle, M. and Acet, M. and Gutfleisch, O. and Gruner, M.E.
    Energy Technology 6 (2018)
    Magnetic refrigeration relies on a substantial entropy change in a magnetocaloric material when a magnetic field is applied. Such entropy changes are present at first-order magnetostructural transitions around a specific temperature at which the applied magnetic field induces a magnetostructural phase transition and causes a conventional or inverse magnetocaloric effect (MCE). First-order magnetostructural transitions show large effects, but involve transitional hysteresis, which is a loss source that hinders the reversibility of the adiabatic temperature change ΔTad. However, reversibility is required for the efficient operation of the heat pump. Thus, it is the mastering of that hysteresis that is the key challenge to advance magnetocaloric materials. We review the origin of the large MCE and of the hysteresis in the most promising first-order magnetocaloric materials such as Ni–Mn-based Heusler alloys, FeRh, La(FeSi)13-based compounds, Mn3GaC antiperovskites, and Fe2P compounds. We discuss the microscopic contributions of the entropy change, the magnetic interactions, the effect of hysteresis on the reversible MCE, and the size- and time-dependence of the MCE at magnetostructural transitions. © 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/ente.201800264
  • L10-FeNi films on Au-Cu-Ni buffer-layer: a high-throughput combinatorial study
    Giannopoulos, G. and Barucca, G. and Kaidatzis, A. and Psycharis, V. and Salikhov, R. and Farle, M. and Koutsouflakis, E. and Niarchos, D. and Mehta, A. and Scuderi, M. and Nicotra, G. and Spinella, C. and Laureti, S. and Varvaro, G.
    Scientific Reports 8 (2018)
    The fct L10-FeNi alloy is a promising candidate for the development of high performance critical-elements-free magnetic materials. Among the different materials, the Au-Cu-Ni alloy has resulted very promising; however, a detailed investigation of the effect of the buffer-layer composition on the formation of the hard FeNi phase is still missing. To accelerate the search of the best Au-Cu-Ni composition, a combinatorial approach based on High-Throughput (HT) experimental methods has been exploited in this paper. HT magnetic characterization methods revealed the presence of a hard magnetic phase with an out-of-plane easy-axis, whose coercivity increases from 0.49 kOe up to 1.30 kOe as the Au content of the Cu-Au-Ni buffer-layer decreases. Similarly, the out-of-plane magneto-crystalline anisotropy energy density increases from 0.12 to 0.35 MJ/m3. This anisotropy is attributed to the partial formation of the L10 FeNi phase induced by the buffer-layer. In the range of compositions we investigated, the buffer-layer structure does not change significantly and the modulation of the magnetic properties with the Au content in the combinatorial layer is mainly related to the different nature and extent of interlayer diffusion processes, which have a great impact on the formation and order degree of the L10 FeNi phase. © 2018, The Author(s).
    view abstract10.1038/s41598-018-34296-9
  • Large uniaxial magnetostriction with sign inversion at the first order phase transition in the nanolaminated Mn2GaC MAX phase
    Novoselova, I.P. and Petruhins, A. and Wiedwald, U. and Ingason, Á.S. and Hase, T. and Magnus, F. and Kapaklis, V. and Palisaitis, J. and Spasova, M. and Farle, M. and Rosen, J. and Salikhov, R.
    Scientific Reports 8 (2018)
    In 2013, a new class of inherently nanolaminated magnetic materials, the so called magnetic MAX phases, was discovered. Following predictive material stability calculations, the hexagonal Mn2GaC compound was synthesized as hetero-epitaxial films containing Mn as the exclusive M-element. Recent theoretical and experimental studies suggested a high magnetic ordering temperature and non-collinear antiferromagnetic (AFM) spin states as a result of competitive ferromagnetic and antiferromagnetic exchange interactions. In order to assess the potential for practical applications of Mn2GaC, we have studied the temperature-dependent magnetization, and the magnetoresistive, magnetostrictive as well as magnetocaloric properties of the compound. The material exhibits two magnetic phase transitions. The Néel temperature is T N ∼ 507 K, at which the system changes from a collinear AFM state to the paramagnetic state. At T t = 214 K the material undergoes a first order magnetic phase transition from AFM at higher temperature to a non-collinear AFM spin structure. Both states show large uniaxial c-axis magnetostriction of 450 ppm. Remarkably, the magnetostriction changes sign, being compressive (negative) above T t and tensile (positive) below the T t . The sign change of the magnetostriction is accompanied by a sign change in the magnetoresistance indicating a coupling among the spin, lattice and electrical transport properties. © 2018 The Author(s).
    view abstract10.1038/s41598-018-20903-2
  • Magnetic properties and structural characterization of layered (Cr 0.5 Mn 0.5 ) 2 AuC synthesized by thermally induced substitutional reaction in (Cr 0.5 Mn 0.5 ) 2 GaC
    Lai, C.-C. and Tao, Q. and Fashandi, H. and Wiedwald, U. and Salikhov, R. and Farle, M. and Petruhins, A. and Lu, J. and Hultman, L. and Eklund, P. and Rosen, J.
    APL Materials 6 (2018)
    The magnetic properties of the new phase (Cr 0.5 Mn 0.5 ) 2 AuC are compared to the known MAX-phase (Cr 0.5 Mn 0.5 ) 2 GaC, where the former was synthesized by thermally induced substitution reaction of Au for Ga in (Cr 0.5 Mn 0.5 ) 2 GaC. The reaction introduced a lattice expansion of ~3% along the c-axis, an enhancement of the coercive field from 30 mT to 140 mT, and a reduction of the Curie temperature and the saturation magnetization. Still, (Cr 0.5 Mn 0.5 ) 2 AuC displays similar features in the magnetic field- and temperature-dependent magnetization curves as previously reported magnetic MAX phases, e.g., (Cr 0.5 Mn 0.5 ) 2 GaC and (Mo 0.5 Mn 0.5 ) 2 GaC. The work suggests a pathway for tuning the magnetic properties of MAX phases. © 2018 Author(s).
    view abstract10.1063/1.5006304
  • Magnetic properties and structural characterization of layered (Cr0.5Mn0.5)2AuC synthesized by thermally induced substitutional reaction in (Cr0.5Mn0.5)2GaC
    Lai, C.-C. and Tao, Q. and Fashandi, H. and Wiedwald, U. and Salikhov, R. and Farle, M. and Petruhins, A. and Lu, J. and Hultman, L. and Eklund, P. and Rosen, J.
    APL Materials 6 (2018)
    The magnetic properties of the new phase (Cr0.5Mn0.5)2AuC are compared to the known MAX-phase (Cr0.5Mn0.5)2GaC, where the former was synthesized by thermally induced substitution reaction of Au for Ga in (Cr0.5Mn0.5)2GaC. The reaction introduced a lattice expansion of ~3% along the c-axis, an enhancement of the coercive field from 30 mT to 140 mT, and a reduction of the Curie temperature and the saturation magnetization. Still, (Cr0.5Mn0.5)2AuC displays similar features in the magnetic field- and temperature-dependent magnetization curves as previously reported magnetic MAX phases, e.g., (Cr0.5Mn0.5)2GaC and (Mo0.5Mn0.5)2GaC. The work suggests a pathway for tuning the magnetic properties of MAX phases. © 2018 Author(s).
    view abstract10.1063/1.5006304
  • Magnetite-Gold nanohybrids as ideal all-in-one platforms for theranostics
    Efremova, M.V. and Naumenko, V.A. and Spasova, M. and Garanina, A.S. and Abakumov, M.A. and Blokhina, A.D. and Melnikov, P.A. and Prelovskaya, A.O. and Heidelmann, M. and Li, Z.-A. and Ma, Z. and Shchetinin, I.V. and Golovin, Y.I. and Kireev, I.I. and Savchenko, A.G. and Chekhonin, V.P. and Klyachko, N.L. and Farle, M. and Majouga, A.G. and Wiedwald, U.
    Scientific Reports 8 (2018)
    High-quality, 25 nm octahedral-shaped Fe3O4 magnetite nanocrystals are epitaxially grown on 9 nm Au seed nanoparticles using a modified wet-chemical synthesis. These Fe3O4-Au Janus nanoparticles exhibit bulk-like magnetic properties. Due to their high magnetization and octahedral shape, the hybrids show superior in vitro and in vivo T2 relaxivity for magnetic resonance imaging as compared to other types of Fe3O4-Au hybrids and commercial contrast agents. The nanoparticles provide two functional surfaces for theranostic applications. For the first time, Fe3O4-Au hybrids are conjugated with two fluorescent dyes or the combination of drug and dye allowing the simultaneous tracking of the nanoparticle vehicle and the drug cargo in vitro and in vivo. The delivery to tumors and payload release are demonstrated in real time by intravital microscopy. Replacing the dyes by cell-specific molecules and drugs makes the Fe3O4-Au hybrids a unique all-in-one platform for theranostics. © 2018, The Author(s).
    view abstract10.1038/s41598-018-29618-w
  • Shaping iron oxide nanocrystals for magnetic separation applications
    Testa-Anta, M. and Liébana-Viñas, S. and Rivas-Murias, B. and Rodríguez González, B. and Farle, M. and Salgueiriño, V.
    Nanoscale 10 (2018)
    Iron oxide nanostructures are attractive for a variety of bio-related applications given their wide range of magnetic properties. Here, we report on the study of the magnetophoretic mobility of octapod-shaped nanocrystals, which we relate to stoichiometry, quality and elongation in the 〈111〉 direction of these cubic structures. This special morphology combines magnetocrystalline anisotropies, increases shape anisotropy and hinders the formation of an epitaxial wüstite-magnetite interface. As a result, one obtains nanocrystals with large magnetic susceptibility and small coercivity, that is, with optimum characteristics for magnetic guidance, separation, and drug delivery, due to the increased magnetophoretic mobility displayed. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8nr05864d
  • Size-selected Fe3O4-Au hybrid nanoparticles for improved magnetism-based theranostics
    Efremova, M.V. and Nalench, Y.A. and Myrovali, E. and Garanina, A.S. and Grebennikov, I.S. and Gifer, P.K. and Abakumov, M.A. and Spasova, M. and Angelakeris, M. and Savchenko, A.G. and Farle, M. and Klyachko, N.L. and Majouga, A.G. and Wiedwald, U.
    Beilstein Journal of Nanotechnology 9 (2018)
    Size-selected Fe3O4-Au hybrid nanoparticles with diameters of 6-44 nm (Fe3O4) and 3-11 nm (Au) were prepared by high temperature, wet chemical synthesis. High-quality Fe3O4 nanocrystals with bulk-like magnetic behavior were obtained as confirmed by the presence of the Verwey transition. The 25 nm diameter Fe3O4-Au hybrid nanomaterial sample (in aqueous and agarose phantom systems) showed the best characteristics for application as contrast agents in magnetic resonance imaging and for local heating using magnetic particle hyperthermia. Due to the octahedral shape and the large saturation magnetization of the magnetite particles, we obtained an extraordinarily high r2-relaxivity of 495 mM-1·s-1 along with a specific loss power of 617 W·gFe-1 and 327 W·gFe-1 for hyperthermia in aqueous and agarose systems, respectively. The functional in vitro hyperthermia test for the 4T1 mouse breast cancer cell line demonstrated 80% and 100% cell death for immediate exposure and after precultivation of the cells for 6 h with 25 nm Fe3O4-Au hybrid nanomaterials, respectively. This confirms that the improved magnetic properties of the bifunctional particles present a next step in magnetic-particle-based theranostics. © 2018 Efremova et al.
    view abstract10.3762/bjnano.9.251
  • Spin-Current Detection via an Interfacial Molecular Paramagnet
    Marzi, T. and Meckenstock, R. and Masur, S. and Farle, M.
    Physical Review Applied 10 (2018)
    Pure spin currents are considered as key processes for future low-dissipation electronics. Current detection schemes use the inverse spin Hall effect for converting the spin current into an electrical voltage. Here we present an alternative, contact-free detection scheme based on a paramagnetic molecular probe layer that in principle allows the integration of spin currents into molecular electronics. In a measurement without the requirement of lithographically tailored samples and electrical contacts, we detect changes of the electron spin resonance in the probe layer when a spin current is injected from a microwave-driven ferromagnetic layer. Tuning the microwave resonance conditions of the ferromagnetic and molecular layer to the same resonance condition, we observe the flow of spin momentum at the interface through the change of the microwave-power-dependent absorption of the adsorbed molecular layer. We use oleic acid as the molecular probe layer on metallic and oxidized iron films. © 2018 American Physical Society.
    view abstract10.1103/PhysRevApplied.10.054002
  • Structural, magnetic and electrical transport properties of non-conventionally prepared MAX phases V2AlC and (V/Mn)2AlC
    Hamm, C.M. and Dürrschnabel, M. and Molina-Luna, L. and Salikhov, R. and Spoddig, D. and Farle, M. and Wiedwald, U. and Birkel, C.S.
    Materials Chemistry Frontiers 2 (2018)
    A plethora of magnetic ground states along with intriguing magnetic properties have been reported in thin films of Mn-containing MAX phases. However, fewer results and therefore less knowledge in the area of bulk magnetic MAX phases exist resulting in many open research questions that still remain unanswered. Synthesis of high quality materials is key and is here achieved for bulk V2AlC and its Mn-doped analogs by means of microwave heating and spark plasma sintering. The obtained materials are carefully characterized by structural and microstructural investigations resulting in an average Mn-content of 2% corresponding to the mean chemical composition of (V0.960.02Mn0.040.02)2AlC in the Mn-doped V2AlC samples. While the parent MAX phase as well as the sample with the nominally lowest Mn-content are obtained essentially single-phase, samples with higher Mn-levels exhibit Mn-rich side phases. These are most likely responsible for the ferromagnetic behavior of the corresponding bulk materials. Besides, we show Pauli paramagnetism of the parent compound V2AlC and a combination of Pauli and Langevin paramagnetism in (V0.960.02Mn0.040.02)2AlC. For the latter, a magnetic moment of mM = 0.2(2) mB per M atom can be extracted. © 2018 Royal Society of Chemistry. All rights reserved.
    view abstract10.1039/c7qm00488e
  • Structure and size dependence of the magnetic properties of Ni@C nanocomposites
    Manukyan, A. and Elsukova, A. and Mirzakhanyan, A. and Gyulasaryan, H. and Kocharian, A. and Sulyanov, S. and Spasova, M. and Römer, F. and Farle, M. and Sharoyan, E.
    Journal of Magnetism and Magnetic Materials 467 (2018)
    Carbon-coated nickel (Ni) nanoparticles, Ni@C nanocomposites, have been synthesized using solid-state pyrolysis of nickel phthalocyanine and metal-free phthalocyanine (NiPc)x· (H2Pc)1−x solid solutions, 0⩽x⩽1. The Ni concentrations in carbon matrix (cNi) of the prepared samples continuously varied in the range of 0–3at.% (0–12wt.%). The average nanoparticle size varied within 4–40 nm range. All samples containing single domain Ni nanoparticles exhibit both ferromagnetic and superparamagnetic properties because of the wide range of size distribution. An abrupt drop of saturation magnetization has been observed with decrease in size of Ni nanoparticles from 40 nm to 12 nm. Nearly linear dependence of saturation magnetization on the nanoparticle surface/volume ratio can be interpreted as a result of contact interaction between Ni nanoparticles and the carbon matrix which provides an electron transfer from carbon matrix to nickel. However, further reductions in nanoparticle size increase magnetization growth of which can apparently contribute to the emergence of the giant paramagnetism due to large orbital moments of conductive electrons. The size effects and surface magnetic anisotropy in Ni@C nanocomposites are revealed in the measurements of coercive field, zero-field cooling (ZFC) susceptibility, blocking temperatures and ferromagnetic resonance spectra. Concentration dependencies of ferromagnetic and electron paramagnetic resonance parameters in Ni@C nanocomposites have also been investigated and their peculiarities highlighted. A correlation between concentration dependencies of FMR and SQUID magnetometry parameters, namely between the g-factor curves – geff, the resonance linewidth – ΔHFMR and coercive field – Hc, have been observed. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmmm.2018.07.056
  • Control of morphology and formation of highly geometrically confined magnetic skyrmions
    Jin, C. and Li, Z.-A. and Kovács, A. and Caron, J. and Zheng, F. and Rybakov, F.N. and Kiselev, N.S. and Du, H. and Blügel, S. and Tian, M. and Zhang, Y. and Farle, M. and Dunin-Borkowski, R.E.
    Nature Communications 8 (2017)
    The ability to controllably manipulate magnetic skyrmions, small magnetic whirls with particle-like properties, in nanostructured elements is a prerequisite for incorporating them into spintronic devices. Here, we use state-of-the-art electron holographic imaging to directly visualize the morphology and nucleation of magnetic skyrmions in a wedge-shaped FeGe nanostripe that has a width in the range of 45-150 nm. We find that geometrically-confined skyrmions are able to adopt a wide range of sizes and ellipticities in a nanostripe that are absent in both thin films and bulk materials and can be created from a helical magnetic state with a distorted edge twist in a simple and efficient manner. We perform a theoretical analysis based on a three-dimensional general model of isotropic chiral magnets to confirm our experimental results. The flexibility and ease of formation of geometrically confined magnetic skyrmions may help to optimize the design of skyrmion-based memory devices. © The Author(s) 2017.
    view abstract10.1038/ncomms15569
  • Controlling the conductivity of Ti3C2 MXenes by inductively coupled oxygen and hydrogen plasma treatment and humidity
    Römer, F.M. and Wiedwald, U. and Strusch, T. and Halim, J. and Mayerberger, E. and Barsoum, M.W. and Farle, M.
    RSC Advances 7 (2017)
    We report on the effects of plasma treatment and humidity on the electrical conductivities of Ti3C2 MXene thin films. The latter-spincoated from a colloidal solution produced by LiF/HCl etching of Ti3AlC2 powders-were 13 nm thick with an area of 6.8 mm2. The changes in the films exposed to hydrogen (H) and oxygen (O) plasmas in vacuum were analyzed by X-ray photoelectron spectroscopy. We find that the film resistivities can be switched reproducibly by plasma treatment between 5.6 μΩm (oxidized state) to 4.6 μΩm (reduced state). Both states show metallic like conductivity. In high vacuum, the film resistivity was 243 Ω; when the relative humidity was 80% the film resistance increased to 6340 Ω, a 26 fold increase. © The Royal Society of Chemistry.
    view abstract10.1039/c6ra27505b
  • Doubling of the magnetic energy product in ferromagnetic nanowires at ambient temperature by capping their tips with an antiferromagnet
    Wang, F.Z. and Salikhov, R. and Spasova, M. and Liébana-Viñas, S. and Bran, C. and Chen, Y.-S. and Vazquez, M. and Farle, M. and Wiedwald, U.
    Nanotechnology 28 (2017)
    We present an approach to prepare free-standing tips of micrometer-long nanowires electrodeposited in anodic aluminum oxide nanopores. Such open tips can be further processed, e.g. for vertical interconnects of functional layers or for tailoring the magnetization reversal of ferromagnetic nanowires. The magnetic switching of nanowires is usually initiated by vortex or domain formation at the nanowire tips. We show that coating the tips of Fe30Co70 nanowires (diameter 40 nm, length 16 μm) with thin antiferromagnetic Fe50Mn50 capping layers (thickness ≈10 nm) leads to magnetic hardening with a more than doubled energy product at ambient temperature. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aa77b7
  • Dynamics of nonergodic ferromagnetic/antiferromagnetic ordering and magnetocalorics in antiperovskite Mn3SnC
    Çaklr, Ö. and Cugini, F. and Solzi, M. and Priolkar, K. and Acet, M. and Farle, M.
    Physical Review B 96 (2017)
    We investigated the time dependence of the magnetic configuration at the mixed magnetic magnetostructural transition in Mn3SnC. The nonergodic nature of the transition involves the stabilization of a final magnetic configuration that involves additional AF ordering which is not present when the transition is initiated and develops only in time. We show the presence of the nonergodicity over a time scale of about 1 hour by field and time-dependent magnetization studies. Two characteristic times related to the transition are observed. We also study the equilibrium thermodynamics under ergodic conditions by heat capacity studies and determine the entropy-change and the adiabatic temperature change around the transition. We find agreement between the indirect and direct methods in determining the adiabatic temperature change and discuss the influence of nonergodic properties on the magnetocaloric effects. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.014436
  • Enhanced spin-orbit coupling in tetragonally strained Fe-Co-B films
    Salikhov, R. and Reichel, L. and Zingsem, B. and Abrudan, R. and Edström, A. and Thonig, D. and Rusz, J. and Eriksson, O. and Schultz, L. and Fähler, S. and Farle, M. and Wiedwald, U.
    Journal of Physics Condensed Matter 29 (2017)
    Tetragonally strained interstitial Fe-Co-B alloys were synthesized as epitaxial films grown on a 20 nm thick Au0.55Cu0.45 buffer layer. Different ratios of the perpendicular to in-plane lattice constant c/a = 1.013, 1.034 and 1.02 were stabilized by adding interstitial boron with different concentrations 0, 4, and 10 at.%, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the total orbital magnetic moment significantly increases with increasing c/a ratio, indicating that reduced crystal symmetry and interstitial B leads to a noticeable enhancement of the effect of spin-orbit coupling (SOC) in the Fe-Co-B alloys. First-principles calculations reveal that the increase in orbital magnetic moment mainly originates from B impurities in octahedral position and the reduced symmetry around B atoms. These findings offer the possibility to enhance SOC phenomena - namely the magnetocrystalline anisotropy and the orbital moment - by stabilizing anisotropic strain by doping 4 at.% B. Results on the influence of B doping on the Fe-Co film microstructure, their coercive field and magnetic relaxation are also presented. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aa7498
  • Formation Mechanism of Laser-Synthesized Iron-Manganese Alloy Nanoparticles, Manganese Oxide Nanosheets and Nanofibers
    Zhang, D. and Ma, Z. and Spasova, M. and Yelsukova, A.E. and Lu, S. and Farle, M. and Wiedwald, U. and Gökce, B.
    Particle and Particle Systems Characterization 34 (2017)
    Laser ablation in liquids (LAL) has emerged as a versatile approach for the synthesis of alloy particles and oxide nanomaterials. However, complex chemical reactions often take place during synthesis due to inevitable atomization and ionization of the target materials and decomposition/hydrolysis of solvent/solution molecules, making it difficult to understand the particle formation mechanisms. In this paper, a possible route for the formation of FeMn alloy nanoparticles as well as MnOx nanoparticles, -sheets, and -fibers by LAL is presented. The observed structural, compositional, and morphological variations are clarified by transmission electron microscopy (TEM). The studies suggest that a reaction between Mn atoms and Fe ions followed by surface oxidation result in nonstoichiometric synthesis of Fe-rich FeMn@FeMn2O4 core-shell alloy particles. Interestingly, a phase transformation from Mn3O4 to Mn2O3 and finally to Ramsdellite γ-MnO2 is accompanied by a morphology change from nanosheets to nanofibers in gradually increasing oxidizing environments. High-resolution TEM images reveal that the particle-attachment mechanism dominates the growth of different manganese oxides. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ppsc.201600225
  • Imaging techniques: Nanoparticle atoms pinpointed
    Farle, M.
    Nature 542 (2017)
    The locations of atoms in a metallic alloy nanoparticle have been determined using a combination of electron microscopy and image simulation, revealing links between the particle's structure and magnetic properties.
    view abstract10.1038/542035a
  • Kinetic arrest of the ferrimagnetic state in indium-doped Mn1.82Co0.18Sb
    Tekgül, A. and Acet, M. and Farle, M. and Ünal, N.
    Journal of Alloys and Compounds 695 (2017)
    Substituting Mn with small amounts of Co in Mn2Sb leads to kinetic arrest and magnetic frustration effects. By adding small amounts of In in place of Sb, the tetragonality and thereby the kinetic arrest properties can be modified and controlled. We investigate the kinetic arrest effect in Mn1.82Co0.18Sb1−xInx with x = 0 and 0.5 at the first-order antiferromagnetic-ferrimagnetic magnetostructural transition. Kinetic arrest is observed in both systems, whereby the arrest is partial under 5 T field for Mn1.82Co0.18Sb and full for Mn1.82Co0.18Sb0.95In0.05. The FI state remains arrested as long as the cooling-field is maintained. © 2016 Elsevier B.V.
    view abstract10.1016/j.jallcom.2016.11.093
  • Low-Temperature Phase c-axis Oriented Manganese Bismuth Thin Films with High Anisotropy Grown from an Alloy Mn55Bi45 Target
    Sabet, S. and Hildebrandt, E. and Römer, F.M. and Radulov, I. and Zhang, H. and Farle, M. and Alff, L.
    IEEE Transactions on Magnetics 53 (2017)
    Manganese bismuth thin films were deposited from a Mn55Bi45 (at.%) alloy target onto glass substrates at room temperature using dc magnetron sputtering. The ferromagnetic low-temperature phase (LTP) of MnBi was formed through a subsequent vacuum annealing step. The resulting thin films were highly c-axis textured. Magnetic measurement shows a maximum saturation magnetization of 600 eμcm3 (0.60 MA/m). A magnetic uniaxial anisotropy energy density of \sim 1.86 {\cdot 10{7}} erg/cm3 (1.86 MJ/m3) was measured by torque magnetometry. The coercive field has a positive temperature coefficient and reaches 12 kOe (1.2 T) and 14 kOe (1.4 T) at 300 K for the out-of-plane and in-plane direction, respectively. Density functional theory calculations have confirmed that the magnetocrystalline anisotropy energy increases with increasing temperature as a result of a spin-reorientation occurring around 100 K. Growing LTP MnBi thin films directly from an alloy Mn55Bi45 target is an important step toward facilitating the synthesis of multilayers for spintronics or in an exchange spring magnet configuration. © 1965-2012 IEEE.
    view abstract10.1109/TMAG.2016.2636817
  • Magnetic properties of nanolaminated (Mo0.5Mn0.5)2GaC MAX phase
    Salikhov, R. and Meshkian, R. and Weller, D. and Zingsem, B. and Spoddig, D. and Lu, J. and Ingason, A.S. and Zhang, H. and Rosen, J. and Wiedwald, U. and Farle, M.
    Journal of Applied Physics 121 (2017)
    The magnetic properties of hexagonal (Mo0.5Mn0.5)2GaC MAX phase synthesized as epitaxial films on MgO (111) substrates with the c-axis perpendicular to the film plane are presented. The analysis of temperature-dependent ferromagnetic resonance (FMR) and magnetometry data reveals a ferro- to paramagnetic phase transition at 220 K. The electrical transport measurements at 5 K show a negative magnetoresistance of 6% in a magnetic field of 9 T. Further analysis confirms the spin-dependent scattering of charge carriers in this layered material. A small perpendicular (c-axis) magnetocrystalline anisotropy energy density (MAE) of 4.5 kJ/m3 at 100 K was found using FMR. Accordingly, (Mo0.5Mn0.5)2GaC behaves similar to the (Cr0.5Mn0.5)2GaC MAX phase as a soft magnetic material. The density functional theory calculations reveal that the sign and the amplitude of the MAE can be very sensitive to (Mo0.5Mn0.5)2GaC lattice parameters, which may explain the measured soft magnetic properties. © 2017 Author(s).
    view abstract10.1063/1.4982197
  • Magnetic Skyrmion Formation at Lattice Defects and Grain Boundaries Studied by Quantitative Off-Axis Electron Holography
    Li, Z.-A. and Zheng, F. and Tavabi, A.H. and Caron, J. and Jin, C. and Du, H. and Kovács, A. and Tian, M. and Farle, M. and Dunin-Borkowski, R.E.
    Nano Letters 17 (2017)
    We use in situ Lorentz microscopy and off-axis electron holography to investigate the formation and characteristics of skyrmion lattice defects and their relationship to the underlying crystallographic structure of a B20 FeGe thin film. We obtain experimental measurements of spin configurations at grain boundaries, which reveal inversions of crystallographic and magnetic chirality across adjacent grains, resulting in the formation of interface spin stripes at the grain boundaries. In the absence of material defects, we observe that skyrmions lattices possess dislocations and domain boundaries, in analogy to atomic crystals. Moreover, the distorted skyrmions can flexibly change their size and shape to accommodate local geometry, especially at sites of dislocations in the skyrmion lattice. Our findings provide a detailed understanding of the elasticity of topologically protected skyrmions and their correlation with underlying material defects. © 2017 American Chemical Society.
    view abstract10.1021/acs.nanolett.6b04280
  • Micromagnetic Simulations for Coercivity Improvement Through Nano-Structuring of Rare-Earth-Free L10-FeNi Magnets
    Kovacs, A. and Fischbacher, J. and Oezelt, H. and Schrefl, T. and Kaidatzis, A. and Salikhov, R. and Farle, M. and Giannopoulos, G. and Niarchos, D.
    IEEE Transactions on Magnetics 53 (2017)
    In this paper, we investigate the potential of tetragonal L10-ordered FeNi as the candidate phase for rare-earth-free permanent magnets considering anisotropy values from recently synthesized, partially ordered FeNi thin films. In particular, we estimate the maximum energy product (BH)max of L10-FeNi nanostructures using micromagnetic simulations. The maximum energy product is limited due to the small coercive field of partially ordered L10-FeNi. Nano-structured magnets consisting of 128 equi-axed, platelet-like, and columnar-shaped grains show a theoretical maximum energy product of 228, 208, and 252 kJm $^{-3}$ , respectively. © 1965-2012 IEEE.
    view abstract10.1109/TMAG.2017.2701418
  • Nanoparticle atoms pinpointed
    Farle, M.
    Nature 542 (2017)
    view abstract10.1038/542035a
  • Room-temperature five-tesla coercivity of a rare-earth-free shell-ferromagnet
    Scheibel, F. and Spoddig, D. and Meckenstock, R. and Gottschall, T. and Çakır, A. and Krenke, T. and Farle, M. and Gutfleisch, O. and Acet, M.
    Applied Physics Letters 110 (2017)
    Ni2MnX-based Heusler (X: main group element), when enriched with Mn, will decompose into ferromagnetic Ni2MnX and antiferromagnetic NiMn when temper-annealed around 650 K. When the starting material is chosen such that the X-composition is about 5 at. % and the annealing takes place in the presence of a magnetic field of about 1 T, the resulting material is a composite of nanoprecipitate strongly pinned shell-ferromagnets with a soft ferromagnetic core embedded in the antiferromagnetic matrix. We show that the shells of the precipitates are so strongly pinned that the estimated field required to fully reorient the spins is in the order of 20 T. We examine in a Ni50.0Mn45.1In4.9 sample the pinning and the magnetic interactions of the precipitate and the matrix with magnetization and ferromagnetic resonance studies carried out in fields ranging up to 14 and 12 T, respectively. © 2017 Author(s).
    view abstract10.1063/1.4983199
  • Shell-ferromagnetic precipitation in martensitic off-stoichiometric Ni-Mn-In Heusler alloys produced by temper-annealing under magnetic field
    Çakır, A. and Acet, M. and Wiedwald, U. and Krenke, T. and Farle, M.
    Acta Materialia 127 (2017)
    The variety of the multifunctional properties of martensitic Ni-Mn based Heusler alloys are related to the presence of a magnetostructural transition. We report here on a new functionality based on a newly observed property. The observed property is that all off-stoichiometric Ni-Mn-based Heuslers, here in the form Ni50Mn50−xInx with 0< x< 25, decompose into predominantly cubic ferromagnetic Ni50Mn25In25 and tetragonal antiferromagnetic NiMn components when temper-annealed. The new functionality is based on magnetic field assisted temper-annealing of a compound with stoichiometry x=5, whereby precipitates of Ni50Mn25In25 with a ferromagnetic shell are formed with spins in the field direction, strongly pinned by the surrounding antiferromagnetic anisotropy, even at temperatures as high as 500 K. The remanent pinning at high temperatures survives any thermal cycling between lowest temperatures and the annealing temperature and any magnetic field cycling between −9 and +9 T. The resulting product can serve as a thermally stable, magnetic-field-proof memory. © 2017
    view abstract10.1016/j.actamat.2017.01.027
  • Shell-ferromagnetism in a Ni-Mn-In off-stoichiometric Heusler studied by ferromagnetic resonance
    Scheibel, F. and Spoddig, D. and Meckenstock, R. and Çakir, A. and Farle, M. and Acet, M.
    AIP Advances 7 (2017)
    Next to the multifunctional properties of Ni-Mn-based Heusler alloys new functionalities related to shell-ferromagnetism are emerging. To understand in more detail the properties of shell-ferromagnetism we examine a decomposed Ni50.0Mn45.1In4.9 off-stoichiometric compound using magnetic resonance techniques which provides details on magnetic interactions. We find that the ferromagnetic resonance profile of the shell-ferromagnetic state is symmetric for positive and negative fields and is independent of the direction of the field-sweep except for the hysteresis observed at small fields. © 2017 Author(s).
    view abstract10.1063/1.4976335
  • The combination of micro-resonators with spatially resolved ferromagnetic resonance
    Schaffers, T. and Meckenstock, R. and Spoddig, D. and Feggeler, T. and Ollefs, K. and Schöppner, C. and Bonetti, S. and Ohldag, H. and Farle, M. and Ney, A.
    Review of Scientific Instruments 88 (2017)
    We present two new and complementary approaches to realize spatial resolution for ferromagnetic resonance (FMR) on the 100 nm-scale. Both experimental setups utilize lithographically fabricated micro-resonators. They offer a detection sensitivity that is increased by four orders of magnitude compared with resonator-based FMR. In the first setup, the magnetic properties are thermally modulated via the thermal near-field effect generated by the thermal probe of an atomic force microscope. In combination with lock-in detection of the absorbed microwave power in the micro-resonator, a spatial resolution of less than 100 nm is achieved. The second setup is a combination of a micro-resonator with a scanning transmission x-ray microscope (STXM). Here a conventional FMR is excited by the micro-resonator while focused x-rays are used for a time-resolved snap-shot detection of the FMR excitations via the x-ray magnetic circular dichroism effect. This technique allows a lateral resolution of nominally 35 nm given by the STXM. Both experimental setups combine the advantage of low-power FMR excitation in the linear regime with high spatial resolution to study single and coupled nanomagnets. As proof-of-principle experiments, two perpendicular magnetic micro-stripes (5 μm × 1 μm) were grown and their FMR excitations were investigated using both setups. © 2017 Author(s).
    view abstract10.1063/1.4996780
  • The FMR Behaviour of Li–Ni Ferrite Prepared by Hydrothermal Method
    Al-Shakarchi, E.K. and Lafta, S.H. and Musa, A. and Farle, M. and Salikhov, R.
    Journal of Superconductivity and Novel Magnetism 30 (2017)
    Magnetic hysteresis loop and ferromagnetic resonance (FMR) behaviour had been studied by considering the variation of the molar ratio of Li0.5−0.5xNixFe2.5−0.5xO4 ferrite nanostructure prepared by the hydrothermal method. The parameters of the ferrite nanostructure include coercivity (Hc), saturation magnetisation (Ms) and magnetic susceptibility. There was low coercivity at x = 0.3, high saturation magnetisation and high susceptibility (χi) at (x = 0.5). These parameters showed an improvement in their values compared with the samples prepared by conventional methods. The FMR analysis indicated that these samples were lossy materials due to having a broad line width ranging from 800 to 925 G. The FMR lines had asymmetric shapes due to anisotropic broadening and conduction mechanism. © 2017 Springer Science+Business Media New York
    view abstract10.1007/s10948-017-4058-9
  • The Production of Cu Nanoparticles on Large Area Graphene by Sputtering and in-Flight Sintering
    Ünlü, C.G. and Acet, M. and Tekgül, A. and Farle, M. and Atakan, Ş. and Lindner, J.
    Crystal Research and Technology 52 (2017)
    We have developed a simple method to synthesize Cu nanoparticles on graphene, which is a composite that is currently investigated for use as biosensors. Firstly, large area graphene (2 × 2 cm2) was prepared by chemical vapor deposition on Cu foils and then transferred onto SiO2 substrates by a transfer process. The Cu nanoparticles were collected on graphene/SiO2 by magnetron sputtering. The presence of graphene was verified by optical microscopy and Raman spectroscopy. The structure of graphene decorated with Cu nanoparticles was determined by scanning and transmission electron microscopy. The results show that the Cu nanoparticles acquire a cubic structure on graphene. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/crat.201700149
  • The reversibility of the inverse magnetocaloric effect in Mn2−xCrxSb0.95Ga0.05
    Tekgül, A. and Acet, M. and Scheibel, F. and Farle, M. and Ünal, N.
    Acta Materialia 124 (2017)
    Mn2−xCrxSb exhibits an antiferromagnetic-ferrimagnetic transition of which the temperature can be adjusted by the Cr concentration. The transition temperature is at room temperature for x = 0.13, but an additional ferromagnetic MnSb impurity phase is usually inevitable at this concentration. To suppress the occurrence of this phase unfavorable for the magnetocaloric effect, we partially replace Sb by Ga. We show that Mn2−xCrxSb0.95Ga0.05 alloys have a narrow transitional hysteresis and exhibit the inverse magnetocaloric effect. We present here results on structural, magnetic, and magnetocaloric properties using isothermal magnetization and direct adiabatic temperature-change measurements. We find in particular for the sample with x = 0.13 a nearly hysteresis-free transition with a 2 K temperature-change around room temperature making it attractive for magnetic cooling technology. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.10.072
  • Thermally induced substitutional reaction of Fe into Mo2GaC thin films
    Lai, C.-C. and Petruhins, A. and Lu, J. and Farle, M. and Hultman, L. and Eklund, P. and Rosen, J.
    Materials Research Letters (2017)
    The first Fe-based MAX phase is realized by solid-state substitution reaction of an Fe/Au/Mo2GaC thin-film diffusion couple, as determined by X-ray diffraction and scanning transmission electron microscopy. Chemical analysis together with elemental mapping reveals that as much as 50 at.% Fe on the A site can be obtained by thermally induced Au and Fe substitution for Ga atomic layers in Mo2GaC. One-sixth of the original Ga is also replaced by Au atoms. When annealing Mo2GaC thin films covered with Fe only, the Mo2GaC phase remains intact, that is, Au acts as a catalyst for the substitution reaction. IMPACT STATEMENT The first direct evidence showing Fe-containing MAX phase, Mo2AC, with Fe ∼50 at.% on the A sites is presented, synthesized by thermally induced Fe and Au substitution reaction catalyzed by Au. © 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group
    view abstract10.1080/21663831.2017.1343207
  • Unified description of collective magnetic excitations
    Zingsem, B.W. and Winklhofer, M. and Meckenstock, R. and Farle, M.
    Physical Review B 96 (2017)
    We derive the linearized ferromagnetic high-frequency susceptibility tensor in explicit form, which can be evaluated for arbitrary micromagnetic energy functionals and without imposing a priori symmetry constraints. For energy functionals defined spatially, our method can be used to describe the magnon dispersion. Our approach is also applicable to magnetic systems in a nonsaturated state and can thus be combined with micromagnetic simulations. To find the equilibrium states of the magnetization, we suggest a trajectory-dependent solver, which allows for fast fitting of experimental ferromagnetic resonance spectra. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.224407
  • A versatile large-scale and green process for synthesizing magnetic nanoparticles with tunable magnetic hyperthermia features
    Simeonidis, K. and Liébana-Viñas, S. and Wiedwald, U. and Ma, Z. and Li, Z.-A. and Spasova, M. and Patsia, O. and Myrovali, E. and Makridis, A. and Sakellari, D. and Tsiaoussis, I. and Vourlias, G. and Farle, M. and Angelakeris, M.
    RSC Advances 6 (2016)
    This work proposes a large-scale synthesis methodology for engineered and functional magnetic nanoparticles (i.e. ferrites, sulfides) designed towards the principles of green and sustainable production combined with biomedical applicability. The experimental setup consists of a two-stage continuous-flow reactor in which single-crystalline nanoparticles are formed by the coprecipitation of metal salts in an aqueous environment. A series of optimized iron-based nanocrystals (Fe3O4, Fe3S4, CoFe2O4 and MnFe2O4) with diameters between 18 and 38 nm has been obtained. The samples were validated as potential magnetic hyperthermia agents by their heating efficiency as determined by specific loss power (SLP) in calorimetric experiments. In an effort to enhance colloidal stability and surface functionality, nanoparticles were coated by typical molecules of biomedical interest in a single step process. Finally, two-phase particle systems have been produced by a two-stage procedure to enhance the heating rate by the effective combination of different magnetic features. Results indicate relatively high SLP values for uncoated nanoparticles (420 W g-1 for Fe3O4) and a reduction of 20-60% in the heat dissipation rate upon covering by functional groups. Eventually, such effect was more than counterbalanced by the magnetic coupling of different phases in binary systems, since SLP was multiplied up to ∼1700 W g-1 for MnFe2O4/Fe3O4 suggesting a novel route to tune the efficiency of magnetic hyperthermia agents. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c6ra09362k
  • Arrangement at the nanoscale: Effect on magnetic particle hyperthermia
    Myrovali, E. and Maniotis, N. and Makridis, A. and Terzopoulou, A. and Ntomprougkidis, V. and Simeonidis, K. and Sakellari, D. and Kalogirou, O. and Samaras, T. and Salikhov, R. and Spasova, M. and Farle, M. and Wiedwald, U. and Angelakeris, M.
    Scientific Reports 6 (2016)
    In this work, we present the arrangement of Fe 3 O 4 magnetic nanoparticles into 3D linear chains and its effect on magnetic particle hyperthermia efficiency. The alignment has been performed under a 40 mT magnetic field in an agarose gel matrix. Two different sizes of magnetite nanoparticles, 10 and 40 nm, have been examined, exhibiting room temperature superparamagnetic and ferromagnetic behavior, in terms of DC magnetic field, respectively. The chain formation is experimentally visualized by scanning electron microscopy images. A molecular Dynamics anisotropic diffusion model that outlines the role of intrinsic particle properties and inter-particle distances on dipolar interactions has been used to simulate the chain formation process. The anisotropic character of the aligned samples is also reflected to ferromagnetic resonance and static magnetometry measurements. Compared to the non-aligned samples, magnetically aligned ones present enhanced heating efficiency increasing specific loss power value by a factor of two. Dipolar interactions are responsible for the chain formation of controllable density and thickness inducing shape anisotropy, which in turn enhances magnetic particle hyperthermia efficiency. © The Author(s) 2016.
    view abstract10.1038/srep37934
  • Characterization of the oleic acid/iron oxide nanoparticle interface by magnetic resonance
    Masur, S. and Zingsem, B. and Marzi, T. and Meckenstock, R. and Farle, M.
    Journal of Magnetism and Magnetic Materials 415 (2016)
    The synthesis of colloidal nanoparticles involves surfactant molecules, which bind to the particle surface and stabilize nanoparticles against aggregation. In many cases these protecting shells also can be used for further functionalization. In this study, we investigated monodisperse single crystalline iron oxide core/shell nanoparticles (FexOy-NPs) in situ covered with an oleic acid layer which showed two electron spin resonance (ESR) signals. The nanoparticles with the ligands attached were characterized by transmission electron microscopy (TEM) and ferro- and paramagnetic resonance (FMR, EPR). Infrared spectroscopy confirmed the presence of the functional groups and revealed that the oleic acid (OA) is chemisorbed as a carboxylate on the iron oxide and is coordinated symmetrically to the oxide atoms. We show that the EPR signal of the OA ligand molecule can be used as a local probe to determine the temperature changes at the surface of the nanoparticle. © 2016 Published by Elsevier B.V.
    view abstract10.1016/j.jmmm.2016.03.045
  • Dynamical Effects of the Martensitic Transition in Magnetocaloric Heusler Alloys from Direct Δtad Measurements under Different Magnetic-Field-Sweep Rates
    Gottschall, T. and Skokov, K.P. and Scheibel, F. and Acet, M. and Zavareh, M.G. and Skourski, Y. and Wosnitza, J. and Farle, M. and Gutfleisch, O.
    Physical Review Applied 5 (2016)
    Large magnetocaloric effects can be obtained in Ni-Mn-based Heusler alloys due to the magnetostructural transition between martensite and austenite. This phase transformation proceeds via nucleation and growth. By direct measurements of the adiabatic temperature change ΔTad using different magnetic-field-sweeping rates from 0.01 up to 1500 T s-1, we study the dynamic behavior of the two Heusler compounds Ni50Mn35In15 and Ni45Mn37In13Co5 transforming near room temperature. From these experiments, we conclude that the nucleation process is rather slow in contrast to the relatively fast movement of the phase boundary between martensite and austenite. This is a limiting factor for cooling concepts operating at frequencies beyond 100 Hz. However, the dynamic effects of the transition are negligible in field rates typically used in magnetic refrigeration. These findings are essential considering the suitability of Heusler compounds for energy-efficient solid-state cooling. © 2016 American Physical Society.
    view abstract10.1103/PhysRevApplied.5.024013
  • Effect of Ni Content on Structural and Magnetic Properties of Li-Ni Ferrites Nanostructure Prepared by Hydrothermal Method
    Al-Shakarchi, E.K. and Lafta, S.H. and Musa, A.M. and Farle, M. and Salikov, R.
    Journal of Superconductivity and Novel Magnetism 29 (2016)
    Li-Ni ferrite with chemical formula Li0.5−0.5xNixFe2.5−0.5xO4 was prepared by hydrothermal method with different Ni contents (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) using metal chlorides, ferrous sulfate and sodium hydroxide as oxidants. The hydrothermal treatment was accomplished at (155 ∘ C) for (3 h). The required analyses of XRD, FTIR, SEM, TEM, EDX and magnetic hysteresis loop were performed to characterize the complete behavior as a function of x. It was found that lattice constant has a small increase as x increased. Crystallite size had a minimum value of about 13 nm at x = 1.0 and maximum value of about 33 nm at x = 0.3. It was also found that XRD density increased as x was increased. The particle size distributions showed that the maximum value is around 22 nm. FTIR analysis showed the presence of two main peaks with some shifting. Nanospheres were the predominant particles beside the presence of low nanorod concentration. M-H loops had super paramagnetic shape. The coercivity had a minimum value at x = 0.5. The magnetic saturation had a maximum value at x = 0.3, and the initial susceptibility χi had a maximum value at x = 0.5. © 2015, Springer Science+Business Media New York.
    view abstract10.1007/s10948-015-3334-9
  • Enhanced magnetocrystalline anisotropy of Fe30Co70 nanowires by Cu additives and annealing
    Palmero, E.M. and Salikhov, R. and Wiedwald, U. and Bran, C. and Spasova, M. and Vázquez, M. and Farle, M.
    Nanotechnology 27 (2016)
    The use of 3d transition metal-based magnetic nanowires (NWs) for permanent magnet applications requires large magnetocrystalline anisotropy energy (MAE), which in combination with the NWs' magnetic shape anisotropy yields magnetic hardening and an enhancement of the magnetic energy product. Here, we report on the significant increase in MAE by 125 kJ m-3 in Fe30Co70 NWs with diameters of 20-150 nm embedded in anodic aluminum oxide templates by adding 5 at.% Cu and subsequent annealing at 900 K. Ferromagnetic resonance (FMR) reveals that this enhancement of MAE is twice as large as the enhancement of MAE in annealed, but undoped NWs. X-ray diffraction (XRD) analysis suggests that upon annealing the immiscible Cu in FeCo NWs causes a crystal reorientation with respect to the NW axis with a considerable distortion of the bcc FeCo lattice. This strain is most likely the origin of the strongly enhanced MAE. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/27/36/365704
  • Exchange bias caused by field-induced spin reconfiguration in Ni-Mn-Sn
    Çaklr, A. and Acet, M. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    Exchange bias is observed in ferromagnetic/antiferromagnetic (FM/AF) layered stacks and in materials with neighboring ferromagnetic and antiferromagnetic granules. The latter is commonly observed in Ni-Mn-based martensitic Heusler alloys. In general, the exchange-bias effect is identified as horizontally shifted hysteresis loop when the system is field cooled from high temperatures. We report here loop shifts not only under field-cooled but also under zero-field-cooled conditions in magnetically granular martensitic Ni50Mn50-xSnx Heusler alloys in the compositional range 13.0≥x≥8.9. Under zero-field-cooled conditions, the initially applied field can carry the system over energy barriers and stabilize a spin-reconfigured state so that a negatively shifted hysteresis loop can also occur here as in the field-cooled state. Spin reconfiguration occurs when the relative size of the AF and FM regions as well as the relative strength of the of AF and FM interactions are in balance. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.094411
  • Functionalized Hybrid Nanomagnets: New Materials for Innovations in Energy Storage and Medical Theranostics
    Farle, M.
    Ieee Transactions on Magnetics 52 (2016)
    view abstract10.1109/TMAG.2016.2617258
  • Gas-phase synthesis of Fe-Bi metastable and dumbbell particles
    Ünlü, C.G. and Li, Z.-A. and Acet, M. and Farle, M.
    Crystal Research and Technology 51 (2016)
    Fe-Bi nanoparticles were prepared in the gas-phase by DC magnetron sputtering and in-fight annealing. The morphological, structural and compositional properties were investigated by High-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and scanning transmission electron microscopy. High-resolution microscopy studies show that primary particles produced without in-flight annealing are spherical with a diameter of about 50 nm. Particles sintered at 773 K acquire a dumbbell structure with Fe-FeO and Bi sections. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/crat.201500329
  • Magnetic anisotropy and relaxation of single Fe/FexOy core/shell- nanocubes: A ferromagnetic resonance investigation
    Terwey, A. and Meckenstock, R. and Zingsem, B.W. and Masur, S. and Derricks, C. and Römer, F.M. and Farle, M.
    AIP Advances 6 (2016)
    In this work a full angle dependent Ferromagnetic Resonance (FMR) investigation on a system of 20 separated Fe/FexOy nanocubes without dipolar coupling is reported. The angular magnetic field dependence of FMR spectra of 20 single particles and 2 dimers were recorded using a microresonator setup with a sensitivity of 106 μB at X-band frequencies. We determine an effective magnetocrystalline anisotropy field of 2K4,eff/M = 50 mT ± 5 mT for selected particles, which is smaller than the one of bulk Fe due to the core shell morphology of the particles. The FMR resonances have a linewidth of 4 mT ± 1 mT, corresponding to a magnetic effective damping parameter α = 0.0045 ± 0.0005 matching the values of high quality iron thin films. Numerical calculations taking into account the different angular orientations of the 24 particles with respect to the external magnetic field yield a good agreement to the experiment. © 2016 Author(s).
    view abstract10.1063/1.4944399
  • Magnetic correlations in the magnetocaloric materials Mn3GaC and Mn3GaC0.85N0.15 studied by neutron polarization analysis and neutron depolarization
    Çakr, Ö. and Acet, M. and Farle, M. and Wildes, A.
    Journal of Physics Condensed Matter 28 (2016)
    Partially substituting carbon by nitrogen in the antiperovskite compound Mn3GaC increases the first order antiferromagnetic/ferromagnetic transition temperature and at the same time causes the high-temperature long-range ferromagnetism to weaken. To show that the weakening is related to the diminishing of ferromagnetic domain formation, we undertake neutron depolarization and neutron polarization analysis experiments on Mn3GaC and Mn3GaC0.85N0.15. Polarization analysis experiments show that strong ferromagnetic correlations are present at high temperatures in the paramagnetic states of both Mn3GaC and Mn3GaC0.85N0.15 and that these correlations vanish in the antiferromagnetic state. Neutron depolarization studies show that above the first order transition temperature, ferromagnetic domain formation is present in Mn3GaC but is absent in Mn3GaC0.85N0.15. The relationship between ferromagnetic domain formation and transitional hysteresis is brought forward for these two important magnetocaloric materials. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/28/13/13LT02
  • Magnetic proximity effect and shell-ferromagnetism in metastable Ni50Mn45Ga5
    Krenke, T. and Çakır, A. and Scheibel, F. and Acet, M. and Farle, M.
    Journal of Applied Physics 120 (2016)
    The present study on magnetic and structural properties of Ni50Mn45Ga5 confirms that structural metastability is an inherent property of Ni50Mn50- xXx Heusler alloys with X as In, Ga, and Sn. The ternary alloy transforms during temper-annealing into a dual-phase composite alloy. The two phases are identified to be cubic L21, Ni50Mn25Ga25, and tetragonal L10 Ni50Mn50. Depending on the annealing temperature, the magnetic-proximity effect giving rise to shell-ferromagnetism has been observed when annealing is carried out under an external magnetic field. The upper and lower remanence values MR+ and MR− have the same sign even at high temperatures. Such alloys can be promising candidates for heat- and magnetic-field-resistant magnetic recording media. © 2016 Author(s).
    view abstract10.1063/1.4972480
  • Mastering hysteresis in magnetocaloric materials
    Gutfleisch, O. and Gottschall, T. and Fries, M. and Benke, D. and Radulov, I. and Skokov, K.P. and Wende, H. and Gruner, M. and Acet, M. and Entel, P. and Farle, M.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374 (2016)
    Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratoryscale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La-Fe-Si-, Heusler- and Fe2 P-type compounds. This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
    view abstract10.1098/rsta.2015.0308
  • Optimum nanoscale design in ferrite based nanoparticles for magnetic particle hyperthermia
    Liébana-Viñas, S. and Simeonidis, K. and Wiedwald, U. and Li, Z.-A. and Ma, Zh. and Myrovali, E. and Makridis, A. and Sakellari, D. and Vourlias, G. and Spasova, M. and Farle, M. and Angelakeris, M.
    RSC Advances 6 (2016)
    The study demonstrates the multiplex enhancement of the magnetic hyperthermia response in ferrites by nanoscale design and tuning without sparing the biocompatibility of iron-oxide. We propose core/shell nanoparticles with a 7-9 nm ferrite core, either magnetically soft MnFe2O4 or hard CoFe2O4, encapsulated by a 2-3 nm Fe3O4 shell providing a core/shell interface. In this case, the exchange interaction between core and shell dramatically affects the macroscopic magnetic behavior and, at the same time, a biocompatible shell prevents interactions of the toxic cores with their environment. The tunable, yet superior, magnetic hyperthermia response is proven by an increase of the specific loss power by a factor of 24 for CoFe2O4-Fe3O4 core/shell particles. This gain is directly connected with the magnetic coupling strength at the core/shell interface and opens the possibility of further optimization. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c6ra17892h
  • Shell-ferromagnetism of nano-Heuslers generated by segregation under magnetic field
    Çaklr, A. and Acet, M. and Farle, M.
    Scientific Reports 6 (2016)
    We report on a new functional property in an AF martensitic Heusler Ni50Mn45In5, which when annealed at high temperatures under a magnetic field, segregates and forms Ni50Mn25In25 Heusler precipitates embedded in a Ni50Mn50 matrix. The precipitates are paramagnetic whereas the matrix is antiferromagnetic. However, the spins at the interface with the Ni50Mn50 matrix align with the field during their nucleation and growth and become strongly pinned in the direction of the applied field during annealing, whereas the core spins become paramagnetic. This shell-ferromagnetism persists up to 600 K and is so strongly pinned that the remanent magnetization remains unchanged, even when the field is reversed or when the temperature is cycled between low temperatures and close to the annealing temperature.
    view abstract10.1038/srep28931
  • Synthesis and structure of strontium ferrite nanowires and nanotubes of high aspect ratio
    Ebrahimi, F. and Ashrafizadeh, F. and Bakhshi, S.R. and Farle, M.
    Journal of Sol-Gel Science and Technology 77 (2016)
    Abstract: Strontium hexaferrite nanowires and nanotubes were synthesized in porous anodic aluminum oxide templates. Different solution-based synthesis techniques (spin coating, vacuum suction, and dip coating) were investigated. Strontium ferrite nanopowders were also synthesized by a similar sol–gel process. The morphology, structure, and composition of the embedded hexaferrite nanostructures were examined by field emission scanning electron microscope, X-ray diffraction, and transmission electron microscopy. Strontium ferrite wires with Fe/Sr ratios from 10 to 12 under different annealing temperatures of 500–700 °C were studied. The results showed that dip coating could produce fine and uniform strontium ferrite nanowires. The ratio of Fe/Sr of 11 and a calcination temperature of 650 °C were found to be optimum conditions. The produced material may be of importance for novel microwave-frequency nanoscale devices. Graphical Abstract: [Figure not available: see fulltext.] © 2015, Springer Science+Business Media New York.
    view abstract10.1007/s10971-015-3902-2
  • Temperature dependence of perpendicular magnetic anisotropy in CoFeB thin films
    Fu, Y. and Barsukov, I. and Li, J. and Gonçalves, A.M. and Kuo, C.C. and Farle, M. and Krivorotov, I.N.
    Applied Physics Letters 108 (2016)
    We study perpendicular magnetic anisotropy in thin films of Ta/Co20Fe60B20/MgO by ferromagnetic resonance and find a linear temperature dependence for the first and second order uniaxial terms from 5 to 300 K. Our data suggest the possible hybridization of Fe-O orbitals at the CoFeB/MgO interface for the origin of the first order anisotropy. However, we also find that non-interfacial contributions to the anisotropy are present. An easy-cone anisotropy is found for the entire temperature range in the narrow region of film thicknesses around the spin reorientation transition 1.2-1.35 nm. © 2016 AIP Publishing LLC.
    view abstract10.1063/1.4945682
  • Thin film synthesis and characterization of a chemically ordered magnetic nanolaminate (V,Mn)3GaC2
    Tao, Q. and Salikhov, R. and Mockute, A. and Lu, J. and Farle, M. and Wiedwald, U. and Rosen, J.
    APL Materials 4 (2016)
    We report on synthesis and characterization of a new magnetic nanolaminate (V,Mn)3GaC2, which is the first magnetic MAX phase of a 312 stoichiometry. Atomically resolved energy dispersive X-ray mapping of epitaxial thin films reveals a tendency of alternate chemical ordering between V and Mn, with atomic layers composed of primarily one element only. Magnetometry measurements reveal a ferromagnetic response between 50 K and 300 K, with indication of a magnetic ordering temperature well above room temperature. © 2016 Author(s).
    view abstract10.1063/1.4961502
  • Tuning the magnetism of ferrite nanoparticles
    Viñas, S.L. and Simeonidis, K. and Li, Z.-A. and Ma, Z. and Myrovali, E. and Makridis, A. and Sakellari, D. and Angelakeris, M. and Wiedwald, U. and Spasova, M. and Farle, M.
    Journal of Magnetism and Magnetic Materials 415 (2016)
    The importance of magnetic interactions within an individual nanoparticle or between adjacent ones is crucial not only for the macroscopic collective magnetic behavior but for the AC magnetic heating efficiency as well. On this concept, single-(MFe2O4 where M=Fe, Co, Mn) and core-shell ferrite nanoparticles consisting of a magnetically softer (MnFe2O4) or magnetically harder (CoFe2O4) core and a magnetite (Fe3O4) shell with an overall size in the 10 nm range were synthesized and studied for their magnetic particle hyperthermia efficiency. Magnetic measurements indicate that the coating of the hard magnetic phase (CoFe2O4) by Fe3O4 provides a significant enhancement of hysteresis losses over the corresponding single-phase counterpart response, and thus results in a multiplication of the magnetic hyperthermia efficiency opening a novel pathway for high-performance, magnetic hyperthermia agents. At the same time, the existence of a biocompatible Fe3O4 outer shell, toxicologically renders these systems similar to iron-oxide ones with significantly milder side-effects. © 2016 Elsevier B.V.
    view abstract10.1016/j.jmmm.2016.02.098
  • An approach for transparent and electrically conducting coatings: A transparent plastic varnish with nanoparticulate magnetic additives
    Beck, G. and Barcikowski, S. and Chakravadhanula, V.S.K. and Comesaña-Hermo, M. and Deng, M. and Farle, M. and Hilgendorff, M. and Jakobi, J. and Janek, J. and Kienle, L. and Mogwitz, B. and Schubert, T. and Stiemke, F.
    Thin Solid Films 595 (2015)
    For the purpose of preparing TCCs (= transparent and electrical conducting coatings), metallic and ferromagnetic nano-additives were dispersed into a transparent varnish and the obtained dispersions were coated on transparent plastic substrates. During hardening of the dispersion the magnetic nano-additives were aligned by a magnetic field. The resulting coatings have electrical pathways along lines of nano-additive chains and are highly transparent in the areas between the lines. Therefore, the electrical conductivity is anisotropic, and it depends on the alignment of the nano-additives (i.e. on the distance between the nano-additives within the chains and the length of the lines) as well as on the thickness of an oxide and/or solvent shell around the nano-additives. The transparency depends also on the alignment and here especially on the thickness and the distance between the formed lines. The quality of the alignment in turn, depends on the magnetic properties and on the size of the particles. We used commercial plastic varnishes, which form electrically isolating (≥ 10− 12 S/m) and transparent (about 90% transparency) coatings, and the following magnetic additives: Co-, Fe-, CoPt3, CoPt3@Au- and Fe@Au-nanoparticles as well as CoNi-nanowires. Coatings with Fe@Au-nanoparticles show the best results in terms of the electrical conductivity (10− 5 S/m–10− 6 S/m) at transparencies above 70%. Furthermore, in addition to the magnetic nano-additives, transparent additives (Al2O3-particles) and non-magnetic, but better conducting additives (carbon-nanotubes) were added to the varnish to increase the transparency and the electrical conductivity, respectively. © 2015 Elsevier B.V.
    view abstract10.1016/j.tsf.2015.10.059
  • Characteristics of 5M modulated martensite in Ni-Mn-Ga magnetic shape memory alloys
    Çaklr, A. and Acet, M. and Righi, L. and Albertini, F. and Farle, M.
    AIP Advances 5 (2015)
    The applicability of the magnetic shape memory effect in Ni-Mn-based martensitic Heusler alloys is closely related to the nature of the crystallographically modulated martensite phase in these materials. We study the properties of modulated phases as a function of temperature and composition in three magnetic shape memory alloys Ni49.8Mn25.0Ga25.2, Ni49.8Mn27.1Ga23.1 and Ni49.5Mn28.6Ga21.9. The effect of substituting Ga for Mn leads to an anisotropic expansion of the lattice, where the b-parameter of the 5M modulated structure increases and the a and c-parameters decrease with increasing Ga concentration. The modulation vector is found to be both temperature and composition dependent. The size of the modulation vector corresponds to an incommensurate structure for Ni49.8Mn25.0Ga25.2 at all temperatures. For the other samples the modulation is incommensurate at low temperatures but reaches a commensurate value of q 0.400 close to room temperature. The results show that commensurateness of the 5M modulated structure is a special case of incommensurate 5M at a particular temperature. © 2015 Author(s).
    view abstract10.1063/1.4932233
  • Correlation between structure and magnetic properties in CoxFe100-x nanowires: The roles of composition and wire diameter
    Bran, C. and Palmero, E.M. and Li, Z.-A. and Del Real, R.P. and Spasova, M. and Farle, M. and Vázquez, M.
    Journal of Physics D: Applied Physics 48 (2015)
    The structural and magnetic characteristics of CoxFe100-x (0 ≤ x ≤ 100) cylindrical nanowire arrays are investigated for two series of nanowires with diameters of 20 and 40 nm, respectively. The crystalline structure evolves with Co content from bcc Fe through a mixed bcc-fcc phase to a final polycrystalline fcc or hcp phase for 40 and 20 nm diameter Co nanowires, respectively. A monocrystalline structure is found only in a few nanowires with a 40 nm diameter. The magnetic characterization under axial magnetic field reveals an increase in coercivity and remanence for increasing Co content as the crystalline structure evolves from bcc Fe to fcc Co. These parameters decrease when hcp Co with a stronger magnetocrystalline anisotropy and nearly perpendicular 'c' axis is formed. Overall higher values are observed in nanowires when the nanowire diameter decreases from 40 to 20 nm. An increase of the total magnetic anisotropy energy density is found with decreasing temperature, especially for Co wires where the strong magnetocrystalline anisotropy plays the most significant role. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/48/14/145304
  • Dependence of the inverse magnetocaloric effect on the field-change rate in Mn3GaC and its relationship to the kinetics of the phase transition
    Scheibel, F. and Gottschall, T. and Skokov, K. and Gutfleisch, O. and Ghorbani-Zavareh, M. and Skourski, Y. and Wosnitza, J. and Çakir, Ö. and Farle, M. and Acet, M.
    Journal of Applied Physics 117 (2015)
    We study the dependence of the magnetocaloric effect on the magnetic field-change-rate the first order magnetostructural transition in Mn<inf>3</inf>GaC by measuring the adiabatic temperature change ΔT at three different time scales: 11 mT s-1, 700 mT s-1, and ∼1000 T s-1. We find that the maximum adiabatic temperature-change of about 5K is reached in the 11 mT s-1 and 700 mT s-1 rates, whereas for the ∼1000 T s-1-rate the transition lags the change in the magnetic field so that the maximum adiabatic temperature-change is not attained. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4922722
  • Electrostatic doping as a source for robust ferromagnetism at the interface between antiferromagnetic cobalt oxides
    Li, Z.-A. and Fontaíña-Troitiño, N. and Kovács, A. and Liébana-Viñas, S. and Spasova, M. and Dunin-Borkowski, R.E. and Müller, M. and Doennig, D. and Pentcheva, R. and Farle, M. and Salgueiriño, V.
    Scientific Reports 5 (2015)
    Polar oxide interfaces are an important focus of research due to their novel functionality which is not available in the bulk constituents. So far, research has focused mainly on heterointerfaces derived from the perovskite structure. It is important to extend our understanding of electronic reconstruction phenomena to a broader class of materials and structure types. Here we report from high-resolution transmission electron microscopy and quantitative magnetometry a robust - above room temperature (Curie temperature TC 蠑 300 K) - environmentally stable- ferromagnetically coupled interface layer between the antiferromagnetic rocksalt CoO core and a 2-4 nm thick antiferromagnetic spinel Co3O4 surface layer in octahedron-shaped nanocrystals. Density functional theory calculations with an on-site Coulomb repulsion parameter identify the origin of the experimentally observed ferromagnetic phase as a charge transfer process (partial reduction) of Co3+ to Co2+ at the CoO/Co3O4 interface, with Co2+ being in the low spin state, unlike the high spin state of its counterpart in CoO. This finding may serve as a guideline for designing new functional nanomagnets based on oxidation resistant antiferromagnetic transition metal oxides.
    view abstract10.1038/srep07997
  • Enhanced biomedical heat-triggered carriers via nanomagnetism tuning in ferrite-based nanoparticles
    Angelakeris, M. and Li, Z.-A. and Hilgendorff, M. and Simeonidis, K. and Sakellari, D. and Filippousi, M. and Tian, H. and Van Tendeloo, G. and Spasova, M. and Acet, M. and Farle, M.
    Journal of Magnetism and Magnetic Materials 381 (2015)
    Biomedical nanomagnetic carriers are getting a higher impact in therapy and diagnosis schemes while their constraints and prerequisites are more and more successfully confronted. Such particles should possess a well-defined size with minimum agglomeration and they should be synthesized in a facile and reproducible high-yield way together with a controllable response to an applied static or dynamic field tailored for the specific application. Here, we attempt to enhance the heating efficiency in magnetic particle hyperthermia treatment through the proper adjustment of the core-shell morphology in ferrite particles, by controlling exchange and dipolar magnetic interactions at the nanoscale. Thus, core-shell nanoparticles with mutual coupling of magnetically hard (CoFe2O4) and soft (MnFe2O4) components are synthesized with facile synthetic controls resulting in uniform size and shell thickness as evidenced by high resolution transmission electron microscopy imaging, excellent crystallinity and size monodispersity. Such a magnetic coupling enables the fine tuning of magnetic anisotropy and magnetic interactions without sparing the good structural, chemical and colloidal stability. Consequently, the magnetic heating efficiency of CoFe2O4 and MnFe2O4 core-shell nanoparticles is distinctively different from that of their counterparts, even though all these nanocrystals were synthesized under similar conditions. For better understanding of the AC magnetic hyperthermia response and its correlation with magnetic-origin features we study the effect of the volume ratio of magnetic hard and soft phases in the bimagnetic core-shell nanocrystals. Eventually, such particles may be considered as novel heating carriers that under further biomedical functionalization may become adaptable multifunctional heat-triggered nanoplatforms. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmmm.2014.12.069
  • Ferromagnetic resonance study of the perpendicular magnetic anisotropy in MgO/CoFeB/ Ta multilayers as a function of annealing temperature
    Aleksandrov, Y. and Fowley, C. and Kowalska, E. and Sluka, V. and Lindner, J. and Farle, M. and Ocker, B. and Fassbender, J. and Deac, A.
    2015 IEEE International Magnetics Conference, INTERMAG 2015 (2015)
    MgO-based magnetic tunnel junctions (MTJs) are currently the structures of choice for magnetic random access memories (MRAMs), as they exhibit extremely high tunnel magnetoresistance (TMR) values due to highly effective spin-dependent tunneling [1, 2]. Initial studies focused on devices with both free and reference layers exhibiting in-plane remnant states [3, 4]. On the other hand, it has been reported that devices having the magnetic layers magnetized perpendicular to the layer interface offer a better trade-off between reducing the writing power and maintaining a thermal stability sufficient for data retention [5, 6]. It has also been recently demonstrated that CoFeB-based MgO-MTJs can exhibit perpendicular magnetic anisotropy (PMA), while maintaining the crystalline quality of the barrier required for achieving high TMR ratios, thus making them good candidates for next generation spin-transfer-torque (STT) MRAM [7]. © 2015 IEEE.
    view abstract10.1109/INTMAG.2015.7156508
  • Inducing high coercivity and anisotropy into strained Fe-Co thin films, towards rare earth free permanent magnets applications
    Giannopoulos, G. and Salikhov, R. and Reichel, L. and Markou, A. and Panagiotopoulos, I. and Farle, M. and Fahler, S. and Psycharis, V. and Niarchos, D.G.
    2015 IEEE International Magnetics Conference, INTERMAG 2015 (2015)
    The present study is focused on our effort towards high performance permanent magnets with no rare earth elements. For this reason, high magnetization FeCo system is proposed as an alternative candidate. High magneto-crystalline anisotropy along with high magnetic moment is required for permanent magnets. According to theoretical calculations FeCo alloy can support large uniaxial magnetic anisotropy energy (MAE), Ku and saturation magnetization Ms [1], while tetragonal distortion can be induced via coherent growth of Fe-Co layers on different substrate or buffer materials. Based on a combinatorial exploration of AuCu underlayer lattice constant [2] we employed AuCu as a buffer layer in our system. Adding a third element, e.g. Carbon, was recently proposed to stabilize the strain in Fe-Co, preventing the complete lattice relaxation of the system [3], while an increased magnetocrystalline anisotropy was observed in FeCoC thin films [4]. We report giant perpendicular anisotropy in our thin magnetic films in the excess of 1MJ/m3, confirmed by FMR measurements and coercivity induction of almost 850Oe in our multilayers films approach. © 2015 IEEE.
    view abstract10.1109/INTMAG.2015.7157630
  • Intermartensitic transitions and phase stability in Ni50Mn50-xSnx Heusler alloys
    Çakir, A. and Righi, L. and Albertini, F. and Acet, M. and Farle, M.
    Acta Materialia 99 (2015)
    Ni-Mn based Heusler alloys are of considerable interest due to their multifunctional properties such as magnetic shape memory, magnetocaloric effect and spintronics. The reason for these multifunctional properties is the presence of a first order martensitic transition and its strong coupling to the magnetization. In this work, one of the outstanding class of martensitic Heuslers, Ni-Mn-Sn, is investigated in relation to magneto-structural phase transitions and the stability of the various crystallographic structures under varying temperature. Temperature-dependent X-ray diffraction, resistance and magnetization measurements on Ni<inf>50</inf>Mn<inf>50-x</inf>Sn<inf>x</inf> alloys are performed in a broad valence electron concentration range 7.91 ≤ (e/a) ≤ 8.34 (5.1 ≤ x ≤ 20.3at.%). The results reveal that in addition to the austenite-martensite transition, further intermartensitic transitions take place with decreasing temperature. Depending on the composition, we observe that the parent martensite phase tends to transform to L1<inf>0</inf> martensite as the ground state phase when the temperature is lowered. A phase diagram of Ni<inf>50</inf>Mn<inf>50-x</inf>Sn<inf>x</inf> is constructed to include intermartensitic phase transition boundaries. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.07.072
  • Kinetic arrest in magnetically inhomogeneous C-deficient Mn3GaC
    Cakir, O. and Acet, M. and Farle, M. and Dias, E. and Priolkar, K.
    Journal of Magnetism and Magnetic Materials 390 (2015)
    Thermal broadening of the first order magnetostructural transition and enhancement in ferromagnetic exchange occurs in carbon deficient Mn3GaC0.9. We show from temperature and field-dependent magnetization measurements that this leads to inhomogeneous magnetism and causes frustration and kinetic arrest effects. The arrested state is deactivated and the system returns to its ground state when the cooling-field is removed. This causes open hysteresis loops with which we study the kinetic arrest effect in this system. (C) 2015 Published by Elsevier B.V.
    view abstract10.1016/j.jmmm.2015.04.084
  • Large magnetic anisotropy in strained Fe/Co multilayers on AuCu and the effect of carbon doping
    Giannopoulos, G. and Salikhov, R. and Zingsem, B. and Markou, A. and Panagiotopoulos, I. and Psycharis, V. and Farle, M. and Niarchos, D.
    APL Materials 3 (2015)
    A tetragonally distorted FeCo structure is obtained in Fe/Co multilayers epitaxially grown on Au<inf>50</inf>Cu<inf>50</inf> buffer using MgO single crystal substrates as a result of the lattice mismatch between the buffer and the FeCo ferromagnetic layer. The presence of large magnetic anisotropy energy (MAE) of the order of 1 MJ/m3 has been confirmed by ferromagnetic resonance. Furthermore, the effect of carbon (C) doping to maintain the tetragonal distortion throughout the thickness of 3 nm FeCo has been investigated. Our study shows that FeCo alloys maintain large magnetic moment and possess high MAE properties that are required for designing permanent magnets. © 2015 Author(s).
    view abstract10.1063/1.4919058
  • Magnetic anisotropy in the (Cr0.5Mn0.5)2Gac MAX phase
    Salikhov, R. and Semisalova, A.S. and Petruhins, A. and Ingason, A.S. and Rosen, J. and Wiedwald, U. and Farle, M.
    Materials Research Letters 3 (2015)
    Magnetic MAX phase (Cr0.5Mn0.5)2GaC thin films grown epitaxially on MgO(111) substrates were studied by ferromagnetic resonance at temperatures between 110 and 300 K. The spectroscopic splitting factor g = 2.00 ± 0.01 measured at all temperatures indicates pure spin magnetism in the sample. At all temperatures we find the magnetocrystalline anisotropy energy to be negligible which is in agreement with the identified pure spin magnetism. © 2015 The Author(s). Published by Taylor & Francis.
    view abstract10.1080/21663831.2015.1036324
  • Magnetic hardening of Fe30Co70nanowires
    Liébana Viñas, S. and Salikhov, R. and Bran, C. and Palmero, E.M. and Vazquez, M. and Arvan, B. and Yao, X. and Toson, P. and Fidler, J. and Spasova, M. and Wiedwald, U. and Farle, M.
    Nanotechnology 26 (2015)
    3d transition metal-based magnetic nanowires (NWs) are currently considered as potential candidates for alternative rare-earth-free alloys as novel permanent magnets. Here, we report on the magnetic hardening of Fe30Co70nanowires in anodic aluminium oxide templates with diameters of 20 nm and 40 nm (length 6 μm and 7.5 μm, respectively) by means of magnetic pinning at the tips of the NWs. We observe that a 3-4 nm naturally formed ferrimagnetic FeCo oxide layer covering the tip of the FeCo NW increases the coercive field by 20%, indicating that domain wall nucleation starts at the tip of the magnetic NW. Ferromagnetic resonance (FMR) measurements were used to quantify the magnetic uniaxial anisotropy energy of the samples. Micromagnetic simulations support our experimental findings, showing that the increase of the coercive field can be achieved by controlling domain wall nucleation using magnetic materials with antiferromagnetic exchange coupling, i.e. antiferromagnets or ferrimagnets, as a capping layer at the nanowire tips. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/26/41/415704
  • Magnetic ordering in magnetic shape memory alloy Ni-Mn-In-Co
    Ollefs, K. and Schöppner, C. and Titov, I. and Meckenstock, R. and Wilhelm, F. and Rogalev, A. and Liu, J. and Gutfleisch, O. and Farle, M. and Wende, H. and Acet, M.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    Structural and magnetic properties across the martensite-austenite phase transitions in the shape memory alloy Ni-Mn-In-Co are studied using complementary experimental techniques: ferromagnetic resonance, macroscopic magnetization measurements, and x-ray magnetic circular dichroism in the temperature range from 5 to 450 K. Ferromagnetic resonance experiments show coexisting antiferromagnetic and ferromagnetic correlations for the martensite phase and ferromagnetic and paramagnetic correlations in the austenite phase. Magnetization measurements reveal spin-glass-like behavior for T<30 K and Ni and Co K-edge x-ray magnetic circular dichroism measurements confirm an assignment of a ferromagnetic resonance line purely to Ni (and Co) for a wide temperature range from 125 to 225 K. Hence a combined analysis of ferromagnetic resonance and x-ray magnetic circular dichroism allows us to attribute particular magnetic resonance signals to individual elemental species in the alloy. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.224429
  • Magnetic phase transitions in Ta/CoFeB/MgO multilayers
    Barsukov, I. and Fu, Y. and Safranski, C. and Chen, Y.-J. and Youngblood, B. and Gonçalves, A.M. and Spasova, M. and Farle, M. and Katine, J.A. and Kuo, C.C. and Krivorotov, I.N.
    Applied Physics Letters 106 (2015)
    We study thin films and magnetic tunnel junction nanopillars based on Ta/Co<inf>20</inf>Fe<inf>60</inf>B<inf>20</inf>/MgO multilayers by electrical transport and magnetometry measurements. These measurements suggest that an ultrathin magnetic oxide layer forms at the Co<inf>20</inf>Fe<inf>60</inf>B<inf>20</inf>/MgO interface. At approximately 160K, the oxide undergoes a phase transition from an insulating antiferromagnet at low temperatures to a conductive weak ferromagnet at high temperatures. This interfacial magnetic oxide is expected to have significant impact on the magnetic properties of CoFeB-based multilayers used in spin torque memories. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4921306
  • Magnetocaloric effect in (La1-xSmx)0.67Pb0.33MnO3 (0 ≤ x ≤ 0.3) manganites near room temperature
    Çetin, S.K. and Acet, M. and Güneş, M. and Ekicibil, A. and Farle, M.
    Journal of Alloys and Compounds 650 (2015)
    Abstract We present an investigation on structure, magnetic and magnetocaloric properties of the perovskite manganites (La<inf>1-x</inf>Sm<inf>x</inf>)<inf>0.67</inf>Pb<inf>0.33</inf>MnO<inf>3</inf> (x = 0, 0.1, 0.2, 0.3) synthesized by sol-gel technique. The XRD patterns show that all synthesized samples have reflections typical of the perovskite structure with orthorhombic symmetry. Thermomagnetic measurements showed that all the samples display a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature decreases with increasing Sm content from 358 K for x = 0-286 K for x = 0.3. We determined the isothermal magnetic entropy changes of all the samples from the magnetization measurements and furthermore measured the adiabatic temperature change of the sample for x = 0.3 directly. The results showed that the adiabatic temperature change, determined using together the entropy change and the specific heat and the value obtained by direct temperature change measurements both give 1.3 K for a magnetic field change of 3 T at about 280 K. We also measured the cyclic adiabatic temperature-change of the sample and the results indicate that the sample undergoes a reversible temperature-change on field-cycling which is essential for magnetic refrigeration. © 2015 Elsevier B.V.
    view abstract10.1016/j.jallcom.2015.07.217
  • Model-independent measurement of the charge density distribution along an Fe atom probe needle using off-axis electron holography without mean inner potential effects
    Migunov, V. and London, A. and Farle, M. and Dunin-Borkowski, R.E.
    Journal of Applied Physics 117 (2015)
    The one-dimensional charge density distribution along an electrically biased Fe atom probe needle is measured using a model-independent approach based on off-axis electron holography in the transmission electron microscope. Both the mean inner potential and the magnetic contribution to the phase shift are subtracted by taking differences between electron-optical phase images recorded with different voltages applied to the needle. The measured one-dimensional charge density distribution along the needle is compared with a similar result obtained using model-based fitting of the phase shift surrounding the needle. On the assumption of cylindrical symmetry, it is then used to infer the three-dimensional electric field and electrostatic potential around the needle with ∼10 nm spatial resolution, without needing to consider either the influence of the perturbed reference wave or the extension of the projected potential outside the field of view of the electron hologram. The present study illustrates how a model-independent approach can be used to measure local variations in charge density in a material using electron holography in the presence of additional contributions to the phase, such as those arising from changes in mean inner potential and specimen thickness. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4916609
  • Splenic red pulp macrophages are intrinsically superparamagnetic and contaminate magnetic cell isolates
    Franken, L. and Klein, M. and Spasova, M. and Elsukova, A. and Wiedwald, U. and Welz, M. and Knolle, P. and Farle, M. and Limmer, A. and Kurts, C.
    Scientific Reports 5 (2015)
    A main function of splenic red pulp macrophages is the degradation of damaged or aged erythrocytes. Here we show that these macrophages accumulate ferrimagnetic iron oxides that render them intrinsically superparamagnetic. Consequently, these cells routinely contaminate splenic cell isolates obtained with the use of MCS, a technique that has been widely used in immunological research for decades. These contaminations can profoundly alter experimental results. In mice deficient for the transcription factor SpiC, which lack red pulp macrophages, liver Kupffer cells take over the task of erythrocyte degradation and become superparamagnetic. We describe a simple additional magnetic separation step that avoids this problem and substantially improves purity of magnetic cell isolates from the spleen.
    view abstract10.1038/srep12940
  • Structure-Correlated Exchange Anisotropy in Oxidized Co80Ni20 Nanorods
    Liébana-Viñas, S. and Wiedwald, U. and Elsukova, A. and Perl, J. and Zingsem, B. and Semisalova, A.S. and Salgueiriño, V. and Spasova, M. and Farle, M.
    Chemistry of Materials 27 (2015)
    Rare earth-free permanent magnets for applications in electro-magnetic devices promise better sustainability and availability and lower prices. Exploiting the combination of shape, magnetocrystalline and exchange anisotropy in 3D-metals can pave the way to practical application of nanomagnets. In this context, we study the structural and magnetic properties of Co<inf>80</inf>Ni<inf>20</inf> nanorods with a mean diameter of 6.5 nm and a mean length of 52.5 nm, prepared by polyol reduction of mixed cobalt and nickel acetates. Structural analysis shows crystalline rods with the crystallographic c-axis of the hexagonal close-packed (hcp) phase parallel to the long axis of the Co<inf>80</inf>Ni<inf>20</inf> alloy rods, which appear covered by a thin oxidized face-centered cubic (fcc) shell. The temperature dependence of the surprisingly high coercive field and the exchange bias effect caused by the antiferromagnetic surface oxide indicate a strong magnetic hardening due to alignment of anisotropy axes. We identify a temperature dependent local maximum of the coercive field at T = 250 K, which originates from noncollinear spin orientations in the ferromagnetic core and the antiferromagnetic shell. This might be useful for building four way magnetic switches as a function of temperature. © 2015 American Chemical Society.
    view abstract10.1021/acs.chemmater.5b00976
  • The structural, magnetic, and magnetocaloric properties of In-doped Mn2- xCrxSb
    Tekgül, A. and Çaklr, Ö. and Acet, M. and Farle, M. and Ünal, N.
    Journal of Applied Physics 118 (2015)
    Mn2- xCrxSb exhibits an antiferromagnetic-ferrimagnetic transition of which the temperature can be changed by controlling the Cr concentration. Increasing the Cr content from x = 0.05 to 0.13 raises the transition temperature from about 180 K up to around room temperature. To suppress the inevitable ferromagnetic MnSb impurity phase, we partially replace Sb by In. Mn2- xCrxSb1- yIny alloys have an antiferromagnetic-ferrimagnetic transition and a narrow transition hysteresis and exhibit the inverse magnetocaloric effect. We examine the magnetocaloric effect from the magnetization and direct adiabatic temperature-change measurements. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4934253
  • Toward broad-band x-ray detected ferromagnetic resonance in longitudinal geometry
    Ollefs, K. and Meckenstock, R. and Spoddig, D. and Römer, F.M. and Hassel, C. and Schöppner, C. and Ney, V. and Farle, M. and Ney, A.
    Journal of Applied Physics 117 (2015)
    An ultrahigh-vacuum-compatible setup for broad-band X-ray detected ferromagnetic resonance (XFMR) in longitudinal geometry is introduced which relies on a low-power, continuous-wave excitation of the ferromagnetic sample. A simultaneous detection of the conventional ferromagnetic resonance via measuring the reflected microwave power and the XFMR signal of the X-ray absorption is possible. First experiments on the Fe and Co L<inf>3</inf>-edges of a permalloy film covered with Co nanostripes as well as the Fe and Ni K-edges of a permalloy film are presented and discussed. Two different XFMR signals are found, one of which is independent of the photon energy and therefore does not provide element-selective information. The other much weaker signal is element-selective, and the dynamic magnetic properties could be detected for Fe and Co separately. The dependence of the latter XFMR signal on the photon helicity of the synchrotron light is found to be distinct from the usual x-ray magnetic circular dichroism effect. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4922248
  • Angular dependent ferromagnetic resonance analysis in a single micron sized cobalt stripe
    Schoeppner, C. and Wagner, K. and Stienen, S. and Meckenstock, R. and Farle, M. and Narkowicz, R. and Suter, D. and Lindner, J.
    Journal of Applied Physics 116 (2014)
    We demonstrate how planar microresonators (PMRs) can be utilized to investigate the angular dependent magnetic resonance response of single magnetic nanostructures. In contrast to alternative detection schemes like electrical or optical detection, the PMR approach provides a classical means of investigating the high frequency dynamics of single magnetic entities, enabling the use of well-established analysis methods of ferromagnetic resonance (FMR) spectroscopy. To demonstrate the performance of the PMR-based FMR setup for angular dependent measurements, we investigate the microwave excited magnons in a single Co stripe of 5 × 1 × 0.02 μm3 and compare the results to micromagnetic simulations. The evolution of excited magnons under rotation of one individual stripe with respect to a static magnetic field is investigated. Besides quasi uniform excitations, we observe magneto-static as well as localized excitations. We find a strong influence of inhomogeneous dynamic and static demagnetizing fields for all modes. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4890515
  • Bolometer detection of magnetic resonances in nanoscaled objects
    Rod, I. and Meckenstock, R. and Zähres, H. and Derricks, C. and Mushenok, F. and Reckers, N. and Kijamnajsuk, P. and Wiedwald, U. and Farle, M.
    Nanotechnology 25 (2014)
    We report on a nanoscaled thermocouple (ThC) as a temperature sensor of a highly sensitive bolometer for probing the dissipative damping of spin dynamics in nanosized Permalloy (Py) stripes. The Au-Pd ThC based device is fabricated by standard electron beam lithography on a 200 nm silicon nitride membrane to minimize heat dissipation through the substrate. We show that this thermal sensor allows not only measurements of the temperature change on the order of a few mK due to the uniform resonant microwave (MW) absorption by the Py stripe but also detection of standing spin waves of different mode numbers. Using a 3D finite element method, we estimate the absorbed MW power by the stripe in resonance and prove the necessity of using substrates with an extremely low heat dissipation like a silicon nitride membrane for successful thermal detection. The voltage responsivity and the noise equivalent power for the ThC-based bolometer are equal to 15 V W-1 and 3 nW Hz-1/2, respectively. The ThC device offers a magnetic resonance response of 1 nV/(μB W) corresponding to a sensitivity of 109 spins and a temperature resolution of 300 μK under vacuum conditions. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/25/42/425302
  • Can commercial ferrofluids be exploited in AC magnetic hyperthermia treatment to address diverse biomedical aspects?
    Angelakeris, M. and Li, Z.-A. and Sakellari, D. and Simeonidis, K. and Spasova, M. and Farle, M.
    EPJ Web of Conferences 75 (2014)
    Multifunctional magnetic nanoparticles are considered as promising candidates for various applications combining diagnosis, imaging and therapy. In the present work, we elaborate on the commercial colloidal solution "FluidMAG" (from Chemicell GmbH) as a possible candidate for magnetic hyperthermia application. The current product is a dispersion of magnetite nanoparticles employed for purification or separation of biotinylated biomolecules from different sources (e.g. blood). Transmission Electron Microscopy showed that the NPs have a spherical shape with mean diameter of 12.3 nm (± 20%), and SQUID magnetometry revealed their superparamagnetic character. Our promising results of the AC hyperthermia efficiency of "FluidMAG" suggest that with the appropriate manipulation it can also be exploited as magnetic hyperthermia agent. © Owned by the authors, published by EDP Sciences, 2014.
    view abstract10.1051/epjconf/20147508002
  • Chemically ordered decahedral FePt nanocrystals observed by electron microscopy
    Li, Z.-A. and Spasova, M. and Ramasse, Q.M. and Gruner, M.E. and Kisielowski, C. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The crystal structure of FePt nanoparticles of mean size of 6 nm produced by gas-phase condensation is characterized using a combination of high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM). These FePt nanoparticles are found to be chemically ordered, decahedral shaped, and Pt enriched at the surfaces. The experimentally determined crystallographic lattice constants and distribution of Fe and Pt atoms are compared with first-principles calculations of ordered decahedral FePt nanoparticles to confirm the discovery of a unique decahedral structure with Fe/Pt ordering and Pt surface segregation. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.161406
  • Dipole-dipole interaction in arrays of Fe/FexOy Core/shell nanocubes probed by ferromagnetic resonance
    Sukhov, A. and Horley, P.P. and Berakdar, J. and Terwey, A. and Meckenstock, R. and Farle, M.
    IEEE Transactions on Magnetics 50 (2014)
    This paper represents a detailed theoretical study of the role of the long-range magnetic dipole-dipole interaction (DDI) evidenced by the ferromagnetic resonance (FMR) spectra for the ordered arrays of cubic nanoparticles. We show that the size of the array essentially controls the stability of the system, allowing us to suppress the intermittent low-field excitations starting from the arrays formed by 6 × 6 nanoparticles. Our numerical simulations allow us to determine the threshold inter-particle distance (around 80/100 nm), after which the DDI becomes negligible so that the FMR spectrum of the nanoparticle arrays becomes the same as the spectrum featured by a single nanoparticle.We also compare our simulations with experimental FMR-spectra of 24 Fe/FexOy-nanocubes irregularly placed on a substrate. © 2014 IEEE.
    view abstract10.1109/TMAG.2014.2329814
  • Effect of a side reaction involving structural changes of the surfactants on the shape control of cobalt nanoparticles
    Comesaña-Hermo, M. and Estivill, R. and Ciuculescu, D. and Li, Z.-A. and Spasova, M. and Farle, M. and Amiens, C.
    Langmuir 30 (2014)
    Cobalt nanoparticles with different sizes and morphologies including spheres, rods, disks, and hexagonal prisms have been synthesized through the decomposition of the olefinic precursor [Co(η3-C 8H13)(η4-C8H12)] under dihydrogen, in the presence of hexadecylamine and different rhodamine derivatives, or aromatic carboxylic acids. UV-vis spectroscopy, X-ray diffraction, low and high resolution transmission electron microscopy, and electron tomography have been used to characterize the nanomaterials. Especially, the Co nanodisks formed present characteristics that make them ideal nanocrystals for applications such as magnetic data storage. Focusing on their growth process, we have evidenced that a reaction between hexadecylamine and rhodamine B occurs during the formation of these Co nanodisks. This reaction limits the amount of free acid and amine, usually at the origin of the formation of single crystal Co rods and wires, in the growth medium of the nanocrystals. As a consequence, a growth mechanism based on the structure of the preformed seeds rather than oriented attachment or template assisted growth is postulated to explain the formation of the nanodisks. © 2014 American Chemical Society.
    view abstract10.1021/la5005165
  • Enhancement of magnetization damping coefficient of permalloy thin films with dilute Nd dopants
    Luo, C. and Feng, Z. and Fu, Y. and Zhang, W. and Wong, P.K.J. and Kou, Z.X. and Zhai, Y. and Ding, H.F. and Farle, M. and Du, J. and Zhai, H.R.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    For spintronics application, which requires fast field switching, it is important to have a kind of soft magnetic material with large damping coefficient. Here, we present the studies of the Nd dopant-level-dependent damping coefficient of Ndx-Py(1-x) thin films (30 nm) in a dilute region utilizing ferromagnetic resonance (FMR). With the Nd content increasing, the film structure was found to be changing from polycrystalline to amorphous when the Nd content is around 3.4%. Meanwhile, the magnetization decreases linearly. Interestingly, we find that along the easy axis, both low coercivity and high hysteresis squareness are simultaneously maintained in the system; i.e., the magnetic softness has been well kept. By theoretical fitting of the angular dependence of the FMR field, the first- and second-order magnetic anisotropy constants, K1 and K2, and the Lande g factor are obtained and discussed quantitatively. The measurements of angular and frequency dependence of the ferromagnetic resonance linewidth, as well as the theoretical fitting by considering the contributions of Gilbert damping, two-magnon scattering, and inhomogeneous broadening, show that the damping coefficient α increases rapidly (about 25-fold) as the Nd content increases to 11.6%, which is mainly due to the enhanced spin-orbit coupling by the Nd additives, supported by x-ray magnetic circular dichroism measurements. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.184412
  • Field-dependent perpendicular magnetic anisotropy in CoFeB thin films
    Barsukov, I. and Fu, Y. and Gonçalves, A.M. and Spasova, M. and Farle, M. and Sampaio, L.C. and Arias, R.E. and Krivorotov, I.N.
    Applied Physics Letters 105 (2014)
    We report ferromagnetic resonance measurements of perpendicular magnetic anisotropy in thin films of Ta/Co20Fe60B20/MgO as a function of the Co20Fe60B20 layer thickness. The first and second order anisotropy terms show unexpectedly strong dependence on the external magnetic field applied to the system during the measurements. We propose strong interfacial spin pinning as a possible origin of the field-dependent anisotropy. Our results imply that high-field anisotropy measurements cannot be directly used for quantitative evaluation of zero-field performance parameters of CoFeB-based devices such as spin torque memory. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4897939
  • Hollow and yolk-shell iron oxide nanostructures on few-layer graphene in li-ion batteries
    Sun, Z. and Xie, K. and Li, Z.A. and Sinev, I. and Ebbinghaus, P. and Erbe, A. and Farle, M. and Schuhmann, W. and Muhler, M. and Ventosa, E.
    Chemistry - A European Journal 20 (2014)
    We report a simple and template-free strategy for the synthesis of hollow and yolk-shell iron oxide (FeOx) nanostructures sandwiched between few-layer graphene (FLG) sheets. The morphology and microstructure of this material are characterized in detail by X-ray diffraction, X-ray absorption near-edge structure, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning and transmission electron microscopy. Its properties are evaluated as negative electrode material for Li-ion batteries and compared with those of solid FeOx/FLG and two commercial iron oxides. In all cases, the content of carbon in the electrode has a great influence on the performance. The use of pristine FLG improves the capacity retention and further enhancement is achieved with the hollow structure. For a low carbon loading of 18wt. %, the presence of metallic iron in the hollow and yolk-shell FeOx/FLG composite significantly enhances the capacity retention, albeit with a relatively lower initial reversible capacity, retaining above 97 % after 120cycles at 1000mA g-1 in the voltage range of 0.1-3.0V. © 2014 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstract10.1002/chem.201303723
  • Kinetic arrest and exchange bias in L10 Ni-Mn-Ga
    Çakir, A. and Acet, M. and Farle, M.
    Physica Status Solidi (B) Basic Research 251 (2014)
    Ni-Mn-Ga Heusler alloys are particularly interesting for their magnetic shape-memory properties. However, more recently, the observation of magnetic glass-like behavior in these alloys, particularly at low Ga concentrations, has extended the interest in the properties of these alloys. We have investigated the thermomagnetic properties of the Ni49.8Mn39.1Ga12.1 Heusler alloy with (e/a)=7.98. In this alloy with relatively low Ga concentration, we find kinetic arrest and exchange-bias effects occurring concurrently. We show that these effects are related to the presence of mixed ferromagnetic and antiferromagnetic interactions and that the two components are magnetically phase-separated. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201451301
  • Mechanism of tailored magnetic anisotropy in amorphous Co 68Fe24Zr8 thin films
    Fu, Y. and Barsukov, I. and Meckenstock, R. and Lindner, J. and Raanaei, H. and Hjörvarsson, B. and Farle, M.
    Applied Physics Letters 104 (2014)
    The mechanism of tailored magnetic anisotropy in amorphous Co 68Fe24Zr8 thin films was investigated by ferromagnetic resonance (FMR) on samples deposited without an applied magnetic field, with an out-of-plane field and an in-plane field. Analysis of FMR spectra profiles, high frequency susceptibility calculations, and statistical simulations using a distribution of local uniaxial magnetic anisotropy reveal the presence of atomic configurations with local uniaxial anisotropy, of which the direction can be tailored while the magnitude remains at an intrinsically constant value of 3.0(2) kJ/m3. The in-plane growth field remarkably sharpens the anisotropy distribution and increases the sample homogeneity. The results benefit designing multilayer spintronic devices based on highly homogeneous amorphous layers with tailored magnetic anisotropy. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4865369
  • Neutron diffraction study of the magnetic-field-induced transition in Mn3GaC
    Çakir, Ö. and Acet, M. and Farle, M. and Senyshyn, A.
    Journal of Applied Physics 115 (2014)
    The antiperovskite Mn3GaC undergoes an isostructural cubic-cubic first order transition from a low-temperature, large-cell-volume antiferromagnetic state to a high-temperature, small-cell-volume ferromagnetic state at around 160 K. The transition can also be induced by applying a magnetic field. We study here the isothermal magnetic-field-evolution of the transition as ferromagnetism is stabilized at the expense of antiferromagnetism. We make use of the presence of the two distinct cell volumes of the two magnetic states as a probe to observe by neutron diffraction the evolution of the transition, as the external magnetic field carries the system from the antiferromagnetic to the ferromagnetic state. We show that the large-volume antiferromagnetic and the small-volume ferromagnetic states coexist in the temperature range of the transition. The ferromagnetic state is progressively stabilized as the field increases. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4862903
  • New approaches for measuring electrostatic potentials and charge density distributions in working devices using off-axis and in-line electron holography
    Dunin-Borkowski, R.E. and Migunov, V. and London, A. and Farle, M. and Tavabi, A.H. and Pozzi, G.
    Microscopy and Microanalysis 20 (2014)
    view abstract10.1017/S143192761400302X
  • Room-temperature ferromagnetism in antiferromagnetic cobalt oxide nanooctahedra
    Fontaíña-Troitiño, N. and Liébana-Viñas, S. and Rodríguez-González, B. and Li, Z.-A. and Spasova, M. and Farle, M. and Salgueiriño, V.
    Nano Letters 14 (2014)
    Cobalt oxide octahedra were synthesized by thermal decomposition. Each octahedron-shaped nanoparticle consists of an antiferromagnetic CoO core enclosed by eight {111} facets interfaced to a thin (∼4 nm) surface layer of strained Co3O4. The nearly perfectly octahedral shaped particles with 20, 40, and 85 nm edge length show a weak room-temperature ferromagnetism that can be attributed to ferromagnetic correlations appearing due to strained lattice configurations at the CoO/Co3O4 interface. © 2014 American Chemical Society.
    view abstract10.1021/nl4038533
  • Spin-wave modes in permalloy/platinum wires and tuning of the mode damping by spin Hall current
    Duan, Z. and Boone, C.T. and Cheng, X. and Krivorotov, I.N. and Reckers, N. and Stienen, S. and Farle, M. and Lindner, J.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    We report measurements of spectral properties of spin-wave modes in permalloy/platinum (Py/Pt) bilayer wires magnetized along two principal in-plane axes. We find that the spin torque arising from the spin Hall current in Pt can significantly reduce the spectral linewidth of the bulk and edge spin-wave modes of the wire magnetized perpendicular to its long axis. The linewidth reduction is strongest for the quasiuniform mode and weakest for the edge mode. Our work demonstrates the importance of extrinsic contributions to spin-wave damping for tuning of magnetization dynamics by spin Hall current. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.024427
  • Splitting of spin-wave modes in thin films with arrays of periodic perturbations: Theory and experiment
    Gallardo, R.A. and Banholzer, A. and Wagner, K. and Körner, M. and Lenz, K. and Farle, M. and Lindner, J. and Fassbender, J. and Landeros, P.
    New Journal of Physics 16 (2014)
    A joint theoretical-experimental study focusing on the description of the ferromagnetic resonance response of thin films in the presence of periodic perturbations introduced on the upper film surface is presented. From the viewpoint of theory, these perturbations may exist in the form of any kind of one- or two-dimensional rectangular defect arrays patterned onto one surface of the magnetic film. Indeed, the defects may be pits or bumps, or ion-implanted regions with a lower saturation magnetization. The complete set of response functions, given by the components of the frequency and wave-vector dependent dynamic magnetic susceptibility tensor of the film exposed to microwave excitation, are provided and are used to explain the experimental data. This allows us to obtain the response of the system due to microwave absorption, from which the zero wave-vector spin-wave modes in the field-frequency spectra, including their intensity, are calculated. Explicit calculations for periodic defects featuring the shape of stripes, dots and rectangles are given in detail, as well as experimental results for stripe-like defects prepared either by topographical depressions or by ion implantation of thin magnetic films. The excellent agreement of the theoretical and experimental results manifests the validity of the presented model. © 2014 IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/16/2/023015
  • Tunable emission properties by ferromagnetic coupling Mn(II) aggregates in Mn-doped CdS microbelts/nanowires
    Kamran, M.A. and Liu, R. and Shi, L.-J. and Li, Z.-A. and Marzi, T. and Schöppner, C. and Farle, M. and Zou, B.
    Nanotechnology 25 (2014)
    Tunable optical emission properties from ferromagnetic semiconductors have not been well identified yet. In this work, high-quality Mn(II)-doped CdS nanowires and micrometer belts were prepared using a controlled chemical vapor deposition technique. The Mn doping could be controlled with time, precursor concentration and temperature. These wires or belts can produce both tunable redshifted emissions and ferromagnetic responses simultaneously upon doping. The strong emission bands at 572, 651, 693, 712, 745, 768, 787 and 803 nm, due to the Mn(II) 4T1(4G) → 6A 1(6s) d-d transition, can be detected and accounted for by the aggregation of Mn ions at Cd sites in the CdS lattice at high temperature. These aggregates with ferromagnetism and shifted luminescence are related to the excitonic magnetic polaron (EMP) and localized EMP formations; this is verified by ab initio calculations. The correlation between aggregation-dependent optical emissions and ferromagnetic responses not only presents a new size effect for diluted magnetic semiconductors (DMSs), but also supplies a possible way to study or modulate the ferromagnetic properties of a DMS and to fabricate spin-related photonic devices in the future. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/25/38/385201
  • Uniaxial anisotropy and its manipulation in amorphous Co 68Fe24Zr8 thin films (invited)
    Fu, Y. and Barsukov, I. and Meckenstock, R. and Lindner, J. and Zhai, Y. and Hjörvarsson, B. and Farle, M.
    Journal of Applied Physics 115 (2014)
    We have proven that the growth of Co68Fe24Zr 8 layers under external field yields a uniaxial anisotropy, defined by the direction of the field. No magnetic coupling is present between Co 68Fe24Zr8 layers when separated by a 3nm of Al70Zr30. The anisotropy axis can therefore be manipulated at will and the direction can be tailored, layer by layer in multilayers, by the choice of the direction of the applied field during growth. The g-factor (2.13) and the anisotropy constant, obtained from ferromagnetic resonance, support the existence of short-range order. The relation between the temperature dependences of magnetic anisotropy and magnetization are partially captured by Callen-Callen power law. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4870591
  • Blocked-micropores, surface functionalized, bio-compatible and silica-coated iron oxide nanocomposites as advanced MRI contrast agent
    Darbandi, M. and Laurent, S. and Busch, M. and Li, Z.-A. and Yuan, Y. and Krüger, M. and Farle, M. and Winterer, M. and Vander Elst, L. and Muller, R.N. and Wende, H.
    Journal of Nanoparticle Research 15 (2013)
    Biocompatible magnetic nanoparticles have been found promising in several biomedical applications for tagging, imaging, sensing and separation in recent years. In this article, a systematic study of the design and development of surface-modification schemes for silica-coated iron oxide nanoparticles (IONP) via a one-pot, in situ method at room temperature is presented. Silica-coated IONP were prepared in a water-in-oil microemulsion, and subsequently the surface was modified via addition of organosilane reagents to the microemulsion system. The structure and the morphology of the as synthesized nanoparticles have been investigated by means of transmission electron microscopy (TEM) and measurement of N2 adsorption-desorption. Electron diffraction and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the IONP structures. Nitrogen adsorption indicates microporous and blocked-microporous structures for the silica-coated and amine functionalized silica-coated IONP, respectively which could prove less cytotoxicity of the functionalized final product. Besides, the colloidal stability of the final product and the presence of the modified functional groups on top of surface layer have been proven by zeta-potential measurements. Owing to the benefit from the inner IONP core and the hydrophilic silica shell, the as-synthesized nanocomposites were exploited as an MRI contrast enhancement agent. Relaxometric results prove that the surface functionalized IONP have also signal enhancement properties. These surface functionalized nanocomposites are not only potential candidates for highly efficient contrast agents for MRI, but could also be used as ultrasensitive biological-magnetic labels, because they are in nanoscale size, having magnetic properties, blocked-microporous and are well dispersible in biological environment. © 2013 Springer Science+Business Media Dordrecht.
    view abstract10.1007/s11051-013-1664-8
  • Extended investigation of intermartensitic transitions in Ni-Mn-Ga magnetic shape memory alloys: A detailed phase diagram determination
    Çakir, A. and Righi, L. and Albertini, F. and Acet, M. and Farle, M. and Aktürk, S.
    Journal of Applied Physics 114 (2013)
    Martensitic transitions in shape memory Ni-Mn-Ga Heusler alloys take place between a high temperature austenite and a low temperature martensite phase. However, intermartensitic transformations have also been encountered that occur from one martensite phase to another. To examine intermartensitic transitions in magnetic shape memory alloys in detail, we carried out temperature dependent magnetization, resistivity, and x-ray diffraction measurements to investigate the intermartensitic transition in Ni50Mn50- xGax in the composition range 12 ≤ x ≤ 25 at. %. Rietveld refined x-ray diffraction results are found to be consistent with magnetization and resistivity data. Depending on composition, we observe that intermartensitic transitions occur in the sequences 7 M → L 1 0, 5 M → 7 M, and 5 M → 7 M → L 1 0 with decreasing temperature. The L1 0 non-modulated structure is most stable at low temperature. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4831667
  • Observation of current-driven oscillatory domain wall motion in Ni 80Fe20/Co bilayer nanowire
    Zhang, W. and Wong, P.K.J. and Yan, P. and Wu, J. and Morton, S.A. and Wang, X.R. and Hu, X.F. and Xu, Y.B. and Scholl, A. and Young, A. and Barsukov, I. and Farle, M. and Van Der Laan, G.
    Applied Physics Letters 103 (2013)
    Direct observation of current-driven oscillatory domain wall motion above the Walker breakdown by x-ray magnetic circular dichroism in photoemission electron microscopy is reported in Ni80Fe20/Co nanowire, showing micrometer-scale displacement at ∼13 MHz. We identify two key factors that enhance the oscillatory motion: (i) increase of the hard-axis magnetic anisotropy field value Ḣ and (ii) increase of the ratio between non-adiabatic spin-transfer parameter to the Gilbert damping coefficient, β/α, which is required to be larger than 1. These findings point to an important route to tune the long-scale oscillatory domain wall motion using appropriate geometry and materials. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4816359
  • Single-step synthesis of monolithic comb-like CdS nanostructures with tunable waveguide properties
    Liu, R. and Li, Z.-A. and Zhang, C. and Wang, X. and Kamran, M.A. and Farle, M. and Zou, B.
    Nano Letters 13 (2013)
    Using a simple in situ seeding chemical vapor deposition (CVD) process, comb-like (branched) monolithic CdS micro/nanostructures were grown. Efficient optical coupling between the backbone and the teeth of the branched architecture is demonstrated by distributing light from an UV-laser-excited spot at one end of the backbone to all branch tips. By varying the deposition conditions, the orientation of the branches with respect to the backbone, their size and density can be tuned as well as the size of the backbone. This in situ seeding CVD method has the potential for a low-cost single-step fabrication of high-quality, micro/nanointegrated photonic devices, with tunable complex waveguiding possibilities. © 2013 American Chemical Society.
    view abstract10.1021/nl401726z
  • Surface morphology and atomic structure of thin layers of Fe3Si on GaAs(001) and their magnetic properties
    Noor, S. and Barsukov, I. and Özkan, M.S. and Elbers, L. and Melnichak, N. and Lindner, J. and Farle, M. and Köhler, U.
    Journal of Applied Physics 113 (2013)
    The structural and magnetic properties of ultrathin near-stoichiometric Fe3Si layers on GaAs(001) are investigated after using scanning tunneling microscopy (STM) analysis to optimize the deposition process. This includes atomic resolution imaging of the surface as measured by STM revealing the atomic ordering and characteristic defects in the topmost layers. Emphasis is laid on connections between the layer morphology and its magnetic properties, which are analysed by in situ MOKE, FMR, and SQUID magnetometry. Upon nucleation, the Fe3Si islands behave like superparamagnetic nanoparticles where we find a quantitative agreement between the size of the nanoparticles and their superspin. At higher coverage, the Fe3Si layers show ferromagnetic behaviour. Here, we investigate the superposition of the magnetocrystalline and the uniaxial anisotropies where the latter can be excluded to be caused by shape anisotropy. Furthermore, an unexpected increase of the magnetic moment towards low coverage can be observed which apart from an increased orbital moment can be attributed to an increased step density. © 2013 American Institute of Physics.
    view abstract10.1063/1.4795163
  • Carbon nanotube bolometer: Transport properties and noise characteristics
    Rod, I. and Wirtz, C. and Kazakova, O. and Panchal, V. and Cox, D. and Zähres, H. and Posth, O. and Lindner, J. and Meckenstock, R. and Farle, M.
    Solid State Phenomena 190 (2012)
    The temperature dependent resistance and the noise characteristics of an individual multiwall carbon nanotube (CNT) decorated with a finite number of magnetic nanocubes are investigated. We show that CNT is a highly sensitive bolometer and can enable measurements of magnetic resonance in a single nanoparticle. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/SSP.190.510
  • Electron transport in partially filled iron carbon nanotubes
    Migunov, V. and Li, Z.-A. and Spasova, M. and Farle, M.
    Solid State Phenomena 190 (2012)
    We report electron transport properties of iron filled multiwalled carbon nanotubes (MWCNT) with outer diameters of 30 to 80 nm and lengths of 1 to 10 μm. Our study is combined with a structural investigation of the iron core using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). It was found that high current densities of 1.8×107A/cm2 increase the conductivity of the MWCNT by a factor of two at 300 K, while the Fe core disappears probably forming defect states in the carbon shells. The enhanced diffusion of iron is most probably the result of local heating of the iron followed by implantation of iron atoms in the nanotube layers. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/SSP.190.498
  • Hysteresis effects in the inverse magnetocaloric effect in martensitic Ni-Mn-In and Ni-Mn-Sn
    Titov, I. and Acet, M. and Farle, M. and González-Alonso, D. and Mañosa, L. and Planes, A. and Krenke, T.
    Journal of Applied Physics 112 (2012)
    The presence of a large inverse magnetocaloric effect around the martensitic transformation in Ni-Mn-Sn and Ni-Mn-In alloys is expected to lead to substantial cooling on applying a magnetic field. However, the occurrence of hysteresis around the transition causes limitations on adiabatic temperature-changes. We study the adiabatic temperature-change in both systems in relation to the hysteresis effects. Ni-Mn-In, having a relatively narrower hysteresis and a greater shift of the characteristic transition temperatures with applied field with respect to Ni-Mn-Sn, shows reversibility in the adiabatic-temperature change related to the inverse magnetocaloric effect when the state of the system is cycled within a minor transitional hysteresis loop. Ni-Mn-Sn does not show reversibility in the inverse magnetocaloric effect under cycling-fields up to 5 T. The reversibility in the adiabatic temperature-change is directly related to the reversibility in the relative amount of austenite and martensite in the sample when the field is cycled. © 2012 American Institute of Physics.
    view abstract10.1063/1.4757425
  • Imaging magnetic responses of nanomagnets by XPEEM
    Sandig, O. and Herrero-Albillos, J. and Römer, F.M. and Friedenberger, N. and Kurde, J. and Noll, T. and Farle, M. and Kronast, F.
    Journal of Electron Spectroscopy and Related Phenomena 185 (2012)
    The Spin-resolved Photoelectron Emission Microscope (SPEEM) is a permanently installed set-up at Helmholtz-Zentrum Berlin (HZB). Due to its specific contrast it is mainly used for magnetic imaging and micro-spectroscopy with quantitative analysis. A crucial point in magnetic imaging is the application of magnetic fields. Many experiments require observation of magnetic responses or the preparation of a certain magnetic state during the measurement. We present a dedicated magnetic sample holder combining magnetic field during imaging with additional temperature control. This set-up enables SPEEM to measure magnetization curves of individual Fe nanocubes (18 nm) 3 in size. If additionally alternating magnetic fields are applied we can image the local magnetic AC susceptibility (χAC) as a function of temperature. The latter is ideally suited to visualize local variations of the Curie temperature (TC) in nano- and microstructures. © 2012 Elsevier B.V.
    view abstract10.1016/j.elspec.2012.07.005
  • In situ multifrequency ferromagnetic resonance and x-ray magnetic circular dichroism investigations on Fe/GaAs(110): Enhanced g-factor
    Römer, F.M. and Möller, M. and Wagner, K. and Gathmann, L. and Narkowicz, R. and Zähres, H. and Salles, B.R. and Torelli, P. and Meckenstock, R. and Lindner, J. and Farle, M.
    Applied Physics Letters 100 (2012)
    We determined the magnetic anisotropy energy and g-factor of an uncapped 10 nm thick Fe/GaAs(110) film using a setup that allows frequency (1.5-26.5 GHz) as well as angular dependent ferromagnetic resonance measurements under ultrahigh vacuum conditions. The g-factor g = 2. 61 ± 0. 1 is unusually high at room temperature and can be interpreted as the result of an increased orbital moment due to strain. This interpretation is supported by more surface sensitive x-ray magnetic circular dichroism measurements which yield g = 2.21 ± 0. 02 measured at remanence. The difference in g may be the result of magnetic field dependent magnetostriction which influences the orbital moment. © 2012 American Institute of Physics.
    view abstract10.1063/1.3687726
  • Influence of Tb doping on the luminescence characteristics of ZnO nanoparticles
    Sharma, A. and Dhar, S. and Singh, B.P. and Kundu, T. and Spasova, M. and Farle, M.
    Journal of Nanoparticle Research 14 (2012)
    Structural and optical properties of the Tb-doped ZnO nanoparticles with average diameter ≈4 nm have been systematically investigated. Our X-ray diffraction studies show a contraction of the ZnO lattice with the increase of the Tb mole-fraction x for x ≤ 0.02 and an expansion beyond x ≈ 0.02. The photoluminescence spectra are found to be comprised of a near band edge ultra violet luminescence (UVL) and a broad green luminescence (GL) band. Under the atmospheric condition, the intensity of the GL band is found to increase with the Tb molefraction over the entire doping range. On the other hand, under the vacuum condition, it has been observed that the GL intensity decreases with the increase of x up to x ≈ 0.02 but further increase of x leads to a gradual revival of the GL emission. Our study suggests that for x ≤ 0.02, GL results due to the physisorption of certain groups on the surface of the nanoparticles (GL-groups). It is also found that in this Tb mole-fraction regime, Tb incorporates mostly on the surface of the nanoparticles and affects the UVL to GL intensity ratio by influencing the attachment of the GL-groups. However, for x > 0.02, GL originates not only from the GL-groups but also from certain point defects, which are likely to be generated due to the incorporation of Tb in the core of the nanoparticles. A simple rate equation model is introduced to get a quantitative understanding about the variation of the density of the centers responsible for the GL emission as a function of x under the atmospheric and the vacuum conditions. © Springer Science+Business Media B.V. 2012.
    view abstract10.1007/s11051-011-0676-5
  • Intrinsic magnetism and collective magnetic properties of size-selected nanoparticles
    Antoniak, C. and Friedenberger, N. and Trunova, A. and Meckenstock, R. and Kronast, F. and Fauth, K. and Farle, M. and Wende, H.
    NanoScience and Technology 79 (2012)
    Using size-selected spherical FePt nanoparticles and cubic Fe/Fe-oxide nanoparticles as examples, we discuss the recent progress in the determination of static and dynamic properties of nanomagnets. Synchroton radiation-based characterization techniques in combination with detailed structural, chemical and morphological investigations by transmission and scanning electron microscopy allow the quantitative correlation between element-specific magnetic response and spin structure on the one hand and shape, crystal and electronic structure of the particles on the other hand. Examples of measurements of element-specific hysteresis loops of single 18 nm sized nanocubes are discussed. Magnetic anisotropy of superparamagnetic ensembles and their dynamic magnetic response are investigated by ferromagnetic resonance as a function of temperature at different microwave frequencies. Such investigations allow the determination of the magnetic relaxation and the extraction of the average magnetic anisotropy energy density of the individual particles. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_11
  • Rapid and surfactant-free synthesis of bimetallic Pt-Cu nanoparticles simply via ultrasound-assisted redox replacement
    Sun, Z. and Masa, J. and Xia, W. and König, D. and Ludwig, Al. and Li, Z.-A. and Farle, M. and Schuhmann, W. and Muhler, M.
    ACS Catalysis 2 (2012)
    The synthesis of bimetallic nanoparticles (NPs) with well-defined morphology and a size of <5 nm remains an ongoing challenge. Here, we developed a facile and efficient approach to the design of bimetallic nanostructures by the galvanic replacement reaction facilitated by high-intensity ultrasound (100 W, 20 kHz) at low temperatures. As a model system, Pt-Cu NPs deposited on nitrogen-doped carbon nanotubes (NCNTs) were synthesized and characterized by spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) inspection shows that the mean diameter of Pt-Cu NPs can be as low as ≈2.8 nm, regardless of the much larger initial Cu particle size, and that a significant increase in particle number density by a factor of 35 had occurred during the replacement process. The concentration of the Pt precursor solution as well as of the size of the seed particles were found to control the size of the bimetallic NPs. Energy dispersive X-ray spectroscopy performed in the scanning TEM mode confirmed the alloyed nature of the Pt-Cu NPs. Electrochemical oxygen reduction measurements demonstrated that the resulting Pt-Cu/NCNT catalysts exhibit an approximately 2-fold enhancement in both mass- and area-related activities compared with a commercial Pt/C catalyst. © 2012 American Chemical Society.
    view abstract10.1021/cs300187z
  • Silver, gold, and alloyed silver-gold nanoparticles: Characterization and comparative cell-biologic action
    Mahl, D. and Diendorf, J. and Ristig, S. and Greulich, C. and Li, Z.A. and Farle, M. and Köller, M. and Epple, M.
    Journal of Nanoparticle Research 14 (2012)
    Silver, gold, and silver-gold-alloy nanoparticles were prepared by citrate reduction modified by the addition of tannin during the synthesis, leading to a reduction in particle size by a factor of three. Nanoparticles can be prepared by this easy waterbased synthesis and subsequently functionalized by the addition of either tris(3-sulfonatophenyl)phosphine or poly(N-vinylpyrrolidone). The resulting nanoparticles of silver (diameter 15-25 nm), gold (5-6 nm), and silver-gold (50:50; 10-12 nm) were easily dispersable in water and also in cell culture media (RPMI + 10 % fetal calf serum), as shown by nanoparticle tracking analysis and differential centrifugal sedimentation. High-resolution transmission electron microscopy showed a polycrystalline nature of all nanoparticles. EDX on single silver-gold nanoparticles indicated that the concentration of gold is higher inside a nanoparticle. The biologic action of the nanoparticles toward human mesenchymal stem cells (hMSC) was different: Silver nanoparticles showed a significant concentration-dependent influence on the viability of hMSC. Gold nanoparticles showed only a small effect on the viability of hMSC after 7 days. Surprisingly, silver-gold nanoparticles had no significant influence on the viability of hMSC despite the silver content. Silver nanoparticles and silver-gold nanoparticles in the concentration range of 5-20 μg mL -1 induced the activation of hMSC as indicated by the release of IL-8. In contrast, gold nanoparticles led to a reduction of the release of IL-6 and IL-8. © Springer Science+Business Media B.V. 2012.
    view abstract10.1007/s11051-012-1153-5
  • Stable single domain Co nanodisks: Synthesis, structure and magnetism
    Comesaña-Hermo, M. and Ciuculescu, D. and Li, Z.-A. and Stienen, S. and Spasova, M. and Farle, M. and Amiens, C.
    Journal of Materials Chemistry 22 (2012)
    Colloidal synthesis of morphologically stable oxide-free cobalt nanodisks (diameter ≈ 21 nm, thickness ≈ 12 nm) has been achieved. The hcp c-axis is found to be perpendicular to the flat side of the disk which is explained by surface energy considerations. Magnetic measurements and comparisons with simulations show that the nanodisks are single magnetic domains with a magnetisation oriented perpendicular to the plane of the disks and reaching that of bulk cobalt, and high anisotropy field. These disks are thus potential candidates for magnetic data storage, magnetic sensor arrays, and cheap permanent magnets. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2jm16751d
  • Tuning magnetic relaxation by oblique deposition
    Barsukov, I. and Landeros, P. and Meckenstock, R. and Lindner, J. and Spoddig, D. and Li, Z.-A. and Krumme, B. and Wende, H. and Mills, D.L. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    Oblique deposition conditions of Si were used to create a periodic compositional defect matrix in Fe 3Si/MgO(001) thin films. The modified growth conditions provoke shadow effects, which lead to a two-magnon scattering channel with twofold symmetry in the film plane. Its axis is controlled by the sample orientation with respect to the Si evaporator. Angular-dependent ferromagnetic resonance data reveal an enhanced magnetic-relaxation rate induced by the dipolar interactions originating from these artificially created defect structures, while magnetic anisotropy is shown to be influenced negligibly. Experimental results agree well with the developed theoretical approach allowing one to distinguish different relaxation channels. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.014420
  • A guideline for atomistic design and understanding of ultrahard nanomagnets
    Antoniak, C. and Gruner, M.E. and Spasova, M. and Trunova, A.V. and Römer, F.M. and Warland, A. and Krumme, B. and Fauth, K. and Sun, S. and Entel, P. and Farle, M. and Wende, H.
    Nature Communications 2 (2011)
    Magnetic nanoparticles are of immense current interest because of their possible use in biomedical and technological applications. Here we demonstrate that the large magnetic anisotropy of FePt nanoparticles can be significantly modified by surface design. We employ X-ray absorption spectroscopy offering an element-specific approach to magnetocrystalline anisotropy and the orbital magnetism. Experimental results on oxide-free FePt nanoparticles embedded in Al are compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allows us to present three rules to achieve desired magnetic properties. In addition, concrete suggestions of capping materials for FePt nanoparticles are given for tailoring both magnetocrystalline anisotropy and magnetic moments. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms1538
  • Effect of microwave irradiation on spin-torque-driven magnetization precession in nanopillars with magnetic perpendicular anisotropy
    Reckers, N. and Cucchiara, J. and Posth, O. and Hassel, C. and Römer, F.M. and Narkowicz, R. and Gallardo, R.A. and Landeros, P. and Zähres, H. and Mangin, S. and Katine, J.A. and Fullerton, E.E. and Dumpich, G. and Meckenstock, R. and Lindner, J. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    The effect of microwave irradiation on the spin-torque-driven magnetization dynamics is studied in (Co/Ni)-based nanopillar spin valves with perpendicular magnetic anisotropy. For this purpose, a setup was developed to measure the ac as well as the dc resistance of the nanopillar under applied fields and injected polarized currents, while irradiating microwaves with varying frequency (6-18 GHz) and power. We find that the microwave irradiation amplifies and maintains the precessional state of the eigenresonance within a larger field range. The experiments are discussed in comparison to micromagnetic as well as macrospin simulations utilizing the nonlinearized Landau-Lifshitz-Gilbert equation. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.184427
  • Element-specific magnetic hysteresis of individual 18 nm Fe nanocubes
    Kronast, F. and Friedenberger, N. and Ollefs, K. and Gliga, S. and Tati-Bismaths, L. and Thies, R. and Ney, A. and Weber, R. and Hassel, C. and Römer, F.M. and Trunova, A.V. and Wirtz, C. and Hertel, R. and Dürr, H.A. and Farle, M.
    Nano Letters 11 (2011)
    Correlating the electronic structure and magnetic response with the morphology and crystal structure of the same single ferromagnetic nanoparticle has been up to now an unresolved challenge. Here, we present measurements of the element-specific electronic structure and magnetic response as a function of magnetic field amplitude and orientation for chemically synthesized single Fe nanocubes with 18 nm edge length. Magnetic states and interactions of monomers, dimers, and trimers are analyzed by X-ray photoemission electron microscopy for different particle arrangements. The element-specific electronic structure can be probed and correlated with the changes of magnetic properties. This approach opens new possibilities for a deeper understanding of the collective response of magnetic nanohybrids in multifunctional materials and in nanomagnetic colloidal suspensions used in biomedical and engineering technologies. © 2011 American Chemical Society.
    view abstract10.1021/nl200242c
  • Frequency dependence of spin relaxation in periodic systems
    Barsukov, I. and Römer, F.M. and Meckenstock, R. and Lenz, K. and Lindner, J. and Hemken To Krax, S. and Banholzer, A. and Körner, M. and Grebing, J. and Fassbender, J. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We show that in the presence of a periodic scattering potential the spin relaxation in ultrathin ferromagnets is not a monotonous function of the frequency, as has been usually assumed taking intrinsic Gilbert and extrinsic two-magnon processes into account. The spin relaxation rate is found to substantially increase at characteristic frequencies related to the periodicity of the magnon scattering potential. We propose a theoretical model which is experimentally confirmed in Ni80Fe20 thin films by artificially introducing different scattering periodicities. As a result, the current general approach for determining spin relaxation parameters in thin films has to be reconsidered. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.140410
  • Magnetocrystalline anisotropy and Gilbert damping in iron-rich Fe 1-xSix thin films
    Barsukov, I. and Mankovsky, S. and Rubacheva, A. and Meckenstock, R. and Spoddig, D. and Lindner, J. and Melnichak, N. and Krumme, B. and Makarov, S.I. and Wende, H. and Ebert, H. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    The magnetocrystalline anisotropy of Fe1-xSix (0≤x≤0.4) epitaxial thin films on MgO(001) was studied by ferromagnetic resonance. The experimental results are in good agreement with theoretical predictions of ab initio electronic structure calculations using the fully relativistic Korringa-Kohn-Rostoker Green's function method within spin-density-functional theory. The Gilbert damping α is found to be isotropic by theory and experiment with a minimum at the composition x=0.2. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.180405
  • Photomodulation of the magnetisation of Co nanocrystals decorated with Rhodamine B
    Comesaña-Hermo, M. and Estivill, R. and Ciuculescu, D. and Amiens, C. and Farle, M. and Batat, P. and Jonusauskas, G. and McClenaghan, N.D. and Lecante, P. and Tardin, C. and Mazeres, S.
    ChemPhysChem 12 (2011)
    How exciting! Upon excitation of Rhodamine B with visible light in magnetic Co nanocrystal-Rhodamine B nanocomposites, electron transfer from the nanocrystal to the dye is evidenced as well as an increase in magnetisation (see picture), affording a new access to photomodulation of the magnetic properties of nanocrystal assemblies. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201100616
  • Planar-defect characteristics and cross-sections of 〈001〉, 〈111〉, and 〈112〉 InAs nanowires
    Li, Z.-A. and Mller, C. and Migunov, V. and Spasova, M. and Farle, M. and Lysov, A. and Gutsche, C. and Regolin, I. and Prost, W. and Tegude, F.-J. and Ercius, P.
    Journal of Applied Physics 109 (2011)
    We report on detailed structural and morphological characterizations of InAs nanowires of 〈001〉, 〈111〉, and 〈112〉 crystallographic directions grown on (001)B InAs wafer substrates using high-resolution transmission electron microscopy. We find that 〈001〉 -oriented InAs nanowires are cubic zincblende-type structure and free of planar defects. The 〈111〉- and 〈112〉-oriented InAs nanowires both have densely twinned (111) planar defects that are perpendicular and parallel to the growth direction, respectively. The cross sections of all three types of InAs nanowires are obtained from 3D reconstructions using electron tomography. The characteristics of the planar defects and the 3D wire shape should provide better estimations of microstructure-relevant physical properties, such as conductivity and Young's modulus of InAs nanowires. © 2011 American Institute of Physics.
    view abstract10.1063/1.3592186
  • Structure, morphology, and aging of Ag-Fe dumbbell nanoparticles
    Elsukova, A. and Li, Z.-A. and Möller, C. and Spasova, M. and Acet, M. and Farle, M. and Kawasaki, M. and Ercius, P. and Duden, T.
    Physica Status Solidi (A) Applications and Materials Science 208 (2011)
    Dumbbell-shaped or Janus-type nanocomposites provide multifunctional properties with various diagnostic and therapeutic applications in biomedicine. We have prepared dumbbell Ag-Fe nanoparticles by magnetron sputtering with subsequent in-flight annealing. Structural properties and chemical compositions of freshly prepared and 5-month aged particles were examined by means of transmission electron microscopy including high-resolution imaging, energy dispersive X-ray spectroscopy, and 3D electron tomography. Fresh particles consist of a faceted Ag part on a Fe-Fe 3O 4 composite particle of more spherical shape. Aging changes the crystallinity and morphology of the particles. The aged nanocomposite consists of a silver spherical particle that is attached to a hollow iron oxide sphere containing one or several silver clusters inside. TEM images of the fresh (a) and aged (b) Ag-Fe nanoparticles. (c) 3D reconstructed image of an aged Ag-Fe particle with color segmentation. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201127104
  • Tailored magnetic anisotropy in an amorphous trilayer
    Fu, Y. and Barsukov, I. and Raanaei, H. and Spasova, M. and Lindner, J. and Meckenstock, R. and Farle, M. and Hjrvarsson, B.
    Journal of Applied Physics 109 (2011)
    An amorphous Co68Fe24Zr8(3 nm)/Al 70Zr30(3 nm)/Co68Fe24Zr 8(3 nm) trilayer system has been investigated using in-plane and out-of-plane angular dependent ferromagnetic resonance at different frequencies. The in-plane magnetic anisotropy is uniaxial, retaining its value of (2.9±0.1)×103J/m3 for each magnetic layer, whereas its direction was tailored independently in an arbitrary manner by applying an external magnetic field during the film deposition. The perpendicular anisotropy constant, supposed to reflect the interface quality, is nearly identical for both layers. Furthermore, the magnetic layers act independently upon each other due to the absence of interlayer coupling. © 2011 American Institute of Physics.
    view abstract10.1063/1.3587189
  • Visualization of spin dynamics in single nanosized magnetic elements
    Banholzer, A. and Narkowicz, R. and Hassel, C. and Meckenstock, R. and Stienen, S. and Posth, O. and Suter, D. and Farle, M. and Lindner, J.
    Nanotechnology 22 (2011)
    The design of future spintronic devices requires a quantitative understanding of the microscopic linear and nonlinear spin relaxation processes governing the magnetization reversal in nanometer-scale ferromagnetic systems. Ferromagnetic resonance is the method of choice for a quantitative analysis of relaxation rates, magnetic anisotropy and susceptibility in a single experiment. The approach offers the possibility of coherent control and manipulation of nanoscaled structures by microwave irradiation. Here, we analyze the different excitation modes in a single nanometer-sized ferromagnetic stripe. Measurements are performed using a microresonator set-up which offers a sensitivity to quantitatively analyze the dynamic and static magnetic properties of single nanomagnets with volumes of (100nm)3. Uniform as well as non-uniform volume modes of the spin wave excitation spectrum are identified and found to be in excellent agreement with the results of micromagnetic simulations which allow the visualization of the spatial distribution of these modes in the nanostructures. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/22/29/295713
  • Composition dependence of exchange stiffness in FexPt 1-x alloys
    Antoniak, C. and Lindner, J. and Fauth, K. and Thiele, J.-U. and Minár, J. and Mankovsky, S. and Ebert, H. and Wende, H. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    The exchange stiffness constants of chemically disordered Fe xPt1-x films with thickness around 50 nm were determined by means of ferromagnetic resonance. It was found to increase with increasing Fe content from 6±4 pJ/m for x=0.27 to 15±4 pJ/m for x=0.67. Theoretical results from fully relativistic and scalar-relativistic band-structure calculations using the Korringa-Kohn-Rostoker method confirm the experimentally obtained values. In addition, determination of the magnetocrystalline anisotropy by angular-dependent measurements of the ferromagnetic resonance gave the possibility to estimate the exchange length that was found to be 40-50 nm for all compositions investigated in this work. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.064403
  • Ferromagnetic resonance in Ni-Mn based ferromagnetic Heusler alloys
    Aksoy, S. and Posth, O. and Acet, M. and Meckenstock, R. and Lindner, J. and Farle, M. and Wassermann, E.F.
    Journal of Physics: Conference Series 200 (2010)
    Ferromagnetic Ni-Mn based Heusler alloys undergo martensitic transformations leading to properties such as magnetic shape memory, magnetic field induced strain and magneto-caloric effects. The occurrence of such effects are closely related to the nature of magnetic interactions around the transition. These interactions can be closely examined by the ferromagnetic resonance (FMR) technique. Here, we report on the results of FMR studies performed at various temperatures in the martensite and austenite states of powder samples and discuss the mixed nature of the magnetic interactions in the martensitic state. © 2010 IOP Publishing Ltd.
    view abstract10.1088/1742-6596/200/9/092001
  • High-resolution X-ray lensless imaging by differential holographic encoding
    Zhu, D. and Guizar-Sicairos, M. and Wu, B. and Scherz, A. and Acremann, Y. and Tyliszczak, T. and Fischer, P. and Friedenberger, N. and Ollefs, K. and Farle, M. and Fienup, J.R. and Stöhr, J.
    Physical Review Letters 105 (2010)
    We demonstrate in the soft x-ray regime a novel technique for high-resolution lensless imaging based on differential holographic encoding. We have achieved superior resolution over x-ray Fourier transform holography while maintaining the signal-to-noise ratio and algorithmic simplicity. We obtain a resolution of 16 nm by synthesizing images in the Fourier domain from a single diffraction pattern, which allows resolution improvement beyond the reference fabrication limit. Direct comparisons with iterative phase retrieval and images from state-of-the-art zone-plate microscopes are presented. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevLett.105.043901
  • Interfacial Cu/ZnO contact by selective photodeposition of copper onto the surface of small ZnO nanoparticles in non-aqueous colloidal solution
    Sliem, M.A. and Hikov, T. and Li, Z.-A. and Spasova, M. and Farle, M. and Schmidt, D.A. and Havenith-Newen, M. and Fischer, R.A.
    Physical Chemistry Chemical Physics 12 (2010)
    Nanoscale copper was selectively photodeposited onto the surface of hexadecylamine (HDA) stabilized (monodispersed not agglomerated) ZnO nanoparticles (NPs) of a diameter of 2-5 nm, which leads to HDA-stabilized Cu/ZnO NPs of varied Cu loading. The particles are soluble in non-polar organic solvents. The line broadening and the red shift of the surface plasmon band of Cu/ZnO NPs relative to HDA-stabilized Cu NPs, the profound decrease of the Cu/ZnO NPs visible photoluminescence at 525 nm, the increase of the UV emission intensity at 365 nm and the enhancement of the Raman scattering (RS) intensity in comparison to the parent ZnO NPs confirmed the interfacial contact between the Cu and ZnO phase. © 2010 the Owner Societies.
    view abstract10.1039/c003861j
  • Magnetic hardness of Fe60Pt40 nanoparticles controlled by surface chemistry
    Serantes, D. and Spasova, M. and Baldomir, D. and Farle, M. and Salgueirino, V.
    Chemistry of Materials 22 (2010)
    Fe60Pt40 nanoparticles stabilized by oleic acid/oleylamine or tetramethylammonium hydroxide and self-assembled in 3D dispersions permit a detailed analysis of the competition of surface, finitesize effects and magnetic interparticle interactions which controls the collective macroscopic magnetic behavior. Temperature dependent magnetometry demonstrates that for FePt nanoparticles with identical size distribution but different surface chemistry, substantial differences of the effective magnetic anisotropy exist and can be understood by comparison with different theoretical models. Finally, a model yielding quantitative data for the competing intrinsic magnetic parameters of complex core-shell nanoparticles is derived. © 2010 American Chemical Society.
    view abstract10.1021/cm1010967
  • Segregation in metastable Fe-Cu nanoparticles
    Yelsukova, A. and Zi-An, L. and Acet, M. and Spasova, M. and Farle, M.
    Journal of Physics: Conference Series 200 (2010)
    Fe-Cu nanoparticles have been prepared by sputtering and subsequent in-flight sintering. Particles deposited onto amorphous carbon are examined by electron diffraction, energy dispersive x-ray line-scans and electron energy loss spectroscopy using high resolution transmission electron microscopy. The results show that non-sintered particles form a metastable Fe-Cu alloy, whereas the sintered particles undergo a spinoidal decomposition leading to an iron-rich core and a Cu-rich shell. The investigations are carried out on particles of various sizes ranging from 5-50 nm. Within this size range, the sintered particles show similar compositional properties. © 2010 IOP Publishing Ltd.
    view abstract10.1088/1742-6596/200/7/072109
  • Spatially resolved measurements of the ferromagnetic phase transition by ac-susceptibility investigations with x-ray photoelectron emission microscope
    Römer, F.M. and Kronast, F. and Heyne, L. and Hassel, C. and Banholzer, A. and Kläui, M. and Meckenstock, R. and Lindner, J. and Farle, M.
    Applied Physics Letters 96 (2010)
    Spatially resolved ac susceptibility measurements on epitaxial Fe films are performed as a function of temperature using a conventional soft-x-ray photoelectron emission microscope. A magnetic contrast is observed at sample locations where the magnetic film undergoes a para/ferromagnetic phase transition. Due to the wedge structure of the Fe film and the thickness dependence of the Curie temperature the spatial extend of the phase transition region and the correlation length can be estimated. © 2010 American Institute of Physics.
    view abstract10.1063/1.3360205
  • Stearate-Based Cu Colloids in Methanol Synthesis: Structural Changes Driven by Strong Metal-Support Interactions
    Schimpf, S. and Rittermeier, A. and Zhang, X. and Li, Z.-A. and Spasova, M. and van den Berg, M.W.E. and Farle, M. and Wang, Y. and Fischer, R.A. and Muhler, M.
    ChemCatChem 2 (2010)
    Metal stearate-stabilized Cu nanoparticles, synthesized by an efficient one-step process, were applied in the continuous liquid-phase synthesis of methanol. After optimizing the reduction procedure, twofold higher rates of methanol formation were found for Cu-Zn colloids, compared to the conventional ternary Cu/ZnO/Al2O3 catalyst applied as fine powder in the liquid phase. Structural changes were investigated as a function of time on stream; after reduction in H2, spherical, well-separated 5-10 nm Cu particles stabilized by a Zn stearate shell were found. Under catalytic high-pressure conditions Zn stearate was hydrolyzed forming ZnO. High-resolution transmission electron microscopy revealed the presence of triangular ZnO prisms with truncated edges. Applying optimized synthesis conditions these triangularly shaped ZnO particles were found to be mostly attached to the spherical Cu particles. The catalytic results and the structural and spectroscopic characterization suggest that these ZnO particles act as a reservoir, releasing ZnOx species, which diffuse onto the Cu particles and promote the catalytic activity. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.200900252
  • Tailoring spin relaxation in thin films by tuning extrinsic relaxation channels
    Barsukov, I. and Meckenstock, R. and Lindner, J. and Moller, M. and Hassel, C. and Posth, O. and Farle, M. and Wende, H.
    IEEE Transactions on Magnetics 46 (2010)
    The importance of extrinsic spin relaxation processes such as magnonmagnon scattering for the overall damping in ferromagnetic thin films has been shown for low relaxation rate systems. Due to its anisotropic behavior, it offers an opportunity for controlling and tailoring the spin relaxation. In this paper, a ferromagnetic resonance study of a system with pure Gilbert damping Fe 94.5Si5.5/MgO(001) is shown. Fe3 Si/MgO(001) systems with native magnonmagnon scattering are discussed. Possibilities for inducing magnonmagnon scattering by volume and surface defects in these systems are presented, offering a method for controlled tailoring of the overall damping in thin films. © 2006 IEEE.
    view abstract10.1109/TMAG.2010.2044482
  • X-ray absorption measurements on nanoparticle systems: Self-assembled arrays and dispersions
    Antoniak, C. and Warland, A. and Darbandi, M. and Spasova, M. and Trunova, A. and Fauth, K. and Aziz, E.F. and Farle, M. and Wende, H.
    Journal of Physics D: Applied Physics 43 (2010)
    X-ray absorption spectroscopy methods are presented as a useful tool to determine local structure, composition and magnetic moments as well as to estimate the effective anisotropy of substrate supported self-assembled arrays of wet-chemically synthesized FePt nanoparticles. A compositional inhomogeneity within the nanoparticles yields reduced magnetic moments with respect to the corresponding bulk material and may also hinder the formation of the chemically ordered L10 phase in FePt nanoparticles. The latter is indicated by a reduced effective anisotropy, which is one order of magnitude smaller than expected from the known value of the corresponding bulk material. As a new approach, measurements of the x-ray absorption near-edge structure of Fe-oxide nanoparticles in dispersion are presented and ageing effects are discussed on the basis of multiplet calculations. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/43/47/474007
  • ferromagnetic materials

  • functionalization and hybrid nanomagnets

  • magnetic materials

  • magnetism

  • nanocrystals

  • spin dynamics and spin transport

  • thin films

  • transmission electron microscopy

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