Dr. Ulf Wiedwald

Experimental Physics
University of Duisburg-Essen


  • Limited Elemental Mixing in Nanoparticles Generated by Ultrashort Pulse Laser Ablation of AgCu Bilayer Thin Films in a Liquid Environment: Atomistic Modeling and Experiments
    Shih, C.-Y. and Chen, C. and Rehbock, C. and Tymoczko, A. and Wiedwald, U. and Kamp, M. and Schuermann, U. and Kienle, L. and Barcikowski, S. and Zhigilei, L.V.
    Journal of Physical Chemistry C (2021)
    Pulsed laser ablation in liquids (PLAL) is a promising technique for the generation of colloidal alloy nanoparticles that are of high demand in a broad range of fields, including catalysis, additive manufacturing, and biomedicine. Many of the applications have stringent requirements on the nanoparticle composition and size distributions, which can only be met through innovations in the PLAL technique guided by a clear understanding of the nanoparticle formation mechanisms. In this work, we undertake a combined computational and experimental study of the nanoparticle formation mechanisms in ultrashort PLAL of Ag/Cu and Cu/Ag bilayer thin films. Experimental probing of the composition of individual nanoparticles and predictions from large-scale atomistic simulations provide consistent evidence of limited mixing between the two components from bilayer films by PLAL. The simulated and experimental distributions of nanoparticle compositions exhibit an enhanced abundance of Ag-rich and Cu-rich nanoparticles, as well as a strongly depressed population of well-mixed alloy nanoparticles. The surprising observation that the nanoscale phase separation of the two components in the bilayer films manifests itself in the sharp departure from the complete quantitative mixing in the colloidal nanoparticles is explained by the complex dynamic interaction between the ablation plume and liquid environment revealed in the simulations of the initial stage of the ablation process. The simulations predict that rapid deceleration of the ablation plume by the liquid environment results in the formation of a transient hot and dense metal region at the front of the plume, which hampers the mixing of the two components and, at the same time, contributes to the stratification of the plume in the emerging cavitation bubble. As a result, nanoparticles of different sizes and compositions are produced in different parts of the emerging cavitation bubble during the first nanoseconds of the ablation process. Notably, the diameters of the largest nanoparticles generated in the simulations of the initial stage of the ablation process are more than twice larger than the thickness of the original bilayer films. This observation provides a plausible scenario for the formation of large nanoparticles observed in the experiments. The conclusion on limited elemental mixing in the nanoparticles is validated in simulations of bilayers with different spatial order of Cu and Ag layers, even though the two systems exhibit some notable quantitative differences mainly related to the different strength of electron-phonon coupling in Cu and Ag. Overall, the results of this study provide new insights into the formation mechanism of bimetallic nanoparticles in ultrashort PLAL from thin bilayer targets and suggest that the formation of alloy nanoparticles from immiscible elements may be hampered for targets featuring distinctive elemental segregation. © 2021 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c09970
  • 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
  • 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
  • Magnetic and Electronic Properties of Highly Mn-Doped β-NaGdF4and β-NaEuF4Nanoparticles with a Narrow Size Distribution
    Schneider, L. and Wehmeier, J. and Wiedwald, U. and Rodewald, J. and Galakhov, V.R. and Udintseva, M.S. and Mesilov, V. and Radu, F. and Luo, C. and Klare, J.P. and Steinhoff, H.-J. and Haase, M. and Kuepper, K.
    Journal of Physical Chemistry C 124 (2020)
    We have performed a detailed study of the magnetic and electronic properties of highly manganese-doped β-NaGdF4 and β-NaEuF4 nanoparticles with a narrow size distribution. XPS as well as XRF experiments confirm the successful doping of 11 ± 2% Mn into β-NaGdF4 and β-NaEuF4 nanoparticles, i.e., a much higher Mn concentration than has been previously reported for the incorporation of bivalent transition metal ions into the β phase of NaREF4 (RE = rare earth) nanocrystals. Owing to the high manganese concentration, we observed quenching of the Mn2+ emission in both materials and even quenching of the red Eu3+ emission in β-NaEuF4:11% Mn particles. We have investigated the Mn-Mn magnetic interactions by means of EPR spectroscopy, and SQUID magnetometry further supports a significant increase of the magnetization of the Mn-doped β-NaEuF4 nanoparticles compared to pure β-NaEuF4. By using element specific X-ray absorption spectroscopy and X-ray magnetic circular dichroism, we show that the magnetism of Mn-doped β-NaEuF4 is dominated by the divalent Mn ions for Mn-doped β-NaEuF4. A parallel alignment of the gadolinium and manganese magnetic moments is found. © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c04639
  • Manipulation of the size and phase composition of yttrium iron garnet nanoparticles by pulsed laser post-processing in liquid
    Hupfeld, T. and Stein, F. and Barcikowski, S. and Gökce, B. and Wiedwald, U.
    Molecules 25 (2020)
    Modification of the size and phase composition of magnetic oxide nanomaterials dispersed in liquids by laser synthesis and processing of colloids has high implications for applications in biomedicine, catalysis and for nanoparticle-polymer composites. Controlling these properties for ternary oxides, however, is challenging with typical additives like salts and ligands and can lead to unwanted byproducts and various phases. In our study, we demonstrate how additive-free pulsed laser post-processing (LPP) of colloidal yttrium iron oxide nanoparticles using high repetition rates and power at 355 nm laser wavelength can be used for phase transformation and phase purification of the garnet structure by variation of the laser fluence as well as the applied energy dose. Furthermore, LPP allows particle size modification between 5 nm (ps laser) and 20 nm (ns laser) and significant increase of the monodispersity. Resulting colloidal nanoparticles are investigated regarding their size, structure and temperature-dependent magnetic properties. © 2020 by the authors.
    view abstract10.3390/molecules25081869
  • Ptychographic imaging and micromagnetic modeling of thermal melting of nanoscale magnetic domains in antidot lattices
    Gräfe, J. and Skripnik, M. and Dieterle, G. and Haering, F. and Weigand, M. and Bykova, I. and Träger, N. and Stoll, H. and Tyliszczak, T. and Vine, D. and Ziemann, P. and Wiedwald, U. and Shapiro, D. and Nowak, U. and Schütz, G. and Goering, E.J.
    AIP Advances 10 (2020)
    Antidot lattices are potential candidates to act as bit patterned media for data storage as they are able to trap nanoscale magnetic domains between two adjacent holes. Here, we demonstrate the combination of micromagnetic modeling and x-ray microscopy. Detailed simulation of these systems can only be achieved by micromagnetic modeling that takes thermal effects into account. For this purpose, a Landau-Lifshitz-Bloch approach is used here. The calculated melting of magnetic domains within the antidot lattice is reproduced experimentally by x-ray microscopy. Furthermore, we compare conventional scanning transmission x-ray microscopy with resolution enhanced ptychography. Hence, we achieve a resolution of 13 nm. The results demonstrate that ptychographic imaging can also recover magnetic contrast in the presence of a strong topological variation and is generally applicable toward magnetic samples requiring ultimate resolution. © 2020 Author(s).
    view abstract10.1063/5.0025784
  • Temperature-controlled magnetic nanoparticles hyperthermia inhibits primary tumor growth and metastases dissemination
    Garanina, A.S. and Naumenko, V.A. and Nikitin, A.A. and Myrovali, E. and Petukhova, A.Y. and Klimyuk, S.V. and Nalench, Y.A. and Ilyasov, A.R. and Vodopyanov, S.S. and Erofeev, A.S. and Gorelkin, P.V. and Angelakeris, M. and Savchenko, A.G. and Wiedwald, U. and Majouga Dr, A.G. and Abakumov, M.A.
    Nanomedicine: Nanotechnology, Biology, and Medicine 25 (2020)
    Magnetic hyperthermia (MHT) is a promising approach for cancer therapy. However, a systematic MHT characterization as function of temperature on the therapeutic efficiency is barely analyzed. Here, we first perform comparative temperature-dependent analysis of the cobalt ferrite nanoparticles-mediated MHT effectiveness in two murine tumors models – breast (4T1) and colon (CT26) cancer in vitro and in vivo. The overall MHT killing capacity in vitro increased with the temperature and CT26 cells were more sensitive than 4T1 when heated to 43 °C. Well in line with the in vitro data, such heating cured non-metastatic CT26 tumors in vivo, while only inhibiting metastatic 4T1 tumor growth without improving the overall survival. High-temperature MHT (>47 °C) resulted in complete 4T1 primary tumor clearance, 25–40% long-term survival rates, and, importantly, more effective prevention of metastasis comparing to surgical extraction. Thus, the specific MHT temperature must be defined for each tumor individually to ensure a successful antitumor therapy. © 2020 Elsevier Inc.
    view abstract10.1016/j.nano.2020.102171
  • 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
  • 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
  • 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
  • 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
  • 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
  • Sol-gel based synthesis and enhanced processability of MAX phase Cr2GaC
    Siebert, J.P. and Bischoff, L. and Lepple, M. and Zintler, A. and Molina-Luna, L. and Wiedwald, U. and Birkel, C.S.
    Journal of Materials Chemistry C 7 (2019)
    MAX phases are typically prepared by high-temperature (oftentimes high-pressure) solid-state methods. Here, we report a new wet chemistry based synthesis technique starting from an aqueous solution of metal nitrates and citric acid to prepare MAX phase Cr2GaC. This solution-processable precursor mixture has the potential to be easily scaled, painted, printed or fabricated onto supports-an advantage that is demonstrated by the formation of hollow carbon microspheres which are decorated with Cr2GaC particles. A small amount of chromium carbide and oxide remains in the product, however, the amount of the latter can be reduced by a larger excess in citric acid in the precursor gel. The transformation mechanism of the initial amorphous gel into highly crystalline and anisotropic MAX phase particles is investigated by detailed thermal analysis. Transmission electron microscopy studies are conducted to elucidate the microstructure of the sol-gel-prepared particles as well as the decorated hollow microspheres. From magnetic susceptibility measurements, the density of states at the Fermi level is deduced reflecting the quality of the Pauli paramagnet Cr2GaC. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9tc01416k
  • Genetically Controlled Lysosomal Entrapment of Superparamagnetic Ferritin for Multimodal and Multiscale Imaging and Actuation with Low Tissue Attenuation
    Massner, C. and Sigmund, F. and Pettinger, S. and Seeger, M. and Hartmann, C. and Ivleva, N.P. and Niessner, R. and Fuchs, H. and de Angelis, M.H. and Stelzl, A. and Koonakampully, N.L. and Rolbieski, H. and Wiedwald, U. and Spasova, M. and Wurst, W. and Ntziachristos, V. and Winklhofer, M. and Westmeyer, G.G.
    Advanced Functional Materials 28 (2018)
    Nanomaterials are of enormous value for biomedical applications because of their customizable features. However, the material properties of nanomaterials can be altered substantially by interactions with tissue thus making it important to assess them in the specific biological context to understand and tailor their effects. Here, a genetically controlled system is optimized for cellular uptake of superparamagnetic ferritin and subsequent trafficking to lysosomes. High local concentrations of photoabsorbing magnetoferritin give robust contrast in optoacoustic imaging and allow for selective photoablation of cells overexpressing ferritin receptors. Genetically controlled uptake of the biomagnetic nanoparticles also strongly enhances third-harmonic generation due to the change of refractive index caused by the magnetite–protein interface of ferritins entrapped in lysosomes. Selective uptake of magnetoferritin furthermore enables sensitive detection of receptor-expressing cells by magnetic resonance imaging, as well as efficient magnetic cell sorting and manipulation. Surprisingly, a substantial increase in the blocking temperature of lysosomally entrapped magnetoferritin is observed, which allows for specific ablation of genetically defined cell populations by local magnetic hyperthermia. The subcellular confinement of superparamagnetic ferritins thus enhances their physical properties to empower genetically controlled interrogation of cellular processes with deep tissue penetration. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201706793
  • 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 Fe@FeOx, Fe@C and α-Fe2 O3 single-crystal nanoblends synthesized by femtosecond laser ablation of Fe in acetone
    Zhang, D. and Choi, W. and Oshima, Y. and Wiedwald, U. and Cho, S.-H. and Lin, H.-P. and Li, Y.K. and Ito, Y. and Sugioka, K.
    Nanomaterials 8 (2018)
    There are few reports on zero-field-cooled (ZFC) magnetization measurements for Fe@FeOx or FeOx particles synthesized by laser ablation in liquids (LAL) of Fe, and the minimum blocking temperature (TB) of 120 K reported so far is still much higher than those of their counterparts synthesized by chemical methods. In this work, the minimum blocking temperature was lowered to 52 K for 4–5 nm α-Fe2 O3 particles synthesized by femtosecond laser ablation of Fe in acetone. The effective magnetic anisotropy energy density (Keff) is calculated to be 2.7–5.4 × 105 J/m3, further extending the Keff values for smaller hematite particles synthesized by different methods. Large amorphous-Fe@α-Fe2 O3 and amorphous-Fe@C particles of 10–100 nm in diameter display a soft magnetic behavior with saturation magnetization (Ms) and coercivities (Hc) values of 72.5 emu/g and 160 Oe at 5 K and 61.9 emu/g and 70 Oe at 300 K, respectively, which mainly stem from the magnetism of amorphous Fe cores. Generally, the nanoparticles obtained by LAL are either amorphous or polycrystalline, seldom in a single-crystalline state. This work also demonstrates the possibility of synthesizing single-crystalline α-Fe2 O3 hematite crystals of several nanometers with (104), (113), (116) or (214) crystallographic orientations, which were produced simultaneously with other products including carbon encapsulated amorphous Fe (a-Fe@C) and Fe@FeOx core-shell particles by LAL in one step. Finally, the formation mechanisms for these nanomaterials are proposed and the key factors in series events of LAL are discussed. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano8080631
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • Ultrasmall Yttrium Iron Garnet Nanoparticles with High Coercivity at Low Temperature Synthesized by Laser Ablation and Fragmentation of Pressed Powders
    Schmitz, T. and Wiedwald, U. and Dubs, C. and Gökce, B.
    ChemPhysChem 18 (2017)
    Pulsed laser ablation of pressed yttrium iron garnet powders in water is studied and compared to the ablation of a single-crystal target. We find that target porosity is a crucial factor, which has far-reaching implications on nanoparticle productivity. Although nanoparticle size distributions obtained by analytical disc centrifugation and transmission electron microscopy (TEM) are in agreement, X-ray diffraction and energy dispersive X-ray analysis show that only nanoparticles obtained from targets with densities close to that of a bulk target lead to comparable properties. Our findings also show why the gravimetrical measurement of nanoparticle productivity is often flawed and needs to be complemented by colloidal productivity measurements. The synthesized YIG nanoparticles are further reduced in size by laser fragmentation to obtain sizes smaller than 3nm. Since the particle diameters are close to the YIG lattice constant, these ultrasmall nanoparticles reveal an immense change of the magnetic properties, exhibiting huge coercivity (0.11 T) and irreversibility fields (8 T) at low temperatures. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201601183
  • 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
  • Combined first-order reversal curve and x-ray microscopy investigation of magnetization reversal mechanisms in hexagonal antidot lattices
    Gräfe, J. and Weigand, M. and Stahl, C. and Träger, N. and Kopp, M. and Schütz, G. and Goering, E.J. and Haering, F. and Ziemann, P. and Wiedwald, U.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    The magnetization reversal in nanoscaled antidot lattices is widely investigated to understand the tunability of the magnetic anisotropy and the coercive field through nanostructuring of thin films. By investigating highly ordered focused ion beam milled antidot lattices with a combination of first-order reversal curves and magnetic x-ray microscopy, we fully elucidate the magnetization reversal along the distinct orientations of a hexagonal antidot lattice. This combination proves especially powerful as all partial steps of this complex magnetization reversal can be identified and subsequently imaged. Through this approach we discovered several additional steps that were neglected in previous studies. Furthermore, by imaging the microscopic magnetization state during each reversal step, we were able to link the coercive and interaction fields determined by the first-order reversal curve method to true microscopic magnetization configurations and determine their origin. © 2016 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 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.93.014406
  • 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
  • Geometric control of the magnetization reversal in antidot lattices with perpendicular magnetic anisotropy
    Gräfe, J. and Weigand, M. and Träger, N. and Schütz, G. and Goering, E.J. and Skripnik, M. and Nowak, U. and Haering, F. and Ziemann, P. and Wiedwald, U.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    While the magnetic properties of nanoscaled antidot lattices in in-plane magnetized materials have widely been investigated, much less is known about the microscopic effect of hexagonal antidot lattice patterning on materials with perpendicular magnetic anisotropy. By using a combination of first-order reversal curve measurements, magnetic x-ray microscopy, and micromagnetic simulations we elucidate the microscopic origins of the switching field distributions that arise from the introduction of antidot lattices into out-of-plane magnetized GdFe thin films. Depending on the geometric parameters of the antidot lattice we find two regimes with different magnetization reversal processes. For small antidots, the reversal process is dominated by the exchange interaction and domain wall pinning at the antidots drives up the coercivity of the system. On the other hand, for large antidots the dipolar interaction is dominating which leads to fragmentation of the system into very small domains that can be envisaged as a basis for a bit patterned media. © 2016 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 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.93.104421
  • Magnetic switching of nanoscale antidot lattices
    Wiedwald, U. and Gräfe, J. and Lebecki, K.M. and Skripnik, M. and Haering, F. and Schütz, G. and Ziemann, P. and Goering, E. and Nowak, U.
    Beilstein Journal of Nanotechnology 7 (2016)
    We investigate the rich magnetic switching properties of nanoscale antidot lattices in the 200 nm regime. In-plane magnetized Fe, Co, and Permalloy (Py) as well as out-of-plane magnetized GdFe antidot films are prepared by a modified nanosphere lithography allowing for non-close packed voids in a magnetic film. We present a magnetometry protocol based on magneto-optical Kerr microscopy elucidating the switching modes using first-order reversal curves. The combination of various magnetometry and magnetic microscopy techniques as well as micromagnetic simulations delivers a thorough understanding of the switching modes. While part of the investigations has been published before, we summarize these results and add significant new insights in the magnetism of exchange-coupled antidot lattices. © 2016 Wiedwald et al.
    view abstract10.3762/bjnano.7.65
  • 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
  • Orientation of FePt nanoparticles on top of a-SiO2/Si(001), MgO(001) and sapphire(0001): Effect of thermal treatments and influence of substrate and particle size
    Schilling, M. and Ziemann, P. and Zhang, Z. and Biskupek, J. and Kaiser, U. and Wiedwald, U.
    Beilstein Journal of Nanotechnology 7 (2016)
    Texture formation and epitaxy of thin metal films and oriented growth of nanoparticles (NPs) on single crystal supports are of general interest for improved physical and chemical properties especially of anisotropic materials. In the case of FePt, the main focus lies on its highly anisotropic magnetic behavior and its catalytic activity, both due to the chemically ordered face-centered tetragonal (fct) L10 phase. If the c-axis of the tetragonal system can be aligned normal to the substrate plane, perpendicular magnetic recording could be achieved. Here, we study the orientation of FePt NPs and films on a-SiO2/Si(001), i.e., Si(001) with an amorphous (a-) native oxide layer on top, on MgO(001), and on sapphire(0001) substrates. For the NPs of an approximately equiatomic composition, two different sizes were chosen: "small" NPs with diameters in the range of 2-3 nm and "large" ones in the range of 5-8 nm. The 3 nm thick FePt films, deposited by pulsed laser deposition (PLD), served as reference samples. The structural properties were probed in situ, particularly texture formation and epitaxy of the specimens by reflection high-energy electron diffraction (RHEED) and, in case of 3 nm nanoparticles, additionally by high-resolution transmission electron microscopy (HRTEM) after different annealing steps between 200 and 650 °C. The L10 phase is obtained at annealing temperatures above 550 °C for films and 600 °C for nanoparticles in accordance with previous reports. On the amorphous surface of a-SiO2/Si substrates we find no preferential orientation neither for FePt films nor nanoparticles even after annealing at 630 °C. On sapphire(0001) supports, however, FePt nanoparticles exhibit a clearly preferred (111) orientation even in the as-prepared state, which can be slightly improved by annealing at 600-650 °C. This improvement depends on the size of NPs: Only the smaller NPs approach a fully developed (111) orientation. On top of MgO(001) the effect of annealing on particle orientation was found to be strongest. From a random orientation in the as-prepared state observed for both, small and large FePt NPs, annealing at 650 °C for 30 min reorients the small particles towards a cube-on-cube epitaxial orientation with a minor fraction of (111)-oriented particles. In contrast, large FePt NPs keep their as-prepared random orientation even after doubling the annealing period at 650 °C to 60 min. © 2016 Schilling et al.
    view abstract10.3762/bjnano.7.52
  • Solvent-surface interactions control the phase structure in laser-generated iron-gold core-shell nanoparticles
    Wagener, P. and Jakobi, J. and Rehbock, C. and Chakravadhanula, V.S.K. and Thede, C. and Wiedwald, U. and Bartsch, M. and Kienle, L. and Barcikowski, S.
    Scientific Reports 6 (2016)
    This work highlights a strategy for the one-step synthesis of FeAu nanoparticles by the pulsed laser ablation of alloy targets in the presence of different solvents. This method allows particle generation without the use of additional chemicals; hence, solvent-metal interactions could be studied without cross effects from organic surface ligands. A detailed analysis of generated particles via transmission electron microscopy in combination with EDX elemental mapping could conclusively verify that the nature of the used solvent governs the internal phase structure of the formed nanoparticles. In the presence of acetone or methyl methacrylate, a gold shell covering a non-oxidized iron core was formed, whereas in aqueous media, an Au core with an Fe 3 O 4 shell was generated. This core-shell morphology was the predominant species found in >90% of the examined nanoparticles. These findings indicate that fundamental chemical interactions between the nanoparticle surface and the solvent significantly contribute to phase segregation and elemental distribution in FeAu nanoparticles. A consecutive analysis of resulting Fe@Au core-shell nanoparticles revealed outstanding oxidation resistance and fair magnetic and optical properties. In particular, the combination of these features with high stability magnetism and plasmonics may create new opportunities for this hybrid material in imaging applications.
    view abstract10.1038/srep23352
  • 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
  • Combined FORC and X-ray microscopy study of magnetisation reversal in antidot lattices
    Grafe, J. and Haering, F. and Stahl, C. and Weigand, M. and Skripnik, M. and Nowak, U. and Ziemann, P. and Wiedwald, U. and Schutz, G. and Goering, E.
    2015 IEEE International Magnetics Conference, INTERMAG 2015 (2015)
    Magnetic nanostructures, that are patterned on the length scale of the dipole and exchange interaction, have gained significant scientific interest in the past years [1-6]. These nanostructures have great potential for technological applications in data processing and storage, and spintronics [1-6]. Magnonic crystals are a class of such nanostructures and are metamaterials with periodically alternating magnetic properties - similar to photonic crystals [2,3,4]. This periodic variation is achieved by creating holes in a magnetic host material to form a so-called antidot lattice. The introduction of the artificial antidot lattice changes the spin wave dispersion in the material and can be used to form a spin wave guide or filter [2,4]. To tune the spin wave dispersion, understanding the magnetisation states and the static magnetic properties is of great importance. These static properties like the anisotropy, the coercivity and the orientation of the easy axes are determined by the hole size and distance, the antidot lattice symmetry and its orientation, and the magnetic host material [1,2,5,6]. Here, we present new insights into the magnetisation reversal behaviour of nanoscaled hexagonal antidot lattices, patterned both in in-plane (Fe) and out-of-plane (GdFe) magnetised thin films. The antidots were prepared by polystyrene self-organisation lithography or FIB milling of the magnetic materials [6]. An approach combining first-order reversal curve (FORC) measurements and x-ray microscopy (XM) with magnetic contrast was used to identify irreversible processes and to subsequently image their microscopic origin. Using a fast laser magneto-optical Kerr effect (MOKE) based FORC technique [7], it was possible to individually measure specific sample areas (spatial resolution <2 μm) and to compare a large number of samples. Subsequent XM investigations allowed to reproduce, localise, and quantify the magnetic states involved in the reversal processes. © 2015 IEEE.
    view abstract10.1109/INTMAG.2015.7156878
  • 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
  • Perpendicular magnetisation from in-plane fields in nano-scaled antidot lattices
    Gräfe, J. and Haering, F. and Tietze, T. and Audehm, P. and Weigand, M. and Wiedwald, U. and Ziemann, P. and Gawroński, P. and Schütz, G. and Goering, E.J.
    Nanotechnology 26 (2015)
    Investigations of geometric frustrations in magnetic antidot lattices have led to the observation of interesting phenomena like spin-ice and magnetic monopoles. By using highly focused magneto-optical Kerr effect measurements and x-ray microscopy with magnetic contrast we deduce that geometrical frustration in these nanostructured thin film systems also leads to an out-of-plane magnetization from a purely in-plane applied magnetic field. For certain orientations of the antidot lattice, formation of perpendicular magnetic domains has been found with a size of several μm that may be used for an in-plane/out-of-plane transducer. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/26/22/225203
  • Role of developing L10 chemical order on the (0 0 1)-texture formation of (Fe1 - XCux)Pt films grown on amorphous substrates
    Scheibel, F. and Haering, F. and Ziemann, P. and Wiedwald, U.
    Journal of Physics D: Applied Physics 48 (2015)
    We study the technologically important (0 0 1)-texture formation in 10 nm thick (Fe0.9Cu0.1)52Pt48 and, as reference, Fe49Pt51 alloy films. The samples are grown on SiO2(200 nm)/Si(0 0 1) substrates at ambient temperature by pulsed laser deposition. Subsequent rapid thermal processing (RTP) at 650 °C for various time steps drives the initially nanocrystalline and chemically disordered films into the tetragonal L10 phase accompanied by a strong (0 0 1)-texture leading to perpendicular magnetic anisotropy. The fundamental role of the chemical order during short-time annealing as an additional source of strain in the films is experimentally addressed. The structural and magnetic results indicate selective grain growth leading to the (0 0 1)-texture. Strongly prolonged annealing, however, leads to a reorientation of grains towards the (1 1 1)-texture pointing to the increasing importance of surface energies when the initial strain has released. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/48/8/085001
  • Solid solution magnetic FeNi nanostrand-polymer composites by connecting-coarsening assembly
    Barcikowski, S. and Baranowski, T. and Durmus, Y. and Wiedwald, U. and Gökce, B.
    Journal of Materials Chemistry C 3 (2015)
    An approach to assemble high aspect ratio nanostrands consisting of magnetic nanowires and their incorporation in a polymer with the aim of tailoring transparent FeNi nanostrand-PMMA-composites is presented. These nanostrands are controllable in length (<600 μm) and width (<12 μm) via process parameters and have an ultra-high aspect ratio (∼160). This rapid and universal method provides flexible and transparent magnetic materials with tunable transparency and magnetic attraction force by adjusting the density of nanoparticles in the composite. These composites can be used as a window coating for shielding radio frequency electromagnetic waves while being transparent in the optical range. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5tc02160j
  • 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
  • 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
  • Growth modes and epitaxy of FeAl thin films on a-cut sapphire prepared by pulsed laser and ion beam assisted deposition
    Yao, X. and Wiedwald, U. and Trautvetter, M. and Ziemann, P.
    Journal of Applied Physics 115 (2014)
    FeAl films around equiatomic composition are grown on a-cut (11 2 0) sapphire substrates by ion beam assisted deposition (IBAD) and pulsed laser deposition (PLD) at ambient temperature. Subsequent successive annealing is used to establish chemical order and crystallographic orientation of the films with respect to the substrate. We find a strongly [110]-textured growth for both deposition techniques. Pole figures prove the successful preparation of high quality epitaxial films by PLD with a single in-plane orientation. IBAD-grown films, however, exhibit three in-plane orientations, all of them with broad angular distributions. The difference of the two growth modes is attributed to the existence of a metastable intermediate crystalline orientation as concluded from nonassisted sputter depositions at different substrate temperatures. The formation of the chemically ordered crystalline B2 phase is accompanied by the expected transition from ferromagnetic to paramagnetic behavior of the films. In accordance with the different thermally induced structural recovery, we find a step-like magnetic transition to paramagnetic behavior after annealing for 1 h at TA 300 °C for IBAD deposition, while PLD-grown films show a gradual decrease of ferromagnetic signals with rising annealing temperatures. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4861377
  • Single core-shell nanoparticle probes for non-invasive magnetic force microscopy
    Uhlig, T. and Wiedwald, U. and Seidenstücker, A. and Ziemann, P. and Eng, L.M.
    Nanotechnology 25 (2014)
    We present an easy, fast and reliable method for the preparation of magnetic force microscopy (MFM) probes based on single Co nanoparticles (NPs). Due to their dipolar character, these magnetic probes open up a new approach for quantitative and non-invasive MFM measurements on the nanometer length scale. To guarantee long-term stability of these tips under ambient conditions, an ultrathin protecting Au shell was grown around the Co NPs through photochemical deposition. Single magnetic particles were firmly attached to standard silicon AFM tips using bifunctional self-assembling molecules. Such probes were tested on longitudinal magnetic recording media and compared to the results as recorded with conventional thin-film MFM tips. Easy data interpretation of the magnetic nanoparticle probes in a point dipole model is shown. Our nanoparticle tips provide excellent endurance for MFM recording, enable non-invasive probing while maintaining a high sensitivity, resolution, and reproducibility. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/25/25/255501
  • Formation of highly ordered alloy nanoparticles based on precursor-filled latex spheres
    Manzke, A. and Plettl, A. and Wiedwald, U. and Han, L. and Ziemann, P. and Schreiber, E. and Ziener, U. and Vogel, N. and Weiss, C.K. and Landfester, K. and Fauth, K. and Biskupek, J. and Kaiser, U.
    Chemistry of Materials 24 (2012)
    An experimental approach is presented, allowing the preparation of substrate supported, hexagonally arranged metallic alloy nanoparticles with narrow size distributions, well-defined interparticle distances, and controlled chemical composition. The method is based on miniemulsion polymerization and isotropic plasma etching. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) colloids-in the present study containing Fe- and Pt-precursor complexes in a predefined ratio-are deposited onto hydrophilic Si/SiO 2 substrates by dip-coating, forming a highly ordered monolayer. Contrary to colloidal lithography, here, precursor-filled polystyrene colloids serve as carriers for the alloy forming elements. After reactive ion etching and annealing, hexagonally ordered arrays of crystalline FePt nanoparticles are formed exhibiting the desired 1:1 Fe-Pt ratio, as revealed by detailed analysis after each preparation step. Formation of stoichiometric binary alloy FePt nanoparticles is confirmed by determining magnetic hysteresis loops, as well as applying aberration-corrected high-resolution transmission electron microscopy. © 2012 American Chemical Society.
    view abstract10.1021/cm203241p
  • Towards quantitative magnetic force microscopy: Theory and experiment
    Häberle, T. and Haering, F. and Pfeifer, H. and Han, L. and Kuerbanjiang, B. and Wiedwald, U. and Herr, U. and Koslowski, B.
    New Journal of Physics 14 (2012)
    We introduce a simple and effective model of a commercial magnetic thin-film sensor for magnetic force microscopy (MFM), and we test the model employing buried magnetic dipoles. The model can be solved analytically in the half-space in front of the sensor tip, leading to a simple 1/R dependence of the magnetic stray field projected to the symmetry axis. The model resolves the earlier issue as to why the magnetic sensors cannot be described reasonably by a restricted multipole expansion as in the point pole approximation: the p oint pole model must be extended to incorporate a 'lower-order' pole, which we term 'pseudo-pole'. The near-field dependence (/ R?1) turns into the well-known and frequently used dipole behavior (/ R?3) if the separation, R, exceeds the height of the sensor. Using magnetic nanoparticles (average diameter 18 nm) embedded in a SiO cover as dipolar point probes, we show that the force gradient-distance curves and magnetic images fit almost perfectly to the proposed model. The easy axis of magnetization of single nanoparticles is successfully deduced from these magnetic images. Our model paves the way for quantitative MFM, at least if the sensor and the sample are independent. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/14/4/043044
  • Tuning the properties of magnetic thin films by interaction with periodic nanostructures
    Wiedwald, U. and Haering, F. and Nau, S. and Schulze, C. and Schletter, H. and Makarov, D. and Plettl, A. and Kuepper, K. and Albrecht, M. and Boneberg, J. and Ziemann, P.
    Beilstein Journal of Nanotechnology 3 (2012)
    The most important limitation for a significant increase of the areal storage density in magnetic recording is the superparamagnetic effect. Below a critical grain size of the used CoCrPt exchange-decoupled granular films the information cannot be stored for a reasonable time (typically ten years) due to thermal fluctuations arbitrary flipping of the magnetization direction. An alternative approach that may provide higher storage densities is the use of so-called percolated media, in which defect structures are imprinted in an exchange-coupled magnetic film. Such percolated magnetic films are investigated in the present work. We employ preparation routes that are based on (i) self-assembly of Au nanoparticles and (ii) homogeneous size-reduction of self-assembled polystyrene particles. On such non-close-packed nanostructures thin Fe films or Co/Pt multilayers are grown with in-plane and outof-plane easy axis of magnetization. The impact of the particles on the magnetic switching behavior is measured by both integral magnetometry and magnetic microscopy techniques. We observe enhanced coercive fields while the switching field distribution is broadened compared to thin-film reference samples. It appears possible to tailor the magnetic domain sizes down to the width of an unperturbed domain wall in a continuous film, and moreover, we observe pinning and nucleation at or close to the imprinted defect structures. © 2012 Wiedwald et al.
    view abstract10.3762/bjnano.3.93
  • Effect of large mechanical stress on the magnetic properties of embedded Fe nanoparticles
    Saranu, S. and Selve, S. and Kaiser, U. and Han, L. and Wiedwald, U. and Ziemann, P. and Herr, U.
    Beilstein Journal of Nanotechnology 2 (2011)
    Magnetic nanoparticles are promising candidates for next generation high density magnetic data storage devices. Data storage requires precise control of the magnetic properties of materials, in which the magnetic anisotropy plays a dominant role. Since the total magneto-crystalline anisotropy energy scales with the particle volume, the storage density in media composed of individual nanoparticles is limited by the onset of superparamagnetism. One solution to overcome this limitation is the use of materials with extremely large magneto-crystalline anisotropy. In this article, we follow an alternative approach by using magneto-elastic interactions to tailor the total effective magnetic anisotropy of the nanoparticles. By applying large biaxial stress to nanoparticles embedded in a non-magnetic film, it is demonstrated that a significant modification of the magnetic properties can be achieved. The stress is applied to the nanoparticles through expansion of the substrate during hydrogen loading. Experimental evidence for stress induced magnetic effects is presented based on temperature-dependent magnetization curves of superparamagnetic Fe particles. The results show the potential of the approach for adjusting the magnetic properties of nanoparticles, which is essential for application in future data storage media. © 2011 Saranu et al.
    view abstract10.3762/bjnano.2.31
  • Magnetic ground-state and systematic X-ray photoreduction studies of an iron-based star-shaped complex
    Kuepper, K. and Taubitz, C. and Taubitz, D. and Wiedwald, U. and Scheurer, A. and Sperner, S. and Saalfrank, R.W. and Kappler, J.-P. and Joly, L. and Ziemann, P. and Neumann, M.
    Journal of Physical Chemistry Letters 2 (2011)
    We revisit the star-shaped iron-based single-molecule magnet [Fe III{FeIII(L)2}3] (2) (exact chemical formula C30H66Fe4N6O12, H2L = N-methyldiethanolamine) in order to clarify some open questions concerning the exact electronic and magnetic properties arising from earlier studies on this specific compound. Namely, we address the internal magnetic structure by applying X-ray magnetic circular dichroism to the Fe L 2,3 edges and carefully investigate radiation photochemistry. We observe an Fe3+ to Fe2+ photoreduction process, taking place under soft X-ray radiation. © 2011 American Chemical Society.
    view abstract10.1021/jz2005013
  • Nanoscaled alloy formation from self-assembled elemental Co nanoparticles on top of Pt films
    Han, L. and Wiedwald, U. and Biskupek, J. and Fauth, K. and Kaiser, U. and Ziemann, P.
    Beilstein Journal of Nanotechnology 2 (2011)
    The thermally activated formation of nanoscale CoPt alloys was investigated, after deposition of self-assembled Co nanoparticles on textured Pt(111) and epitaxial Pt(100) films on MgO(100) and SrTiO3(100) substrates, respectively. For this purpose, metallic Co nanoparticles (diameter 7 nm) were prepared with a spacing of 100 nm by deposition of precursor-loaded reverse micelles, subsequent plasma etching and reduction on flat Pt surfaces. The samples were then annealed at successively higher temperatures under a H2 atmosphere, and the resulting variations of their structure, morphology and magnetic properties were characterized. We observed pronounced differences in the diffusion and alloying of Co nanoparticles on Pt films with different orientations and microstructures. On textured Pt(111) films exhibiting grain sizes (20-30 nm) smaller than the particle spacing (100 nm), the formation of local nanoalloys at the surface is strongly suppressed and Co incorporation into the film via grain boundaries is favoured. In contrast, due to the absence of grain boundaries on high quality epitaxial Pt(100) films with micron-sized grains, local alloying at the film surface was established. Signatures of alloy formation were evident from magnetic investigations. Upon annealing to temperatures up to 380 °C, we found an increase both of the coercive field and of the Co orbital magnetic moment, indicating the formation of a CoPt phase with strongly increased magnetic anisotropy compared to pure Co. At higher temperatures, however, the Co atoms diffuse into a nearby surface region where Pt-rich compounds are formed, as shown by element-specific microscopy. © 2011 Han et al.
    view abstract10.3762/bjnano.2.51
  • Precise chemical, electronic, and magnetic structure of binuclear complexes studied by means of X-ray spectroscopies and theoretical methods
    Kuepper, K. and Benoit, D.M. and Wiedwald, U. and Mögele, F. and Meyering, A. and Neumann, M. and Kappler, J.-P. and Joly, L. and Weidle, S. and Rieger, B. and Ziemann, P.
    Journal of Physical Chemistry C 115 (2011)
    We investigate two planar complexes MnNi and CoNi (see Scheme 1) by X-ray photoelec-tron spectroscopy (XPS) and ultralow-temperature X-ray magnetic circular dichroism (XMCD). In this way the valence states as well as the presence of uncompensated magnetic moments are obtained. The magnetism has been probed at a temperature of 0.6 K in order to reveal the magnetic ground state properties. We find that divalent Ni ions are in a diamagnetic low spin ground state in both complexes; however, in MnNi a small fraction of divalent nickel high-spin ions leads to a residual XMCD signal, indicating parallel spin alignment with the Mn spins. Mn and Co are found to be in a divalent high-spin configuration in both compounds. Theoretically, we address the energetic ordering of the different possible spin states of the binuclear complexes using (zeroth-order) relativistic approximation density functional calculations and a triple-ζ quality basis set. These results show that intermediate-spin states are often favored over low-spin states for most both metal combinations, in qualitative agreement with our experimental observations. © 2011 American Chemical Society.
    view abstract10.1021/jp2069804
  • Thermally driven solid-phase epitaxy of laser-ablated amorphous AlFe films on (0001)-oriented sapphire single crystals
    Trautvetter, M. and Wiedwald, U. and Paul, H. and Minkow, A. and Ziemann, P.
    Applied Physics A: Materials Science and Processing 102 (2011)
    Solid-phase epitaxy is demonstrated for the metallic binary alloy AlFe. Stoichiometric thin films are deposited at ambient temperature onto c-cut sapphire by pulsed laser deposition (PLD), resulting in smooth amorphous films as revealed by X-ray diffraction (XRD) and atomic force microscopy (AFM). By annealing at 600°C, still smooth epitaxial AlFe films are obtained exhibiting the B2 phase with the (110) direction parallel to the substrate normal and an in-plane orientation as given by AlFe[001]||Al2O 3[112̄0]. While ferromagnetism is observed for the amorphous phase, the formation of the B2 structure is accompanied by paramagnetic behavior, confirming the high structural quality. © Springer-Verlag 2010.
    view abstract10.1007/s00339-010-5972-x
  • Dissolution kinetics of Si into Ge (111) substrate on the nanoscale
    Balogh, Z. and Erdélyi, Z. and Beke, D.L. and Wiedwald, U. and Pfeiffer, H. and Tschetschetkin, A. and Ziemann, P.
    Thin Solid Films 519 (2010)
    In this paper we present experiments and simulations on the dissolution of Si into single crystalline Ge(111) substrates. The interface shift during the dissolution was tracked by X-ray Photoelectron Spectroscopy. It was obtained that the interface remained sharp and shifted according to anomalous kinetics similarly to our previous measurement in the Si/amorphous-Ge system. The interface shift, x, can be described by a power function of time x ∞ t kc with a kinetic exponent, kc, of 0.85 ± 0.1, larger than the one measured for the amorphous system (0.7 ± 0.1). Both exponents, however, are different from the kc = 0.5 Fickian (parabolic) value and it is interpreted as a nanoscale diffusional anomaly caused by the strong composition dependence of the diffusion coefficients. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2010.08.146
  • Fabrication of two-dimensional Au@FePt core-shell nanoparticle arrays by photochemical metal deposition
    Härtling, T. and Uhlig, T. and Seidenstücker, A. and Bigall, N.C. and Olk, P. and Wiedwald, U. and Han, L. and Eychmüller, A. and Plettl, A. and Ziemann, P. and Eng, L.M.
    Applied Physics Letters 96 (2010)
    In this report, we experimentally demonstrate that single platinum nanoparticles exhibit the necessary catalytic activity for the optically induced reduction of H [AuCl4] complexes to elemental gold. This finding is exploited for the parallel Au encapsulation of FePt nanoparticles arranged in a self-assembled two-dimensional array. Magnetic force microscopy reveals that the thin gold layer formed on the FePt particles leads to a strongly increased long-term stability of their magnetization under ambient conditions. © 2010 American Institute of Physics.
    view abstract10.1063/1.3425670
  • Identification of magnetic properties of few nm sized FePt crystalline particles by characterizing the intrinsic atom order using aberration corrected S/TEM
    Biskupek, J. and Jinschek, J.R. and Wiedwald, U. and Bendele, M. and Han, L. and Ziemann, P. and Kaiser, U.
    Ultramicroscopy 110 (2010)
    Hard-magnetic nanomaterials like nanoparticles of FePt are of great interest because of their promising potential for data storage applications. The magnetic properties of FePt structures strongly differ whether the crystal phases are face centered cubic (fcc) or face centered tetragonal (fct). We evaluated aberration corrected HRTEM, electron diffraction and aberration corrected HAADF-STEM as methods to measure the chemical degree of order S that describes the ordering of Pt and Fe atoms within the crystals unit cells. S/TEM experiments are accompanied by image calculations. The findings are compared with results obtained from X-ray diffraction on a FePt film. Our results show that STEM is a reasonable fast approach over HRTEM and electron diffraction to locally determine the chemical degree of order S. © 2010 Elsevier B.V.
    view abstract10.1016/j.ultramic.2010.02.043
  • Monodispersed NiO nanoflowers with anomalous magnetic behavior
    Ge, M.Y. and Han, L.Y. and Wiedwald, U. and Xu, X.B. and Wang, C. and Kuepper, K. and Ziemann, P. and Jiang, J.Z.
    Nanotechnology 21 (2010)
    Nickel oxide (NiO) nanoflowers, prepared by thermal decomposition, exhibit anomalous magnetic properties far below the blocking temperature, i.e., a cusp in both the zero-field-cooled and field-cooled curves at about 21 K. Detailed characterization discloses that the individual NiO nanoflower consists of porous crystals with holes (1.0-1.5 nm in size) inside. We believe that the low temperature magnetic feature observed here could be a new kind of spin transition for the uncompensated spins around the holes and will trigger more studies in other nanostructured antiferromagnetic materials. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/21/42/425702
  • Narrowly size distributed zinc-containing poly(acrylamide) latexes via inverse miniemulsion polymerization
    Kobitskaya, E. and Ekinci, D. and Manzke, A. and Plettl, A. and Wiedwald, U. and Ziemann, P. and Biskupek, J. and Kaiser, U. and Ziener, U. and Landfester, K.
    Macromolecules 43 (2010)
    Polyacrylamide nanoparticles containing zinc nitrate were prepared via inverse miniemulsion polymerization using ultrasound emulsification. The effects of sonication time, mode of sonication, nature and type of emulsifier, amount of zinc salt, solvent in the dispersed phase, nature of dispersed and continuous phases, and type of initiator on the nucleation mechanism, conversion, molecular mass of polymer, and size distribution of the latex particles were investigated. The results showed that an increase in sonication time up to 4 min and using an amphiphilic polymeric surfactant with a relatively short hydrophilic part improved both the monodispersity and the stability of the zinc-containing latexes. An increase in viscosity of the continuous phase (changed by means of different nonpolar solvents) and decrease in viscosity of the dispersed phase (varied by the amount of water) had also a positive effect on the monodispersity. At the same time, the average diameter of the particles in the range of 225 nm changed only marginally. The use of either highly hydrophilic (ammonium persulfate) or highly hydrophobic (2,2′-azobis(2- methylbutyronitrile)) initiators, and the transfer from miniemulsion polymerization to dispersion, precipitation, or a combination of several polymerization types by the modification of the dispersed and continuous medium spread the polydispersity of the latex particles and impaired the stability. Samples with small content of salt were used for unconventional nanolithography by subjecting a highly ordered layer of the nanoparticles to a plasma etching process. Highly ordered arrays of particles containing ZnO nanocrystals were observed. © 2010 American Chemical Society.
    view abstract10.1021/ma902553a
  • Planar Au/TiO2 model catalysts: Fabrication, characterization and catalytic activity
    Eyrich, M. and Kielbassa, S. and Diemant, T. and Biskupek, J. and Kaiser, U. and Wiedwald, U. and Ziemann, P. and Bansmann, J.
    ChemPhysChem 11 (2010)
    Different types of planar Au/TiO2 model catalysts are produced on TiO2(110) single-crystal substrates and thin TiO2 films on Ru(0001) by physical vapor deposition of gold under ultrahigh-vacuum (UHV) conditions or by micelle-based chemical routes. Both the Au nanoparticles and the support are characterized by surface-science-based methods (such as atomic force microscopy and X-ray photoelectron spectroscopy) as well as by transmission electron microscopy. Finally, the activity of the model catalysts in the CO oxidation reaction is analyzed in a microflow reactor. Au/TiO2(110) model catalysts with a stoichiometric TiO2(110) support exhibit only a low catalytic activity compared to those with a reduced crystal and Au/TiO2 model catalysts with thin TiO2 films on Ru(0001) as a substrate. The possible influence of Ti interstitials in the reduced TiO2(110) substrates on the CO oxidation activity is discussed.© 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstract10.1002/cphc.200900942
  • Preparation and characterization of supported magnetic nanoparticles prepared by reverse micelles
    Wiedwald, U. and Han, L. and Biskupek, J. and Kaiser, U. and Ziemann, P.
    Beilstein Journal of Nanotechnology 1 (2010)
    Monatomic (Fe, Co) and bimetallic (FePt and CoPt) nanoparticles were prepared by exploiting the self-organization of precursor loaded reverse micelles. Achievements and limitations of the preparation approach are critically discussed. We show that selfassembled metallic nanoparticles can be prepared with diameters d = 2-12 nm and interparticle distances D = 20-140 nm on various substrates. Structural, electronic and magnetic properties of the particle arrays were characterized by several techniques to give a comprehensive view of the high quality of the method. For Co nanoparticles, it is demonstrated that magnetostatic interactions can be neglected for distances which are at least 6 times larger than the particle diameter. Focus is placed on FePt alloy nanoparticles which show a huge magnetic anisotropy in the L10 phase, however, this is still less by a factor of 3-4 when compared to the anisotropy of the bulk counterpart. A similar observation was also found for CoPt nanoparticles (NPs). These results are related to imperfect crystal structures as revealed by HRTEM as well as to compositional distributions of the prepared particles. Interestingly, the results demonstrate that the averaged effective magnetic anisotropy of FePt nanoparticles does not strongly depend on size. Consequently, magnetization stability should scale linearly with the volume of the NPs and give rise to a critical value for stability at ambient temperature. Indeed, for diameters above 6 nm such stability is observed for the current FePt and CoPt NPs. Finally, the long-term conservation of nanoparticles by Au photoseeding is presented. © 2010 Wiedwald et al; licensee Beilstein-Institut.
    view abstract10.3762/bjnano.1.5
  • Preparation, properties and applications of magnetic nanoparticles
    Wiedwald, U. and Ziemann, P.
    Beilstein Journal of Nanotechnology 1 (2010)
    view abstract10.3762/bjnano.1.4
  • ferromagnetism

  • magnetism

  • MAX phase

  • nanomagnetics

  • nanoparticles

  • storage devices

  • theranostics

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