Highly symmetric periodic domain patterns were obtained in Co2MnGe-Heusler microstripes as a result of the competition between growth-induced in-plane magnetic anisotropy and shape anisotropy. Zero field magnetic configurations and magnetic field-induced domain wall (DW) motion were studied by magnetic force microscopy-image technique for two different cases: dominant uniaxial- and dominant cubic in-plane anisotropy. We implemented a magneto-optical Kerr effect susceptometer to investigate the DW dynamics of periodic domain structures by measuring the in-phase and out-of-phase components of the Kerr signal as a function of magnetic field frequency and amplitude. The DW dynamics for fields applied transversally to the long stripe axis was found to be dominated by viscous slide motion. We used the inherent symmetry/periodicity properties of the magnetic domain structure to fit the experimental results with a theoretical model allowing to extract the DW mobility for the case of transverse DWs (μ TDW = 1.1 m s-1 Oe-1) as well as for vortex-like DWs (μ VDW = 8.7 m s-1 Oe-1). Internal spin structure transformations may cause a reduction of DW mobility in TDWs as observed by OMMFF simulations. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

We report a clear manifestation of the negative contribution to the magnetoresistance due to domain walls in Co2MnGe-Heusler submicron zigzag wires in which the domain structure, domain size and domain wall density can be well controlled. The magnetic behavior of these systems results from the interplay between the intrinsic magneto-crystalline (K4) anisotropy, growth induced uniaxial (KU = 4.7x103 J/m3) anisotropy and shape anisotropy (KS), as observed by magnetic-force microscopy (MFM) and longitudinal Kerr hysteresis loop measurements. Magnetoresistance measurements were performed by the four-point method under a field applied in the plane of the wires at a temperature of 300 K. In these structures, domain wall-creation and annihilation occur in a coherent way. As a result, clear jumps of the resistance are detected during the transition from single-domain- to multi-domain states. At room temperature a value RDW = -2.5 mΩ was obtained; this result is the same order of magnitude as other experimental and theoretical findings. The negative resistive contribution due to the domain wall is also discussed and compared with the existing theoretical models. © 2015 The Authors. Published by Elsevier B.V.

High resolution transmission electron microscopy, nano-beam electronic diffraction, energy dispersive X-rays scanning spectroscopy, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) techniques are used in view of comparing (static and dynamic) magnetic and structural properties of Co 2MnGe(13 nm)/Al2O3(3 nm)/Co(13 nm) tunnel magnetic junctions (TMJs), deposited on various single crystalline substrates (a-plane sapphire, MgO(100) and Si(111)). They allow for providing a correlation between these magnetic properties and the fine structure investigated at atomic scale. The Al2O3 tunnel barrier is always amorphous and contains a large concentration of Co atoms, which, however, is significantly reduced when using a sapphire substrate. The Co layer is polycrystalline and shows larger grains for films grown on a sapphire substrate. The VSM investigation reveals in-plane anisotropy only for samples grown on a sapphire substrate. The FMR spectra of the TMJs are compared to the obtained ones with a single Co and Co2MnGe films of identical thickness deposited on a sapphire substrate. As expected, two distinct modes are detected in the TMJs while only one mode is observed in each single film. For the TMJ grown on a sapphire substrate, the FMR behavior does not significantly differ from the superposition of the individual spectra of the single films, allowing for a conclusion that the exchange coupling between the two magnetic layers is too small to give rise to observable shifts. For TMJs grown on a Si or on a MgO substrate, the resonance spectra reveal one mode which is nearly identical to the obtained one in the single Co film, while the other observed resonance shows a considerably smaller intensity and cannot be described using the magnetic parameters appropriate to the single Co2MnGe film. The large Co concentration in the Al2O3 interlayer prevents for a simple interpretation of the observed spectra when using Si or MgO substrates. © 2013 Elsevier B.V.

We studied asymmetric superconducting spin valves with the layer sequence V/Fe/V/Fe/CoO, grown on MgO(100). These thin film heterostructures exhibit a superconducting spin valve (SSV) effect, that is, a difference, ΔT s, between the superconducting transition temperatures Ts with the magnetization directions of the two Fe layers oriented antiparallel and parallel. For pure Fe layers in the SSV we observe a maximum shift ΔTs = 24 mK. On replacing the Fe layers with FeV alloy layers, the SSV effect was increased to a maximum value of ΔTs = 35 mK. This is one of the largest shifts observed in SSV devices up to now, but still an order of magnitude smaller than the SSV effect observed in V layers grown on epitaxial Fe/V superlattices previously. Our results suggest a dominant importance of the Fe/V interface quality for the amplitude of the SSV effect. © 2013 IOP Publishing Ltd.

Cu2MnAl films of different thicknesses (50, 70, and 100 nm) were grown by UHV RF-sputtering on a-plane sapphire or on MgO (100) substrates. Their structural and static magnetic properties have been studied by X-rays diffraction (XRD) and by vibrating sample magnetometry (VSM), respectively. The Cu2MnAl films exhibit a (100) and (110)-texture when grown on MgO and sapphire substrates, respectively. The best growth quality and the higher magnetization at saturation were obtained for the films grown on MgO. Dynamic magnetic properties were investigated using micro-strip line ferromagnetic resonance (MS-FMR). From the resonance measurements varying the direction and the amplitude of the in-plane and out-of-plane applied magnetic fields we derive the effective magnetization, the Landé g-factor (g = 2.11), the exchange constant (Aex = 0.34 μerg cm-1) and the magnetic anisotropy terms. The in-plane anisotropy can be described as a superposition of two terms showing a small twofold and a dominant fourfold symmetry. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Magnetic properties of Co2MnSi thin films of 20 nm and 50 nm in thickness grown by radio frequency sputtering on a-plane sapphire substrates have been studied. X-ray diffraction (XRD) revealed that the cubic 110 Co2MnSi axis is normal to the substrate and that well defined preferential in-plane orientations are present. The static magnetic properties were studied at room temperature by conventional magneto-optical Kerr effect (MOKE), transverse bias initial inverse susceptibility and torque (TBIIST) MOKE. The dynamic magnetic properties were investigated by micro-strip ferromagnetic resonance (MS-FMR) at room temperature. The resonance and TBIIST measurements versus the direction of the in-plane applied magnetic field reveal that the in-plane anisotropy results from the superposition of a two-fold and a four-fold symmetry. The directions of the principal axes of the twofold anisotropy are sample dependent. The angular dependence of remanent normalized magnetizations and coercive fields, studied by MOKE are analyzed within the frame of a coherent rotation model. A good agreement is observed between the field anisotropy values obtained from MSFMR and from TBIIST data. Frequency and angular dependence of FMR linewidth has been studied. Apparent damping coefficient of 0.0112 has been measured for 50 nm thick sample. © 2013 Owned by the authors, published by EDP Sciences.

We study static and dynamic magnetic properties of Co2MnGe (13 nm)/Al2O3 (3 nm)/Co (13 nm) tunnel magnetic junctions, deposited on various single crystalline substrates (a-plane sapphire, MgO(100), Si(111)). The results are compared to the magnetic properties of Co and of Co2MnGe single films lying on sapphire substrates. X-rays diffraction always shows (110) orientation of the Co2MnGe films. Structural observations obtained by high resolution transmission electron microscopy confirmed the high quality of the tunnel magnetic junction grown on sapphire. Our vibrating sample magnetometry measurements reveal in-plane anisotropy only in samples grown on a sapphire substrate. Depending on the substrate, the ferromagnetic resonance spectra of the tunnel magnetic junctions, studied by the microstrip technique, show one or two pseudo-uniform modes. In the case of MgO and of Si substrates only one mode is observed: it is described by magnetic parameters (g-factor, effective magnetization, in-plane magnetic anisotropy) derived in the frame of a simple expression of the magnetic energy density; these parameters are practically identical to those obtained for the Co single film. With a sapphire substrate two modes are present: one of them does not appreciably differ from the observed mode in the Co single film while the other one is similar to the mode appearing in the Co2MnGe single film: their magnetic parameters can thus be determined independently, using a classical model for the energy density in the absence of interlayer exchange coupling. Copyright © 2013 American Scientific Publishers.

Cu2MnAl Heusler alloy films were grown on MgO (001) substrates by using the ion beam sputtering technique. The films were post-annealed at varying temperatures in order to investigate the influence of annealing on crystal structure and magnetic properties. The structural properties of Cu 2MnAl films have been investigated by using x-ray diffraction (XRD) and magnetic properties have been investigated by both vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) techniques. The experimental data indicates that the crystal structure of the films strongly depends on the annealing temperature. When the films were annealed at 200 °C, the saturation magnetization (Ms = 250 emu/cm3) achieved its maximum and the coercive field (Hc 7 Oe) reached its minimum with B2 ordered structure. In addition, FMR results have revealed that the Cu2MnAl film annealed 200 °C has the highest effective magnetization. The combination of structural and magnetic characterization indicates that the optimum growth temperature is 200 °C for the Cu2MnAl Heusler alloy films on MgO substrates. © Springer Science+Business Media, LLC 2011.

Using time-resolved x-ray resonant magnetic scattering we report on the precessional dynamics of spin valve systems with parallel (P) and antiparallel (AP) orientation of the ferromagnetic layers separated by a nonmagnetic spacer layers. Previously we observed in Co/Cu/Ni 81Fe 19(Py) spin valve systems an increase of the magnetic damping parameter in Py with changing magnetization direction of Py and Co layers from P to AP orientation. We attributed this finding to the configurational dependence of the spin pumping effect. Here we extend our earlier findings by investigating the temperature dependence of the spin pumping effect and possible other causes for the configurational dependence of the damping parameter, such as domain wall induced coupling or magnetic dipole coupling. The main focus is on Co/Cu/Py trilayers and on Co 2MnGe/V/Py trilayers with spin valve properties. © 2012 American Physical Society.

We report on the structural and magnetic properties of exchange-biased Fe/CoO bilayers grown on MgO (001) substrates by using rf-sputtering. For varying Fe thicknesses (4 nm, 10 nm, and 20 nm) the ferromagnetic resonance (FMR) spectra of bilayers have been studied as a function of temperature at X-band frequency. The resonance lines of FMR spectra have a relatively small linewidth indicating a high crystallinity of the Fe films. The roomtemperature FMR data also show that the easy axis of Fe is in the film plane and parallel to the [110] crystallographic direction of MgO substrate. In addition, M versus H loops were recorded at selected temperatures by using VSM magnetometry. The VSM measurements indicate that the Fe thickness and temperature dependence of exchangebias properties are in good agreement with the previous results on similar systems. However, the blocking temperature of the exchange-biased system is strongly reduced compared to the bulk values. This reduction in the blocking temperature is explained by both the thickness and superstoichiometric structure of antiferromagnetic CoO layer. © Springer Science+Business Media, LLC 2011.

Co 2MnSi (CMS) films of different thicknesses (20, 50, and 100 nm) were grown by radio frequency (RF) sputtering on a-plane sapphire substrates. Our X-rays diffraction (XRD) study shows that, in all the samples, the cubic 〈110〉 CMS axis is normal to the substrate and that six well defined preferential in-plane orientations are present. Static and dynamic magnetic properties were investigated using vibrating sample magnetometry (VSM) and microstrip line ferromagnetic resonance (MS-FMR), respectively. From the resonance measurements versus the direction and the amplitude of an applied magnetic field, most of the magnetic parameters are derived, i.e.: the magnetization, the gyromagnetic factor, the exchange stiffness coefficient, and the magnetic anisotropy terms. The in-plane anisotropy results from the superposition of two terms showing a twofold and a fourfold symmetry, respectively. The observed behavior of the hysteresis loops is in agreement with this complex form of the in-plane anisotropy. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The effects of the ferromagnetic (FM) Co layer thickness on magnetic anisotropies and exchange bias (EB) properties of Co/CoO bilayers have been investigated by using ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. The temperature and thickness dependence of EB were studied in temperature range of 4-320 K. FMR and VSM measurements show that Co/CoO bilayers exhibit negative exchange bias just below the blocking temperature (T B). Room-temperature in-plane FMR measurements reveal that the Co layer was epitaxially grown on MgO substrate with four-fold magneto-crystalline symmetry. The data also show that the easy axis of magnetization is in the film plane and parallel to the (110) crystallographic directions of MgO substrate in all samples. © Springer Science+Business Media, LLC 2011.

Exchange-biased CoO/Fe bilayer grown on MgO (0 0 1) substrate by sputtering, studied by variable angle and temperature ferromagnetic resonance. Room temperature in-plane measurements reveal that the Fe layer was epitaxially grown on MgO substrate with a fourfold cubic symmetry. The data also show that the easy axis of magnetization is in the film plane and makes an angle of 45° with the [1 0 0] crystallographic direction of MgO substrate. The low temperature data exhibit a sudden onset of a field cooling-induced and shifted cubic anisotropy below the Nel temperature of CoO. This results in a twofold uniaxial or fourfold cubic symmetry for in-plane magnetic anisotropy depending on a field cooling direction. Low temperature measurements also present a reduction in the resonance fields due to the antiferromagnetic/ferromagnetic coupling. The developed theoretical model perfectly simulates the experimental data of coupled CoO/Fe bilayer. © 2010 Elsevier B.V. All rights reserved.

The disorder-order transition of a highly defective A2-ordered Cu 2MnAl film on MgO(0 0 1) upon annealing at 600 K was monitored by means of x-ray absorption spectroscopy (XAS) at the Cu and Mn L2,3 edges. Additionally, x-ray magnetic circular dichroism (XMCD) was employed to determine element-specific orbital and spin resolved magnetic moments of the Cu and Mn atoms. A small induced total magnetic moment of ≈0.04 0.01μB per atom was detected at the Cu sites, whereas a total magnetic moment of 3.57 0.52μB is carried by the Mn atoms. The experimental XAS and XMCD spectra of Cu agree reasonably with the results from ab initio calculations, magnetic moments derived by the sum rules are in accordance with the calculations. © 2011 IOP Publishing Ltd.

We have prepared thin films of the ferromagnetic Heusler alloy Co 2MnGe on A-plane Al2O3 substrates exhibiting growth-induced, superimposed fourfold and uniaxial magnetic anisotropies, the symmetry being determined by the single-crystalline Al2O3 substrate. The magnitude of the uniaxial anisotropy compared to the cubic anisotropy can be tuned over a wide range by the film thickness and the growth conditions. On submicrometer-wide stripes of Co2MnGe prepared by electron beam lithography we studied magnetic domain patterns by magnetic force microscopy. For stripes with a sufficiently large uniaxial anisotropy and with the easy axis oriented perpendicular to the stripe axis, we find perfectly regular domain patterns with the magnetization direction perpendicular to the stripe axis and alternating from domain to domain. The highly regular and controllable domain patterns in Co2MnGe nanostripes could be useful for magnetic storage devices and applications related to spin transfer torque. © 2011 American Physical Society.

We have studied the magnetotransport properties of thin films of the Heusler compounds Cu2MnAl, Co2MnGe, and Co2MnSi prepared by sputtering on substrates at room temperature. By stepwise annealing at high temperatures, we transform the as-prepared, weakly magnetic, nanocrystalline state to the fully ordered, crystalline state via several intermediate steps. At the phase boundary between the nanocrystalline state and the long-range-ordered crystalline state, we observe a change of the electrical resistivity from that of a strongly disordered metal with negative dρ/dT to that of a normal metal with positive dρ/dT. The high-field magnetoresistance (MR) for Cu2MnAl is large, negative, and isotropic and is mainly due to static spin disorder scattering. For Co2MnGe and Co 2MnSi a corresponding sizable spin disorder MR is missing. At low fields Co2MnGe and Co2MnSi exhibit a conventional anisotropic MR, whereas the low-field MR for Cu2MnAl is negative and isotropic. In the nanocrystalline phase of Co2MnGe and Co 2MnSi we find a large anomalous Hall effect (AHE), which can be attributed to very effective skew scattering at the nanocrystalline grain boundaries. For Cu2MnAl, the AHE behavior is very unusual, indicating a situation with the contributions of spin-up and spin-down electrons to the anomalous Hall voltage nearly compensating each other. © 2011 American Physical Society.

We have studied Josephson junctions with barriers prepared from the Heusler compound Cu2 MnAl. In the as-prepared state the Cu2 MnAl layers are nonferromagnetic and the critical Josephson current density jc decreases exponentially with the thickness of the Heusler layers dF. On annealing the junctions at 240°C the Heusler layers develop ferromagnetic order and we observe a dependence jc (dF) with jc strongly enhanced and weakly thickness dependent in the thickness range 7.0< dF < 10.6nm. We interpret this feature as an indication of a triplet component in the superconducting pairing function generated by the specific magnetization profile inside thin Cu2 MnAl layers. © 2010 The American Physical Society.

Magnetic properties of Co2 MnGe thin films of different thicknesses (13, 34, 55, 83, 100, and 200 nm), grown by rf sputtering at 400 °C on single crystal sapphire substrates, were studied using vibrating sample magnetometry and conventional or microstrip line ferromagnetic resonance. Their behavior is described assuming a magnetic energy density showing twofold and fourfold in-plane anisotropies with some misalignment between their principal directions. For all the samples, the easy axis of the fourfold anisotropy is parallel to the c -axis of the substrate while the direction of the twofold anisotropy easy axis varies from sample to sample and seems to be strongly influenced by the growth conditions. Its direction is most probably monitored by the slight unavoidable miscut angle of the Al2 O 3 substrate. The twofold in-plane anisotropy field Hu is almost temperature independent, in contrast with the fourfold field H 4 which is a decreasing function of the temperature. Finally, we study the frequency dependence of the observed line-width of the resonant mode and we conclude to a typical Gilbert damping constant α value of 0.0065 for the 55-nm-thick film. © 2010 American Institute of Physics.

X-ray amorphous thin films of the Heusler alloys Cu2MnAl and Co2MnSi have been prepared by magnetron sputter deposition at room temperature. In the amorphous state the Cu2MnAl phase is non-ferromagnetic; Co2MnSi is weakly ferromagnetic with a ferromagnetic Curie temperature of 170 K. By solid-state crystallization at high temperatures strong ferromagnetic order and high Curie temperatures are established in both alloys. The saturation magnetization of the Co 2MnSi alloy reaches 5.1μB/f.u. at 4 K, corresponding to 100% of the theoretical value; for Cu2MnAl we obtain 2.8μB/f.u. at 4 K, which corresponds to 87.5% of the theoretical value. In samples of the Co2MnSi phase with optimum saturation magnetization Bragg reflections as indicators of a long-range chemical order are missing, whereas for the Cu2MnAl phase Bragg reflections confirm epitaxial quality and long-range L21 order. © 2010 IOP Publishing Ltd.