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International audience; In this work, the effect of the loading path on the multicracking of Nickel thin films on Kapton® substrate was studied thanks to an experimental set-up combining controlled biaxial deformation, x-ray diffraction and digital image correlation. Samples were biaxially stretched up to 10% strain following either a single equibiaxial path or a complex one consisting of loading successively along each of the axes of the cruciform specimen. While the first path leads to a mud-crack pattern (random), the second leads to a roman-bricks one (square). Moreover, the in situ x-ray diffraction experiments show that the stress field developed in the thin film during the multicracking is clearly dependent on the loading path. By combining the study of stresses and x-ray diffraction peaks linewidth, we evidenced mechanical domains related to initiation of cracks and their multiplication for each loading path. Moreover, stress evolution in the thin film during mud-crack pattern formation is significantly smoother than in the case of roman-bricks one as represented in the plane stress space.

The control of localized magnetic modes has been obtained in Ni60Fe40 square lattice (600 nm) antidot arrays. This has been performed by tailoring the magnetoelastic field at the scale of the antidot primitive cell. The corresponding heterogeneous strain field distributions have been generated by a PZT substrate and enhanced by the incorporation of a supporting compliant layer. It has been highlighted by a differentiated variation of magnetic energy directly due to the local magnetoelastic field felt by each magnetic mode, probed by ferromagnetic resonance spectroscopy. A modeling, involving micromagnetic simulations (to locate the magnetic modes), full-field simulations (to evaluate the strain field distributions), and an analytical model generally dedicated to continuous film that we have extended to those magnetic modes, shows a good agreement with the experimental data. This approach is very promising to develop multichannel systems with simultaneous and differentiated controlled frequencies in magnetic devices.

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Co20Fe60B20 thin films of various thicknesses (3 nm $\le t_{\text {CFB}} \le 14$ nm) have been sputtered on thermally oxidized Si substrates and capped with 8 nm-thick Ir or Ru layers and then annealed at 400 °C. A vibrating sample magnetometer has been used to measure the CoFeB thickness dependence of the saturation magnetic moment per unit area in order to straightforwardly determine the magnetization at saturation and the magnetic dead layer thickness, found to be 1115 ± 50 emu/cm3 (930 ± 50 emu/cm3) and 1.53 nm (0.76 nm) for the as-deposited CoFeB/Ir (CoFeB/Ru) sytems, respectively. These values change to 1185 ± 50 emu/cm3 (1110±50 emu/cm3) and 2.93 nm (1.62 nm) for CoFeB/Ir (CoFeB/Ru) films annealed at 400 °C. Microstrip ferromagnetic resonance (MS-FMR) was used to investigate the Gilbert damping parameter and the magnetic anisotropy as a function of the CoFeB thickness. While the in-plane anisotropy field for the as-deposited films does not show a regular behavior versus the CoFeB thickness, a clear linear behavior of this field versus the reciprocal CoFeB effective thickness can be observed for the annealed samples. Moreover, the effective magnetization varies linearly with the inverse effective thickness of CoFeB due to the perpendicular interface anisotropy. This surface anisotropy constant, which was estimated to be 0.84 and 0.89 erg/cm2 for the as-deposited CoFeB films capped with Ir or Ru, respectively, reinforces the perpendicular easy axis. The corresponding anisotropy constants for the annealed Ir and Ru capped films are found to be 1.07 and 0.57 erg/cm2, respectively. Moreover, MS-FMR results revealed that the damping constant increases linearly with the reciprocal effective thickness of CoFeB, most probably due to spin pumping. The effective spin mixing conductance of the as-deposited (annealed) CoFeB/Ru and CoFeB/Ir has been estimated to be 12.93 nm−2 (22.4 nm−2) and 29.3 nm−2 (43.85 nm−2), respectively.

This article presents a study whose purpose is to elucidate the damage effects in thin films on their magnetic response. Co40Fe40B20 and Ni80Fe20 films of different nanometric thicknesses were stretched by more than 10% and in situ probed by atomic force microscopy measurements to determine their irreversible mechanical behavior (multi-cracking, buckling). Once these phenomena have been well identified, magnetic behavior of these stretched systems has been studied by ferromagnetic resonance to measure resulting magnetic anisotropy and damping evolutions. All of these experimental studies show that the magnetic properties are mainly affected by the stresses generated during the damage but not by the local discontinuities induced by the numerous cracks and buckles. This is in particular confirmed by the almost zero sensitivity to the damage of the magnetic properties of Ni80Fe20 alloy which is known for its vanishing magnetostriction.

This work presents the effect of large strains (up to 20%) on the behavior of magnetic nanowires (Ni80Fe20) deposited on a Kapton substrate. The multicracking phenomenon was followed by in situ tensile tests combined with atomic force microscopy measurements. These measurements show, on the one hand, a delay in crack initiation relative to the nonpatterned thin film and, on the other hand, a saturation of the length of the nanowire fragments. The latter makes it possible to retain the initial magnetic anisotropy measured after deformation by ferromagnetic resonance. In addition, the ferromagnetic resonance line profile (intensity, width) is minimally affected by the numerous cracks, which is explained by the small variation in magnetic anistropy and the low magnetostriction coefficient of Ni80Fe20.

Replacing materials based on rare-earth elements in current permanent magnets is a real scientific, economic and environmental challenge. Ferromagnetic 3d transition metals seem an apt direction to go in this field, due to their high residual magnetization and thermal stability. In order to improve their coercive behavior, nanostructured magnets based on the assembly of high-aspect-ratio nanoparticles ( i.e. cobalt based nanorods and nanowires) have recently been proposed. In these, the nanoparticle morphology itself drives the magnetization reversal mechanism. This purely geometrical effect seems to obscure the effects of structural defects, although it is clear that high magnetocrystalline energy is required to maintain a stable orientation of the magnetic moment inside the nanoparticles. We present here an experimental study whose aim is to distinguish the role of the stacking faults from the effects of shape and morphology on the magnetization reversal mechanism in cobalt-based nanowires. Coercive field results have been obtained on Co 80 Ni 20 nanowires synthesized by a polyol process. Through accurate control of shape and morphology, it was possible to discard the effects of shape and thus to highlight the influence of crystal defects on the magnetism of Co 80 Ni 20 nanowires. A micromagnetic study, consistent with the experimental analyses, is also presented. The results discussed in this work clearly show that even if the morphological characteristics are conducive to a high coercive field, the presence of numerous stacking faults has the opposite effect and leads to materials with a significantly lower coercive field than expected, which is not suitable for permanent magnet applications.

25 nm to 50 nm Co2FeAl (CFA) thick wire arrays with varying widths and spacing have been patterned from continuous CFA films deposited on MgO(001) using e-beam lithography and Ar ion milling. Magneto-optical Kerr effect, transverse bias initial inverse susceptibility and torque measurements reveal that the in-plane magnetic anisotropy of the wires is dominantly monitored by a uniaxial term, in contrast with the continuous films where it is governed by the superposition of a fourfold term and of a smaller uniaxial term. The microstrip ferromagnetic resonance spectra performed using a magnetic field H, applied in the plane of the studied sample along various directions, or perpendicularly to this plane, gave us access to various quantized modes originating from the patterning. In addition, Brillouin light scattering also exhibits quantized modes. A large part of the experimental observations can be quantitatively interpreted as resulting from the demagnetizing terms induced by the geometrical patterning. However, the presented model, simply built on the effect of the demagnetizing field, is not able to give account of all the quantized modes present in the resonance spectra. When H is parallel to the wires, a more complete description is used: it considers the wave-vector quantization induced by the patterning. For the magnetic modes concerned by both approaches, the correspondence between the 2 models is easily established. When H is not parallel to the wires quantitative descriptions of the behavior of the field dependence of the observed modes still can often be performed. Finally, in all the studied patterned samples, the uniform magnetic mode, termed “film mode”, relative to the parent continuous film is observed by ferromagnetic resonance: such a behavior, which has been reported previously, remains to be completely interpreted.

The mechanical properties of CoCrCuFeNi thin films deposited by magnetron sputtering on Kapton substrates are studied in this paper. Measurements by confocal microscopy during tensile tests allow the analysis of multi-cracking and subsequent buckling of these thin films. Different multi-cracking models (Beuth, Xia & Hutchinson, Franck) allow to analyze the fragmentation experiments and to determine the fracture toughness. It increases with the film thickness and thus does not seem to be an intrinsic property of the material, but linked to the nanocrystalline nature of the films. Finally, the adhesion between CoCrCuFeNi thin films and Kapton is found to be comparable to that of other metal/polymer systems.

This work presents the effect of large strains (up to 20%) on the behavior of magnetic nanowires (Ni

Co2FeAl (CFA) thin films of various thicknesses (1 nm $\le d \le 10$ nm) have been grown on (001) MgO single-crystal substrates and annealed at different temperatures ( $T_{a}$ ). Their magnetic properties have been studied by vibrating sample magnetometry, broadband microstrip ferromagnetic resonance (MS-FMR), and Brillouin light scattering (BLS). The effective magnetization, deduced from the fit of MS-FMR and BLS measurements, linearly decreases with the inverse thickness ( $1/d$ ) for all the annealing temperatures. This allows quantifying perpendicular surface anisotropy constants of 1.46, 1.71, and 1.29 erg/cm2 for $T_{a} = 215$ °C, 265 °C, and 315 °C, respectively. The perpendicular magnetization is achieved for the CFA films below a critical thickness that increases with increasing $T_{a}$ . Moreover, the fourfold anisotropy fields, measured in the in-plane magnetized films, increase with $T_{a}$ and decrease with respect to the CFA inverse thickness.

We study the influence of applied in-plane elastic strains on the static magnetic configuration of a 530 nm magnetostrictive FeCuNbSiB ( Finemet ® ) thin film. The in-plane strains are induced via the application of a voltage to a piezoelectric actuator on which the film/substrate system was glued. A quantitative characterization of the voltage dependence of the induced-strain at the surface of the film was performed using a digital image correlation technique. Magnetic Force Microscopy (MFM) images at remanence (H=0 Oe and U=0 V) clearly reveal the presence of weak stripe domains. The effect of the voltage-induced strain shows the existence of a voltage threshold value for the strike configuration break. For a maximum strain of e XX ~ 0.5 × 10 − 3 we succeed in destabilizing the stripes configuration helping the setting up of a complete homogeneous magnetic pattern.

The interface perpendicular magnetic anisotropy (PMA) is known to be induced at the interface between a ferromagnetic metal (FM) layer and a non-magnetic (NM) heavy-metal layer such as Platinum or Palladium (Pd) in a FM/NM bilayer or multilayer film [1–3].

International audience; The phenomena associated with strain-mediated multiferroic behavior have been quantitatively studied by combining synchrotron x-ray diffraction (XRD), ferromagnetic resonance (FMR) and macroscopic strain measurements (digital image correlation) in a magnetoelectric composite (Ni film on PZT substrate). The ferroelectric and piezoelectric strains have been studied by analyzing the evolution of x-ray patterns (peak intensities and shifts), while the magnetic response is studied by FMR (energy shift of the resonance), during in situ application of electrical voltage. Being given the intrinsic selective phase nature of XRD, the lattice strains have also been measured in the Ni film and have been found to be comparable to the macroscopic one (full transmission from the PZT substrate). The PZT strain has been successfully separated into two contributions, i.e. ferroelectric and lattice strains, the first one being five times higher than the second one. Moreover the last is itself composed of two sub-contributions: pure piezoelectric effect and ferroelectric strain in surrounding grains (strain transfer at grain boundaries). Finally, through in situ FMR measurements, we showed that the ferroelectric contribution to the strain-mediated magnetoelectric response is ten times higher than the pure piezoelectric one.

We study static and dynamic magnetic properties of Co2MnGe (13 nm)/Al2O3 (3 nm)/Co (13 nm) tunnel magnetic junctions (TMJ), 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 a (110) orientation of the Co2MnGe films. Structural observations obtained by high resolution transmission electron microscopy confirmed the high quality of the TMJ 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 TMJs, 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.

The fundamental properties of 2D arrays of magnetic nanowires and nanotubes (NWs and NTs) have been well described theoretically, but real systems are subject to certain flaws and imperfections which strongly affect the mechanism and dynamics of magnetization reversal. Therefore, before exploiting the vast potential of magnetic NWs and NTs, there is a need to investigate the materials parameters as well as the field dependent magnetic properties in real systems. Amongst the various issues at stake for a comprehensive understanding of these arrays is the influence of long-range dipolar interactions because these interactions strongly influence the magnetization reversal and thus the switching field distributions (SFD) which plays a significant role for information storage. Particularly, the width of the SFD is important since smaller values of this parameter leads to less recording errors and it is also a measure of the quality of the recording media. Consequently, the understanding and evaluation of the distinct parameters influencing the interactions and the intrinsic SFD in the magnetization reversal process of such arrays is critical for the development of magnetic recording media. It is shown that the intrinsic SFD mostly originates from nonuniformities of the geometrical parameters such as the aspect ratio and the shape of the magnetic nanostructures. This chapter provides an overview of the MFMcharacterization studies undertaken to understand the reversal behavior of 2D arrays of magnetic NWs and NTs at the nanoscale and the motivations to investigate isolated single domain NWs and NTs.

We present in this paper the structural and magnetic properties of high aspect ratio Co nanoparticles (~10) at high temperatures (up to 623 K) using in situ X ray diffraction (XRD) and SQUID characterizations. We show that the anisotropic shapes, the structural and texture properties are preserved up to 500 K. The coercivity can be modelled by u0Hc=2(Kmc+Kshape)/Ms with Kmc the magnetocrystalline anisotropy constant, Kshape the shape anisotropy constant and Ms the saturation magnetization. Hc decreases linearly when the temperature is increased due to the loss of the Co magnetocrystalline anisotropy contribution. At 500K, 50% of the room temperature coercivity is preserved corresponding to the shape anisotropy contribution only. We show that the coercivity drop is reversible in the range 300 - 500 K in good agreement with the absence of particle alteration. Above 525 K, the magnetic properties are irreversibly altered either by sintering or by oxidation., 8 pages, 7 figures, submitted to Journal of Solid State Chemistry

$\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{F}{\mathrm{e}}_{0.5}$ (CoFe) ultrathin films of various thicknesses $(0.8\phantom{\rule{0.16em}{0ex}}\mathrm{nm}\ensuremath{\le}{t}_{\mathrm{CoFe}}\ensuremath{\le}1.6\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$ have been grown by sputtering on (001) MgO single crystal or Si/${\mathrm{SiO}}_{2}$ substrates, using Pt as capping or buffer layers, respectively. The x-ray diffraction revealed an in-plane epitaxial (isotropic) growth of Pt on MgO (Si). Their magnetic properties have been studied by vibrating sample magnetometry and Brillouin light scattering (BLS) in the Damon-Eshbach geometry. Vibrating sample magnetometry characterizations show that films grown on MgO are in-plane magnetized, while films deposited on Si are perpendicularly magnetized for CoFe thickness below 1.4 nm. The BLS measurements reveal a pronounced nonreciprocal spin waves propagation, which increases with decreasing CoFe thickness. This nonreciprocity was attributed to an interfacial Dzyaloshinskii-Moriya interaction (DMI) induced by Pt interface with CoFe. Moreover, the DMI sign has been found to depend on the stacks order: it is positive (negative) for CoFe/Pt (Pt/CoFe). The effective thickness dependence of the DMI effective constant shows two regimes due to the degradation of the interfaces as the CoFe thickness decreases. We thus show that the magnetic dead layer should be taken into account to precisely determine the surface DMI constant ${D}_{\mathrm{s}}$. Therefore, for the thickest samples, the surface DMI constants are nearly opposite: $\ensuremath{-}1.27$ and $1.32\phantom{\rule{0.16em}{0ex}}\mathrm{pJ}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ for Pt/CoFe and its reversed system, respectively.

The progress of magnetization reversal of weakly packed ferromagnetic Ni80Fe20 nanowire arrays of different diameters (40, 50, 70 and 100 nm) electrodeposited in polycarbonate membranes was studied by magnetic force microscopy (MFM). For such a low packing density of nanomagnets, the dipolar interactions between neighbouring wires can be neglected. The intrinsic switching field distribution has been extracted from in situ MFM images and its width was found to be considerably smaller than for densely packed nanowire arrays., 4 pages, 5 figures. To appear in Journal of Superconductivity and Novel Magnetism

Co20Fe60B20/Pt systems with variable thicknesses of Co20Fe60B20 and of Pt have been sputtered and then annealed at various temperatures (Ta) up to 300 °C. Microstrip line ferromagnetic resonance (MS-FMR) has been used to investigate Co20Fe60B20 and Pt thickness dependencies of the magnetic damping enhancement due to the spin pumping. Using diffusion and ballistic models for spin pumping, the spin mixing conductance and the spin diffusion length have been deduced from the Co20Fe60B20 and the Pt thickness dependencies of the Gilbert damping parameter α of the Co20Fe60B20/Pt heterostructures, respectively. Within the ballistic simple model, both the spin mixing conductance at the CoFeB/Pt interface and the spin-diffusion length of Pt increase with the increasing annealing temperature and show a strong enhancement at 300 °C annealing temperature. In contrast, the spin mixing conductance, which increases with Ta, shows a different trend to the spin diffusion length when using the diffusion model. Moreover, MS-...

Co2MnGe films of 30 and 50 nm in thickness were grown by RF-sputtering. Their magnetic anisotropies, dynamic properties and the different excited spin wave modes have been studied using conventional ferromagnetic resonance (FMR) and Microstrip line FMR (MS-FMR). From the in-plane and the out-of-plane resonance field values, the effective magnetization (4πMeff) and the g-factor are deduced. These values are then used to fit the in-plane angular-dependence of the uniform precession mode and the field-dependence of the resonance frequency of the uniform mode and the first perpendicular standing spin wave to determine the in-plane uniaxial, the four-fold anisotropy fields, the exchange stiffness constant and the magnetization at saturation. The samples exhibit a clear predominant four-fold magnetic anisotropy besides a smaller uniaxial anisotropy. This uniaxial anisotropy is most probably induced by the growth conditions.

Perpendicular magnetic anisotropy (PMA) is considered as one of the highly desirable properties of magnetic thin films for modern spintronic devices. In fact, the integration of materials with PMA reduces the critical current densities and enhances the thermal stability [1] in spin transfer torque based magnetic random access memories (STT-MRAM). Moreover, the large anisotropy values in such materials yields narrow domain walls making them good candidates for high-density STT-MRAM involving the propagation of magnetic domain walls. Co/Pt and Co/Pd mutilayers have been recognized many years ago as perpendicularly magnetized materials [2]. However, their application as electrodes in magnetic tunnel junctions (TMJ) and in giant magnetoresistance (GMR) devices has been limited by the reduced magnetoresitsance (TMR or GMR) values [2] and the high Gilbert damping parameter due to the spin-orbit coupling (SOC) of the Pt or Pd. Recently, strong PMA in ferromagnetic thin films (FM), Co, CoFe, and CoFeB when interfaced with an oxide, has been demonstrated [3-5] due to the hybridization of the O 2p and the FM (e.g. Fe) 3d orbitals, together with a degeneracy lift induced by SOC. Therefore, designing heterostructures presenting this interface PMA and incorporating FM that combines high spin polarisation and low Gilbert damping such as Co 2 FeAl (CFA) full Heusler alloy would be an attractive step towards building suitable materials for the above-mentioned spintronic applications.

Microstrip line ferromagnetic resonance (MS-FMR) has been used to investigate the dependence of the magnetic damping enhancement on the thickness of Co20Fe60B20 and Pt due to spin pumping. Samples with variable thicknesses of Co20Fe60B20 (Pt) are used to determine the spin mixing conductance (spin diffusion length) via the thickness dependence of the Gilbert damping parameter α of Co20Fe60B20/Pt heterostructures. The results obtained from the analysis of the MS-FMR measurements reveal that α increases linearly with the inverse thickness of Co20Fe60B20 for films capped with a 10 nm thick Pt layer, while the variation of α versus the Pt thickness of the 4 nm thick Co20Fe60B20 layer has an exponential behaviour. The experimental data was analysed using a ballistic or a diffusive spin transport model for spin pumping, which includes the effective spin mixing conductance of the CoFeB/Pt interface and the spin-diffusion length of Pt. The estimated values are 37.5 nm−2 (spin mixing conductance) and 1.7 nm when the ballistic transport model is used, and 56.75 nm−2 and 2.2 nm when the experimental data are analysed using a more realistic model based on a diffusive transport. Moreover, MS-FMR measurements reveal that the effective magnetization varies linearly with the Co20Fe60B20 inverse thickness due to the perpendicular interface anisotropy. This anisotropy, estimated to be 1.31 erg cm−2, reinforces the perpendicular magnetization easy axis.

In our work, we study the physical origins of the hydrogen gas induced reduction of the strength of perpendicular magnetic anisotropy (PMA) at the interface of cobalt and palladium layers. To this end, we grow these films on flexible substrates and carry out ferromagnetic resonance (FMR) measurements in the presence of two different stimuli—hydrogen gas and elastic strain. Exposing the samples to H2 results in a downshift of the FMR field. On the contrary, FMR measurements carried out in the presence of an externally applied predominantly tensile elastic stress show an up-shift in the resonance field consistent with negative values of the saturation magnetostriction coefficient for our samples. Qualitative analysis of these results demonstrates that the magneto-elastic contribution to the hydrogen-induced change in PMA is very small and is of the opposite sign to the electronic contribution related to the influence of hydrogen ions on the hybridisation of cobalt and palladium orbitals at the interface.

We have studied the magnetomechanical properties of thin films deposited on polyimide substrate. Particularly, we studied the effect of annealing in the accessible temperature range (0–375 °C) allowed by the polymer substrate. Complementary techniques have been used to characterize the elastic, magnetic and magnetoelastic properties of annealed samples (Brillouin light scattering and tensile tests combined with in situ ferromagnetic resonance). Below 200 °C, no significant evolutions of the properties have been found excepted the magnetic damping parameter. In contrast, above 200 °C, strong variations are observed (+15% for Young's modulus, for magnetostriction coefficient and +500% for the uniaxial magnetic anisotropy field). This is in correlation with the x-ray diffraction peak sharpening observed in the studied thin films. This is most probably due to exceeding of the glass transition temperature of the thin film. The complete data analysis allowed an estimation of the thermal stability of alloy thin films.

We have studied the electro-mechanical properties of 20 nm thick CoFeB thin films deposited on Kapton® substrate by electrical resistivity measurements and atomic force microscopy observations during in situ tensile tests. We show in this paper that the amorphous CoFeB thin film has a brittle behavior with crack initiation at 1.6%. Moreover, the partial reversibility of the resistivity curve depends on the maximum applied strain because of the plastic deformation of the Kapton® up to 5%. Finally, the adhesion energy between CoFeB and Kapton® has been estimated from the geometry of buckling induced by the transverse compressive stress occurring during the fragmentation test. This study shows that adhesion is good enough for considering this system for potential applications in flexible spintronics applications.

A systematic micromagnetic study of the morphological characteristic effects over the magnetic static properties of Co-based complex shaped nanowires is presented. The relevance of each characteristic size (i.e. length L, diameter d, and size of the nanowires head T) and their critical values are discussed in the coercive field optimization goal. Our results strongly confirms that once the aspect ratio (L/d) of the nanowire is bigger than around 10, the length is no more the pertinent parameter and instead the internal diameter and the shape of the nanowires play a key role. We attribute this behavior to the non uniform distribution of the demagnetizing field which is localized in the nanowires head and acts as a nucleation point for the incoherent magnetization reversal. Finally, angular dependence of the magnetization are simulated and compared to the case of a prolate spheroid for all considered morphologies., 7 pages, 6 figures

This paper reports the development of a methodology combining microstrip ferromagnetic resonance (MS-FMR) and digital image correlation (DIC) in order to silmuteanously measure the voltage-induced strains and the magnetic resonance in artificial magnetoelectric heterostructures (magnetic films/piezoelectric substrate? or magnetic films/flexible substrate/piezoelectric actuator ?). The overall principle of the technique and the related analytical modelling are described. It is powerful to estimate the magnetostriction coefficient of ferromagnetic thin films and can be used to determine the effective magnetoelectric coefficient of the whole heterostructures in addition to the piezoelectric coefficient related to the in-plane voltage-induced strains. This methodology can be applied to system for which the strains are well transmitted at the different interfaces., 7 pages, 6 figures

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10 nm and 50 nm thick Co 2 FeAl (CFA) thin films have been deposited on thermally oxidized Si(001) substrates by magnetron sputtering using a Tantalum cap layer and were then ex-situ annealed at 415°C, 515°C and 615°C during 15 minutes in vacuum. X-rays diffraction indicates that films CFA are polycrystalline and exhibit an in-plane isotropy growth. Ferromagnetic resonance measurements, using a microstrip line (MS-FMR), reveal a huge interfacial perpendicular magnetic anisotropy and small in-plane uniaxial anisotropy both annealing temperature-dependent. The MS-FMR data also allow concluding that the gyromagnetic factor remains constant and that the exchange stiffness constant increases with annealing temperature. Finally, the FMR linewidth decreases with increasing annealing temperature due to the enhancement of the chemical order, and allow deriving a very low intrinsic damping parameter (1.3×10 −3 at 615°C).

The correlation between magnetic and structural properties of Co_{2} FeAl (CFA) thin films of different thickness (10 nm, Comment: 5 pages, 4 figures, submitted to J. Mag. Mag. Mat

Polarized small-angle neutron scattering (PSANS) experimental results obtained on arrays of ferromagnetic Co nanowires ($\phi\approx13$ nm) embedded in self-organized alumina (Al$_{2}$O$_{3}$) porous matrices are reported. The triangular array of aligned nanowires is investigated as a function of the external magnetic field with a view to determine experimentally the real space magnetization $\vec{M}(\vec{r})$ distribution inside the material during the magnetic hysteresis cycle. The observation of field-dependentSANSintensities allows us to characterize the influence of magnetostatic fields. The PSANS experimental data are compared to magnetostatic simulations. These results evidence that PSANS is a technique able to address real-space magnetization distributions in nanostructured magnetic systems. We show that beyond structural information (shape of the objects, two-dimensional organization) already accessible with nonpolarized SANS, using polarized neutrons as the incident beam provides information on the magnetic form factor and stray fields \textgreek{m}0Hd distribution in between nanowires., Comment: 13 pages, 10 figures, submitted to Phys. Rev. B

Ferromagnetic metal CoNi-based nano-objects have been synthesized in a polyol media within different elaboration conditions in order to drive their morphology (i.e. enhancing their length-to-diameter ratio ﴾d/L﴿, and changing the diameter d ratio over edge T width ﴾d/L﴿). Transmission Electron Microscopy (TEM) studies revealed unexpected effects on the Co80Ni20 nano-objects arising from the magnetic field assisted synthesis. This gave us the opportunity to compare this latter to coming from the variation of Ruthenium (III) chloride hydrate nucleating agent concentration. A Co80Ni20 anisotropic particles elaboration was successfully achieved under zero magnetic field assisted synthesis, while an important percentage of isotropic nanoparticles appeared immediately under the application of a small magnetic field (i.e. H > 500 Oe). In the first case we were able to sharply drive both the aspect ratio and head morphology of nanowires (T and ﴾d/T﴿). The good crystallinity and structures symmetry of all our samples have been proved by X-Ray Diffraction (XRD) pattern analysis. Magnetic static properties showed a ferromagnetic standard behavior with a coercive field efficiency which was strongly dependent on shape parameters. The magnetic static behavior was studied within a standard Stoner-Wohlfart model as a function of the observed morphologies. Our observations are fully consistent with a shape anisotropy origin behavior of the enhanced coercivity measured as function of the decreasing ﴾d/L﴿ ratio. However, they revealed the presence of contributions to the global effective anisotropy coming from other complex terms then the shape one (i.e. conic head impressiveness, dipolar interactions and magnetocrystalline anisotropy).

It is shown experimentally that collective Bloch spin waves are able to propagate in a dense periodic array of nanowires with step-modulated thickness along the periodicity direction. The spin wave dispersion (frequency versus wave vector k) was measured using the Brillouin light scattering technique by sweeping the wave vector perpendicularly to the wire length. Remarkably, the mode measured at the lowest frequency exhibits an oscillating dispersion and its frequency is up-shifted with respect to the homogeneous-thickness wires of the same width. The modes located at higher frequencies have negligible dependencies on the wave number, i.e. are practically dispersionless. Complementary ferromagnetic resonance measurements enabled us to independently measure the whole set of modes at k = 0, showing a good agreement with the Brillouin light scattering data. These results have been successfully reproduced in a numerical simulation employing a two-dimensional Green's function description of the dynamic dipole field of the precessing magnetization. The theory also allowed visualizing the non-trivial distribution of dynamic magnetization across the wire cross-section and estimating the Brillouin light scattering cross-section. The analysis of these intensities suggests complicated magneto-optical coupling between the light and the dynamic magnetization in the arrays of nanowires with step-modulated thickness. This work can stimulate the design, tailoring, and characterization of three-dimensional magnonic crystals. © 2014 IOP Publishing Ltd.

Bending effect on the magnetic anisotropy in 20 nm Co$_{2}$FeAl Heusler thin film grown on Kapton\textregistered{} has been studied by ferromagnetic resonance and glued on curved sample carrier with various radii. The results reported in this letter show that the magnetic anisotropy is drastically changed in this system by bending the thin films. This effect is attributed to the interfacial strain transmission from the substrate to the film and to the magnetoelastic behavior of the Co$_{2}$FeAl film. Moreover two approaches to determine the in-plane magnetostriction coefficient of the film, leading to a value that is close to $\lambda^{CFA}=14\times10^{-6}$, have been proposed., Comment: 4 pages, 4 figures

Co2FeAl (CFA) thin films with thickness varying from 10 nm to 115 nm have been deposited on MgO(001) substrates by magnetron sputtering and then capped by Ta or Cr layer. X-rays diffraction (XRD) revealed that the cubic $[001]$ CFA axis is normal to the substrate and that all the CFA films exhibit full epitaxial growth. The chemical order varies from the $B2$ phase to the $A2$ phase when decreasing the thickness. Magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer measurements show that, depending on the field orientation, one or two-step switchings occur. Moreover, the films present a quadratic MOKE signal increasing with the CFA thickness, due to the increasing chemical order. Ferromagnetic resonance, MOKE transverse bias initial inverse susceptibility and torque (TBIIST) measurements reveal that the in-plane anisotropy results from the superposition of a uniaxial and of a fourfold symmetry term. The fourfold anisotropy is in accord with the crystal structure of the samples and is correlated to the biaxial strain and to the chemical order present in the films. In addition, a large negative perpendicular uniaxial anisotropy is observed. Frequency and angular dependences of the FMR linewidth show two magnon scattering and mosaicity contributions, which depend on the CFA thickness. A Gilbert damping coefficient as low as 0.0011 is found., 11 pages, 11 figures, to appear in Phys. Rev B

In the present work, we report on the in situ magnetic force microscopy (MFM) study of the magnetization reversal in two-dimensional arrays of ferromagnetic Ni80Fe20 and Co55Fe45 nanowires(NW) with different diameters (40, 50, 70 and 100 nm) deposited inside low porosity (P, 6 pages, 4 figures, To appear in Journal of Applied Physics

International audience; Polarized SANS results obtained on ferromagnetic Co nanowires embedded in alumina (Al2O3) porous matrices are reported. The triangular arrays of aligned nanowires are investigated as a function of the external magnetic field in order to determine experimentally the real space magnetization M' ('r) distribution inside the material during the magnetic hys- teresis cycle. The observation of field-dependent SANS intensities allows us to characterize the influence of stray fields. Micromagnetic calculations are compared to the SANSPOL experimental data. These results show that SANSPOL is a technique able to address real-space magnetization distributions in magnetic nanosystems.

HRTEM, 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 Co2MnGe (13 nm)/Al2O3 (3 nm)/Co (13 nm) tunnel magnetic junctions (TMJ), 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 concluding 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., Comment: 11 pages, 10 figures, Thin Solid Films

We present the study of the dynamic properties of CoFeB (CFB) based magnetic tunnel junctions (MTJ) by Brillouin light scattering spectroscopy and micro-strip ferromagnetic resonance technique as well as magnetoresistance measurements. Two different MTJs: single and double barrier systems have been investigated. The dynamic studies revealed mostly similar perpendicular anisotropy field for both CoFeB layers for the single barrier MTJ with the thickest CFB layers, while two sensibly different perpendicular anisotropy fields were revealed in the case of the double barrier MTJ with the thickest CFB soft and hard layers. Modeling the spin wave frequencies behavior allows us the analysis of the experimental data and the derivation of the pertinent magnetic parameters. Our results suggest an interfacial origin of the anisotropy. Therefore, by changing the thickness of the perpendicularly magnetized CFB layers, it is possible to obtain a free and pinned layer usually ensured by the use of synthetic antiferromagnetic material. The tunnel magnetoresistance has been measured in elliptical elements of double barrier MTJ of different sizes. Strong size-dependent switching fields and antiparallel state being more unstable in the smallest device are observed. Patterning effect, lowering the anisotropies around the edge areas and increasing the dipolar coupling that favors the parallel state in the case of stacked layers, are probably the origin.

Magnetization switching by current induced spin–orbit torques (SOTs) in heavy metal/ferromagnetic metal/oxide structures is of great research interest due to its potential applications in the field of low power consumption spintronic devices. Here, we study the Slonczewski-like and the field-like SOT effective fields in β-W/Co2FeAl/MgO structures showing perpendicular magnetic anisotropy (PMA). We characterize the SOT effective fields using harmonic Hall voltage measurements and we point out the essential role of the planar Hall effect corrections. We estimate that for bulk β-W an effective spin Hall angle as large as 0.3 ± 0.03 and a spin diffusion length of 2.2 ± 0.3 nm. Moreover, we demonstrate SOT-induced magnetization switching for charge current densities of the order of 106 A cm−2.

In this paper a unique expression of the anisotropy field induced by any multiaxial stress state in a magnetic thin film and probed by ferromagnetic resonance is derived. This analytical development has been made using the uniaxial equivalent stress concept, for which correspondances between definitions given by different authors in the literature is found. The proposed model for the anisotropy field has been applied to thin films (25 nm) stressed both by piezoelectric actuation (non-equi-biaxial) or by bending tests (uniaxial) and measured with a broadband ferromagnetic resonance technique. The overall exprimental data can be easily plotted on a unique graph from which the magnetostriction coefficient has been estimated.

The residual magnetic uniaxial anisotropy of thin films deposited on flexible substrates was studied to elucidate its main origins. For that purpose, we sputtered 20 nm thick films of Co40Fe40B20 and Ni80Fe20 on Kapton® substrates. After measuring magnetomechanical properties of each system, we have shown that the weak residual anisotropy in the Ni80Fe20 film, compared to that of Co40Fe40B20, is perfectly correlated to its low magnetostriction coefficient. As a consequence, we conclude that this residual anisotropy is undoubtedly due to the unavoidable bending of the system, even if it is invisible with the naked eye (radius of several centimeters).