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International audience; The low power manipulation of magnetization is currently a highly sought-after objective in spintronics. Non ferromagnetic large spin-orbit coupling heavy metal (NM) / ferromagnet (FM) heterostructures offer interesting elements of response to this issue, by granting the manipulation of the FM magnetization by the NM spin Hall effect (SHE) generated spin current. Additional functionalities, such as the electric field control of the spin current generation, can be offered using multifunctional ferromagnets. We have studied the spin current transfer processes between Pt and the multifunctional magnetoelectric Ga0.6Fe1.4O3 (GFO). In particular, via angular dependent magnetotransport measurements, we were able to differentiate between magnetic proximity effect (MPE)-induced anisotropic magnetoresistance (AMR) and spin Hall magnetoresistance (SMR). Our analysis shows that SMR is the dominant phenomenon at all temperatures and is the only one to be considered near room temperature, with a magnitude comparable to those observed in Pd/YIG or Pt/YIG heterostructures. These results indicate that magnetoelectric GFO thin films show promises for achieving an electric-field control of the spin current generation in NM/FM oxide-based heterostructures.

International audience; Magnetic Surface Acoustic Wave (MSAW) devices combine a magnetostrictive material with a piezoelectric material to enable the sensing/measurement of magnetic fields by electrically generated SAWs. In such systems, the response is strongly dependent on the field direction with respect to the magnetic anisotropy of the magnetic material, which could give rise to detrimental hysteretic behaviour. The challenge in the current study is to control the shape of MSAW responses and remove the hysteresis in the magneto-acoustic response. We report especially on the magnetic response of a MSAW device with perpendicular magnetic anisotropy, rarely reported in literature. On the basis of the MSAW response with in-plane magnetic anisotropy studied previously, the MSAW signal with out-of-plane magnetization could be understood for all the field configurations. The final response is hysteresis free, which is suitable for interrogation of out of the plane magnetic fields. Such a technology could therefore complete the "in-plane" sensing technology to build up 3D sensors.

We report on the epitaxial growth of ${\mathrm{PrVO}}_{3}$ and ${\mathrm{LaVO}}_{3}$ by molecular beam epitaxy on (001)-oriented ${\mathrm{SrTiO}}_{3}$ substrates. We show that a high control of the deposition is achieved and leads to a highly reproducible layer-by-layer growth mode. We evidence also the effect of epitaxial strain on the magnetic properties of the epitaxial films. High-resolution transmission electron microscopy and scanning transmission electron microscopy observations reveal that an antipolar motion of the rare-earth atoms exists in both ${\mathrm{LaVO}}_{3}$ and ${\mathrm{PrVO}}_{3}$ thin films. The investigation of the structural distortions and atomic displacements in the vanadate thin films may be generalized to other orthorhombic perovskite oxides and contributes to the search of ferroelectricity by design in symmetry-breaking magnetic oxide superlattices.

International audience; A temperature compensated magnetic field sensor based on the combination of CoFeB ferromagnetic thin films and Quartz/ZnO Love waveguide platform is developed and optimized. The Love wave is a shear horizontal guided wave and therefore provides an optimal interaction with magnetisation in the magneto-elastic thin film resulting in higher acoustic wave magneto-elastic coupling compared to the conventional Rayleigh wave based devices. ST-cut Quartz was chosen as substrate, ZnO as insulating layer for Love wave generation and temperature coefficient of frequency (TCF) compensation and CoFeB as the magnetostrictive layer sensitive to magnetic field. Experimental results show a magneto-acoustic sensitivity of 15.53 MHz/T with almost zero TCF.

The strongly magnetostrictive TbFe

International audience; This work aims at studying the interaction between surface acoustic waves (SAW) and micro-structured magnetostrictive layers under a magnetic field with a perspective to develop magnetic field sensors. The impact of the competition between the strong intrinsic magnetic anisotropy of the magnetic material and the shape anisotropy of the interdigitated transducer (IDT) fingers introduced by the micro-structuration is investigated. Therefore, the macroscopic and microscopic magnetic properties of the IDT and their influence on the magneto-acoustic response are studied. A SAW resonator with the IDTs made of the magnetostrictive thin film was elaborated and the magnetic surface acoustic wave (MSAW) response under a magnetic field was performed and discussed. Depending on the energy balance, the anisotropy gets modified and a correlation with the MSAW sensitivity to an externally applied magnetic field is made.

International audience; The TbFe2 compound has been deposited by Molecular Beam Epitaxy on Lithium Niobate (LN) substrates with different orientations (LN Z-, 128 Y-and 41 Y-cuts). Despite the challenging growth on these unconventional substrates, crystalline TbFe2 films (as a single orientated domain or with a limited number of orientations) of reasonable structural quality could be obtained after the deposition of a Mo buffer layer. Detailed and combined RHEED and X-ray analysis permitted to unravel the complex Mo and TbFe2 crystal orientations and to reveal common 3D Orientation Relationships (OR) between the different lattices, whatever the initial LN cut. The magnetic properties and especially the magnetic anisotropy have been investigated in taking magnetocrystalline, magnetoelastic and magnetostatic contributions into account. These promising results on the epitaxial growth of hybrid piezoelectric/ magnetostrictive crystalline system constitute an important step towards the control of magnetization via electrically generated static and/or dynamic strains, and towards the development of magnetic sensors based on Surface Acoustic Wave devices.

In previous studies [1,2] we have shown the possibility to realize wireless magnetic SAW sensors with a delay line configuration using layered structures. SAW devices in resonator configuration with Ni interdigital transducers (IDT) on the substrate were also investigated by Kadota et al. [3]. The physical phenomena behind sensor behavior is still unclear and the interpretation of the results is not fully in line with the proposed theoretical models. This work aims to understand the physics and the interaction between acoustic waves and magnetostrictive layers under magnetic fields. Here, we investigate multilayer (LiNbO3/(TbCo2/FeCo)) SAW structures with different geometries and configurations.

International audience; While surface acoustic wave (SAW) sensors have been used to measure temperature, pressure, strains, and low magnetic fields, the capability to measure bipolar fields and high fields is lacking. In this paper, we report magnetic surface acoustic wave sensors that consist of interdigital transducers made of a single magnetostrictive material, either Ni or TbFe 2 , or based on exchange-biased (Co=IrMn) multilayers. By controlling the ferromagnet magnetic properties, high-field sensors can be obtained with unipolar or bipolar responses. The issue of hysteretic response of the ferromagnetic material is especially addressed, and the control of the magnetic properties ensures the reversible behavior in the SAW response.

International audience; The strongly magnetostrictive TbFe2 compound has been epitaxially grown on Z-cut Lithium Niobate (LiNbO3) substrates after the deposition of various buffer layers (Mo, Ti and Ti/Mo). Detailed and combined RHEED and X-ray analysis permitted to unravel the in-plane and relative Orientation Relationships (OR) of the different materials in the system. Despite the use of different templates with different structural orders, similar final OR are eventually found between the piezoelectric substrate and the magnetic layer. The structural and magnetic properties are analyzed in order to get a TbFe2 layer of optimum quality to build a magnetostrictive/piezoelectric hybrid system with efficient strain mediated coupling. Such systems are of interest for the development of magnetic sensors as well as for the electric control of magnetization.

International audience; We investigate the heat transport through a rare earth multilayer system composed of yttrium (Y), dysprosium (Dy), and niobium (Nb) by ultrafast x-ray diffraction. This is an example of a complex heat flow problem on the nanoscale, where several different quasiparticles carry the heat and conserve a nonequilibrium for more than 10 ns. The Bragg peak positions of each layer represent layer-specific thermometers that measure the energy flow through the sample after excitation of the Y top layer with fs-laser pulses. In an experiment-based analytic solution to the nonequilibrium heat transport problem, we derive the individual contributions of the spins and the coupled electron-lattice system to the heat conduction. The full characterization of the spatiotemporal energy flow at different starting temperatures reveals that the spin excitations of antiferromagnetic Dy speed up the heat transport into the Dy layer at low temperatures, whereas the heat transport through this layer and further into the Y and Nb layers underneath is slowed down. The experimental findings are compared to the solution of the heat equation using macroscopic temperature-dependent material parameters without separation of spin and phonon contributions to the heat. We explain why the simulated energy density matches our experiment-based derivation of the heat transport, although the simulated thermoelastic strain in this simulation is not even in qualitative agreement.

We investigate the structural and thermal nonequilibrium dynamics including the spin-reordering dynamics of an antiferromagnetic Dysprosium (Dy) nanolayer after excitation with fs-laser pulses by means of ultrafast structural and magnetic x-ray diffraction experiments.

We report a simultaneous investigation of $3d$ and $4f$ magnetic moments by exploring the $\mathrm{Fe}\ensuremath{-}{L}_{2,3}$ and $\mathrm{Dy}\ensuremath{-}{M}_{4,5}$ electron energy-loss edges of a $\mathrm{DyF}{\mathrm{e}}_{2}/\mathrm{YF}{\mathrm{e}}_{2}$ superlattice using the energy-loss magnetic chiral dichroism (EMCD) technique. Specific EMCD sum rules for ${M}_{4,5}$ edges were established and carefully applied to the dichroic signal at $\mathrm{Dy}\ensuremath{-}{M}_{4,5}$ edges, giving an orbital to the effective spin moment ratio of $5.1\ifmmode\pm\else\textpm\fi{}1.8$. With dynamic diffraction effects considered, the opposite signs of $\mathrm{Fe}\ensuremath{-}{L}_{3}$ and $\mathrm{Dy}\ensuremath{-}{M}_{5}$ dichroic peaks unambiguously indicate the antiparallel alignment of net Fe $3d$ and Dy $4f$ moments. The EMCD technique is shown to be an effective tool to locally characterize the $4f$ moment of rare earth elements and study $3d\ensuremath{-}4f$ moment coupling.

The reflectometry curves of an Nb(4 nm)/YFe2(40 nm, 〈110〉)/Fe(1.5 nm)/Nb(50 nm) sample measured in the synchrotron radiation of right- and left-handed circular polarization for a set of wavelengths near the Y L 2,3 absorption edges have been used to determine the energy dependences of the component magnitudes for the X-ray susceptibility tensor of YFe2 near the yttrium absorption edges, including off-diagonal magnetic additives. Our result is in good agreement with the normalized experimental absorption and circular dichroism spectra and their Kramers-Kronig transformations.

[ DyFe2/YFe2] superlattices have been grown by Molecular Beam Epitaxy. The magnetic properties of this hard/soft composite system, the components of which are exchange-coupled at the interfaces, have been investigated in the 10 K–300 K temperature range, with a specific attention paid to the influence of the soft and hard materials thicknesses. In order to unravel the very rich magnetization reversal processes, conventional susceptibility and magnetization measurements have been combined with element-selective X-ray Magnetic Circular Dichroism analysis. The superlattice with thin individual thicknesses ([ 1nm DyFe2/4 nm YFe2] 70) reverses as a unique giant ferrimagnetic block in which the exchange-favoured antiparallel arrangement between net magnetizations is maintained under a magnetic field. In the superlattices with large individual thicknesses ([ 10 nm DyFe2/13 nm YFe2] 18 and [ 10 nm DyFe2/20 nm YFe2] 13), the expected exchange spring behaviour is observed: the soft YFe2 layers reverse for positive bias fields, followed by the irreversible switch of the hard DyFe2 layers. In the case of intermediate thickness for the individual DyFe2 layers ([ 3 nm DyFe2/12 nm YFe2] 22, [ 5 nm DyFe2/20 nm YFe2] 13, [ 7 nm DyFe2/28 nm YFe2] 10), the magnetization reversal process strongly depends on temperature. In particular, an unusual magnetization reversal process occurs in the high temperature range where it becomes easier to reverse the hard DyFe2 layers for positive fields, while maintaining the dominant YFe2 magnetization along the field direction.

Bulk Sm 1-x Gd x Al 2 (0.003

The epitaxial growth of (1 1 0) and (1 1 1) SmAl 2 thin films has been achieved on (1 1 0)Nb ∥ (1 1 2¯ 0) sapphire by molecular beam epitaxy. The successful fabrication of this compound as thin films is the first and promising step towards the growth of thin films of the ternary compound Sm 1− x Gd x Al 2 , a zero magnetization ferromagnet of high interest for magnetic nanodevices. We report especially on the influence of the deposition temperature on the growth direction of SmAl 2 : in-situ RHEED analysis and ex situ large angle X-ray scattering experiments show that the growth direction evolves from [1 1 1] to [1 1 0] when the deposition temperature increases from 510 to 620 °C. The (1 1 0) films stabilized at higher temperature present twins rotated by 70° about the surface normal, whereas the (1 1 1) films are twin-free single domains. Results are correlated to the film morphology at the beginning of growth, observed by AFM, which depends also significantly on the deposition temperature. The strains measured parallel to the growth direction depend slightly on the film thickness and are larger than those expected from the substrate thermal contraction after deposition.

In order to investigate magnetic phenomena in Nd and Sm layers separately, resonant x-ray magnetic scattering experiments have been performed to study Nd/Sm(001) superlattices with different relative layers thickness. The samples were grown using molecular beam epitaxy, and optimized to yield dhcp Sm growth and thus a coherent dhcp stacking across the Nd/Sm superlattices. The magnetic phases in Sm layers are very close to the ones evidenced in dhcp thick films. In contrast, the magnetism in Nd layers shows strong differences with the bulk case. In superlattices with a large Sm thickness (>8 nm), no magnetic scattering usually associated with Nd magnetic structure was detected. In superlattices with smaller Sm thickness (

The propagation of Nd long range magnetic order in the hexagonal and cubic sublattices has been investigated in double hexagonal compact Nd/Sm(001) superlattices by resonant x-ray magnetic scattering at the Nd L(2) absorption edge. For a superlattice with 3.7 nm thick Sm layers, the magnetic structure of the hexagonal sublattice propagates coherently through several bilayers, whereas the order in the cubic sublattice remains confined to single Nd blocks. For a superlattice with 1.4 nm thick Sm layers, the magnetic structures of both sublattices appear to propagate coherently through the superlattice. This is the first observation (i) of the long range coherent propagation of Nd order on the cubic sites between Nd blocks and (ii) of a different thickness dependence of the propagation of the Nd magnetic phases associated with the hexagonal and cubic sublattices. The propagation of the Nd magnetic order through Sm is interpreted in terms of generalized susceptibility of the Nd conduction electrons.

In order to determine the strain tensor in a 375 nm thick Eu(110) epitaxial thin film, we have developed a new method, based on the accurate determination of the lattice vectors by high resolution X-ray diffraction. We show that a biaxial strain model gives a good representation of the state of the strains field in the film.

Single crystal Eu(1 1 0) films exhibit an atypical magnetic arrangement, different from the bulk three helices configuration: at low temperature, under zero field, only two magnetic domains are present since the magnetic helix that propagates along the in-plane [0 0 1] axis disappears below a given temperature. This paper presents the study of the stability of the magnetic domains under magnetic field using neutron scattering experiments and magnetisation measurements on a 375 nm thick Eu(1 1 0) film. The helix propagating along [0 0 1] is restored by the application of an external field along the [0 0 1] direction, which gives rise to a supplemental Zeeman contribution.

A detailed structural analysis of the epitaxial growth of europium using in situ reflection high-energy electron diffraction has been performed for two kinds of buffer layers with different symmetry: (1 1 0)niobium (Nb) and (0 0 0 1)yttrium (Y). Moreover, by varying the preparation conditions of the (1 1 0)Nb layer we get either a reconstructed or an unreconstructed Nb surface, which turns out to have a strong influence on the Eu growth: the reconstructed (1 1 0)Nb surface favours the stabilisation of an Eu hexagonal surface structure, as well as the (0 0 0 1)Y buffer, whereas a pure unreconstructed (1 1 0)Nb surface enables the growth of single crystalline bcc (1 1 0)Eu films. With increasing Eu thickness, the relaxation of the metastable Eu hexagonal surface obtained on (0 0 0 1)Y or on reconstructed (1 1 0)Nb gives rise to a multidomain structure that drastically depends on the nature of the underlying buffer layer.

Magnetic anisotropy measurements performed in a (110) (Tb0.27Dy0.73)Fe2 (Terfenol-D) film epitaxially grown on a sapphire substrate are presented. The magnetic torque curves have been determined by using a vectorial vibrating sample magnetometer, which allows us to measure the angular dependence of magnetization components parallel, , and perpendicular, , to the applied field up to 2 T. The fourfold symmetry associated with the cubic structure within the (110) plane is clearly observed. The analysis of the experimental torque has been carried out considering magnetocrystalline anisotropy up to sixth order and magnetoelastic energy up to second order; so, the magnetocrystalline anisotropy constants in the (110) plane of the film, K1 and K2, have been obtained. This allows us to determine the direction of the magnetization easy axis for (110) Terfenol-D thin-film: it is at RT, passes through at 140 K and then changes to at 40 K. It was completely impossible to explain the angular dependence of the experimental magnetic torque without including shear and tetragonal magnetoelastic stress parameters, b2 and b1, respectively. This confirms the paramount role of the strain in the determination of the magnetic properties in this kind of Terfenol-D thin film.

The spectral dependence of the Bragg peak position under conditions of extremely asymmetric diffraction has been analyzed in the kinematical and dynamical approximations of the diffraction theory. Simulations have been performed for the L 3 absorption edge of yttrium in a single-crystal YFe2 film; they have shown that the magneto-optical constants (or, equivalently, the dispersion corrections to the atomic scattering factor) for hard X-rays can be determined from this dependence. Comparison with the experimental data obtained for a Nb(4 nm)/YFe2(40 nm〈110〉)/Fe(1.5 nm)/Nb(50 nm)/sapphire sample at the European Synchrotron Radiation Facility has been made.

International audience; Magnetic and magneto-transport properties of [Co/Pt]/MgO/[Co/Pt]CoTb magnetic tunnel junctions have been investigated. Depending on the thickness of the Pt layer, temperature, and field history, the [Co/Pt]CoTb hard layer shows complex behaviors that can be reproduced by micromagnetic calculations. The magnetic tunnel junctions appear to exhibit conventional behavior but also exchange bias and spring magnet and domain duplication phenomena that have been observed combining magnetometry and interface-sensitive spin-dependent tunnel transport.

The epitaxial growth of Fe on a bcc Nb(1 1 0) buffer has been studied at room temperature and at 550 °C, using reflection high energy electron diffraction and Auger electron spectroscopy. At room temperature, the growth occurs in several steps that have been clearly identified in increasing the Fe deposited thickness: (i) two-dimensional growth of a strained Fe layer (ii) intermixing between Fe and Nb with the formation of an alloy (iii) stabilization of a pure Fe layer. After an annealing process up to 550 °C, another alloy is formed, that exhibits a rectangular surface mesh with twins in the (1 1 0) plane. A single domain alloy, with a similar surface structure, can be also obtained by directly growing Fe on the Nb(1 1 0) buffer maintained at 550 °C.

High quality single crystal (0 0 1) Sm films and (0 0 1) Sm/Nd superlattices have been prepared by molecular beam epitaxy. The epitaxial growth is controlled by in situ RHEED measurement and the crystallographic structure is determined ex situ from large angle X-ray scattering. We show how the crystal structure of several-thousands-angstroms-thick samarium films can be tailored by the elaboration conditions (i.e. the deposition of a thin Nd template layer): either a nine hexagonal close-packed plane stacking sequence (“Sm-structure”), as bulk samarium, or a four hexagonal close-packed plane (dhcp) structure are obtained. Moreover, this unusual dhcp structure for pure samarium has also been stabilized in nanoscale (0 0 1) Sm/Nd superlattices that thus present a coherent dhcp stacking over several bilayers. Both dhcp Sm films and coherent dhcp Sm/Nd superlattices are of broad interest for investigations of the link between structure and magnetism in rare earth epitaxied systems.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; We report on the genuine origin of the anisotropy of the magnetization M in rare-earth (RE) metals and RE-based alloys. Taking Ho-based layered nanostructures as testing ground, we prove that the anisotropy of M is substantial despite that the sixfold magnetic anisotropy constant K-6(6) vanishes, which contradicts the established wisdom [E. R. Callen and H. B. Callen, J. Phys. Chem. Solids 16, 310 (1960)]. Furthermore, we show that the symmetric anisotropic contributions to M and K-6(6) vary with temperature distinctively from one another, which indicates that both anisotropic effects are unrelated and stem from dissimilar microscopic sources. Our findings are discussed according to the theory [R. J. Elliott and M. F. Thorpe, J. Appl. Phys. 39, 802 (1968)] that predicts the emergence of symmetric anisotropic indirect-exchange terms under the presence of orbital moments. We show evidence that the anisotropy of M is caused by the indirect-exchange coupling among localized 4f magnetic moments mediated by spin-orbit coupled conduction electrons, which ultimately generates a spatially nonuniform spin polarization that replicates the lattice symmetry.

We report on the study of the magnetoelastic behavior of some rare-earth based thin films and superlattices (SL`s). Magnetoelastic stress (MS) measurements (by using a cantilever capacitive technique) within a wide range of temperatures (10-300 K) and magnetic fields (up to 12 T) have been performed. We derive expressions relating the cantilever curvatures and the magnetoelastic stresses in anisotropic thin films and SL`s (for cubic symmetry) deposited onto crystalline substrates. The magnetoelastic energy associated to the interfaces and the epitaxial stress dependence of the volume MS has been investigated by studying the basal plane MS in Hon/Lu₁₅ and in Ho₁₀/Ym SL`s: we obtain interface MS even higher than the volume ones and the effect in bulk`s MS of the epitaxial strain is large. In Dy/Y and Er/Lu SL's we also deduce the MS contributions but, for Er/Lu, incomplete saturation leads to inconclusive results. Although the latter case also happens in Ho/Tm SL`s, MS clearly shows anisotropy competition. In TbFe₂ (t) / YFe₂(1000 Å) (300 Å < t < 1300 Å) epitaxial bilayers, we determine all the MS allowed by the symmetry and show that epitaxial stress strongly modifies the tetragonal MS. The thermal dependence of MS parameters is also analysed.

Single crystal RFe2(110) films were grown by molecular beam epitaxy to a total thickness of 1000 A at different substrate temperatures ranging from 450 ° C to 660 ° C. The first stages of growth and the surface morphology of the deposited layers have been studied using Reflection High Energy Electron Diffraction (RHEED) and Atomic Force Microscopy (AFM). The growth is first strained but further deposit induces the formation of three-dimensional fully relaxed islands. Subsequently, the morphology of the RFe2(110) nanosystems evolves from anisotropic dots to a smooth surface, as a function of the preparation parameters, i.e. nominal thickness and substrate temperature. It also depends on the rare earth involved in the compound.

Original composite systems with two isomorphous Laves phases $({\mathrm{DyFe}}_{2}$ and ${\mathrm{YFe}}_{2})$ have been elaborated by molecular-beam epitaxy. These superlattices combine the hard magnetic properties of ${\mathrm{DyFe}}_{2}$ with the soft ones of ${\mathrm{YFe}}_{2}.$ Moreover, they exhibit high single-crystal quality and constitute model systems to investigate the influence of the relative energy terms on the magnetization reversal. The magnetization measurements show that it is thus possible to tailor the magnetic configuration and the magnetization reversal process. Particularly interesting behaviors have been observed in the superlattice $[{\mathrm{DyFe}}_{2}(50\AA{})/{\mathrm{YFe}}_{2}(130\AA{})],$ which presents spring ferrimagnet characteristics, together with an exchange bias of the hysteresis loop. The bias can be either positive or negative, depending on the cooling-field value and, therefore, shows memory of the magnetic history of the superlattice. The spring-magnet loop appears to be reversible over the (-7.5 T/+7.5 T) field range at low temperature.

The magnetic ordering of samarium in a 5000 A thick (001) Sm film and in a (001) Sm/Y superlattice has been investigated using resonant x-ray magnetic scattering at the samarium L3 edge. In both samples, the hexagonal sublattice orders below TN = 106 K. Magnetic resonances of dipolar and quadrupolar origin are observed above and below the edge respectively and allow one to study the 5d and 4f magnetism separately. In the film, the cubic sublattice also shows long range magnetic order below 12 K. The relative contributions of the 4f and 5d electrons to the magnetic moments on the hexagonal sites are the same for T < 12 K, 12 K < T < TN and close to TN . In the superlattice, the Y and Sm layers mainly retain their own bulk crystallographic structure (i.e. hcp and `Sm structure' respectively). The hexagonal sublattice of the Sm layers shows the same magnetic structure as in the thick film, but the energy dependence of the resonance is different. There is no coherence of the magnetic structure through the Y spacer layers.

Epitaxial Sm thin films and Sm/Y superlattices have been grown with a high single-crystal quality. The complex nine hexagonal close-packed planes stacking sequence has been successfully obtained in a 5000 \AA{}-thick film, whose magnetic behavior presents a long-range antiferromagnetic order of the hexagonal sites below 100 K, as in the bulk element. However, from neutron scattering, the relative magnetic and nuclear contributions indicate a significant enhancement of the magnetic moment compared to the bulk value. For the superlattice, the neutron-scattering results show neither long-range structural coherence of the stacking sequence along the c direction nor a coherent magnetic ordering at low temperatures.

Single-crystalline $R{\mathrm{Fe}}_{2}(110)$ compounds $(R=\mathrm{Y},$ Sm, Gd, Tb, ${\mathrm{Dy}}_{0.7}{\mathrm{Tb}}_{0.3},$ Dy, Er, and Lu) have been grown by molecular-beam epitaxy. Compared to the bulk compounds, the thin films exhibit modifications of the magnetic anisotropy, related to the strains induced during deposition. We present here a detailed determination of in-plane and out-of-plane parameters using x-ray and neutron diffraction. $R{\mathrm{Fe}}_{2}$ films and $R{\mathrm{Fe}}_{2}{/\mathrm{Y}\mathrm{F}\mathrm{e}}_{2}$ bilayers have been deposited on a Nb buffer covered with Fe. Both systems are strained compared to bulk compounds: they are expanded in the plane of epitaxy and compressed along the growth direction. For single $R{\mathrm{Fe}}_{2}$ films, the strains do not depend on the lattice parameters of the corresponding bulk compounds, i.e., on the large mismatch between the film and the buffer (around 10%). The sign and the values of the strains are explained with a model of differential thermal contraction between the film and the substrate. For $R{\mathrm{Fe}}_{2}$ films involved in $R{\mathrm{Fe}}_{2}{/\mathrm{Y}\mathrm{F}\mathrm{e}}_{2}$ bilayers, the mismatch between $R{\mathrm{Fe}}_{2}$ and the ${\mathrm{YFe}}_{2}$ layer is smaller and the strains in the $R{\mathrm{Fe}}_{2}$ films do depend on the bulk lattice parameters. Their evolution can be explained with an elastic model.

A detailed reflection high-energy electron diffraction study of the first stages of the niobium growth on (1120)s sapphire is presented for several substrate temperatures. It is shown that the niobium film exhibits an hexagonal surface structure when the deposited thickness is smaller than a critical value, which, depending on the substrate temperature, varies between 5 and 15 A. For thicknesses larger than this critical thickness, the surface hexagonal structure relaxes to the (110) bcc niobium structure. The hexagonal surface structure is observed for high substrate temperatures (820-410oC) but does not appear when the substrate temperature is 270oC. The epitaxial relationships between the substrate, the surface hexagonal structure of niobium and the cubic niobium phase are presented.

Neutron diffraction experiments and macroscopic magnetization measurements show evidence of the stabilization of the helical magnetic phase over large temperature ranges for Tb and alloy films grown epitaxially on yttrium. In particular, the temperature of the transition between the helical and the ferromagnetic states is shifted from 220 K for bulk terbium to 160 K for a pure terbium film grown epitaxially on yttrium, despite the low stability of the helical phase in the bulk element. The decrease of the Curie temperature is due to the negative c-axis strain induced by the epitaxial growth on yttrium. The epitaxial strains also induce modifications of the Fermi surface, which leads to an increase of the turn angle even at . At low temperature, a long-wavelength-modulated phase, whose origin still remains to be explained, has been observed.

X-ray magnetic circular dichroism experiments have been performed up to \ifmmode\pm\else\textpm\fi{}17 T to investigate the magnetic configuration and magnetization reversal of an original zero-magnetization ferromagnet Sm${}_{1\ensuremath{-}x}$Gd${}_{x}$Al${}_{2}$ ($x$ $=$ 0.028), both as a single epitaxial layer and as a pinning layer in an exchange-coupled system. The Sm${}_{0.972}$Gd${}_{0.028}$Al${}_{2}$ single layer appears to exhibit an extraordinary large coercivity that exceeds 20 T below its magnetic compensation (${T}_{\mathrm{comp}}$). Despite such huge magnetic stability in the single layer, interface exchange coupling in the bilayer drives the formation of domains in Sm${}_{0.972}$Gd${}_{0.028}$Al${}_{2}$ and their reversal upon field, both below and at magnetic compensation. Increasing the cooling field yields the increase in exchange-favored domains in Sm${}_{0.972}$Gd${}_{0.028}$Al${}_{2}$, surprisingly also at ${T}_{\mathrm{comp}}$, whereas, Zeeman energy does not favor this specific orientation neither at compensation nor during the cooling process. We propose a possible scenario for those domains' formation and highlight the way the external magnetic field may tune the magnetic configuration in such a zero-magnetization ferromagnet. The cooling field also consequently influences the SmAl${}_{2}$ magnetization reversal which is biased by pinned magnetic components in Sm${}_{0.972}$Gd${}_{0.028}$Al${}_{2}$; the bias field is satisfactorily explained in considering different pinned contributions with opposite magnetic orientations.

We present the first epitaxial growth of some (110) rare earth-Fe2 (DyFe2, TbFe2 and Dy0.7Tb0.3Fe2 known as terfenol-D) thin films on (110) Nb(11−20) sappire by molecular beam epitaxy. The epitaxy is initiated by the deposition of a thin layer of iron on niobium. The structures are investigated by RHEED and X-ray scattering. Depending on the thermal treatment of the iron thin layer, the films of RE-Fe2, epitaxially grown on it, are either single crystals or present twins related by a 110° rotation about the surface normal. The growth of epitaxial thin films of these compounds is of interest because of the magnetic and magnetostrictive properties these materials may exhibit.

In-plane and out-of-plane lattice parameters of Nb and Eu have been measured as a function of temperature between 10 and 300 K in the Al2O3(112¯0)∕Nb(110)∕Eu(110) epitaxial system. It is shown that the Nb lattice is clamped in the growth plane to the sapphire substrate, in the whole temperature range. On its own, the Eu lattice is totally free to expand isotropically above a clamping temperature Tcl, below which in-plane parameters also freeze to an almost constant value. This well-defined clamping temperature strongly depends on the Eu thickness and likely is related to the thermal mobility of interface defects.

Element selective x-ray magnetic circular dichroism measurements have been performed to unravel the complex magnetization reversal process occurring in antiferromagnetically exchange-coupled Laves phase superlattices. The separate investigation of each magnetic compound in a [DyFe2 (50 A)/YFe2 (200 A)] superlattice has enlightened a drastic thermal variation towards an unexpected high-temperature regime, where the magnetization in the hard layers first reverses in positive field, whereas the magnetization in the soft ones remains stuck along the field direction. This transfer of the interface magnetic twists towards the hard layers permits us to explain the thermal evolution of the coercive field.

The L / S ratio of samarium has been determined by X-ray magnetic scattering from a thick epitaxial film, via the azimuthal dependence of the scattered intensity. This new method allows a direct determination of L / S in antiferromagnetic systems where charge scattering is not negligible at the position of the magnetic signal. The Russell–Saunders predictions fall within the accuracy of our results.