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We present an experimental study of vortex dynamics in magnetic nanocontacts based on pseudo spin valves comprising the ${\mathrm{Co}}_{2}\mathrm{MnGe}$ Heusler compound. The films were grown by molecular beam epitaxy, where precise stoichiometry control and tailored stacking order allowed us to define the bottom ferromagnetic layer as the reference layer, with minimal coupling between the free and reference layers. 20-nm diameter nanocontacts were fabricated using a nanoindentation technique, leading to self-sustained gyration of the vortex generated by spin-transfer torques above a certain current threshold. By combining frequency- and time-domain measurements, we show that different types of spin-transfer induced dynamics related to different modes associated to the magnetic vortex configuration can be observed, such as mode hopping, mode coexistence, and mode extinction appearing in addition to the usual gyration mode.

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.

We present an experimental study of spin-torque driven vortex self-oscillations in magnetic nanocontacts. We find that above a certain threshold in applied currents, the vortex gyration around the nanocontact is modulated by relaxation oscillations, which involve periodic reversals of the vortex core. This modulation leads to the appearance of commensurate but also more interestingly here, incommensurate states, which are characterized by devil's staircases in the modulation frequency. We use frequency- and time-domain measurements together with advanced time-series analyses to provide experimental evidence of chaos in incommensurate states of vortex oscillations, in agreement with theoretical predictions., Comment: 5 pages, 4 figures

The reduction of magnetic damping is one of the biggest challenges in low-energy-consumption spintronics and magnonics, in the pursuit of $e.g.$ low switching current for spin-transfer-torque-based technology, and long-range spin-wave propagation. This experimental study highlights ultralow damping values in high-quality epitaxial Co${}_{2}$Mn$Z$ ($Z$ = Al, Si, Ga, Ge, Sn, Sb) Heusler half-metallic magnets. As predicted theoretically, these ultralow values are intrinsically coupled to the underlying electronic structure. The width of the spin gap, as well as the location of the Fermi energy within it, play key roles in the relaxation of precessing magnetization.

International audience; Study of resonant tunneling through multi-metallic quantum well (QW) structure is not only important for the fundamental understanding of quantum transport but also for the great potential to generate advanced function-alities of spintronic devices. However, it remains challenging to engineer such a structure due to the short electron phase coherence length in metallic QW system. Here, we demonstrate the successful fabrication of double-QW structure in a single fully epitaxial magnetic tunnel junction (MTJ) heterostructure, where two Fe QW layers are sandwiched between three MgAlO x tunnel barriers. We show clear evidence of the coherent resonant tunneling through the discrete QW states in the two QWs. The coherent resonant tunneling condition is fulfilled only when the middle barrier between the two QWs is thin enough and available QW states are present simultaneously in both QWs under a certain bias. Compared to the single QW structure, the resonant tunneling in double-QW MTJ produces strong conductivity oscillations with much narrower peak width (about half) owing to the enhanced energy filtering effect. This study presents a comprehensive understanding of the resonant tunneling mechanism in MTJ with multiple QWs, which is essential for future development of new spintronic devices operating in the quantum tunneling regime. R esonant tunneling in quantum well (QW) structures has been extensively studied because of its importance in the field of nanoelectronic science and technology. 1 Recently, in the metallic QW system the combination of the tunneling magnetoresistance (TMR) 2,3 effect with resonant tunneling through metallic QW states in magnetic tunnel junctions (MTJs) has triggered considerable interest in the new functionality of spintronic devices. 4− 8 In these structures, the discrete spin-dependent QW states can be formed in the middle metallic QW layer sandwiched between two barriers. The QW potential barrier can be formed either by metallic layer using the symmetry-dependent Bloch state orbital band structure, 9− 12 or by a double oxide tunneling barriers with a much higher barrier height for better electron confinement. 13− 15 Because the QW states are often formed in the majority spin channel, 4,8 the conductance in parallel magnetic configuration can be enhanced when the injector Fermi level is aligned with the QW state at resonance, resulting in an enhanced TMR ratio. 4 A record high resonant TMR (≈ 4000%) has been reported for a double barrier MTJ measured with point-contact technique at 4.2 K. 16 In semiconductor materials, multi-QW structures have already been extensively studied, 17 such as resonant-tunneling diode, 18 multi-quantum well light-emitting diode and laser, 19 and so forth. When the thickness of the barrier and QW is thin enough, the multi-QWs form the superlattice structure and the

Heusler magnetic alloys offer a wide variety of electronic properties very promising for spintronics and magnonics. Some alloys exhibit a spin gap in their band structure at the Fermi energy, the so-called half-metal magnetic (HMM) behavior. This particular property leads to two very interesting properties for spintronics, i.e., fully polarized current together with ultra-low magnetic damping, two key points for spin-transfer-torque based devices. This Tutorial gives experimental details to grow and characterize Heusler Co2MnZ compounds in thin films (Z = Al, Si, Ga, Ge, Sn, Sb) by using molecular beam epitaxy in order to get the proper predicted electronic properties. A first part of this Tutorial is dedicated to control the stoichiometry as best as possible with some methods to test it. The chemical ordering within the lattice was examined by using electron diffraction during growth, regular x-ray diffraction, and scanning transmission electron microscopy. In particular, standard x-ray diffraction is carefully analyzed depending on the chemical ordering in the cubic cell and shown to be inefficient to distinguish several possible phases, on the contrary to electron microscopy. The electronic properties, i.e., magnetic moment, spin polarization, and magnetic damping were reviewed and discussed according to the stoichiometry of the films and also theoretical predictions. Polycrystalline films were also analyzed, and we show that the peculiar HMM properties are not destroyed, a good news for applications. A clear correlation between the spin polarization and the magnetic damping is experimentally demonstrated. At least, our study highlights the major role of stoichiometry on the expected properties.

Engineering of magnetic materials for developing better spintronic applications relies on the control of two key parameters: the spin polarization and the Gilbert damping, responsible for the spin angular momentum dissipation. Both of them are expected to affect the ultrafast magnetization dynamics occurring on the femtosecond timescale. Here, engineered Co2 MnAlx Si1- x Heusler compounds are used to adjust the degree of spin polarization at the Fermi energy, P, from 60% to 100% and to investigate how they correlate with the damping. It is experimentally demonstrated that the damping decreases when increasing the spin polarization from 1.1 × 10-3 for Co2 MnAl with 63% spin polarization to an ultralow value of 4.6 × 10-4 for the half-metallic ferromagnet Co2 MnSi. This allows the investigation of the relation between these two parameters and the ultrafast demagnetization time characterizing the loss of magnetization occurring after femtosecond laser pulse excitation. The demagnetization time is observed to be inversely proportional to 1 - P and, as a consequence, to the magnetic damping, which can be attributed to the similarity of the spin angular momentum dissipation processes responsible for these two effects. Altogether, the high-quality Heusler compounds allow control over the band structure and therefore the channel for spin angular momentum dissipation.

International audience; We investigated the current-induced domain-wall (DW) depinning for various applied magnetic fields on a well-indentified single pinning site in epitaxial Co/Ni-based spin-valve wire of micronic width. The DW depinning process occurs with thermal activation involving a single energy barrier associated with a single pinning site. By measuring the DW depinning probability for various positive and negative applied fields (H+,H−) and currents (I+,I−), we built a map highlighting regions where spin-transfer torque (STT) effect, Joule heating, and Oersted field dominate. We then propose a method to quantify characteristic parameters of both adiabatic and nonadiabatic components of STT despite the presence of other effects due to current injection. The suitability of the method is validated by the fact the extracted values are close to those obtained previously on single [Co/Ni] layer where Oersted field and Joule effects were negligible.

In this paper we analyze in details the electronic properties of (Co/Ni) multilayers, a model system for spintronics devices. We use magneto-optical Kerr (MOKE), spin-polarized photoemission spectroscopy (SRPES), x-ray magnetic circular dichroism (XMCD) and anomalous surface diffraction experiments to investigate the electronic properties and perpendicular magnetic anisotropy (PMA) in [Co(x)/Ni(y)] single-crystalline stacks grown by molecular beam epitaxy., 11 pages, 11 figures, 1 Table

Single-crystal Heusler atomic-scale superlattices that have been predicted to exhibit perpendicular magnetic anisotropy and half-metallicity have been successfully grown by molecular beam epitaxy. Superlattices consisting of full-Heusler ${\mathrm{Co}}_{2}\mathrm{MnAl}$ and ${\mathrm{Fe}}_{2}\mathrm{MnAl}$ with one- to three-unit-cell periodicity were grown on GaAs (001), MgO (001), and Cr (001)/MgO (001). Electron-energy-loss spectroscopy maps confirmed clearly segregated epitaxial Heusler layers with high cobalt or high iron concentrations for samples grown near room temperature on GaAs (001). Superlattice structures grown with an excess of aluminum had significantly lower thin-film shape anisotropy and resulted in an out-of-plane spin reorientation transition at temperatures below 200 K for samples grown on GaAs (001). Synchrotron-based spin-resolved photoemission spectroscopy found that the superlattice structure improves the Fermi-level spin polarization near the $X$ point in the bulk Brillouin zone. Stoichiometric ${\mathrm{Co}}_{2}\mathrm{MnAl}$ terminated superlattice grown on MgO (001) had a spin polarization of 95%, while a pure ${\mathrm{Co}}_{2}\mathrm{MnAl}$ film had a spin polarization of only 65%.

International audience; Exchange magnetic coupling between Fe electrodes through a thin MgO interlayer in epitaxial junctions has been investigated as a function of temperature, MgO thickness, and interface quality. Depending on the MgO thickness, which has been varied from 1.5 to 4 monolayers, two opposite temperature dependences are clearly disentangled. For a thin MgO spacer, the main component decreases with temperature following a metalliclike behavior. On the contrary, for the thickest MgO layers, the main component increases with temperature, following an Arrhenius law. Moreover, the insertion of a monoatomic roughness at the bottom MgO interface, induced by the addition of a fraction of a Fe monolayer, exacerbates the metallic features as an oscillatory behavior from antiferromagnetic to ferromagnetic is observed. These results allow questioning the simple tunneling mechanism usually invoked for MgO coupling, and suggest a crossover behavior of the thin MgO spacer from metallic to insulating with a progressive opening of the gap.

International audience; The combined effect of magnetic field and current on domain wall motion is investigated in epitaxial [Co/Ni] microwires. Both thermally activated and flow regimes are found to be strongly affected by current. All experimental data can be understood by taking into account both adiabatic and nonadiabatic components of the spin transfer torque, the parameters of which are extracted. In the precessional flow regime, it is shown that the domain wall can move in the electron flow direction against a strong applied field, as previously observed. In addition, for a large range of applied magnetic field and injected current, a stochastic domain wall displacement after each pulse is observed. Two-dimensional micromagnetic simulations, including some disorder, show a random fluctuation of the domain wall position that qualitatively matches the experimental results.

International audience; Te capping layer on topological insulator (TI) surface provides an efficient way for the protection of surface from oxidation. However, when using Te to protect the surface of (Bi1−xSbx)2Te3 ternary topological insulators, it is still unclear for the influence of Te on the surface chemical composition and the surface state bands after the procedure of capping and desorption of Te. Here, we have performed a systematic study of the surface morphology, crystallinity, chemical composition as well as the band structure of (Bi1−xSbx)2Te3 after the desorption of the Te capping layer. Our results confirm a good recovery of the surface state bands of the (Bi1−xSbx)2Te3 ternary topological insulators with different Bi and Sb compositions. The chemical composition of (Bi1−xSbx)2Te3 remains almost unchanged after the desorption of Te. This study proves that the Te capping layer works as a suitable protection for ternary (Bi1−xSbx)2Te3 TI layers, allowing for ex-situ transfer of TI samples in air. This opens the way for the development of metal/TI hybrid structures for advanced spintronic applications.

Half metal magnets are of great interest in the field of spintronics because of their potential full spin polarization at the Fermi level (E F) and low magnetization damping. The high Curie temperature and the predicted 0. 7 eV minority spin gap make the Co 2 MnSi Heusler compound very promising for applications. We investigated the half-metallic magnetic character of this compound using spin-resolved photoemission , ab initio calculation , and ferromagnetic resonance. At the surface of Co 2 MnSi , a gap in the minority spin channel is observed , leading to 100% spin polarization. However , this gap is 0. 3 eV below E F , and a minority spin state is observed at E F. We show that a minority spin gap at E F can nevertheless be recovered either by changing the chemical composition of the compound or by covering the surface by Mn , MnSi , or MgO. This spin-gap recovery results in extremely small damping coefficients , reaching values as low as 7 × 10 −4 .

To examine the influence of the barrier quality in fully epitaxial Fe/MgO/Fe(001) magnetic tunnel junctions (MTJs), we propose to use Fe-V alloys as magnetic electrodes. This leads to a reduced misfit with MgO. We actually observe, by high-resolution electron microscopy (HREM) and local strain measurements, that the misfit dislocations density in the MgO barrier is lower when it is grown on Fe-V(001). This improvement of the crystalline quality of the MgO barrier actually leads to a significant increase of the tunnel magneto-resistance (TMR), despite the loss of spin polarization (SP) in these alloys, which was measured by spin-polarized X-ray photoelectron spectroscopy (SR-XPS).

International audience; The formation conditions of single magnetic bubbles through in-plane field demagnetizationare investigated in an array of Co/Ni circular dots by magnetic force microscopy andcompared to micromagnetic calculations. We demonstrate high success rates in nucleatingstable bubbles. The efficiency of single bubble formation is found to depend not only on thedot size, material thickness and intrinsic material parameters but also on the bubble nucleationpath. Experimental phase diagrams and micromagnetic calculations highlight the influenceof the starting in-plane field amplitude and dipolar interactions in stabilizing the bubble.The identification of a systematic procedure for controlling nucleation of single bubbles,multidomain states or a uniform state is important from a technological point of view, openinga path toward the realization of reprogrammable magnonic crystals for the control of spinwavepropagation.

International audience; In this paper the effect of oxygen on the vanadium homoepitaxial growth process is analyzed by using Auger spectroscopy, electron diffraction and scanning tunneling microscopy. As the oxygen induced 1 × 5 surface structure got a lattice spacing 6% different from the pure V lattice, relaxation is observed by electron diffraction during the growth. The average in-plane lattice spacing is thus shown to be proportional to the oxygen surface concentration. The surface lattice relaxation is observed to exponentially vary with the number of deposited atomic planes. A kinetic model is proposed and allows us to explain these observations. Furthermore, it helps us to distinguish two regimes depending on growth temperature. At high temperature, the oxygen surface concentration during growth is due to oxygen upward diffusion from the underneath V layer. For lower temperature however, this upward diffusion is not efficient and another source of oxygen contamination is evidenced. When the oxygen surface concentration is sufficient, a spectacular self-organization is observed at the surface by surface microscopy. Ribbons shape islands are observed and are tentatively explained as a consequence of oxygen surface concentration and stress induced by the surface reconstruction. © 2016 Elsevier B.V. All rights reserved.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; Interplay between voltage induced magnetic anisotropy transition and voltage induced atomic diffusion is studied in epitaxial V/Fe (0.7 nm)/ MgO/ Fe(5 nm)/Co/Au magnetic tunnel junction where thin Fe soft electrode has in-plane or out-of-plane anisotropy depending on the sign of the bias voltage. We investigate the origin of the slow resistance variation occurring when switching bias voltage in opposite polarity. We demonstrate that the time to reach resistance stability after voltage switching is reduced when increasing the voltage amplitude or the temperature. A single energy barrier of about 0.2 eV height is deduced from temperature dependence. Finally, we demonstrate that the resistance change is not correlated to a change in soft electrode anisotropy. This conclusion contrasts with observations recently reported on analogous systems.

WOS:000368442100010; International audience; We report on the magnetization at the Cr/MgO interface, which we studied through two complementary techniques: angle-resolved photoemission spectroscopy and polarized neutron reflectivity. We experimentally observe an enhanced interface magnetization at the interface, yet with values much smaller than the ones reported so far by theoretical and experimental studies on Cr(001) surfaces. Our findings cast some doubts on the interpretations on previous works and could be useful in antiferromagnetic spin torque studies.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al., Interfacial effects on spin and symmetry filtering in single-crystal Fe(001)/MgO/Fe magnetic tunnel junctions are investigated with the insertion of a Fe monoatomic step at the bottom MgO interface. After annealing, the atomically flat bottom electrode is covered by a fractional part of a Fe monoatomic layer resulting in two-dimensional Fe islands that are separated for low coverages and percolated above around half a monolayer. The magnetotransport properties of the junctions are studied as a function of this Fe sublayer coverage that is varied from 0 to 1 monolayer. Surprisingly, the magnetoresistance ratio exhibits a maximum for a coverage around half a monolayer. Tunneling spectroscopy experiments performed at low temperature allow relating this result to the decrease of the contribution of the interfacial resonance state to the conductance of the junction., C.T. acknowledges the following projects: SPINCHAT (ANR-07-BLAN-341), POS CCE ID.574, code SMIS-CSNR 12467, and the Exploratory Research Project, “SPINTAIL” PN-II-ID-PCE-2012-4-0315, No. 23/29.08.2013.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; Magnetic and magneto-transport properties of amorphous Al2O3-based magnetic tunnel junctions (MTJ) having two Co/Ni multilayer electrodes exhibiting perpendicular magnetic anisotropy (PMA) are presented. An additional Co/Pt multilayer is required to maintain PMA in the top Co/Ni electrode. Slight stacking variations lead to dramatic magnetic changes due to dipolar interactions between the top and bottom electrodes. Tunnel magneto-resistance (TMR) of up to 8% at 300K is measured for the MTJ with two PMA electrodes. The TMR value increases when the top PMA electrode is replaced by an in-plane magnetized Co layer. These observations can be attributed to significant intermixing in the top Co/Ni electrode.

The control of magnetic domain wall (DW) motion under the action of an electrical current is of great interest for the development of new data storage electronic devices such as magnetic racetrack memories1 or logic devices2. In this context, materials with perpendicular magnetic anisotropy (PMA) are particularly attractive3,4 since they exhibit very narrow domain walls compatible with high density storage as well as spin-orbits effect that can improve the efficiency of current-induced domain wall motion5,6. However, even if the efficiency of current driven DW motion can be enhanced, the threshold current is still limited by the presence of structural defects in the materials. Particularly, the strong interaction of narrow DWs with random nanoscale inhomogeneities can lead to a so-called thermally activated creep motion for Hdep is the depinning field. This creep regime has been observed in various ultra thin films with PMA such as for instance Co/Pt7,8, CoFe or CoFeB9. Particularly, a ln(v) versus H−1/4 dependence has been found consistent with the propagation of a 1D domain wall in a 2D weak random disorder. As these films are usually deposited by sputtering, the random disorder originates in particular from crystalline texture, interface intermixing or grain boundaries, which induce a distribution of PMA on the nanoscale. In epitaxial systems, the nature, density and distribution of structural inhomogeneities can be very different, which may give rise to a different mechanism of domain wall motion. This has been shown for example in L 1 0 FePt films with PMA10 where extended 3D microtwins induced by a relaxation process generate a dendritic like motion distinct from the creep mechanism.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; We propose a conceptually new way to gather information on the electron bands of buried metal-(semiconductor-) insulator interfaces. The bias dependence of low frequency noise in Fe1-xVx/MgO/Fe(0 < x < 0.25) tunnel junctions shows clear anomalies at specific applied voltages, reflecting electron tunneling to the band edges of the magnetic electrodes. The change in magnitude of these noise anomalies with the magnetic state allows evaluating the degree of spin mixing between the spin polarized bands at the ferromagnet-insulator interface. Our results are in qualitative agreement with numerical calculations.

Double barrier heterostructures are model systems for the study of electron tunneling and discrete energy levels in a quantum well (QW). Until now resonant tunneling phenomena in metallicQW have been observed for limited thicknesses (1-2 nm) under which electron phase coherence is conserved. In the present study we show evidence of QW resonance states in Fe QW up to12 nmthick and at room temperature in fully epitaxial doubleMgAlOxbarrier magnetic tunnel junctions. The electron phase coherence displayed in this QWis of unprecedented quality because ofa homogenous interface phase shift due to the small lattice mismatch at the Fe/MgAlOx interface. The physical understanding of the critical role of interface strain on QW phase coherence will greatly promote the development of the spin-dependent quantum resonant tunneling applications., 16 pages, 3 figures 1 table

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; The value and sign of V/Fe interface anisotropy are investigated. Epitaxial V/Fe/V/Au layers with different iron thicknesses were grown on single crystalline (001) MgO substrate by ultra high vacuum molecular beam epitaxy. Magnetometry was used to measure magnetization and out anisotropy field. From these values, we quantify the number of dead layers due to V/Fe or Fe/V interfaces, and compare it with the literature. We deduce that dead layers occur mostly at the bottom V/Fe interface. An average value for V/Fe and Fe/V interface anisotropy around 0 +/- 0.1 erg/cm(2) (mJ/m(2)) was thus deduced.

${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}$ epitaxial layers were grown by molecular beam epitaxy and controlled using in situ characterization tools (electron diffraction, x-ray photoelectron, and Auger spectroscopies) in order to use these alloys in magnetic tunnel junctions. Thick films are shown to be BCC up to $x=70$%, whereas thin films grown on Fe(001) are BCC over the whole Co concentration range up to a critical thickness. Fe/Co intermixing is also examined in detail. The tunnel magnetoresistance and conductance of MgO-based magnetic tunnel junctions built with BCC ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}$ electrodes show peculiar voltage dependencies. Their interpretation relies on spin- and symmetry-resolved photoemission measurements and ab initio calculations. The bulk and interfacial occupied states detected by photoemission are compared to the calculated ones. Experimental results and ab initio calculations are in good agreement. Finally, the experimental conductance curves are compared to the autoconvolution of the $\ensuremath{\Delta}1$ density of states in which both bulk and interface states are taken into account.

International audience; The study, tailoring, and control of the coupling between magnetic layers triggered the development of spintronics and has been up to now focused on systems comprising at least one ferromagnetic layer. Here we present a system where two antiferromagnetic layers separated by an ultrathin tunnel barrier interact with each other. We have studied with neutron and x-ray diffraction two sets of Cr/MgO/Cr trilayers, with either thin (five monolayers or less) or thick (eight monolayers or more) MgO layers, along with isolated Cr layers used as references. While the behavior of the trilayers with thick MgO layers can be explained by the strain state of each layer, that of trilayers with thin barriers cannot be related to the well known behavior of isolated chromium layers. We indeed report in these samples the presence of anomalous magnetic phases (modified propagation vector, direction of spins and propagation vectors inconsistent with the strain state of the sample) that we ascribe to the existence of a tunnel magnetic coupling between the AF layers through the insulating barrier. This unexpected finding opens opportunities for spintronics using solely antiferromagnets.

The critical temperature of thin Fe layers on Ir(100) is measured through M\"o{\ss}bauer spectroscopy as a function of the layer thickness. From a phenomenological finite-size scaling analysis, we find an effective shift exponent lambda = 3.15 +/- 0.15, which is twice as large as the value expected from the conventional finite-size scaling prediction lambda=1/nu, where nu is the correlation length critical exponent. Taking corrections to finite-size scaling into account, we derive the effective shift exponent lambda=(1+2\Delta_1)/nu, where Delta_1 describes the leading corrections to scaling. For the 3D Heisenberg universality class, this leads to lambda = 3.0 +/- 0.1, in agreement with the experimental data. Earlier data by Ambrose and Chien on the effective shift exponent in CoO films are also explained., Comment: Latex, 4 pages, with 2 figures, to appear in Phys. Rev. Lett.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; The ability of the full Heusler alloy Co1.5Fe1.5Ge(001) (CFG) to be a Half-Metallic Magnetic (HMM) material is investigated. Epitaxial CFG(001) layers were grown by molecular beam epitaxy. The results obtained using electron diffraction, X-ray diffraction, and X-ray magnetic circular dichroism are consistent with the full Heusler structure. The pseudo-gap in the minority spin density of state typical in HMM is examined using spin-resolved photoemission. Interestingly, the spin polarization found to be negative at E-F in equimolar CoFe(001) is observed to shift to positive values when inserting Ge in CoFe. However, no pseudo-gap is found at the Fermi level, even if moderate magnetization and low Gilbert damping are observed as expected in HMM materials. Magneto-transport properties in MgO-based magnetic tunnel junctions using CFG electrodes are investigated via spin and symmetry resolved photoemission.

Superlattice studies are important to synthesise 3d metals in new crystalline structure which may exhibit exotic magnetic properties. The challenge is to relate these properties to the details of the chemical gradient and the local strain. We report here on the use of Diffraction Anomalous Fine Structure (DAFS) spectroscopy to separate the XAFS-like information about Fe atoms located at different crystallographic sites of an Fe/Ir(100) superlattice. We obtained the Fe nearest neighbor distances at the Ir-Fe interfaces by the use of a new crystallographic-based analysis of the DAFS data.

The anharmonicity of the potential well confining the position of the magnetic vortex core is measured dynamically with a Magnetic Resonance Force Microscope (MRFM). The stray field of the MRFM tip is used to displace the core position away from the well minimum. Anharmonicity is then inferred from the relative frequency shift induced on the eigen-frequency of the vortex core translational mode. Traces of these shifts are recorded while scanning the tip above an isolated nanodot, patterned out of a single crystal FeV film. An analytical framework is proposed to analyze the data. It results in a quantitative measurement of the anharmonic coefficient found to be positive and 50% of the parabolic contribution. This calibrates the tunability of the gyrotropic mode by external magnetic fields. In our sample, we observe a variation of the eigen-frequency as high as +10% for a displacement of the vortex core to about one third of the nanodot radius., 5 pages, 4 figures

Equipe 101 : Nanomagnétisme et électronique de spin; International audience; The transport properties of magnetic tunnel junctions (MTJs) are very sensitive to interface modifications. In this work we investigate both experimentally and theoretically the effect of asymmetric barrier modifications on the bias dependence of tunneling magnetoresistance (TMR) in single crystal Fe/MgO-based MTJs with (i) one crystalline and one rough interface, and (ii) with a monolayer of O deposited at the crystalline interface. In both cases we observe an asymmetric bias dependence of TMR and a reversal of its sign at large bias. We propose a general model to explain the bias dependence in these and similar systems reported earlier. The model predicts the existence of two distinct TMR regimes: (i) a tunneling regime when the interface is modified with layers of a different insulator, and (ii) a resonant regime when thin metallic layers are inserted at the interface. We demonstrate that in the tunneling regime, negative TMR is due to the high voltage which overcomes the exchange splitting in the electrodes, while the asymmetric bias dependence of TMR is due to the interface transmission probabilities. In the resonant regime, inversion of TMR could happen at zero voltage depending on the alignment of the resonance levels with the Fermi surfaces of the electrodes. Moreover, the model predicts a regime in which TMR has different signs at positive and negative bias, suggesting possibilities of combining memory with logic functions.

International audience; Owing to their bcc structures and the low lattice misfit with MgO, FeV alloys are interesting for MgO-based magnetic tunnel junctions. We use vector network analyzer ferromagnetic resonance to measure the magnetization, anisotropy, exchange stiffness, and damping of epitaxial FeV/MgO thin alloys of various V contents. The low magnetization, very high exchange stiffness (23 pJ/m) and very low effective damping (

International audience; We perform an extensive experimental spectroscopic study of the collective spin-wave dynamics occurring in a pair of magnetic nano-disks coupled by the magneto-dipolar interaction. For this, we take advantage of the stray field gradient produced by the magnetic tip of a ferromagnetic resonance force microscope (f-MRFM) to continuously tune and detune the relative resonance frequencies between two adjacent nano-objects. This reveals the anti-crossing and hybridization of the spin-wave modes in the pair of disks. At the exact tuning, the measured frequency splitting between the binding and anti-binding modes precisely corresponds to the strength of the dynamical dipolar coupling $\Omega$. This accurate f-MRFM determination of $\Omega$ is measured as a function of the separation between the nano-disks. It agrees quantitatively with calculations of the expected dynamical magneto-dipolar interaction in our sample.

International audience; We explore the origins of perpendicular magnetic anisotropy in epitaxial and textured Co/Ni(111) superlattices using a combination of thin-film growth, structural characterization, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. Transmission electron microscopy and x-ray diffraction experiments allow us to show that the " bulk " magnetoelastic contribution to the total magnetic energy is small compared to the interface anisotropy. The magnetic properties are studied by using XMCD at the Co and Ni L 2,3 edges. Hysteresis loops performed at the Co L 3 edge confirm the perpendicular magnetization for Co thicknesses up to four monolayers. The spin and orbital moments were deduced using the XMCD sum rules. The results are explained by considering two kinds of magnetic moments for Co, distinguishing the interfaces from the rest of the layers. Both effective spin and orbital moments of Co atoms are found to be enhanced at the Co-Ni interfaces, whereas the magnetic moment of Co surrounded by Co is similar to the bulk value. Ab initio calculations allow us to show a strong enhancement of the dipole operator contribution on Co atoms at the interface that is partly responsible for this high effective spin moment at the interface. Such a moment enhancement is not observed for Ni, with the dipole operator contribution being close to zero. Finally, we observed a very surprising proportionality between the effective spin and orbital moments, independent of the absorption edge or deposition technique used. We assign this peculiar behavior to the fact that the magnetic dipole operator involved in the sum rules is closely linked to the increase of the Co orbital moment at the interface. Based on XMCD results obtained on both molecular beam epitaxy and sputter-deposited samples, this link allows us to show the extreme sensitivity of the perpendicular anisotropy with the chemical ordering at the interface.

In combining spin-and symmetry-resolved photoemission, magnetotransport measurements and ab initio calculations we detangled the electronic states involved in the electronic transport in Fe 1Ax Co x ð001Þ=MgO=Fe 1Ax Co x ð001Þ magnetic tunnel junctions. Contrary to previous theoretical predictions , we observe a large reduction in TMR (from 530 to 200% at 20 K) for Co content above 25 atomic % as well as anomalies in the conductance curves. We demonstrate that these unexpected behaviors originate from a minority spin state with A 1 symmetry that exists below the Fermi level for high Co concentration. Using angle-resolved photoemission, this state is shown to be a two-dimensional state that occurs at both Fe 1Ax Co x ð001Þ free surface, and more importantly at the interface with MgO. The combination of this interface state with the peculiar density of empty states due to chemical disorder allows us to describe in details the complex conduction behavior in this system. Since the discovery of giant magnetoresistance (GMR) in spin valves in 1988 [1], a new branch of physics referred to as spintronics has considerably developed. The discovery of the large tunnel magnetoresistance (TMR) in 1995 [2], the prediction of the spin-transfer mechanism in 1996 [3,4], and the demonstration of spin-dependent coherent tunnel-ing in MgO-based epitaxial MTJs in 2001-2004 [5–10], have largely contributed to developments in this field. Currently, a number of new areas are being explored, such as rf oscillators, devices and memories based on the spin-transfer-torque effect, electric field assisted switching , magnonics, or spincaloritronics [11]. In addition, industrial-scale devices such as magnetic recording heads already use the exceptional electrical properties of GMR and TMR. The technology transfer from research to industry continues today, with MRAM demonstrators based on MgO-based MTJs [12] and rf oscillators using spintronics devices. While commercialization as well as broad utilization into various areas of research has been rapid for spin-tronic devices, in many cases a full understanding of the underlying physics is lacking. MgO-based MTJs with FeCo or FeCoB electrodes are a striking example of this situation. FeCoðBÞ=MgO=FeCoðBÞð001Þ multilayers, fabricated by molecular beam epitaxy (MBE) or sputtering deposition are widely utilized for their high spin current injection efficiency and exceptional electrical sensitivity to any change in the magnetic configuration of the electrodes. Because of the huge TMR predicted by ab initio calculation for the equimolar and B2 ordered Fe 0:5 Co 0:5 alloy and for pure bcc Co (1000%–6000% at 0 K [13]), bcc FeCo(001) electrodes are now extensively used in MTJ fabrication. However, the situation is not so clear regarding the reported results. First, large TMR were actually obtained on MBE grown Fe=bcc Co=MgO=Co=Feð001Þ [14]. However, a heating of the whole stacking up to 250 C during 30 minutes suggest a possible alloying between Fe and Co. On the other hand, contrary to expectations, epitaxial Fe 0:5 Co 0:5 =MgO=Feð001Þ and Fe=MgO=Feð001Þ MTJs exhibit the same TMR [15]. It should be noted that the B2 order assumed in Ref. [13] is not observed. Finally, reported TMR of sputtered FeCo=MgO=FeCoð001Þ MTJs present a nonmonotonic dependence as a function of the Co concentration with a maximum around 25% of Co [16]. The detailed effect of Co alloying into Fe on the spin-dependent tunneling remains therefore obscure. In this Letter, we explain quantitatively the unexpected transport properties observed in FeCo=MgO=FeCoð001Þ MTJs. We demonstrate that transport measurements alone are not sufficient to complete the current understanding of this system, and that spin-, symmetry-, and angle-resolved photoemission, together with DFT calculations taking into account the chemical disorder, offer a unique path to probing directly the tunneling electrons. We use a specific photoemission experiment to untangle the different Bloch waves responsible for the conduction along (001) as a function of their symmetry (A 1 or A 5) and spin state (majority " or minority #), in contrast to standard transport measurements where all these contributions are mixed. bcc MgO-based MTJs with Fe 1Ax Co x ð001Þ electrodes were grown by coevaporation using MBE. The epitaxial relationship, growth mode, and surface flatness were controlled using reflection high energy electron diffraction (RHEED). In addition, the evaporation rates of the Co and Fe sources, and consequently the alloys stoichiometry, were accurately controlled by recording the intensity

Thermoelectric effects in magnetic nanostructures and the so-called spin caloritronics are attracting much interest. Indeed it provides a new way to control and manipulate spin currents, which are key elements of spin-based electronics. Here we report on a giant magnetothermoelectric effect in a magnetic tunnel junction. The thermovoltage in this geometry can reach 1 mV. Moreover a magnetothermovoltage effect could be measured with ratio similar to the tunnel magnetoresistance ratio. The Seebeck coefficient can then be tuned by changing the relative magnetization orientation of the two magnetic layers in the tunnel junction. Therefore, our experiments extend the range of spintronic devices application to thermoelectricity and provide a crucial piece of information for understanding the physics of thermal spin transport.

The electronic structure and the magnetic moment of metallic iron are strongly modified near the Fe(100)/Co( bcc ) interface. This effect has been evaluated from the iron L 2,3 energy loss near edge structure recorded for Fe/Co superlattices grown by molecular beam epitaxy with different periods. The interface-induced modification of the intensity ratio I ( L 3 )/ I ( L 2 ) which has been measured by electron energy loss spectroscopy is in good agreement with the enhancement of the magnetic moment calculated from first principles. This shows that the intensity ratio can be used to obtain information on magnetic moments at a nanometer scale in a transmission electron microscope.

Picosecond laser ultrasonic techniques for acoustic wave generation and detection have been employed to probe shear acoustic waves in liquid glycerol at gigahertz frequencies. The experimental approach uses a unique laser pulse shaping technique and a crystallographically canted metal layer to generate frequency-tunable transverse acoustic waves, and uses time-domain coherent Brillouin scattering to detect the waves after they propagate through a liquid layer and into a solid substrate. A linear frequency dependence is found for both the shear speed and attenuation rate in glycerol.

We will review recent progress on the generation and detection of picosecond shear acoustic waves. Examples will be shown in which the transverse isotropic symmetry of the sample structure is broken in order to permit shear wave generation through sudden laser heating. As an illustration of the technique, picosecond longitudinal and shear acoustic waves have been successfully employed to probe structural dynamics in nano-sized solids (gold nano-crystals assemblies) and nano-sized liquids (glycerol and water).

The purpose of this paper is to report some new experimental and theoretical results about the analysis of in-plane lattice spacing oscillations during two-dimensional (2D) homo and hetero epitaxial growth. The physical origin of these oscillations comes from the finite size of the strained islands. The 2D islands may thus relax by their edges, leading to in-plane lattice spacing oscillations during the birth and spread of these islands. On the one hand, we formulate the problem of elastic relaxation of a coherent 2D epitaxial deposits by using the concept of point forces and demonstrate that the mean deformation in the islands exhibits an oscillatory behaviour. On the other hand, we calculate the intensity diffracted by such coherently deposited 2D islands by using a mean model of a pile-up of weakly deformed layers. The amplitude of in-plane lattice spacing oscillations is found to depend linearly on the misfit and roughly linearly on the nucleation density. We show that the nucleation density may be approximated from the full-width at half maximum of the diffracted rods at half coverages. The predicted dependence of the in-plane lattice spacing oscillations amplitude with the nucleation density is thus experimentally verified on V/Fe(001), Mn/Fe(001), Ni/Fe(001), Co/Cu(001) and V/V(001)., 39 pages, 10 figures

International audience; This work is dedicated to the study of magnetization reversal processes in a ferromagnetically exchange coupled ferrimagnetic/ferrimagnetic bilayer system made of a soft magnetic GdFe layer and a hard TbFe layer. Regular magnetization measurements and x-ray magnetic circular dichroism XMCD measurements performed at the ESRF on a GdFe 100 nm/TbFe 3 nm bilayer at different temperatures are presented and compared. The XMCD measurements allowed us to study separately the magnetic contribution of Gd, Tb, and Fe at the system interface. Evidence of the creation of an interface domain wall and its compression and pinning are given. In addition to quasistatic measurements, the interface magnetization reversal was also studied using XMCD aftereffect measurements. Both magnetization and XMCD measurements are explained by considering the formation of lateral domains during the interface magnetization reversal.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; The strong impact of molecular beam epitaxy growth and Synchrotron Radiation characterization tools in the understanding of fundamental issues in nanomagnetism and spintronics is illustrated through the example of fully epitaxial MgO-based Magnetic Tunnel Junctions (MTJs). If ab initio calculations predict very high tunnel magnetoresistance (TMR) in such devices, some discrepancy between theory and experiments still exists. The influence of imperfections in real systems has thus to be considered like surface contaminations, structural defects, unexpected electronic states, etc. The influence of possible oxygen contamination at the Fe/MgO(001) interface is thus studied, and is shown to be not so detrimental to TMR as predicted by ab initio calculations. On the contrary, the decrease of dislocations density in the MgO barrier of MTJs using Fe1-xVx electrodes is shown to significantly increase TMR. Finally, unexpected transport properties in Fe1-XCox/MgO/Fe1-XCox (001) are presented. With the help of spin and symmetry resolved photoemission and ab initio calculation, the TMR decrease for Co content higher than 25% is shown to come from the existence of an interface state and the shift of the empty Delta 1 minority spin state towards the Fermi level.

Clinical studies have shown that low doses of aspirin (300 mg/day) inhibit thromboxane A2 production and platelet aggregation but preserve prostacyclin synthesis. In contrast, high doses of aspirin (1,000 mg/day) suppress the synthesis of both eicosanoids. Because the consequences of aspirin administration have never been investigated on coronary vasomotor tone in vivo, we investigated the effects of low and high doses of aspirin on systemic and coronary hemodynamics under basal conditions and after myocardial reactive hyperemia in conscious dogs. Dogs were instrumented with a Doppler flow probe and a hydraulic occluder. Coronary blood flow was measured in the conscious state at baseline and during myocardial reactive hyperemia after 10, 20, and 30 s of coronary occlusion. Thromboxane B2 serum concentrations, an index of platelet aggregation, decreased by90% after long-term i.v. administration of aspirin, 100 mg/day for 7 days (low dose). Neither systemic and coronary hemodynamics nor reactive hyperemia were affected by the drug. After combined administration of this low dose of aspirin and of the nitric oxide synthase (NOS) inhibitor, N(omega)-nitro-L-arginine (L-NNA, 30 mg/kg/day/7 days), reactive hyperemia decreased to the same extent as when L-NNA was administered alone. After administration of a unique high-dose aspirin (1,000 mg, i.v.), myocardial reactive hyperemia was markedly reduced, and this effect was still observed after previous blockade of NOS and cyclooxygenase by L-NNA and diclofenac, respectively. Thus long-term treatment with a low antiaggregant dose of aspirin does not alter the ability of coronary vessels to dilate during myocardial reactive hyperemia in conscious dogs. In contrast, short-term administration of a high antiinflammatory dose of aspirin severely blunts myocardial reactive hyperemia through a mechanism that is independent of both cyclooxygenase and nitric oxide metabolic pathways.

Equipe 101 : Nanomagnétisme et électronique de spin; International audience; Single-crystalline V/Fe(0.7 nm)/MgO(1.2nm)/Fe(20 nm) magnetic tunnel junctions are studied to quantify the influence of an electric field on the Fe/MgO interface magnetic anisotropy. The thinnest Fe soft layer has a perpendicular magnetic anisotropy (PMA), whereas the thickest Fe layer acts as sensor for magnetic anisotropy changes. When electrons are added at the PMA Fe/MgO interface (negative voltage), no anisotropy changes are observed. For positive voltage, the anisotropy constant decreases with increasing bias voltage. A huge 1150 fJV(-1) m(-1) anisotropy variation with field is observed and the magnetization is found to turn from out-of-plane to in-plane of the sample with the applied voltage.

Nous présentons dans ce papier sans prétendre être exhaustifs tant le thème est vaste, quelques techniques d'obtention des couches minces les plus utilisées dans les laboratoires de recherche, les grands types et processus de croissance notamment pour les couches minces obtenues par évaporation sous vide, l'épitaxie et quelques mécanismes de relaxation des contraintes.

International audience; Les propriétés magnétiques de films épitaxiés de Mn de 1 à 10 monocouches sur des couches tampons de Fe bcc (001) sont étudiées par dichroïsme magnétique circulaire en X mou. Nous montrons tout d'abord que la croissance de Mn sur Fe(001) est bi-dimensionnelle et pseudomorphe au plan (001) du fer. Un ordre ferromagnétique est observé jusqu'à deux plans atomiques de Mn et le couplage interfacial entre les moments de Mn et Fe est ferromagnétique. Ce résultat est en désaccord avec les conclusions d'une expérience similaire récente. Nous montrons que cette différence provient de la préparation des échantillons.

Les propriétés magnétiques du Fer dans les superréseaux Fe / Ir (100) sont étudiées par spectrométrie Mössbauer avec 57Fe. Les échantillons, préparés par épitaxie par jet moléculaire, ont un nombre de monocouches de fer ( 57Fe et/ou 56Fe) variant de 3 à 8 et ce pour des épaisseurs en iridium comprises entre 2 et 30Å. Pour la série à 4 plans où le fer est de structure tétragonale centrée (TC), on observe l'apparition d'un moment magnétique significatif dans les couches centrales de fer au dessus d'un seuil de volume critique, alors que le fer en contact avec l'iridium reste faiblement magnétique. Pour les épaisseurs de fer supérieures à 4 plans, la transition de phase TC - CC contrainte produit de nouveaux sites magnétiques. Une disymétrie marquée entre l'interface Ir/Fe inférieure et l'interface Fe/Ir supérieure, le long de la direction de croissance, est mise en évidence. Enfin, les effets de relaxation magnétique et le régime d'onde de spin, observés respectivement pour les phases TC et CC contrainte, suggèrent une faible corrélation magnétique de couche à couche.