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International audience; The family of Co-based Heusler compounds contains promising candidates for spintronic applications regarding their predicted Half-Metal-Magnetic nature, ultra-low magnetic damping coefficients, high curie temperatures and tunable electronic properties. Here we focused on the transport properties of Co2MnSi thin films with thickness in the range of 4-44 nm exhibiting magnetic damping in the 10-4 range. The goals of this study are to examine the impact of the peculiar electronic band structure on the transport properties, to identify the temperature-dependent scattering process, and to extract robust conduction parameters to exploit this material in magnetoelectric devices. In order to undoubtedly correlate all results, the full study has been performed on the same series of samples. Scanning transmission electron microscopy experiments were performed to check the chemically-ordered L21 phase in our films, and also allowed us to identify misfit dislocations generated at the interface with the substrate. The variation of the resistivity with film thickness was measured at different temperatures. The results are examined under the Fuchs and Sondheimer model which allowed us to extract the electron mean free path in Co2MnSi in the temperature range 5-300 K. Values for the residual resistivity, Debye temperature, and distance between the Fermi energy and the conduction band for minority spins were obtained from the fit of the resistivity versus temperatures curves. A negative AMR ratio was measured for all the samples which confirmed the Half-metallic nature of the Co2MnSi films. The determination of the ordinary Hall coefficient alloys allowed us to extract the carrier concentration and carrier mobility and their dependency on the temperature. Finally, scaling of the anomalous Hall coefficient with the longitudinal resistivity was performed indicating that skew scattering is the dominant temperature-dependent scattering mechanism in our films.

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.

International audience

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.

Objectives We aimed to compare the use of nine different cardiac troponin (cTn) assays (2 cTnT and 7 cTnI) for the diagnosis of NSTEMI in a single multi-centre population. Design and methods One hundred and fifty-eight patients were included (mean age 60 years, SD 17 years), including 23 patients (14%) with NSTEMI. Results The analytical comparison highlighted a large heterogeneity of cTn assays, as reflected by percentages of patients with detectable cTn, correlation coefficients, Passing-Bablok comparisons and concordance coefficients. Correlations within cTnI assays were good and correlation within cTnT assays was excellent. Diagnostic performances demonstrated that each cTn assay has specific threshold values. Furthermore, some assays (HS-cTnI and T, cTnI-Pathfast and cTnI-Centaur) indicated high sensitivity and negative predictive value using the limit of detection (LoD) diagnostic strategy. For the latter assays, a significant increase in specificity was found when using the 99th percentile or the H0-H3 strategies, in comparison to the LoD strategy. When applying the European Society of Cardiology H0-H3 algorithm, comparable diagnostic performances were obtained. Conclusion All 9 cTn assays indicated overall good diagnostic performances for the diagnosis of NSTEMI in emergency departments when the recommended algorithm based on the variation of cTn value between two measurements at admission and 3 h later was used., Highlights • We compare nine troponin assays for the diagnosis of NSTEMI. • Each cTn assay owns specific thresholds for the ESC H0-H3 algorithm. • All assays present high specificity for NSTEMI diagnosis using H0-H3 algorithm. • Assay-specific threshold and delta change values for each cTn assays are mandatory.

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 Co

International audience; Spin-polarization and magnetic damping are measured for several polycrystalline films with each of them being made of a different single Co$_2$ Mn-based Heusler compound. As several epitaxial Co$_2$ Mn-based Heusler compounds are shown to be half-metal magnetic materials with full spin-polarization and ultralow magnetic damping, we explore here these properties but in polycrystalline films. Co$_2$ MnSi, Co$_2$ MnGe, and Co$_2$ MnGa thin films were grown on glass substrates and analyzed in situ by electron diffraction and spin-resolved photoemission and ex situ by transmission electron microscopy and ferromagnetic resonance. Despite the polycrystalline state of the films, they still exhibit high spin polarizations and very low magnetic damping coefficients. The latter are at least of the same order as the best epitaxial films using regular ferromagnetic materials. The key point to achieve such properties is to control the Heusler stoichiometry as best as possible.

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.

The epitaxial growth of MgO on Fe 1 − x V x buffer layers with adjustable lattice parameter is studied by electron diffraction (RHEED) in real time. At the onset of plastic relaxation in the MgO layer, a clear splitting of the diffraction rods is observed in 〈110〉 directions, as well as an increase in their length in the 〈100〉 directions. Splitting along 〈100〉 is also made visible through image background subtraction. These features originate from the surface strain above misfit dislocations, as previously proposed to account for satellite spots in LEED measurements. This explanation is supported by simulations of the diffraction patterns using kinematic diffraction theory. Observation of the diffraction rods splitting is shown to be a powerful way to check the presence of dislocations in MgO tunnel barriers and to accurately determine the critical thickness of plastic relaxation.

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).

Recently, the study of interactions between electron spins and heat currents has given rise to the field of “Spin Caloritronics”. Experimental studies of these interactions have shown a possibility to combine the use of heat and light to power magnetic tunnel junction (MTJ) devices. Here we present a careful study of an MTJ device on Si substrate that can be powered entirely by light. We analyze the influence of the material properties, device geometry, and laser characteristics on the electric response of the sample. We demonstrate that by engineering the MTJ and its electrical contact, a large photovoltage reaching 100 mV can be generated. This voltage originates from the Si substrate and depends on the MTJ magnetic configuration. Finally, we discuss the origin of the photo-voltage in terms of Seebeck and photovoltaic effects.

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.

Recently light induced effects in magnetic nanostructures have attracted much interest. [1-5] Indeed it provides a new way to control and manipulate magnetization and spin currents, which are key elements of spin-based electronics [4]. For example, all optical helicity dependent magnetization switching have been demonstrated and could be applied for magnetic recording [5].

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.

Single-crystal magnetic tunnel junctions employing bcc (100) Fe electrodes and MgO(100) insulating barrier are elaborated by molecular beam epitaxy. The magneto-transport properties are investigated in two extreme regimes. First, for extremely small MgO thickness, we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunnelling of the minority spin interfacial resonance state. Second, for large MgO barrier thickness, the tunnel transport validates specific spin filtering effects in terms of symmetry of the electronic Bloch function and symmetry-dependent wavefunction attenuation in the single-crystal barrier. Within this framework, we present giant tunnel magnetoresistive effects at room temperature (125–160%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the spin filtering. We point out imperfect filtering effects and a strong implication of the minority surface state of Fe on the low voltage variation of tunnel magnetoresistance. The insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance and activates a resonant tunnelling mechanism via the interfacial resonance state.

Electronic and magnetic properties of bcc Co, Fe and Mn(0 0 1) epitaxial monolayers in contact with a single-crystalline MgO(0 0 1) film were studied using X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements. The XPS and XAS analysis clearly evidenced the weak hybridization between the MgO barrier and Fe or Co. On the contrary, a net oxidization of the Mn layer in contact with the MgO layer was observed. The magnetic properties were characterized by probing the XMCD signal of a unique atomic plane of transition metal in contact with MgO. The total magnetic moment per Co and Fe atoms were observed to increase compared to the bulk at the metal/oxide interface. Finally, Mn at the interface with MgO does not present any ferromagnetic behavior. This was assumed to be a consequence of the Mn oxidization.

Single-crystal magnetic tunnel junctions employing bcc (1 0 0) Fe electrodes and MgO(1 0 0) insulating barrier are elaborated by molecular beam epitaxy. Two extreme regimes have been investigated. First, for extremely thin MgO thickness we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunneling of the minority spin interfacial resonance state. Second, for large MgO barrier thickness, the tunnel transport validates specific spin filtering effect in terms of symmetry of the electronic Bloch function and symmetry-dependent wave function attenuation in the single-crystal barrier. Within this framework, we present giant tunnel magnetoresistive effects at room temperature (125–150%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the filtering. We show that the insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance. Moreover, we provide experimental evidence for the electronic interfacial resonance states contribution to the spin polarized tunnel transport.

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

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.

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.

In this paper, the role of oxygen on surface reconstruction and growth of (0 0 1) vanadium is investigated. By using electron diffraction, in-plane lattice spacing oscillations are observed during the layer by layer vanadium growth. The amplitude of these oscillations which are due to elastic relaxation at the edges of the two-dimensional (2D) growing islands, are sometimes very large and thus cannot be explained by size effect in homo-epitaxy. We observe that the oscillation amplitude is clearly correlated with the presence of oxygen on the initial surface, as checked by Auger spectroscopy. As the presence of oxygen induces surface reconstructions, the electron diffraction experiments also allow us to demonstrate that these superstructures are compressed compared to an oxygen free 1×1 V surface. The peculiar morphology of these surface reconstructions, investigated by scanning tunnelling microscopy, may be explained by this compressive strain.

In this paper, the single-crystalline (0 0 1) V surface is studied by electron diffraction, Auger spectroscopy, and scanning tunnelling microscopy (STM). (1×1), (5×5) (and sometimes 6×6) surface atomic arrangement are observed in correlation with the oxygen surface concentration. For less than 0.15 ml of oxygen, no reconstruction is detected. The (5×5) surface structure is observed for an oxygen coverage in the range 0.15–1.2 ml. However, a pseudo-(1×1) surface structure can also be observed near 1 ml of oxygen. The STM analysis demonstrates that the (5×5) reconstruction results in fact from a mixing of (5×1) and (1×5) domains. At the atomic scale, the images show that the electronic density is modified over two parallel atomic lines every five but also six atomic distances. For the pseudo-(1×1) reconstructed surface, these lines are still present, but, depending on the density of vacancies, (6×1) up to (12×1) periodicities can be observed locally. We find that the periodicity linearly increases with the number of vacancies. The extension of this linear dependence up to the lack of vacancies leads to the (5×1) periodicity. These surface reconstructions lead to unusual reflection high-energy electron diffraction (RHEED) patterns, that are perfectly explained by the surface morphology. Finally, RHEED measurement of the in-plane lattice parameter allows us to demonstrate that the (5×5) reconstructed surface is compressed. This behaviour allows us to propose a model for the (0 0 1) V surface reconstructions induced by oxygen.

This study is dedicated to the growth of bcc Mn by molecular beam epitaxy, in order to look at the magnetic properties of bcc Mn near room temperature. For this purpose, Mn is deposited on bcc MxV1-x(001) alloy lattices (M = Fe or Nb) for which the lattice spacing is tunable by varying the concentration x. We first show that the parameter of the MxV1-x alloy's buffer layers can be adjusted from 2.95 A to 3.3 A depending on x and M. Three different structures in Mn films grown on these buffer layers are observed depending on the in-plane spacing of the initial MxV1-x lattice. Thick Mn films are always found to grow epitaxially in the Mnα structure. For moderate thicknesses larger than 4 atomic planes, Mn grows in an unidentified structure. Finally, up to four deposited atomic planes, Mn is found to grow in a tetragonal structure close to a bcc one on Fe(001), FexV1-x(001) and NbxV1-x(001) for \(x \le 25\% \). This tetragonal structure is shown to be a distorsion of a Mn bcc structure with α =2.92 A. Except for ultra-thin Mn films deposited on Fe(001), no macroscopic magnetization is detected in our strained bcc Mn samples. These results are compared to theoretical predictions.

The present paper is devoted to the analysis of in-plane lattice spacing oscillations (IPLSOs) during two-dimensional hetero- and homoepitaxial growth on (001) surfaces of cubic metals using reflection high-energy electron diffraction (RHEED). Our aim was to check systematically the agreement between experimental results and theoretical predictions. We have selected the most significant heteroepitaxial systems B/A with a misfit positive (V/Fe, Mn/Fe), negative (Ni/Fe, Ni/V, Co/Cu) or close to zero (Fe/Cu, Co/Ni). The phase relationships between the different oscillating physical parameters [RHEED intensity, full-width at half maximum (FWHM) of diffraction streaks and in-plane lattice spacing] are examined, as well as the azimuthal dependence of the elastic relaxation process. A general good agreement between experimental data and theory was observed. However, for some systems (Ni/Fe, V/Fe), we prove a strong anisotropy of the phenomenon along the [10] and [11] azimuths of the square surface lattice. We also focus on the case of homoepitaxial growth and search for a convincing demonstration of IPLSOs in this specific case. The influence of substrate temperature and azimuthal dependance are also investigated. Surprisingly, very large relaxation effects are observed and seem to be correlated to the occurrence of surface reconstruction and/or surface contamination. Finally, an anisotropic character was again observed on V and Nb (001) surfaces. The morphology of the two-dimensional islands during growth or/and anisotropic surface reconstructions could explain such anisotropy.

X-ray-diffraction measurements have been performed on ${\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}(001)$ thin films strained in ${\mathrm{I}\mathrm{r}/\mathrm{F}\mathrm{e}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}/\mathrm{Ir}(001)$ sandwiches and in $[{\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}/\mathrm{Ir}{]}_{20}$ superlattices with $x=0.7$ and $x=0.9.$ These concentrations were chosen with regard to the magnetic properties of the alloys. Superconducting quantum interference device measurements showed a ferromagnetic state above $x\ensuremath{\approx}0.75$ and an antiferromagnetic or a nonmagnetic one below. The samples have been investigated by using the atomic contrast available through anomalous diffraction and by recording reciprocal space maps (RSM). The (111) RSM, sensitive to both in-plane and out-of-plane parameters, has been studied as a function of the grazing incidence angle. It has revealed that the $[{\mathrm{Fe}}_{0.9}{\mathrm{Mn}}_{0.1}/\mathrm{Ir}{]}_{20}$ superlattice has a complex structure with two phases stacked along the growth axis whose origin stems from the existence of the two cubic structures (bcc and fcc) of the bulk alloys. In each case, we have shown that the ${\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}$ alloys are strained in a bct structure. This tetragonalization allows us to investigate the magnetic properties throughout a continuous transformation from a bcc phase $(c/a=1)$ to a fcc one $(c/a=\sqrt{2}).$ Together with x-ray resonant magnetic reflectivity measurements, our results show that the tetragonalization plays a dominant role on the magnetic state observed for the investigated samples. For $c/a$ in the range 1.2--1.3, the Fe atoms are found to be in a ferromagnetic low-spin state.

Dans ce travail, l’EXAFS est utilise pour determiner la structure de films de Mn deposes sur des reseaux d’iridium d’orientation (111) et (100) par epitaxie par jets moleculaires. Comme Mn peut adopter differentes configurations cristallographiques avec de tres faibles differences de distances entre premiers et deuxiemes voisins, l’examen, meme quantitatif, du premier pic de la transformee de Fourier des oscillations EXAFS ne suffit pas pour determiner la structure du Mn. Une modelisation des oscillations, prenant en compte les effets de diffusion multiple, devient necessaire. Les structures du Mn sont alors determinees par simple comparaison entre les oscillations EXAFS calculees et experimentales. Dans les films minces de Mn, nous avons trouve du Mn dans une structure trigonale sur le reseau de surface d’lr(111) et tetragonale sur le reseau de surface d’lr(100). En depots epais, Mn relaxe vers sa structure de Mn α. Dans cet article, nous montrons de plus que la superstructure ( √ x √3) R30° observee sur du Mn epitaxie sur lr(111) provient d’un melange de structure cfc et Mn α, et non de la presence de phases de Laves isotypes de Cu2 Mg ou Zn2 Mg comme l’ont suggere certains auteurs. Nous etudions egalement la structure de superreseaux du type Fex Mn1-x /lr(100) en fonction de la composition de l’alliage FeMn. Nous montrons en particulier comment la procedure d’analyse decrite ci-dessus nous permet de suivre pas a pas une evolution de la structure de ces alliages contraints par Ir en fonction de leur composition, et de mettre en evidence une transition structurale pour 70 % de fer dans l’alliage.

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.

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