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2017


  • A. Agbelele, D. Sando, C. Toulouse, C. Paillard, R. - D. Johnson, R. Rüffer, A. - F. Popkov, C. Carretero, P. Rovillain, J. - M. Le Breton, B. Dkhil, M. Cazayous, Y. Gallais, M. - A. Méasson, A. Sacuto, P. Manuel, A. - K. Zvezdin, A. Barthélémy, J. Juraszek, M. Bibes, Strain and Magnetic Field Induced Spin‐Structure Transitions in Multiferroic BiFeO3. Advanced Materials. 29, 1602327 (2017).
    Résumé : The magnetic‐field‐dependent spin ordering of strained BiFeO3 films is determined using nuclear resonant scattering and Raman spectroscopy. The critical field required to destroy the cycloidal modulation...

  • F. - A. Araujo, L. Piraux, Spin-Transfer-Torque Driven Vortex Dynamics in Electrodeposited Nanowire Spin-Valves. SPIN. 07, 1740007 (2017).

  • P. Barate, S. - H. Liang, T. T. Zhang, J. Frougier, B. Xu, P. Schieffer, M. Vidal, H. Jaffrès, B. Lépine, S. Tricot, F. Cadiz, T. Garandel, J. - M. George, T. Amand, X. Devaux, M. Hehn, S. Mangin, B. Tao, X. F. Han, Z. - G. Wang, X. Marie, Y. Lu, P. Renucci, Bias Dependence of the Electrical Spin Injection into GaAs from Co-Fe-B/MgO Injectors with Different MgO Growth Processes. Physical Review Applied. 8, 054027 (2017).
    Résumé : In view of future spintronic and spin-optronic devices, such as spin light-emitting diodes and spin lasers, it is important to understand the dependence of electrical spin injection on bias in bipolar devices based on GaAs. Focusing on the efficiency of Co-Fe-B/MgO spin injectors, the authors investigate the influence of the MgO growth process on this bias dependence, using MgO tunnel barriers fabricated by different techniques. The bias dependence of spin injection efficiency, and the compromise between high intensity and high circular polarization of electroluminescence, depend strongly on the characteristics of the interface, which can be engineered by the growth process.
    Mots-clés : Semiconductors and Spintronics.

  • S. Boyn, J. Grollier, G. Lecerf, B. Xu, N. Locatelli, S. Fusil, S. Girod, C. Carretero, K. Garcia, S. Xavier, J. Tomas, L. Bellaiche, M. Bibes, A. Barthélémy, S. Saïghi, V. Garcia, Learning through ferroelectric domain dynamics in solid-state synapses. Nature Communications. 8, 14736 (2017).
    Résumé : Accurate modelling of memristor dynamics is essential for the development of autonomous learning in artificial neural networks. Through a combined theoretical and experimental study of the polarization switching process in ferroelectric memristors, Boynet al.

  • L. Bégon-Lours, V. Rouco, A. Sander, J. Trastoy, R. Bernard, E. Jacquet, K. Bouzehouane, S. Fusil, V. Garcia, A. Barthélémy, M. Bibes, J. Santamaría, J. - E. Villegas, High-Temperature-Superconducting Weak Link Defined by the Ferroelectric Field Effect. Physical Review Applied. 7, 064015 (2017).
    Résumé : Josephson junctions are the cornerstones of numerous applications, and fabricating them from high-temperature superconductors has received much attention in recent years. The authors exploit the local field-effect doping of a cuprate superconducting film by a ferroelectric overlayer, where domain structure is ``written'' at will using an atomic force microscope. The key advantage of this approach is that it creates devices that are planar and can be reconfigured by ``rewriting'' the ferroelectric.

  • M. Cabero, K. Nagy, F. Gallego, A. Sander, M. Rio, F. A. Cuellar, J. Tornos, D. Hernandez-Martin, N. M. Nemes, F. Mompean, M. Garcia-Hernandez, A. Rivera-Calzada, Z. Sefrioui, N. Reyren, T. Feher, M. Varela, C. Leon, J. Santamaria, Modified magnetic anisotropy at LaCoO3/La0.7Sr0.3MnO3 interfaces. APL Materials. 5, 096104 (2017).
    Résumé : Controlling magnetic anisotropy is an important objective towards engineering novel magnetic device concepts in oxide electronics. In thin film manganites, magnetic anisotropy is weak and it is primarily determined by the substrate, through induced structural distortions resulting from epitaxial mismatch strain. On the other hand, in cobaltites, with a stronger spin orbit interaction, magnetic anisotropy is typically much stronger. In this paper, we show that interfacing La0.7Sr0.3MnO3 (LSMO) with an ultrathin LaCoO3 (LCO) layer drastically modifies the magnetic anisotropy of the manganite, making it independent of the substrate and closer to the magnetic isotropy characterizing its rhombohedral structure. Ferromagnetic resonance measurements evidence a tendency of manganite magnetic moments to point out-of-plane suggesting non collinear magnetic interactions at the interface. These results may be of interest for the design of oxide interfaces with tailored magnetic structures for new oxide devices.

  • S. Cervera, M. Trassinelli, M. Marangolo, C. Carretero, V. Garcia, S. Hidki, E. Jacquet, E. Lamour, A. Lévy, S. Macé, C. Prigent, J. - P. Rozet, S. Steydli, D. Vernhet, Modulating the phase transition temperature of giant magnetocaloric thin films by ion irradiation. Physical Review Materials. 1, 065402 (2017).
    Résumé : Magnetic refrigeration based on the magnetocaloric effect at room temperature is one of the most attractive alternatives to the current gas compression/expansion method routinely employed. Nevertheless, in giant magnetocaloric materials, optimal refrigeration is restricted to the narrow temperature window of the phase transition (${T}_{c}$). In this work, we present the possibility of varying this transition temperature in the same giant magnetocaloric material by ion irradiation. We demonstrate that the transition temperature of iron rhodium thin films can be tuned by the bombardment of ions of ${\mathrm{Ne}}^{5+}$ with varying fluences up to ${10}^{14}$ ions ${\mathrm{cm}}^{\ensuremath{-}2}$, leading to optimal refrigeration over a large 270\char21{}380 K temperature window. The ${T}_{c}$ modification is found to be due to the ion-induced disorder and to the density of new pointlike defects. The variation of the phase transition temperature with the number of incident ions opens new perspectives in the conception of devices using giant magnetocaloric materials.

  • J. - Y. Chauleau, E. Haltz, C. Carretero, S. Fusil, M. Viret, Multi-stimuli manipulation of antiferromagnetic domains assessed by second-harmonic imaging. Nature Materials. 16, 803-807 (2017).
    Résumé : Sub-coercive electric fields and sub-picosecond light pulses are shown to enable low-power manipulation of antiferromagnetic domains in the multiferroic BiFeO3.

  • M. Collet, O. Gladii, M. Evelt, V. Bessonov, L. Soumah, P. Bortolotti, S. O. Demokritov, Y. Henry, V. Cros, M. Bailleul, V. E. Demidov, A. Anane, Spin-wave propagation in ultra-thin YIG based waveguides. Applied Physics Letters. 110, 092408 (2017).
    Résumé : Spin-wave propagation in microfabricated 20 nm thick, 2.5 μm wide Yttrium Iron Garnet (YIG) waveguides is studied using propagating spin-wave spectroscopy (PSWS) and phase resolved micro-focused Brillouin Light Scattering (μ-BLS) spectroscopy. We demonstrate that spin-wave propagation in 50 parallel waveguides is robust against microfabrication induced imperfections and extract spin-wave propagation parameters for the Damon-Eshbach configuration in a wide range of excitation frequencies. As expected from its low damping, YIG allows for the propagation of spin waves over long distances; the attenuation lengths is 25 μm at μ 0 H = 45 mT. Moreover, direct mapping of spin waves by μ-BLS allows us to reconstruct the spin-wave dispersion relation and to confirm the multi-mode propagation in the waveguides, glimpsed by propagating spin-wave spectroscopy.
    Mots-clés : Magnonics.

  • M. Collet, R. Mattana, J. - B. Moussy, K. Ollefs, S. Collin, C. Deranlot, A. Anane, V. Cros, F. Petroff, F. Wilhelm, A. Rogalev, Investigating magnetic proximity effects at ferrite/Pt interfaces. Applied Physics Letters. 111, 202401 (2017).
    Résumé : Spintronic devices based on pure spin currents have drawn a lot of attention during the last few years for low energy device design. One approach to generate pure spin currents is to combine a metallic or insulating ferromagnetic layer with a non-magnetic metallic layer with a large spin-orbit coupling. A recent controversy has arisen in the possible role of magnetic proximity effects at ferromagnetic/non-magnetic interfaces, which can hamper the understanding of pure spin current generation mechanisms. While magnetic proximity effects have been frequently observed at ferromagnetic metal/non-magnetic interfaces, there are only a few studies on ferromagnetic insulator/non-magnetic interfaces. Regarding the use of ferromagnetic insulators, the focus has been mainly on yttrium iron garnet (YIG). However, investigation of induced magnetic moments at YIG/Pt interfaces has engendered contradictory results. Here, we propose to study insulating ferrites for which electronic and magnetic properties can be modulate...

  • M. Collet, L. Soumah, P. Bortolotti, M. Munoz, V. Cros, A. Anane, Spin Seebeck effect in nanometer-thick YIG micro-fabricated strips. AIP Advances. 7, 055924 (2017).
    Résumé : We have investigated the spin Seebeck effect (SSE) generated by current induced-heating in ultra-thin yttrium iron garnet film (20 nm) covered by an 8 nm thick Pt layer. By passing current in the Pt layer, an out-of-plane temperature gradient is established that, in turn, generates an out-of-equilibrium magnons population. The resulting pure spin current is detected using the inverse spin Hall effect (ISHE) measured in the Pt electrode. A lock-in detection scheme is used to separate the SSE signal from other magneto-galvanic effect. Indeed, the SSE signal is obtained as the second harmonic voltage response, while spin Hall magnetoresistance (SMR) is measured as the first harmonic response to the ac excitation current. Interestingly, the amplitude of the SSE in such thin YIG film is comparable to what has been reported for much thicker films.
    Mots-clés : Magnonics.

  • O. Copie, J. Varignon, H. Rotella, G. Steciuk, P. Boullay, A. Pautrat, A. David, B. Mercey, P. Ghosez, W. Prellier, Chemical Strain Engineering of Magnetism in Oxide Thin Films. Advanced Materials. 29 (2017), doi:10.1002/adma.201604112.

  • A. Cornia, P. Seneor, Spintronics: The molecular way. Nature Materials. 16, 505-506 (2017).
    Résumé : Molecular spintronics is an interdisciplinary field at the interface between organic spintronics, molecular magnetism, molecular electronics and quantum computing, which is advancing fast and promises large technological payoffs.

  • V. E. Demidov, S. Urazhdin, G. de Loubens, O. Klein, V. Cros, A. Anane, S. O. Demokritov, Magnetization oscillations and waves driven by pure spin currents. Physics Reports. 673, 1-31 (2017).

  • S. Deng, H. Butt, K. Jiang, B. Dlubak, P. - R. Kidambi, P. Seneor, S. Xavier, A. - K. Yetisen, Graphene nanoribbon based plasmonic Fresnel zone plate lenses. RSC Advances. 7, 16594-16601 (2017).
    Résumé : A graphene-based metamaterial lens is theoretically proposed by combining plasmonic nanoribbons with Fresnel Zone Plate (FZP) architecture to realize wavelength-selective and tunable lensing. The plasmonic FZP lens shows higher focal intensity and efficiency compared to conventional FZP. As compared with normal graphene FZP, the lensing effect of the plasmonic FZP can be enhanced by 83 times. When compared with Au thin film based FZP lenses, the graphene plasmonic lenses can achieve comparable lensing effects, but with a thinner geometry and with an additional advantage of being wavelength selective and tuneable. The analyses of selectivity and tunability of the plasmonic lens show that the plasmonic lens functions as a filter with broadband incident light or as a switch which can be turned on and off via changing the Fermi levels. The diffraction between neighboring graphene nanoribbons and the effect of the substrate on the lensing effect is also discussed. The plasmonic effect of the nanoribbons only contributes to the focal intensity without affecting the diffraction properties of Fresnel zone plate lenses such as focal lengths. This plasmonic FZP lens is an ideal combination of near and far field optics. However, the complex interaction of diffractions within and between the FZP rings could lead to a significant change of the lensing effect, which opens the possibility of creating innovative graphene metamaterial devices. The findings in this work can be used for developing wavelength-selective electro-optical applications operating in the infrared and terahertz ranges.

  • O. d’Allivy Kelly, M. Collet, E. Jacquet, R. Mattana, F. Petroff, P. Bortolotti, V. Cros, A. Anane, F. Wilhelm, K. Ollefs, A. Rogalev, Magnetic Proximity Effect Free Spin Hall Magnetoresistance in YIG/Pd. SPIN. 07, 1740005 (2017).

  • A. Fert, N. Reyren, V. Cros, Magnetic skyrmions: advances in physics and potential applications. Nature Reviews Materials. 2, 17031 (2017).
    Résumé : Magnetic skyrmions are topologically protected spin whirls that hold promise for applications because they can be controllably moved, created and annihilated. In this Review, the underlying physics of the stabilization of skyrmions at room temperature and their prospectives

  • T. Fördös, H. Jaffrès, K. Postava, M. - S. Seghilani, A. Garnache, J. Pištora, H. - J. Drouhin, Eigenmodes of spin vertical-cavity surface-emitting lasers with local linear birefringence and gain dichroism. Physical Review A. 96, 043828 (2017).
    Résumé : We present a general method for the modeling of semiconductor lasers such as a vertical-cavity surface-emitting laser and a vertical-external-cavity surface-emitting laser containing multiple quantum wells and involving anisotropies that may reveal (i) a local linear birefringence due to the strain field at the surface or (ii) a birefringence in quantum wells due to phase amplitude coupling originating from the reduction of the biaxial ${D}_{2d}$ symmetry group to the ${C}_{2v}$ symmetry group at the III-V ternary semiconductor interfaces. From a numerical point of view, a scattering S-matrix recursive method is implemented using a gain or amplification tensor derived analytically from the Maxwell-Bloch equations. It enables one to model the properties of the emission (threshold, polarization, and mode splitting) from the laser with multiple quantum well active zones by searching for the resonant eigenmodes of the cavity. The method is demonstrated on real laser structures and is presently used for the extraction of optical permittivity tensors of surface strain and quantum wells in agreement with experiments. The method can be generalized to find the laser eigenmodes in the most general case of circular polarized pumps (unbalance between the spin-up and spin-down channels) and/or dichroism allowing an elliptically polarized light emission as recently demonstrated experimentally when the linear birefringence is almost compensated [Joly et al., Opt. Lett. 42, 651 (2017)].
    Mots-clés : Semiconductors and Spintronics.

  • M. Geahel, I. Jouanny, D. Gorse-Pomonti, M. Poirier-Quinot, J. Briatico, C. - J. van der Beek, Edge Contamination, Bulk Disorder, Flux Front Roughening, and Multiscaling in Type II Superconducting Thin Films. Condensed Matter. 2, 27 (2017).
    Résumé : We have investigated the effect of different types of disorder on the propagation, roughness, and scaling properties of magnetic flux fronts in a type II superconductor. A progression from the usual (Kardar–Parisi–Zhang-type) scaling to multiscaling is observed as the disorder strength is increased. A hierarchy of disorder strengths is established for YBa 2 Cu 3 O 7 − δ thin films. The results cast light on the physical origin of the roughening of flux fronts, and they are of interest for the design and elimination of flux noise in microscopic superconducting thin-film devices.
    Mots-clés : Supra.

  • J. Gonzalez Sutter, L. Neñer, H. Navarro, G. Leyva, S. Fusil, K. Bouzehouane, N. Haberkorn, M. Sirena, Oxygen influence in the magnetic and the transport properties of ferroelectric/ferromagnetic heterostructures. Thin Solid Films. 639, 42 (2017).
    Résumé : Oxygen vacancies in oxides nanostructures are the origin of many intriguing phenomena. We have studied the influence of the oxygen pressure in the tunneling properties of a ferroelectric barrier, Ba0.25Sr0.75TiO3 (BSTO), grown over a ferromagnetic electrode. A phenomenological model description was used to obtain critical information about the structure and electrical properties of ultra-thin BSTO layers using conductive atomic force microscopy. The BSTO layers present good insulation properties. Reducing the oxygen content increases the conductivity of the samples. The tunneling of the current carriers is probably the main conduction mechanism for samples with higher barrier thicknesses.

  • I. Gross, W. Akhtar, V. Garcia, L. J. Martínez, S. Chouaieb, K. Garcia, C. Carretero, A. Barthélémy, P. Appel, P. Maletinsky, J. - V. Kim, J. - Y. Chauleau, N. Jaouen, M. Viret, M. Bibes, S. Fusil, V. Jacques, Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer. Nature. 549, 252-256 (2017).
    Résumé : Although ferromagnets have many applications, their large magnetization and the resulting energy cost for switching magnetic moments bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem, and often have extra functionalities: non-collinear spin order may break space-inversion symmetry and thus allow electric-field control of magnetism, or may produce emergent spin–orbit effects that enable efficient spin–charge interconversion. To harness these traits for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems. Here, using a non-invasive, scanning single-spin magnetometer based on a nitrogen–vacancy defect in diamond, we demonstrate real-space visualization of non-collinear antiferromagnetic order in a magnetic thin film at room temperature. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 70 nanometres, consistent with values determined by macroscopic diffraction. In addition, we take advantage of the magnetoelectric coupling present in BiFeO3 to manipulate the cycloid propagation direction by an electric field. Besides highlighting the potential of nitrogen–vacancy magnetometry for imaging complex antiferromagnetic orders at the nanoscale, these results demonstrate how BiFeO3 can be used in the design of reconfigurable nanoscale spin textures.

  • F. Hellman, A. Hoffmann, Y. Tserkovnyak, G. Beach, E. Fullerton, C. Leighton, A. - H. MacDonald, D. - C. Ralph, D. - A. Arena, H. Dürr, P. Fischer, J. Grollier, J. - P. Heremans, T. Jungwirth, A. - V. Kimel, B. Koopmans, I. - N. Krivorotov, S. - J. May, A. Petford-Long, J. - M. Rondinelli, N. Samarth, I. - K. Schuller, A. - N. Slavin, M. D. Stiles, O. Tchernyshyov, A. Thiaville, B. - L. Zink, Interface-induced phenomena in magnetism. Reviews of Modern Physics. 89, 025006 (2017).
    Résumé : This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin-transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange-spring magnets, the spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and noncollinear spin textures, nonlinear dynamics including spin-transfer torque and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

  • A. Joly, G. Baili, M. Alouini, J. - M. George, I. Sagnes, G. Pillet, D. Dolfi, Compensation of the residual linear anisotropy of phase in a vertical-external-cavity-surface-emitting laser for spin injection. Optics Letters. 42, 651-654 (2017).
    Résumé : We report on the compensation of the linear anisotropy of phase in a vertical-external-cavity surface-emitting laser from 21 to 0.5 mrad with an intracavity PLZT electro-optical ceramic. It allows dynamic and accurate control of the laser linear anisotropy, as well as dynamic control of the laser polarization eigenstates. At the birefringence compensation point, we observe an elliptical polarization state with 41° of ellipticity, rotated from its initial position of 32°. The experimental observations are in close agreement with the theoretical predictions. Finally, we are able to demonstrate control of the polarization state with spin injection.

  • J. - V. Kim, M. - W. Yoo, Current-driven skyrmion dynamics in disordered films. Applied Physics Letters. 110, 132404 (2017).
    Résumé : A theoretical study of the current-driven dynamics of magnetic skyrmions in disordered perpendicularly magnetized ultrathin films is presented. The disorder is simulated as a granular structure, in which the local anisotropy varies randomly from grain to grain. The skyrmion velocity is computed for different disorder parameters and ensembles. Similar behavior is seen for spin-torques due to in-plane currents and the spin Hall effect, where a pinning regime can be identified at low currents with a transition towards the disorder-free case at higher currents, similar to domain wall motion in disordered films. Moreover, a current-dependent skyrmion Hall effect and fluctuations in the core radius are found, which result from the interaction with the pinning potential.

  • M. Kreissig, R. Lebrun, F. Protze, K. - J. Merazzo, J. Hem, L. Vila, R. Ferreira, M. - C. Cyrille, F. Ellinger, V. Cros, U. Ebels, P. Bortolotti, Vortex spin-torque oscillator stabilized by phase locked loop using integrated circuits. AIP Advances. 7, 056653 (2017).
    Résumé : Spin-torque nano-oscillators (STO) are candidates for the next technological implementation of spintronic devices in commercial electronic systems. For use in microwave applications, improving the noise figures by efficient control of their phase dynamics is a mandatory requirement. In order to achieve this, we developed a compact phase locked loop (PLL) based on custom integrated circuits (ICs) and demonstrate that it represents an efficient way to reduce the phase noise level of a vortex based STO. The advantage of our approach to phase stabilize STOs is that our compact system is highly reconfigurable e.g. in terms of the frequency divider ratio N, RF gain and loop gain. This makes it robust against device to device variations and at the same time compatible with a large range of STOs. Moreover, by taking advantage of the natural highly non-isochronous nature of the STO, the STO frequency can be easily controlled by e.g. changing the divider ratio N.

  • P. Laczkowski, Y. Fu, H. Yang, J. - C. Rojas-Sanchez, P. Noel, V. T. Pham, G. Zahnd, C. Deranlot, S. Collin, C. Bouard, P. Warin, V. Maurel, M. Chshiev, A. Marty, J. - P. Attane, A. Fert, H. Jaffrès, L. Vila, J. - M. George, Large enhancement of the spin Hall effect in Au by side-jump scattering on Ta impurities. Physical Review B. 96, 140405 (2017).
    Résumé : We present measurements of the spin Hall effect (SHE) in AuW and AuTa alloys for a large range of W or Ta concentrations by combining experiments on lateral spin valves and ferromagnetic-resonance/spin-pumping techniques. The main result is the identification of a large enhancement of the spin Hall angle (SHA) by the side-jump mechanism on Ta impurities, with a SHA as high as $+0.5$ (i.e., $50%$) for about 10% of Ta. In contrast, the SHA in AuW does not exceed $+0.15$ and can be explained by intrinsic SHE of the alloy without significant extrinsic contribution from skew or side-jump scattering by W impurities. The AuTa alloys, as they combine a very large SHA with a moderate resistivity (smaller than $85\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\text{cm}$), are promising for spintronic devices exploiting the SHE.

  • R. Lebrun, J. Grollier, F. - A. Araujo, P. Bortolotti, V. Cros, A. Hamadeh, X. de Milly, Y. Li, G. De Loubens, O. Klein, S. Tsunegi, H. Kubota, K. Yakushiji, A. Fukushima, S. Yuasa, Driven energy transfer between coupled modes in spin-torque oscillators. Physical Review B. 95, 134444 (2017).
    Résumé : The mutual interaction between the different eigenmodes of a spin-torque oscillator can lead to a large variety of physical mechanisms from mode hopping to multimode generation, that usually reduce their performances as radio-frequency devices. To tackle this issue for the future applications, we investigate the properties of a model spin-torque oscillator that is composed of two coupled vortices with one vortex in each of the two magnetic layers of the oscillator. In such double-vortex system, the remarkable properties of energy transfer between the coupled modes, one being excited by spin transfer torque while the second one being damped, result into an alteration of the damping parameters. As a consequence, the oscillator nonlinear behavior is concomitantly drastically impacted. This efficient coupling mechanism, driven mainly by the dynamic dipolar field generated by the spin transfer torque induced motion of the vortices, gives rise to an unusual dynamical regime of self-resonance excitation. These results show that mode coupling can be leveraged for controlling the synchronization process as well as the frequency tunability of spin-torque oscillators.

  • R. Lebrun, S. Tsunegi, P. Bortolotti, H. Kubota, A. S. Jenkins, M. Romera, K. Yakushiji, A. Fukushima, J. Grollier, S. Yuasa, V. Cros, Mutual synchronization of spin torque nano-oscillators through a long-range and tunable electrical coupling scheme. Nature Communications. 8, 15825 (2017).
    Résumé : The spintronics based complex network is promising for next generation computing systems but hampered by short-range spin-wave coupling. The authors make progress by achieving long range and tunable mutual synchronization of two spin-torque oscillators with improved emission
    Mots-clés : Spinorbitronics.

  • W. Legrand, D. Maccariello, N. Reyren, K. Garcia, C. Moutafis, C. Moreau-Luchaire, S. Collin, K. Bouzehouane, V. Cros, A. Fert, Room-Temperature Current-Induced Generation and Motion of sub-100 nm Skyrmions. Nano Letters. 17, 2703-2712 (2017).
    Résumé : Magnetic skyrmions are nanoscale windings of the spin configuration that hold great promise for technology due to their topology-related properties and extremely reduced sizes. After the recent observation at room temperature of sub-100 nm skyrmions stabilized by interfacial chiral interaction in magnetic multilayers, several pending questions remain to be solved, notably about the means to nucleate individual compact skyrmions or the exact nature of their motion. In this study, a method leading to the formation of magnetic skyrmions in a micrometer-sized track using homogeneous current injection is evidenced. Spin-transfer-induced motion of these small electrical-current-generated skyrmions is then demonstrated and the role of the out-of-plane magnetic field in the stabilization of the moving skyrmions is also analyzed. The results of these experimental observations of spin torque induced motion are compared to micromagnetic simulations reproducing a granular type, nonuniform magnetic multilayer in order to address the particularly important role of the magnetic inhomogeneities on the current-induced motion of sub-100 nm skyrmions for which the material grains size is comparable to the skyrmion diameter.
    Mots-clés : Spinorbitronics.

  • Y. Li, X. de Milly, F. - A. Araujo, O. Klein, V. Cros, J. Grollier, G. De Loubens, Probing Phase Coupling Between Two Spin-Torque Nano-Oscillators with an External Source. Physical Review Letters. 118, 247202 (2017).
    Résumé : Phase coupling between auto-oscillators is central for achieving coherent responses such as synchronization. Here we present an experimental approach to probe it in the case of two dipolarly coupled spin-torque vortex nano-oscillators using an external microwave field. By phase locking one oscillator to the external source, we observe frequency pulling on the second oscillator. From coupled phase equations we show analytically that this frequency pulling results from concerted actions of oscillator-oscillator and source-oscillator couplings. The analysis allows us to determine the strength and phase shift of coupling between two oscillators, yielding important information for the implementation of large interacting oscillator networks.
    Mots-clés : Spinorbitronics.

  • S. Liang, H. Yang, P. Renucci, B. Tao, P. Laczkowski, S. Mc-Murtry, G. Wang, X. Marie, J. - M. George, S. Petit-Watelot, A. Djeffal, S. Mangin, H. Jaffrès, Y. Lu, Electrical spin injection and detection in molybdenum disulfide multilayer channel. Nature Communications. 8, 14947 (2017).
    Résumé : MoS2 is a promising two-dimensional candidate for opto-electronic and spintronic applications. Here, the authors report electrical spin injection and detection in a few-layered MoS2 channel, demonstrating that the spin diffusion length is at least 235 nm in MoS2conduction band.
    Mots-clés : Semiconductors and Spintronics.

  • X. Lin, L. Su, Z. Si, Y. Zhang, A. Bournel, Y. Zhang, J. - O. Klein, A. Fert, W. Zhao, Gate-Driven Pure Spin Current in Graphene. Physical Review Applied. 8, 034006 (2017).
    Résumé : Manipulating spin current rather than electric current is considered a promising route to ultralow-power ``beyond CMOS'' devices. The authors propose a graphene-based demultiplexer that enables voltage-controlled distribution and propagation of pure spin current. Gate-driven pure spin current would allow multiple logic functions to be cascaded without spin-charge conversion, for efficient long-distance transport, so this device could be a building block for reconfigurable spin-logic circuits.


  • N. Locatelli, V. Cros, dans Introduction to Magnetic Random-Access Memory, B. Dieny, R. B. Goldfarb, K. - J. Lee, Éd. (John Wiley & Sons, Inc., 2017; http://onlinelibrary.wiley.com/doi/10.1002/9781119079415.ch1/summary)p. 1-28.
    Résumé : Spintronics is a merger of magnetism and electronics. It uses the spin of electrons in addition to their charge to obtain new properties and uses these properties in innovative devices. This chapter introduces three major spintronics phenomena, which form the basis of most spintronics devices: giant magnetoresistance, tunneling magnetoresistance, and spin-transfer torque.

  • S. Menshawy, A. S. Jenkins, K. J. Merazzo, L. Vila, R. Ferreira, M. - C. Cyrille, U. Ebels, P. Bortolotti, J. Kermorvant, V. Cros, Spin transfer driven resonant expulsion of a magnetic vortex core for efficient rf detector. AIP Advances. 7, 056608 (2017).
    Résumé : Spin transfer magnetization dynamics have led to considerable advances in Spintronics, including opportunities for new nanoscale radiofrequency devices. Among the new functionalities is the radiofrequency (rf) detection using the spin diode rectification effect in spin torque nano-oscillators (STNOs). In this study, we focus on a new phenomenon, the resonant expulsion of a magnetic vortex in STNOs. This effect is observed when the excitation vortex radius, due to spin torques associated to rf currents, becomes larger than the actual radius of the STNO. This vortex expulsion is leading to a sharp variation of the voltage at the resonant frequency. Here we show that the detected frequency can be tuned by different parameters; furthermore, a simultaneous detection of different rf signals can be achieved by real time measurements with several STNOs having different diameters. This result constitutes a first proof-of-principle towards the development of a new kind of nanoscale rf threshold detector.

  • S. Oyarzún, F. Rortais, J. - C. Rojas-Sanchez, F. Bottegoni, P. Laczkowski, C. Vergnaud, S. Pouget, H. Okuno, L. Vila, J. - P. Attane, C. Beigné, A. Marty, S. Gambarelli, C. Ducruet, J. Widiez, J. - M. George, H. Jaffrès, M. Jamet, Spin–Charge Conversion Phenomena in Germanium. Journal of the Physical Society of Japan. 86, 011002 (2017).
    Résumé : The spin–orbit coupling relating the electron spin and momentum allows for spin generation, detection and manipulation. It thus fulfils the three basic functions of the spin field-effect-transistor made of semiconductors. In this paper, we review our recent results on spin–charge conversion in bulk germanium and at the Ge(111) surface. We used the spin pumping technique to generate pure spin currents to be injected into bulk germanium and at the Fe/Ge(111) interface. The mechanism for spin–charge conversion in bulk germanium is the spin Hall effect and we could experimentally determine the spin Hall angle θSHE, i.e., the spin–charge conversion efficiency, in heavily doped n-type and p-type germanium. We found very small values at room temperature: θSHE ≈ (1–2) × 10−3 in n-Ge and θSHE ≈ (6–7) × 10−4 in p-Ge. Moreover, we pointed out the essential role of spin dependent scattering on ionized impurities in the spin Hall effect mechanism. We concluded that the spin Hall effect in bulk germanium is too weak to produce large spin currents, whereas a large Rashba effect (>100 meV) at Ge(111) surfaces covered with heavy metals could generate spin polarized currents. We could indeed demonstrate a giant spin-to-charge conversion in metallic states at the Fe/Ge(111) interface due to the Rashba coupling. We generated very large charge currents by direct spin pumping into the interface states from 20 K to room temperature. By this, we raise a new paradigm: the possibility to use the spin–orbit coupling for the development of the spin-field-effect-transistor.

  • S. Peng, W. Zhao, J. Qiao, L. Su, J. Zhou, H. Yang, Q. Zhang, Y. Zhang, C. Grezes, P. Amiri, K. Wang, Giant interfacial perpendicular magnetic anisotropy in MgO/CoFe/capping layer structures. Applied Physics Letters. 110, 072403 (2017).
    Résumé : Magnetic tunnel junction based on the CoFeB/MgO/CoFeB structures is of great interest due to its application in the spin-transfer-torque magnetic random access memory (STT-MRAM). Large interfacial perpendicular magnetic anisotropy (PMA) is required to achieve high thermal stability. Here, we use the first-principles calculations to investigate the magnetic anisotropy energy (MAE) of the MgO/CoFe/capping layer structures, where the capping materials include 5d metals Hf, Ta, Re, Os, Ir, Pt, and Au and 6p metals Tl, Pb, and Bi. We demonstrate that it is feasible to enhance PMA by using proper capping materials. Relatively large PMA is found in the structures with the capping materials of Hf, Ta, Os, Ir, and Pb. More importantly, the MgO/CoFe/Bi structure gives rise to giant PMA (6.09 mJ/m2), which is about three times larger than that of the MgO/CoFe/Ta structure. The origin of the MAE is elucidated by examining the contributions to MAE from each atomic layer and orbital. These findings provide a comprehens...

  • M. Piquemal-Banci, R. Galceran, M. - B. Martin, F. Godel, A. Anane, F. Petroff, B. Dlubak, P. Seneor, 2D-MTJs: introducing 2D materials in magnetic tunnel junctions. Journal of Physics D: Applied Physics. 50, 203002 (2017).

  • D. Preziosi, A. Sander, A. Barthélémy, M. Bibes, Reproducibility and off-stoichiometry issues in nickelate thin films grown by pulsed laser deposition. AIP Advances. 7, 015210 (2017).
    Résumé : Rare-earth nickelates are strongly correlated oxides displaying a metal-to-insulator transition at a temperature tunable by the rare-earth ionic radius. In PrNiO3 and NdNiO3, the transition is very sharp and shows an hysteretic behavior akin to a first-order transition. Both the temperature at which the transition occurs and the associated resistivity change are extremely sensitive to doping and therefore to off-stoichiometry issues that may arise during thin film growth. Here we report that strong deviations in the transport properties of NdNiO3 films can arise in films grown consecutively under nominally identical conditions by pulsed laser deposition; some samples show a well-developed transition with a resistivity change of up to five orders of magnitude while others are metallic down to low temperatures. Through a detailed analysis of in-situ X-ray photoelectron spectroscopy data, we relate this behavior to large levels of cationic off-stoichoimetry that also translate in changes in the Ni valence an...

  • I. Rashid, H. Butt, A. - K. Yetisen, B. Dlubak, J. - E. Davies, P. Seneor, A. Vechhiola, F. Bouamrane, S. Xavier, Wavelength-Selective Diffraction from Silica Thin-Film Gratings. ACS Photonics. 4, 2402-2409 (2017).
    Résumé : A reflective diffraction grating with a periodic square-wave profile will combine the effects of thin-film interference with conventional grating behavior when composed of features having a different refractive index than that of the substrate. A grating period of 700–1300 nm was modeled and compared for both silicon (Si) and silicon dioxide (SiO2) to determine the behavior of light interaction with the structures. Finite element analysis was used to study nanostructures having a multirefractive index grating and a conventional single material grating. A multimaterial grating has the same diffraction efficiency as that of a grating formed in a single material, but had the advantage of having an ordered relationship between the grating dimensions (thickness and period) and the intensity of reflected and diffracted optical wavelengths. We demonstrate a color-selective feature of the modeled SiO2 grating by fabricating samples with grating periods of 800 and 1000 nm, respectively. A high diffraction efficiency was measured for the green wavelength region as compared to other colors in the spectrum for 800 nm grating periodicity; whereas wavelengths within the red region of spectrum interfered constructively for the grating with 1000 nm periodicity resulting a higher efficiency for red color bandwidth. The results show that diffraction effects can be enhanced by the thin-film interference phenomenon to produce color selective optical devices.

  • N. Reyren, K. Bouzehouane, J. - Y. Chauleau, S. Collin, A. Fert, S. Finizio, K. Garcia, S. Hughes, N. Jaouen, W. Legrand, D. Maccariello, S. McFadzean, S. McVitie, C. Moutafis, H. Popescu, J. Raabe, C. a. F. Vaz, V. Cros, (International Society for Optics and Photonics, 2017)vol. 10357p. 1035724.
    Résumé : Sub-100-nm skyrmions are stabilized in magnetic metallic multilayers and observed using transmission electron microscopy, magnetic force microscopy, scanning transmission X-ray microscopy and X-ray resonant magnetic scattering. All these advanced imaging techniques demonstrate the presence of 'pure' Neel skyrmion textures with a determined chirality. Combining these observations with electrical measurements allows us to demonstrate reproducible skyrmion nucleation using current pulses, and measure their contribution to the transverse resistivity to detect them electrically. Once nucleated, skyrmions can be moved using charge currents. We find predominantly a creep-like regime, characterized by disordered skyrmion motion, as observed by atomic force microscopy and scanning transmission X-ray microscopy. These observations are explained qualitatively and to some extent quantitatively by the presence of crystalline grains of about 20nm lateral size with a distribution of magnetic properties.

  • F. Rortais, C. Vergnaud, A. Marty, L. Vila, J. - P. Attane, J. Widiez, C. Zucchetti, F. Bottegoni, H. Jaffrès, J. - M. George, M. Jamet, Non-local electrical spin injection and detection in germanium at room temperature. Applied Physics Letters. 111, 182401 (2017).
    Résumé : Non-local carrier injection/detection schemes lie at the very foundation of information manipulation in integrated systems. This paradigm consists in controlling with an external signal the channel where charge carriers flow between a “source” and a well separated “drain.” The next generation electronics may operate on the spin of carriers in addition to their charge and germanium appears as the best hosting material to develop such a platform for its compatibility with mainstream silicon technology and the predicted long electron spin lifetime at room temperature. In this letter, we demonstrate injection of pure spin currents (i.e., with no associated transport of electric charges) in germanium, combined with non-local spin detection at 10 K and room temperature. For this purpose, we used a lateral spin valve with epitaxially grown magnetic tunnel junctions as spin injector and spin detector. The non-local magnetoresistance signal is clearly visible and reaches ≈15 mΩ at room temperature. The electron sp...
    Mots-clés : Semiconductors and Spintronics.

  • M. Sushruth, J. - P. Fried, A. Anane, S. Xavier, C. Deranlot, V. Cros, P. - J. Metaxas, Chirality-mediated bistability and strong frequency downshifting of the gyrotropic resonance of a magnetic vortex due to dynamic destiffening. Physical Review B. 96, 060405 (2017).
    Résumé : The gyrotropic resonance frequency of a magnetic vortex, widely studied for applications in data storage and radio-frequency signal processing, increases with the vortex core's stiffness. In traditional disk-shaped elements, this stiffness increases if the core is shifted towards the element's edges. Here, the authors show that introducing a flat edge into a disk locally de-stiffens the core, resulting in the resonant frequency strongly decreasing when the core approaches the element's flat edge. By controllably displacing the core within the disk one can thus both increase or decrease the core's resonant frequency with respect to its value when unshifted. This has the effect of more than doubling the accessible range of resonant frequencies. However, more fundamentally, the above properties lead to a chirality-mediated bistability: for a given finite static in-plane magnetic field, one of two values of gyrotropic frequencies can be observed depending on the vortex chirality.

  • J. Torrejon, M. Riou, F. - A. Araujo, S. Tsunegi, G. Khalsa, D. Querlioz, P. Bortolotti, V. Cros, K. Yakushiji, A. Fukushima, H. Kubota, S. Yuasa, M. D. Stiles, J. Grollier, Neuromorphic computing with nanoscale spintronic oscillators. Nature. 547, 428-431 (2017).
    Résumé : Neurons in the brain behave as nonlinear oscillators, which develop rhythmic activity and interact to process information. Taking inspiration from this behaviour to realize high-density, low-power neuromorphic computing will require very large numbers of nanoscale nonlinear oscillators. A simple estimation indicates that to fit 108 oscillators organized in a two-dimensional array inside a chip the size of a thumb, the lateral dimension of each oscillator must be smaller than one micrometre. However, nanoscale devices tend to be noisy and to lack the stability that is required to process data in a reliable way. For this reason, despite multiple theoretical proposals and several candidates, including memristive and superconducting oscillators, a proof of concept of neuromorphic computing using nanoscale oscillators has yet to be demonstrated. Here we show experimentally that a nanoscale spintronic oscillator (a magnetic tunnel junction) can be used to achieve spoken-digit recognition with an accuracy similar to that of state-of-the-art neural networks. We also determine the regime of magnetization dynamics that leads to the greatest performance. These results, combined with the ability of the spintronic oscillators to interact with each other, and their long lifetime and low energy consumption, open up a path to fast, parallel, on-chip computation based on networks of oscillators.

  • J. Varignon, M. N. Grisolia, J. Íñiguez, A. Barthélémy, M. Bibes, Complete phase diagram of rare-earth nickelates from first-principles. npj Quantum Materials. 2, 21 (2017).
    Résumé : The structural, electronic and magnetic properties of AMO3 perovskite oxides, where M is a 3d transition metal, are highly sensitive to the geometry of the bonds between the metal-d and oxygen-p ions (through octahedra rotations and distortions) and to their level of covalence. This is particularly true in rare-earth nickelates RNiO3 that display a metal–insulator transition with complex spin orders tunable by the rare-earth size, and are on the border line between dominantly ionic (lighter elements) and covalent characters (heavier elements). Accordingly, computing their ground state is challenging and a complete theoretical description of their rich phase diagram is still missing. Here, using first-principles simulations, we successfully describe the electronic and magnetic experimental ground state of nickelates. We show that the insulating phase is characterized by a split of the electronic states of the two Ni sites (i.e., resembling low-spin 4+ and high-spin 2+) with a concomitant shift of the oxygen-2p orbitals toward the depleted Ni cations. Therefore, from the point of view of the charge, the two Ni sites appear nearly identical whereas they are in fact distinct. Performing such calculations for several nickelates, we built a theoretical phase diagram that reproduces all their key features, namely a systematic dependence of the metal–insulator transition with the rare-earth size and the crossover between a second to first order transition for R = Pr and Nd. Finally, our results hint at strategies to control the electronic and magnetic phases of perovskite oxides by fine tuning of the level of covalence. A new theoretical approach provides a complete phase diagram of rare-earth nickelates, reproducing the key features seen in experiments. Transition metal oxides with a pervoskite crystal structure exhibit a broad range of behaviours due to a complex the interplay between lattice, electronic and magnetic degrees of freedom. Rare-earth nickelates are a particularly interesting class of perovskite oxide that undergo a highly tunable (and potentially exploitable) metal-insulator transition. Theoretically describing the insulating phase of nickelates, however, is far from trivial and two seemingly distinct descriptions have emerged. A team of researchers led by Manuel Bibes at Unité Mixte de Physique CNRS/Thales use first-principles simulations to somewhat reconcile these conflicting visions, fully describing the electronic and magnetic ground state of nickelates, as well as hinting at strategies for tuning these fascinating materials.

  • D. - C. Vaz, E. Lesne, A. Sander, H. Naganuma, E. Jacquet, J. Santamaria, A. Barthélémy, M. Bibes, Tuning Up or Down the Critical Thickness in LaAlO3/SrTiO3 through In Situ Deposition of Metal Overlayers. Advanced Materials. 29 (2017), doi:10.1002/adma.201700486.
    Résumé : The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 and SrTiO3 has attracted much attention from the oxide electronics community. One of its hallmark features is the existence...

  • D. Vodenicarevic, N. Locatelli, F. - A. Araujo, J. Grollier, D. Querlioz, A Nanotechnology-Ready Computing Scheme based on a Weakly Coupled Oscillator Network. Scientific Reports. 7, 44772 (2017).
    Résumé : With conventional transistor technologies reaching their limits, alternative computing schemes based on novel technologies are currently gaining considerable interest.

  • D. Vodenicarevic, N. Locatelli, A. Mizrahi, J. S. Friedman, A. - F. Vincent, M. Romera, A. Fukushima, K. Yakushiji, H. Kubota, S. Yuasa, S. Tiwari, J. Grollier, D. Querlioz, Low-Energy Truly Random Number Generation with Superparamagnetic Tunnel Junctions for Unconventional Computing. Physical Review Applied. 8, 054045 (2017).
    Résumé : Random number generation is critical for many emerging computing schemes, but the associated energy consumption and circuit area are major bottlenecks. This study exploits the stochastic behavior of superparamagnetic tunnel junctions, magnetic nanodevices that, due solely to thermal noise, will switch randomly between two well-defined states. These tunnel junctions can produce high-quality, truly random bit streams, with an energy efficiency that is orders of magnitude better than the state of the art. The authors furthermore develop an example that highlights the utility of superparamagnetic random number generation for low-energy probabilistic computing.

  • B. Xu, V. Garcia, S. Fusil, M. Bibes, L. Bellaiche, Intrinsic polarization switching mechanisms in BiFeO3. Physical Review B. 95, 104104 (2017).
    Résumé : A first-principles-based effective Hamiltonian technique is used to investigate the polarization switching mechanisms in two polymorphic phases of ${\mathrm{BiFeO}}_{3}$ having no defects. The switching mechanism is homogeneous for any switching field in the rhombohedral phase, while in the supertetragonal phase it changes from the classical nucleation and domain-wall motion to nucleation-limited switching with virtually no propagation, and then to homogeneous switching with increasing electric field. The first two inhomogeneous switching mechanisms of the supertetragonal phase of ${\mathrm{BiFeO}}_{3}$ are thus intrinsic in nature, and can be well described by the classical and nucleation-limited switching models, respectively. The reason behind their absence in the rhombohedral phase is also indicated. Moreover, the field-induced changes of switching mechanism within the supertetragonal phase are further elucidated from an energetic point of view.

  • M. - W. Yoo, V. Cros, J. - V. Kim, Current-driven skyrmion expulsion from magnetic nanostrips. Physical Review B. 95, 184423 (2017).
    Résumé : We study the current-driven skyrmion expulsion from magnetic nanostrips using micromagnetic simulations and analytic calculations. We explore the threshold current density for the skyrmion expulsion, and show that this threshold is determined by the critical boundary force as well as the spin-torque parameters. We also find the dependence of the critical boundary force on the magnetic parameters; the critical boundary force decreases with increasing the exchange stiffness and perpendicular anisotropy constants, while it increases with increasing Dzyaloshinskii-Moriya interaction and saturation magnetization constants. Using a simple model describing the skyrmion and locally-tilted edge magnetization, we reveal the underlying physics of the dependence of the critical boundary force on the magnetic parameters based on the relation between the scaled Dzyaloshinskii-Moriya-interaction parameter and the critical boundary force. This work provides a fundamental understanding of the skyrmion expulsion and the interaction between the skymion and boundaries of devices and shows that the stability of the skyrmion in devices can be related to the scaled Dzyaloshinskii-Moriya-interaction parameter of magnetic materials.

  • H. Yu, S. - D. Brechet, P. Che, F. - A. Vetro, M. Collet, S. Tu, Y. - G. Zhang, Y. Zhang, T. Stueckler, L. Wang, H. Cui, D. Wang, C. Zhao, P. Bortolotti, A. Anane, J. - P. Ansermet, W. Zhao, Thermal spin torques in magnetic insulators. Physical Review B. 95, 104432 (2017).
    Résumé : The damping of spin waves transmitted through a two-port magnonic device implemented on a yttrium iron garnet thin film is shown to be proportional to the temperature gradient imposed on the device. The sign of the damping depends on the relative orientation of the magnetic field, the wave vector, and the temperature gradient. The observations are accounted for qualitatively and quantitatively by using an extension of the variational principle that leads to the Landau-Lifshitz equation. All parameters of the model can be obtained by independent measurements.
    Mots-clés : Magnonics.

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