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2017


  • A. Agbelele, D. Sando, C. Toulouse, C. Paillard, R. - D. Johnson, R. Rüffer, A. - F. Popkov, C. Carrétéro, 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).

  • S. Boyn, J. Grollier, G. Lecerf, B. Xu, N. Locatelli, S. Fusil, S. Girod, C. Carrétéro, 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. 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, 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

  • 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.

  • 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.


  • 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...

  • 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. 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.

  • 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.

2016


  • A. Accioly, N. Locatelli, A. Mizrahi, D. Querlioz, L. - G. Pereira, J. Grollier, J. - V. Kim, Role of spin-transfer torques on synchronization and resonance phenomena in stochastic magnetic oscillators. Journal of Applied Physics. 120, 093902 (2016).
    Résumé : A theoretical study on how synchronization and resonance-like phenomena in superparamagnetic tunnel junctions can be driven by spin-transfer torques is presented. We examine the magnetization of a superparamagnetic free layer that reverses randomly between two well-defined orientations due to thermal fluctuations, acting as a stochastic oscillator. When subject to an external ac forcing, this system can present stochastic resonance and noise-enhanced synchronization. We focus on the roles of the mutually perpendicular damping-like and field-like torques, showing that the response of the system is very different at low and high frequencies. We also demonstrate that the field-like torque can increase the efficiency of the current-driven forcing, especially at sub-threshold electric currents. These results can be useful for possible low-power, more energy efficient applications.

  • A. Agbelele, D. Sando, I. C. Infante, C. Carretero, S. Jouen, J. - M. Le Breton, A. Barthélémy, B. Dkhil, M. Bibes, J. Juraszek, Insight into magnetic, ferroelectric and elastic properties of strained BiFeO3 thin films through Mössbauer spectroscopy. Applied Physics Letters. 109, 042902 (2016).
    Résumé : We have studied the magnetic order of highly strained (001)-oriented BiFeO3 (BFO) thin films using 57Fe Conversion Electron Mössbauer Spectrometry. From 90 K to 620 K the films exhibit a collinear antiferromagnetic structure, in contrast with the cycloidal structure observed in bulk BFO. Moreover, we find that both the planar magnetic anisotropy for compressive strain and out-of-plane anisotropy for tensile strain persist from 90 K up to the Néel temperature (T N), which itself shows only a weak strain dependence. An analysis of the line asymmetry of the paramagnetic doublet for temperatures above T N is used to reveal the strain-dependent rotation of the polarization direction, consistent with previous observations. Our results show that the lattice dynamics in BFO films are strongly strain-dependent, offering avenues toward acoustic phonon devices. Finally, we use the versatility of Mössbauer spectroscopy technique to reveal various multi-property features including magnetic states, polarization direction and elastic strain.

  • A. Anane, B. Dlubak, H. Idzuchi, H. Jaffres, M. - B. Martin, Y. Otani, P. Seneor, A. Fert, dans Handbook of Spintronics (2016; http://link.springer.com/referenceworkentry/10.1007/978-94-007-6892-5_27)p. 681-706.

  • F. - A. Araujo, J. Grollier, Controlling the synchronization properties of two dipolarly coupled vortex based spin-torque nano-oscillators by the intermediate of a third one. Journal of Applied Physics. 120, 103903 (2016).
    Résumé : In this paper, we propose to control the strength of phase-locking between two dipolarly coupled vortex based spin-torque nano-oscillators by placing an intermediate oscillator between them. We show through micromagnetic simulations that the strength of phase-locking can be largely tuned by a slight variation of current in the intermediate oscillator. We develop simplified numerical simulations based on analytical expressions of the vortex core trajectories that will be useful for investigating large arrays of densely packed spin-torque oscillators interacting through their stray fields.

  • I. T. Bae, H. Naganuma, T. Ichinose, K. Sato, Thickness dependence of crystal and electronic structures within heteroepitaxially grown BiFeO3 thin films. Physical Review B. 93, 064115 (2016).
    Résumé : Crystal and electronic structures of $\mathrm{BiFe}{\mathrm{O}}_{3}$ thin films ($\ensuremath{\sim}10$ and \ensuremath{\sim}300 nm) grown on $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrate have been investigated in terms of $\mathrm{BiFe}{\mathrm{O}}_{3}$ film thickness dependence using the advanced transmission electron microscopy (TEM) technique. Electron diffraction patterns of both $\mathrm{BiFe}{\mathrm{O}}_{3}$ thin films acquired along ${[011]}_{\mathrm{SrTi}{\mathrm{O}}_{3}}$ cross sections clearly exhibited the existence of extra Bragg's reflections which are absent in that from $\mathrm{SrTi}{\mathrm{O}}_{3}$. Structure factor calculations unambiguously revealed that the electron diffraction pattern corresponds to the [211] net pattern of rhombohedral $\mathrm{BiFe}{\mathrm{O}}_{3}$. High-resolution TEM images combined with multislice simulation also demonstrated that the crystalline structure of both $\mathrm{BiFe}{\mathrm{O}}_{3}$ films is rhombohedral. Electron energy loss spectroscopy results for both $\mathrm{BiFe}{\mathrm{O}}_{3}$ thin films showed spectra with the characteristics of bulk $\mathrm{BiFe}{\mathrm{O}}_{3}$, i.e., rhombohedral. The lattice mismatch of 2.5% between $\mathrm{BiFe}{\mathrm{O}}_{3}$ and $\mathrm{SrTi}{\mathrm{O}}_{3}$ found in a particular epitaxial relationship is considered to be the reason that $\mathrm{BiFe}{\mathrm{O}}_{3}$ can grow by maintaining its bulk crystalline, i.e., rhombohedral, structure.

  • C. Barraud, K. Bouzehouane, C. Deranlot, D. - J. Kim, R. Rakshit, S. Shi, J. Arabski, M. Bowen, E. Beaurepaire, S. Boukari, F. Petroff, P. Seneor, R. Mattana, Phthalocyanine based molecular spintronic devices. Dalton Transactions. 45, 16694-16699 (2016).
    Résumé : Molecular spintronics is an effervescent field of research, which aims at combining spin physics and molecular nano-objects. In this article, we show that phthalocyanine molecules integrated in magnetic tunnel junctions (MTJs) can lead to magnetoresistance effects of different origins. We have investigated cobalt and manganese phthalocyanine molecule based magnetic tunnel junctions. CoPc MTJs exhibit both tunneling magnetoresistance (TMR) and tunneling anisotropic magnetoresistance (TAMR) effects of similar magnitude. However, for MnPc MTJs, a giant TAMR dominates with ratios up to ten thousands of percent. Strong features visible in the conductance suggest that spin–flip inelastic electron tunneling processes occur through the Mn atomic chain formed by the MnPc stacks. These results show that metallo-organic molecules could be used as a template to connect magnetic atomic chains or even a single magnetic atom in a solid-state device.

  • C. Blouzon, J. - Y. Chauleau, A. Mougin, S. Fusil, M. Viret, Photovoltaic response around a unique 180° ferroelectric domain wall in single-crystalline BiFeO3. Physical Review B. 94, 094107 (2016).
    Résumé : Using an experimental setup designed to scan a submicron sized light spot and collect the photogenerated current through larger electrodes, we map the photovoltaic response in ferroelectric $\mathrm{BiFe}{\mathrm{O}}_{3}$ single crystals. We study the effect produced by a unique 180\ifmmode^\circ\else\textdegree\fi{} ferroelectric domain wall (DW) and show that the photocurrent maps are significantly affected by its presence and shape. The effect is large in its vicinity and in the Schottky barriers at the interface with the Au electrodes, but no extra photocurrent is observed when the illuminating spot touches the DW, indicating that this particular entity is not the heart of specific photoelectric properties. Using 3D modeling, we argue that the measured effect is due to the spatial distribution of internal fields which are significantly affected by the charge of the DW due to its distortion.

  • S. Boyn, A. - M. Douglas, C. Blouzon, P. Turner, A. Barthélémy, M. Bibes, S. Fusil, J. - M. Gregg, V. Garcia, Tunnel electroresistance in BiFeO3 junctions: size does matter. Applied Physics Letters. 109, 232902 (2016).
    Résumé : In ferroelectric tunnel junctions, the tunnel resistance depends on the polarization orientation of the ferroelectric tunnel barrier, giving rise to tunnel electroresistance. These devices are promising to be used as memristors in neuromorphic architectures and as non-volatile memory elements. For both applications, device scalability is essential, which requires a clear understanding of the relationship between polarization reversal and resistance change as the junction size shrinks. Here we show a robust tunnel electroresistance in BiFeO3-based junctions with diameters ranging from 1200 to 180 nm. We demonstrate that the tunnel electroresistance and the corresponding fraction of reversed ferroelectric domains change drastically with the junction diameter: while the micron-size junctions display a reversal in less than 10% of the area, the smallest junctions show an almost complete polarization reversal. Modeling the electric-field distribution, we highlight the critical role of the bottom electrode resistance which significantly diminishes the actual electric field applied to the ferroelectric barrier in the mixed polarization state. A polarization-dependent critical electric field below which further reversal is prohibited is found to explain the large differences between the ferroelectric switchability of nano- and micron-size junctions. Our results indicate that ferroelectric junctions are downscalable and suggest that specific junction shapes facilitate complete polarization reversal.

  • S. Boyn, J. Sampaio, V. Cros, J. Grollier, A. Fukushima, H. Kubota, K. Yakushiji, S. Yuasa, Twist in the bias dependence of spin torques in magnetic tunnel junctions. Physical Review B. 93, 224427 (2016).
    Résumé : The spin torque in magnetic tunnel junctions possesses two components that both depend on the applied voltage. Here, we develop a method for the accurate extraction of this bias dependence from experiments over large voltage ranges. We study several junctions with different magnetic layer structures of the top electrode. Our results obtained on junctions with symmetric CoFeB electrodes agree well with theoretical calculations. The bias dependences of asymmetric samples, with top electrodes containing NiFe, however, are twisted compared to the quadratic form generally assumed. Our measurements reveal the complexity of spin-torque mechanisms at large bias.

  • F. Y. Bruno, S. Boyn, S. Fusil, S. Girod, C. Carretero, M. Marinova, A. Gloter, S. Xavier, C. Deranlot, M. Bibes, A. Barthélémy, V. Garcia, Millionfold Resistance Change in Ferroelectric Tunnel Junctions Based on Nickelate Electrodes. Advanced Electronic Materials. 2 (2016), doi:10.1002/aelm.201500245.

  • V. - E. Campbell, M. Tonelli, I. Cimatti, J. - B. Moussy, L. Tortech, Y. Dappe, E. Rivière, R. Guillot, S. Delprat, R. Mattana, P. Seneor, P. Ohresser, F. Choueikani, E. Otero, F. Koprowiak, V. Chilkuri, N. Suaud, N. Guihéry, A. Galtayries, F. Miserque, M. - A. Arrio, P. Sainctavit, T. Mallah, Engineering the magnetic coupling and anisotropy at the molecule–magnetic surface interface in molecular spintronic devices. Nature Communications. 7, 13646 (2016).
    Résumé : Controlling the magnetic response of a molecular device is important for spintronic applications. Here the authors report the self-assembly, magnetic coupling, and anisotropy of two transition metal complexes bound to a ferrimagnetic surface, and probe the role of the nature of the transition metal ion.

  • M. Collet, X. de Milly, O. d’Allivy Kelly, V. V. Naletov, R. Bernard, P. Bortolotti, J. Ben Youssef, V. E. Demidov, S. O. Demokritov, J. - L. Prieto, M. Munoz, V. Cros, A. Anane, G. de Loubens, O. Klein, Generation of coherent spin-wave modes in yttrium iron garnet microdiscs by spin–orbit torque. Nature Communications. 7, 10377 (2016).

  • M. Cubukcu, J. Sampaio, K. Bouzehouane, D. Apalkov, A. V. Khvalkovskiy, V. Cros, N. Reyren, Dzyaloshinskii-Moriya anisotropy in nanomagnets with in-plane magnetization. Physical Review B. 93, 020401 (2016).

  • V. E. Demidov, M. Evelt, V. Bessonov, S. O. Demokritov, J. L. Prieto, M. Muñoz, J. B. Youssef, V. V. Naletov, G. De Loubens, O. Klein, M. Collet, P. Bortolotti, V. Cros, A. Anane, Direct observation of dynamic modes excited in a magnetic insulator by pure spin current. Scientific Reports. 6, 32781 (2016).
    Résumé : Excitation of magnetization dynamics by pure spin currents has been recently recognized as an enabling mechanism for spintronics and magnonics, which allows implementation of spin-torque devices based on low-damping insulating magnetic materials.

  • E. Dremetsika, B. Dlubak, S. P. Gorza, C. Ciret, M. - B. Martin, S. Hofmann, P. Seneor, D. Dolfi, S. Massar, P. Emplit, P. Kockaert, Measuring the nonlinear refractive index of graphene using the optical Kerr effect method. Optics Letters. 41, 3281-3284 (2016).
    Résumé : By means of the ultrafast optical Kerr effect method coupled to optical heterodyne detection (OHD-OKE), we characterize the third-order nonlinear response of graphene and compare it to experimental values obtained by the Z-scan method on the same samples. From these measurements, we estimate a negative nonlinear refractive index for monolayer graphene, n2=−1.1×10−13  m2/W. This is in contradiction to previously reported values, which leads us to compare our experimental measurements obtained by the OHD-OKE and the Z-scan method with theoretical and experimental values found in the literature and to discuss the discrepancies, taking into account parameters such as doping.

  • M. Evelt, V. E. Demidov, V. Bessonov, S. O. Demokritov, J. L. Prieto, M. Munoz, J. B. Youssef, V. V. Naletov, G. De Loubens, O. Klein, M. Collet, K. Garcia-Hernandez, P. Bortolotti, V. Cros, A. Anane, High-efficiency control of spin-wave propagation in ultra-thin yttrium iron garnet by the spin-orbit torque. Applied Physics Letters. 108, 172406 (2016).
    Résumé : We study experimentally with submicrometer spatial resolution the propagation of spin waves in microscopic waveguides based on the nanometer-thick yttriumirongarnet and Pt layers. We demonstrate that by using the spin-orbit torque, the propagation length of the spin waves in such systems can be increased by nearly a factor of 10, which corresponds to the increase in the spin-wave intensity at the output of a 10 μm long transmission line by three orders of magnitude. We also show that, in the regime, where the magnetic damping is completely compensated by the spin-orbit torque, the spin-wave amplification is suppressed by the nonlinear scattering of the coherent spin waves from current-induced excitations.
    Mots-clés : Magnonics.

  • N. Figueiredo-Prestes, J. Zarpellon, H. - F. Jurca, V. Fernandes, J. Varalda, W. - H. Schreiner, D. - H. Mosca, P. F. P. Fichtner, Z. - E. Fabrim, K. Bouzehouane, C. Deranlot, J. - M. George, Stabilization of perpendicular magnetic anisotropy in CeO2 films deposited on Co/Pt multilayers. RSC Advances. 6, 56785-56789 (2016).
    Résumé : Materials with perpendicular magnetic anisotropy (PMA) are of great interest as they have potential applications in high-density non-volatile memories, spin logic devices, and other spintronics applications. To attain perpendicular anisotropy, a number of material systems have been explored as ferromagnetic electrodes. Here, we use (Co/Pt)-multilayered films with PMA covered by a gold spacer-layer to induce the perpendicular magnetization in a ferromagnetic layer of cerium oxide and to control the reversible switching of its magnetization. The origin of the room-temperature ferromagnetism observed in nanocrystalline cerium oxide films remains controversial, but their wide energy band-gap and their transparency to visible light attracts attention for possible applications in magneto-optical devices. A weak magnetic stray field of 40 Oe emanates from the (Co/Pt)-multilayered film and permeates the gold spacer layer. Using a simple micromagnetic model based on the Stoner–Wohlfarth magnetization mechanism, the strength of the magnetic coupling between the ferromagnetic layers is estimated to be 18 μJ m−2. This magnetic coupling, which is almost independent of temperature, is sufficient to promote the reversible switching of perpendicular magnetization states in the field range of a only few hundred Oersteds at room temperature.

  • R. Galceran, I. Fina, J. Cisneros-Fernández, B. Bozzo, C. Frontera, L. López-Mir, H. Deniz, K. - W. Park, B. - G. Park, L. Balcells, X. Martí, T. Jungwirth, B. Martínez, Isothermal anisotropic magnetoresistance in antiferromagnetic metallic IrMn. Scientific Reports. 6, 35471 (2016).
    Résumé : Antiferromagnetic spintronics is an emerging field; antiferromagnets can improve the functionalities of ferromagnets with higher response times, and having the information shielded against external magnetic field.

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