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2016


  • T. Ichinose, H. Naganuma, T. Miyazaki, M. Oogane, Y. Ando, T. Ueno, N. Inami, K. Ono, Effect of annealing on Curie temperature and phase transition in La0.55Sr0.08Mn0.37O3 epitaxial films grown on SrTiO3 (100) substrates by reactive radio frequency magnetron sputtering. Materials Characterization. 118, 37–43 (2016).
    Résumé : Mn-poor LaSrMnO3 (LSMO) epitaxial films were grown on SrTiO3 (100) substrates by radio frequency magnetron sputtering in an argon and oxygen gas mix, and then the samples were annealed in air at various temperatures (Ta). 2 theta-chi X-ray diffraction mapping, nano-beam diffraction analysis through transmission electron microscopy, and electron back scatter diffraction through scanning electron microscopy revealed that the crystal symmetry of the LSMO films changed from monoclinic/orthorhombic to rhombohedral on annealing in air. Curie temperature (TC) of the LSMO films was found to increase with increasing Ta, and become higher than the room temperature at Ta ≥ 861 °C, indicating that the cause of these changes was the filling of oxygen and the transition of the crystal symmetry into rhombohedral.

  • A. S. Jenkins, R. Lebrun, E. Grimaldi, S. Tsunegi, P. Bortolotti, H. Kubota, K. Yakushiji, A. Fukushima, G. De Loubens, O. Klein, S. Yuasa, V. Cros, Spin-torque resonant expulsion of the vortex core for an efficient radiofrequency detection scheme. Nature Nanotechnology. 11, 360–364 (2016).
    Résumé : The spin transfer-induced resonant expulsion of the vortex core in magnetic tunnel junctions forms the basis of a novel integrated high-frequency threshold detector.
    Mots-clés : Spinorbitronics.

  • A. Joly, J. Frougier, G. Baili, M. Alouini, J. - M. George, I. Sagnes, D. Dolfi, (International Society for Optics and Photonics, 2016)p. 97551E–97551E–10.
    Mots-clés : Semiconductors and Spintronics.

  • A. Joly, G. Baili, M. Alouini, J. - M. George, I. Sagnes, A. Sirbu, D. Dolfi, dans 2016 Conference on Lasers and Electro-Optics (cleo) (Ieee, New York, 2016)p. .
    Résumé : We report the first efficient spin injection in a CW optically pumped VECSEL, obtained with phase anisotropy dynamic control. A threshold reduction of 1.5 % and an ellipticity of 75% are obtained at RT. Theoretical and experimental investigation of the polarization eigenstates is presented.

  • I. - P. Krug, H. Doganay, F. Nickel, D. - M. Gottlob, C. - M. Schneider, A. Morelli, D. Preziosi, I. Lindfors-Vrejoiu, R. Laskowski, N. Barrett, Interface-mediated ferroelectric patterning and Mn valency in nano-structured PbTiO3/La0.7Sr0.3MnO3. Journal of Applied Physics. 120, 095304 (2016).
    Résumé : We employed a multitechnique approach using piezo-force response microscopy and photoemission microscopy to investigate a self-organizing polarization domain pattern in PbTiO3/La0.7Sr0.3MnO3 (PTO/LSMO) nanostructures. The polarization is correlated with the nanostructure morphology as well as with the thickness and Mn valence of the LSMO template layer. On the LSMO dots, the PTO is upwards polarized, whereas outside the nanodots, the polarization appears both strain and interface roughness dependent. The results suggest that the electronic structure and strain of the PTO/LSMO interface contribute to determining the internal bias of the ferroelectric layer.

  • S. Lequeux, J. Sampaio, V. Cros, K. Yakushiji, A. Fukushima, R. Matsumoto, H. Kubota, S. Yuasa, J. Grollier, A magnetic synapse: multilevel spin-torque memristor with perpendicular anisotropy. Scientific Reports. 6, 31510 (2016).
    Résumé : Memristors are non-volatile nano-resistors which resistance can be tuned by applied currents or voltages and set to a large number of levels. Thanks to these properties, memristors are ideal building blocks for a number of applications such as multilevel non-volatile memories and artificial nano-synapses, which are the focus of this work.

  • E. Lesne, Y. Fu, S. Oyarzun, J. - C. Rojas-Sanchez, D. C. Vaz, H. Naganuma, G. Sicoli, J. - P. Attané, M. Jamet, E. Jacquet, J. - M. George, A. Barthélémy, H. Jaffrès, A. Fert, M. Bibes, L. Vila, Highly efficient and tunable spin-to-charge conversion through Rashba coupling at oxide interfaces. Nature Materials. 15, 1261–1266 (2016).
    Résumé : The Rashba effect at the LaAlO3/SrTiO3 interface is shown to enable large and gate-tunable spin-to-charge conversion through the inverse Rashba-Edelstein effect.The spin current is injected, through spin pumping, from a NiFe film.
    Mots-clés : Spinorbitronics.

  • P. - M. Levy, A. Fert, The longevity of Jacques Friedel's model of the virtual bound state. Comptes Rendus Physique. 17, 447–454 (2016).
    Résumé : We illustrate the continuing pertinence of Friedel's model of the virtual bound state to describe electron scattering in metals. This model has been applied to such disparate studies as the chirality of spin interactions in metals, and the spin Hall effect caused by scattering from impurities with spin–orbit coupling.

  • X. Li, A. Gloter, D. Preziosi, dans European Microscopy Congress 2016: Proceedings (Wiley‐VCH Verlag GmbH & Co. KGaA, Wiley‐VCH Verlag GmbH & Co. KGaA, 2016)p. .

  • T. Liu, V. Puliafito, F. Montaigne, S. Petit, C. Deranlot, S. Andrieu, O. Ozatay, G. Finocchio, T. Hauet, Reproducible formation of single magnetic bubbles in an array of patterned dots. J. Phys. D: Appl. Phys. 49, 245002 (2016).
    Résumé : The formation conditions of single magnetic bubbles through in-plane field demagnetization are investigated in an array of Co/Ni circular dots by magnetic force microscopy and compared to micromagnetic calculations. We demonstrate high success rates in nucleating stable bubbles. The efficiency of single bubble formation is found to depend not only on the dot size, material thickness and intrinsic material parameters but also on the bubble nucleation path. Experimental phase diagrams and micromagnetic calculations highlight the influence of the starting in-plane field amplitude and dipolar interactions in stabilizing the bubble. The identification of a systematic procedure for controlling nucleation of single bubbles, multidomain states or a uniform state is important from a technological point of view, opening a path toward the realization of reprogrammable magnonic crystals for the control of spin-wave propagation.

  • Y. Liu, L. C. Phillips, R. Mattana, M. Bibes, A. Barthélémy, B. Dkhil, Large reversible caloric effect in FeRh thin films via a dual-stimulus multicaloric cycle. Nat Commun. 7, 11614 (2016).
    Résumé : Giant magnetocaloric materials are promising for solid-state refrigeration, as an alternative to hazardous gases used in conventional cooling devices. A giant magnetocaloric effect was discovered near room temperature in near-equiatomic FeRh alloys some years before the benchmark study in Gd5Si2Ge2 that launched the field. However, FeRh has attracted significantly less interest in cooling applications mainly due to irreversibility in magnetocaloric cycles associated with the large hysteresis of its first-order metamagnetic phase transition. Here we overcome the irreversibility via a dual-stimulus magnetic-electric refrigeration cycle in FeRh thin films via coupling to a ferroelectric BaTiO3 substrate. This experimental realization of a multicaloric cycle yields larger reversible caloric effects than either stimulus alone. While magnetic hysteretic losses appear to be reduced by 96% in dual-stimulus loops, we show that the losses are simply transferred into an elastic cycle, contrary to common belief. Nevertheless, we show that these losses do not necessarily prohibit integration of FeRh in practical refrigeration systems. Our demonstration of a multicaloric refrigeration cycle suggests numerous designs for efficient solid-state cooling applications.

  • B. Madon, D. C. Pham, J. - E. Wegrowe, D. Lacour, M. Hehn, V. Polewczyk, A. Anane, V. Cros, Anomalous and planar Righi-Leduc effects in Ni80Fe20 ferromagnets. Phys. Rev. B. 94, 144423 (2016).
    Résumé : In this paper, we report experimental evidence of anomalous and planar Righi-Leduc effects on $\textbackslashmathrm{NiFe}$. The Righi-Leduc effect is the thermal analog of the Hall effect, in which the electric current is replaced by the heat current and the electric field by the temperature gradient. When the material is ferromagnetic, it is well known that there are two other contributions to the Hall voltage which depend on the orientation of the magnetization. These two extra contributions are called the anomalous Hall effect when the magnetization is out of the plane of the sample and the planar Hall effect when the magnetization is in the plane of the sample. In the same way, an anomalous and a planar Righi-Leduc effects are shown to appear when a transverse temperature gradient is generated by a heat current.

  • P. - J. Metaxas, M. Albert, S. Lequeux, V. Cros, J. Grollier, P. Bortolotti, A. Anane, H. Fangohr, Resonant translational, breathing, and twisting modes of transverse magnetic domain walls pinned at notches. Physical Review B. 93, 054414 (2016).
    Résumé : We study resonant translational, breathing, and twisting modes of transverse magnetic domain walls pinned at notches in ferromagnetic nanostrips. We demonstrate that a mode's sensitivity to notches depends strongly on the mode's characteristics. For example, the frequencies of modes that involve lateral motion of the wall are the most sensitive to changes in the notch intrusion depth, especially at the narrow, more strongly confined end of the domain wall. In contrast, the breathing mode, whose dynamics are concentrated away from the notches is relatively insensitive to changes in the notches' sizes. We also demonstrate a sharp drop in the translational mode's frequency towards zero when approaching depinning which is confirmed, using a harmonic oscillator model, to be consistent with a reduction in the local slope of the notch-induced confining potential at its edge.

  • A. Mizrahi, N. Locatelli, J. Grollier, D. Querlioz, Synchronization of electrically coupled stochastic magnetic oscillators induced by thermal and electrical noise. Phys. Rev. B. 94, 054419 (2016).
    Résumé : Superparamagnetic tunnel junctions are nanostructures that auto-oscillate stochastically under the effect of thermal noise. Recent works showed that despite their stochasticity, such junctions possess a capability to synchronize to subthreshold voltage drives, in a way that can be enhanced or controlled by adding noise. In this work, we investigate a system composed of two electrically coupled junctions, connected in series to a periodic voltage source. We make use of numerical simulations and of an analytical model to demonstrate that both junctions can be phase locked to the drive, in phase or in antiphase. This synchronization phenomenon can be controlled by both thermal and electrical noises, although the two types of noises induce qualitatively different behaviors. Namely, thermal noise can stabilize a regime where one junction is phase locked to the drive voltage while the other is blocked in one state; on the contrary, electrical noise causes the junctions to have highly correlated behaviors and thus cannot induce the latter. These results open the way for the design of superparamagnetic tunnel junctions that can perform computation through synchronization, and which harvest the largest part of their energy consumption from thermal noise.

  • A. Mizrahi, N. Locatelli, R. Lebrun, V. Cros, A. Fukushima, H. Kubota, S. Yuasa, D. Querlioz, J. Grollier, Controlling the phase locking of stochastic magnetic bits for ultra-low power computation. Scientific Reports. 6, 30535 (2016).
    Résumé : When fabricating magnetic memories, one of the main challenges is to maintain the bit stability while downscaling. Indeed, for magnetic volumes of a few thousand nm3, the energy barrier between magnetic configurations becomes comparable to the thermal energy at room temperature.

  • A. Montanaro, S. Mzali, J. - P. Mazellier, O. Bezencenet, C. Larat, S. Molin, L. Morvan, P. Legagneux, D. Dolfi, B. Dlubak, P. Seneor, M. - B. Martin, S. Hofmann, J. Robertson, A. Centeno, A. Zurutuza, Thirty Gigahertz Optoelectronic Mixing in Chemical Vapor Deposited Graphene. Nano Lett. 16, 2988–2993 (2016).
    Résumé : The remarkable properties of graphene, such as broadband optical absorption, high carrier mobility, and short photogenerated carrier lifetime, are particularly attractive for high-frequency optoelectronic devices operating at 1.55 μm telecom wavelength. Moreover, the possibility to transfer graphene on a silicon substrate using a complementary metal-oxide-semiconductor-compatible process opens the ability to integrate electronics and optics on a single cost-effective chip. Here, we report an optoelectronic mixer based on chemical vapor-deposited graphene transferred on an oxidized silicon substrate. Our device consists in a coplanar waveguide that integrates a graphene channel, passivated with an atomic layer-deposited Al2O3 film. With this new structure, 30 GHz optoelectronic mixing in commercially available graphene is demonstrated for the first time. In particular, using a 30 GHz intensity-modulated optical signal and a 29.9 GHz electrical signal, we show frequency downconversion to 100 MHz. These results open promising perspectives in the domain of optoelectronics for radar and radio-communication systems.

  • C. Moreau-Luchaire, C. Moutafis, N. Reyren, J. Sampaio, C. a. F. Vaz, N. V. Horne, K. Bouzehouane, K. Garcia, C. Deranlot, P. Warnicke, P. Wohlhüter, J. - M. George, M. Weigand, J. Raabe, V. Cros, A. Fert, Additive interfacial chiral interaction in multilayers for stabilization of small individual skyrmions at room temperature. Nat Nano. 11, 444–448 (2016).
    Résumé : Facing the ever-growing demand for data storage will most probably require a new paradigm. Nanoscale magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin films in nature. We designed cobalt-based multilayered thin films in which the cobalt layer is sandwiched between two heavy metals and so provides additive interfacial Dzyaloshinskii–Moriya interactions (DMIs), which reach a value close to 2 mJ m–2 in the case of the Ir\textbarCo\textbarPt asymmetric multilayers. Using a magnetization-sensitive scanning X-ray transmission microscopy technique, we imaged small magnetic domains at very low fields in these multilayers. The study of their behaviour in a perpendicular magnetic field allows us to conclude that they are actually magnetic skyrmions stabilized by the large DMI. This discovery of stable sub-100 nm individual skyrmions at room temperature in a technologically relevant material opens the way for device applications in the near future. View full text
    Mots-clés : Spinorbitronics.

  • S. Mzali, A. Montanaro, S. Xavier, B. Servet, J. - P. Mazellier, O. Bezencenet, P. Legagneux, M. Piquemal-Banci, R. Galceran, B. Dlubak, P. Seneor, M. - B. Martin, S. Hofmann, J. Robertson, C. - S. Cojocaru, A. Centeno, A. Zurutuza, Stabilizing a graphene platform toward discrete components. Applied Physics Letters. 109, 253110 (2016).
    Résumé : We report on statistical analysis and consistency of electrical performances of devices based on a large scale passivated graphene platform. More than 500 graphene field effect transistors (GFETs) based on graphene grown by chemical vapor deposition and transferred on 4 in. SiO2/Si substrates were fabricated and tested. We characterized the potential of a two-step encapsulation process including an Al2O3 protection layer to avoid graphene contamination during the lithographic process followed by a final Al2O3 passivation layer subsequent to the GFET fabrication. Devices were investigated for occurrence and reproducibility of conductance minimum related to the Dirac point. While no conductance minimum was observed in unpassivated devices, 75% of the passivated transistors exhibited a clear conductance minimum and low hysteresis. The maximum of the device number distribution corresponds to a residual doping below 5 × 1011 cm−2 (0.023 V/nm). This yield shows that GFETs integrating low-doped graphene and exhi...

  • S. Ouanani, J. Kermorvant, C. Ulysse, M. Malnou, Y. Lemaitre, B. Marcilhac, C. Feuillet-Palma, N. Bergeal, D. - G. Crété, J. Lesueur, High-Tc superconducting quantum interference filters (SQIFs) made by ion irradiation. Supercond. Sci. Technol. 29, 094002 (2016).

  • S. Oyarzún, A. K. Nandy, F. Rortais, J. - C. Rojas-Sánchez, M. - T. Dau, P. Noël, P. Laczkowski, S. Pouget, H. Okuno, L. Vila, C. Vergnaud, C. Beigné, A. Marty, J. - P. Attané, S. Gambarelli, J. - M. George, H. Jaffrès, S. Blügel, M. Jamet, Evidence for spin-to-charge conversion by Rashba coupling in metallic states at the Fe/Ge(111) interface. Nature Communications. 7, 13857 (2016).
    Résumé : Engineering the interaction between spin and charge is important for the creation of spintronics devices. Here, the authors show that the Rashba effect at a single crystalline Fe/Ge(111) interface produces enhanced spin-charge conversion, which could help develop a spin field-effect-transistor.
    Mots-clés : Spinorbitronics.

  • M. Piquemal-Banci, R. Galceran, S. Caneva, M. - B. Martin, R. - S. Weatherup, P. - R. Kidambi, K. Bouzehouane, S. Xavier, A. Anane, F. Petroff, A. Fert, J. Robertson, S. Hofmann, B. Dlubak, P. Seneor, Magnetic tunnel junctions with monolayer hexagonal boron nitride tunnel barriers. Applied Physics Letters. 108, 102404 (2016).
    Résumé : We report on the integration of atomically thin 2D insulating hexagonal boron nitride (h-BN) tunnel barriers into Co/h-BN/Fe magnetic tunnel junctions(MTJs). The h-BN monolayer is directly grown by chemical vapor deposition on Fe. The Conductive Tip Atomic Force Microscopy (CT-AFM) measurements reveal the homogeneity of the tunnel behavior of our h-BN layers. As expected for tunneling, the resistance depends exponentially on the number of h-BN layers. The h-BN monolayer properties are also characterized through integration into complete MTJ devices. A Tunnel Magnetoresistance of up to 6% is observed for a MTJ based on a single atomically thin h-BN layer.

  • C. Rinaldi, J. - C. Rojas-Sanchez, R. N. Wang, Y. Fu, S. Oyarzun, L. Vila, S. Bertoli, M. Asa, L. Baldrati, M. Cantoni, J. - M. George, R. Calarco, A. Fert, R. Bertacco, Evidence for spin to charge conversion in GeTe(111). APL Materials. 4, 032501 (2016).
    Résumé : GeTe has been predicted to be the father compound of a new class of multifunctional materials,ferroelectric Rashba semiconductors, displaying a coupling between spin-dependent k-splitting and ferroelectricity. In this paper, we report on epitaxial Fe/GeTe(111) heterostructures grown by molecular beam epitaxy. Spin-pumping experiments have been performed in a radio-frequency cavity by pumping a spin current from the Fe layer into GeTe at the Fe ferromagnetic resonance and detecting the transverse charge current originated in the slab due to spin-to-charge conversion. Preliminary experiments indicate that a clear spin to charge conversion exists, thus unveiling the potential of GeTe for spin-orbitronics.
    Mots-clés : Spinorbitronics.

  • J. - C. Rojas-Sanchez, P. Laczkowski, J. Sampaio, S. Collin, K. Bouzehouane, N. Reyren, H. Jaffrès, A. Mougin, J. - M. George, Perpendicular magnetization reversal in Pt/[Co/Ni]3/Al multilayers via the spin Hall effect of Pt. Applied Physics Letters. 108, 082406 (2016).
    Résumé : We experimentally investigate the current-induced magnetization reversal in Pt/[Co/Ni]3/Al multilayers combining the anomalous Hall effect and magneto-optical Kerr effect techniques in crossbar geometry. The magnetization reversal occurs through nucleation and propagation of a domain of opposite polarity for a current density of the order of 3 × 1011 A/m2. In these experiments, we demonstrate a full control of each stage: (i) the Ørsted field controls the domain nucleation and (ii) domain-wall propagation occurs by spin torque from the Pt spin Hall effect. This scenario requires an in-plane magnetic field to tune the domain wall center orientation along the current for efficient domain wall propagation. Indeed, as nucleated, domain walls are chiral and Néel-like due to the interfacial Dzyaloshinskii-Moriya interaction.
    Mots-clés : Spinorbitronics.

  • J. - C. Rojas-Sanchez, S. Oyarzun, Y. Fu, A. Marty, C. Vergnaud, S. Gambarelli, L. Vila, M. Jamet, Y. Ohtsubo, A. Taleb-Ibrahimi, P. Le Fevre, F. Bertran, N. Reyren, J. - M. George, A. Fert, Spin to Charge Conversion at Room Temperature by Spin Pumping into a New Type of Topological Insulator: α-Sn Films. Physical Review Letters. 116, 096602 (2016).
    Résumé : We present results on spin to charge current conversion in experiments of resonant spin pumping into the Dirac cone with helical spin polarization of the elemental topological insulator (TI) α-Sn. By angle-resolved photoelectron spectroscopy (ARPES), we first check that the Dirac cone (DC) at the α-Sn (0 0 1) surface subsists after covering Sn with Ag. Then we show that resonant spin pumping at room temperature from Fe through Ag into α-Sn layers induces a lateral charge current that can be ascribed to the inverse Edelstein effect by the DC states. Our observation of an inverse Edelstein effect length much longer than those generally found for Rashba interfaces demonstrates the potential of TIs for the conversion between spin and charge in spintronic devices. By comparing our results with data on the relaxation time of TI free surface states from time-resolved ARPES, we can anticipate the ultimate potential of the TI for spin to charge conversion and the conditions to reach it.
    Mots-clés : Spinorbitronics.

  • M. Romera, P. Talatchian, R. Lebrun, K. - J. Merazzo, P. Bortolotti, L. Vila, J. - D. Costa, R. Ferreira, P. - P. Freitas, M. - C. Cyrille, U. Ebels, V. Cros, J. Grollier, Enhancing the injection locking range of spin torque oscillators through mutual coupling. Applied Physics Letters. 109, 252404 (2016).
    Résumé : We investigate how the ability of the vortex oscillation mode of a spin-torque nano-oscillator to lock to an external microwave signal is modified when it is coupled to another oscillator. We show experimentally that the mutual electrical coupling can lead to locking range enhancements of a factor 1.64. Furthermore, we analyze the evolution of the locking range as a function of the coupling strength through experiments and numerical simulations. By uncovering the mechanisms at stake in the locking range enhancement, our results will be useful for designing spin-torque nano-oscillator arrays with high sensitivities to external microwave stimuli.
    Mots-clés : Spinorbitronics.

  • F. Rortais, S. Oyarzún, F. Bottegoni, J. - C. Rojas-Sanchez, P. Laczkowski, A. Ferrari, C. Vergnaud, C. Ducruet, C. Beigné, N. Reyren, A. Marty, J. - P. Attane, L. Vila, S. Gambarelli, J. Widiez, F. Ciccacci, H. Jaffrès, J. - M. George, M. Jamet, Spin transport in p-type germanium. J. Phys.: Condens. Matter. 28, 165801 (2016).
    Résumé : We report on the spin transport properties in p-doped germanium (Ge-p) using low temperature magnetoresistance measurements, electrical spin injection from a ferromagnetic metal and the spin pumping-inverse spin Hall effect method. Electrical spin injection is carried out using three-terminal measurements and the Hanle effect. In the 2–20 K temperature range, weak antilocalization and the Hanle effect provide the same spin lifetime in the germanium valence band (≈1 ps) in agreement with predicted values and previous optical measurements. These results, combined with dynamical spin injection by spin pumping and the inverse spin Hall effect, demonstrate successful spin accumulation in Ge. We also estimate the spin Hall angle ${{\textbackslashtheta}_{\textbackslashtext{SHE}}}$ in Ge-p (6–$7\textbackslashtimes {{10}ˆ{-4}}$ ) at room temperature, pointing out the essential role of ionized impurities in spin dependent scattering.
    Mots-clés : Semiconductors and Spintronics.

  • F. Rortais, C. Vergnaud, C. Ducruet, C. Beigné, A. Marty, J. - P. Attane, J. Widiez, H. Jaffrès, J. - M. George, M. Jamet, Electrical spin injection in silicon and the role of defects. Phys. Rev. B. 94, 174426 (2016).
    Résumé : Three-terminal devices, where the same ferromagnetic electrode is used for electrical spin injection and detection, is a very easy and powerful tool to probe the spin properties in nonmagnetic materials. For instance, it has been intensively used to study spin injection and detection in silicon. However the interpretation of the magnetoresistance signals observed experimentally is still under debate. In particular, a controversy has been raised about the experimental spin signal which is orders of magnitude larger than the predicted value. Recently, Song et al. [Phys. Rev. Lett. 113, 047205 (2014)] proposed that the magnetoresistance signal measured using the Hanle effect in a three-terminal geometry is due to defects or impurities in the tunnel barrier separating the ferromagnetic electrode from the silicon channel. It has also been supported by the experimental work of Txoperena et al. [Phys. Rev. Lett. 113, 146601 (2014)]. In this study, we perform electrical spin injection/detection measurements using three-terminal devices in different silicon films and study the role of defects. For this purpose, we use the tunneling inelastic spectroscopy to measure the Hanle effect and control the presence of defects in the tunnel barrier. Contrary to previous reports, we demonstrate that defects have no significant contribution to the spin signal. From a comparison with capacitance-voltage measurements in $n$-doped germanium in which interface states contribute to the spin signal, we also conclude on the presence of interface states in silicon.
    Mots-clés : Semiconductors and Spintronics.

  • J. Sampaio, A. V. Khvalkovskiy, M. Kuteifan, M. Cubukcu, D. Apalkov, V. Lomakin, V. Cros, N. Reyren, Disruptive effect of Dzyaloshinskii-Moriya interaction on the magnetic memory cell performance. Applied Physics Letters. 108, 112403 (2016).
    Résumé : In order to increase the thermal stability of a magnetic random access memory cell, materials with high spin-orbit interaction are often introduced in the storage layer. As a side effect, a strong Dzyaloshinskii-Moriya interaction (DMI) may arise in such systems. Here, we investigate the impact of DMI on the magnetic cell performance, using micromagnetic simulations. We find that DMI strongly promotes non-uniform magnetization states and non-uniform switching modes of the magnetic layer. It appears to be detrimental for both the thermal stability of the cell and its switching current, leading to considerable deterioration of the cell performance even for a moderate DMI amplitude.
    Mots-clés : Spinorbitronics.

  • D. Sando, Y. Yang, E. Bousquet, C. Carrétéro, V. Garcia, S. Fusil, D. Dolfi, A. Barthélémy, P. Ghosez, L. Bellaiche, M. Bibes, Large elasto-optic effect and reversible electrochromism in multiferroic BiFeO3. Nature Communications. 7, 10718 (2016).
    Résumé : The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology—a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we apply epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption—reminiscent of piezochromism—which we show can be manipulated by an electric field. This constitutes an electrochromic effect that is reversible, remanent and not driven by defects. These findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses.

  • A. Soumyanarayanan, N. Reyren, A. Fert, C. Panagopoulos, Emergent phenomena induced by spin–orbit coupling at surfaces and interfaces. Nature. 539, 509–517 (2016).

  • M. Sushruth, J. - P. Fried, A. Anane, S. Xavier, C. Deranlot, M. Kostylev, V. Cros, P. - J. Metaxas, Electrical measurement of magnetic-field-impeded polarity switching of a ferromagnetic vortex core. Phys. Rev. B. 94, 100402 (2016).
    Résumé : Vortex core polarity switching in NiFe disks has been evidenced using an all-electrical magnetoresistive rectification scheme. Simulation and experiments yield a consistent rectified signal loss when driving core gyration at high powers. With increasing power, the frequency range over which the loss occurs grows and the resonance downshifts in frequency, consistent with nonlinear core dynamics and periodic core polarity switching induced by the core reaching its critical velocity. Core-polarity-dependent rectification signals enable an independent verification of the switched core polarity. We also demonstrate the ability to impede core polarity switching by displacing the core towards the disk's edge where an increased core stiffness reduces the core velocity.

  • T. Taniguchi, J. Grollier, M. - D. Stiles, dans Spintronics IX (International Society for Optics and Photonics, 2016)vol. 9931p. 99310W.
    Résumé : We propose an experimental scheme to determine the spin-transfer torque efficiency excited by the spin-orbit interaction in ferromagnetic bilayers from the measurement of the longitudinal magnetoresistace. Solving a diffusive spin-transport theory with appropriate boundary conditions gives an analytical formula of the longitudinal charge current density. The longitudinal charge current has a term that is proportional to the square of the spin-transfer torque efficiency and that also depends on the ratio of the film thickness to the spin diffusion length of the ferromagnet. Extracting this contribution from measurements of the longitudinal resistivity as a function of the thickness can give the spin-transfer torque efficiency.

  • B. S. Tao, P. Barate, J. Frougier, P. Renucci, B. Xu, A. Djeffal, H. Jaffrès, J. - M. George, X. Marie, S. Petit-Watelot, S. Mangin, X. F. Han, Z. G. Wang, Y. Lu, Electrical spin injection into GaAs based light emitting diodes using perpendicular magnetic tunnel junction-type spin injector. Applied Physics Letters. 108, 152404 (2016).
    Résumé : Remanent electrical spin injection into an InGaAs/GaAs based quantum well light emitting diode is realized by using a perpendicularly magnetized MgO/CoFeB/Ta/CoFeB/MgO spin injector. We demonstrate that the Ta interlayer plays an important role to establish the perpendicular magnetic anisotropy and the thickness of Ta interlayer determines the type of exchange coupling between the two adjacent CoFeB layers. They are ferromagnetically or antiferromagnetically coupled for a Ta thickness of 0.5 nm or 0.75 nm, respectively. A circular polarized electroluminescence (Pc) of about 10% is obtained at low temperature and at zero magnetic field. The direction of the electrically injected spins is determined only by the orientation of the magnetization of the bottom CoFeB layer which is adjacent to the MgO/GaAs interface. This work proves the critical role of the bottom CoFeB/MgO interface on the spin-injection and paves the way for the electrical control of spin injection via magnetic tunnel junction-type spin injector.
    Mots-clés : Semiconductors and Spintronics.

  • S. Tatay, M. Galbiati, S. Delprat, C. Barraud, K. Bouzehouane, S. Collin, C. Deranlot, E. Jacquet, P. Seneor, R. Mattana, F. Petroff, Self-assembled monolayers based spintronics: from ferromagnetic surface functionalization to spin-dependent transport. Journal of Physics: Condensed Matter. 28, 094010 (2016).
    Résumé : Chemically functionalized surfaces are studied for a wide range of applications going from medicine to electronics. Whereas non-magnetic surfaces have been widely studied, functionalization of magnetic surfaces is much less common and has almost never been used for spintronics applications. In this article we present the functionalization of La 2/3 Sr 1/3 MnO 3 , a ferromagnetic oxide, with self-assembled monolayers for spintronics. La 2/3 Sr 1/3 MnO 3 is the prototypical half-metallic manganite used in spintronics studies. First, we show that La 2/3 Sr 1/3 MnO 3 can be functionalized by alkylphosphonic acid molecules. We then emphasize the use of these functionalized surfaces in spintronics devices such as magnetic tunnel junctions fabricated using a nano-indentation based lithography technique. The observed exponential increase of tunnel resistance as a function of alkyl chain length is a direct proof of the successful connection of molecules to ferromagnetic electrodes. For all alkyl chains studied we obtain stable and robust tunnel magnetoresistance, with effects ranging from a few tens to 10 000%. These results show that functionalized electrodes can be integrated in spintronics devices and open the door to a molecular engineering of spintronics.

  • B. B. Tian, J. L. Wang, S. Fusil, Y. Liu, X. L. Zhao, S. Sun, H. Shen, T. Lin, J. L. Sun, C. G. Duan, M. Bibes, A. Barthélémy, B. Dkhil, V. Garcia, X. J. Meng, J. H. Chu, Tunnel electroresistance through organic ferroelectrics. Nat Commun. 7, 11502 (2016).
    Résumé : Organic electronics is emerging for large-area applications such as photovoltaic cells, rollable displays or electronic paper. Its future development and integration will require a simple, low-power organic memory, that can be written, erased and readout electrically. Here we demonstrate a non-volatile memory in which the ferroelectric polarisation state of an organic tunnel barrier encodes the stored information and sets the readout tunnel current. We use high-sensitivity piezoresponse force microscopy to show that films as thin as one or two layers of ferroelectric poly(vinylidene fluoride) remain switchable with low voltages. Submicron junctions based on these films display tunnel electroresistance reaching 1,000% at room temperature that is driven by ferroelectric switching and explained by electrostatic effects in a direct tunnelling regime. Our findings provide a path to develop low-cost, large-scale arrays of organic ferroelectric tunnel junctions on silicon or flexible substrates.

  • S. Tsunegi, E. Grimaldi, R. Lebrun, H. Kubota, A. S. Jenkins, K. Yakushiji, A. Fukushima, P. Bortolotti, J. Grollier, S. Yuasa, V. Cros, Self-Injection Locking of a Vortex Spin Torque Oscillator by Delayed Feedback. Scientific Reports. 6, 26849 (2016).
    Résumé : The self-synchronization of spin torque oscillators is investigated experimentally by re-injecting its radiofrequency (rf) current after a certain delay time.
    Mots-clés : Spinorbitronics.

  • F. Töpler, A. Hönemann, K. Tauber, D. - V. Fedorov, M. Gradhand, I. Mertig, A. Fert, Nonlocal anomalous Hall effect in ternary alloys based on noble metals. Phys. Rev. B. 94, 140413 (2016).
    Résumé : We present a theoretical study of the nonlocal anomalous Hall effect induced by heavy-metal impurities in dilute magnetic alloys based on noble metals. The results of our first-principles calculations are shown in comparison to those obtained within a model consideration via Matthiessen's rule. Based on the transport properties of the constituent binary alloys, we reveal optimal host-impurity combinations to enhance the phenomenon. In particular, this allows us to explain experimental findings showing a strong effect in Cu-based alloys but a vanishing effect in the case of the Au host.

  • A. Vecchiola, P. Chrétien, S. Delprat, K. Bouzehouane, O. Schneegans, P. Seneor, R. Mattana, S. Tatay, B. Geffroy, Y. Bonnassieux, D. Mencaraglia, F. Houzé, Wide range local resistance imaging on fragile materials by conducting probe atomic force microscopy in intermittent contact mode. Applied Physics Letters. 108, 243101 (2016).
    Résumé : An imaging technique associating a slowly intermittent contact mode of atomic force microscopy (AFM) with a home-made multi-purpose resistance sensing device is presented. It aims at extending the widespread resistance measurements classically operated in contact mode AFM to broaden their application fields to soft materials (molecular electronics, biology) and fragile or weakly anchored nano-objects, for which nanoscale electrical characterization is highly demanded and often proves to be a challenging task in contact mode. Compared with the state of the art concerning less aggressive solutions for AFM electrical imaging, our technique brings a significantly wider range of resistance measurement (over 10 decades) without any manual switching, which is a major advantage for the characterization of materials with large on-sample resistance variations. After describing the basics of the set-up, we report on preliminary investigations focused on academic samples of self-assembled monolayers with various thicknesses as a demonstrator of the imaging capabilities of our instrument, from qualitative and semi-quantitative viewpoints. Then two application examples are presented, regarding an organic photovoltaic thin film and an array of individual vertical carbon nanotubes. Both attest the relevance of the technique for the control and optimization of technological processes.

  • L. G. Vivas, J. Rubín, A. I. Figueroa, F. Bartolomé, L. M. García, C. Deranlot, F. Petroff, L. Ruiz, J. M. González-Calbet, S. Pascarelli, N. - B. Brookes, F. Wilhelm, M. Chorro, A. Rogalev, J. Bartolomé, Perpendicular magnetic anisotropy in granular multilayers of CoPd alloyed nanoparticles. Phys. Rev. B. 93, 174410 (2016).
    Résumé : Co-Pd multilayers obtained by Pd capping of pre-deposited Co nanoparticles on amorphous alumina are systematically studied by means of high-resolution transmission electron microscopy, x-ray diffraction, extended x-ray absorption fine structure, SQUID-based magnetometry, and x-ray magnetic circular dichroism. The films are formed by CoPd alloyed nanoparticles self-organized across the layers, with the interspace between the nanoparticles filled by the non-alloyed Pd metal. The nanoparticles show atomic arrangements compatible with short-range chemical order of $\textbackslashmathrm{L}{1}_{0}$ strucure type. The collective magnetic behavior is that of ferromagnetically coupled particles with perpendicular magnetic anisotropy, irrespective of the amount of deposited Pd. For increasing temperature three magnetic phases are identified: hard ferromagnetic with strong coercive field, soft-ferromagnetic as in an amorphous asperomagnet, and superparamagnetic. Increasing the amount of Pd in the system leads to both magnetic hardness increment and higher transition temperatures. Magnetic total moments of 1.77(4) ${\textbackslashensuremath{\textbackslashmu}}_{B}$ and 0.45(4) ${\textbackslashensuremath{\textbackslashmu}}_{B}$ are found at Co and Pd sites, respectively, where the orbital moment of Co, 0.40(2) ${\textbackslashensuremath{\textbackslashmu}}_{B}$, is high, while that of Pd is negligible. The effective magnetic anisotropy is the largest in the capping metal series (Pd, Pt, W, Cu, Ag, Au), which is attributed to the interparticle interaction between de nanoparticles, in addition to the intraparticle anisotropy arising from hybridization between the $3d\textbackslashchar21{}4d$ bands associated to the Co and Pd chemical arrangement in a $\textbackslashmathrm{L}{1}_{0}$ structure type.

  • L. - G. Vivas, A. I. Figueroa, F. Bartolomé, J. Rubin, L. - M. Garcia, C. Deranlot, F. Petroff, L. Ruiz, J. - M. Gonzalez-Calbet, N. - B. Brookes, F. Wilhelm, A. Rogalev, J. Bartolomé, dans Journal of Magnetism and Magnetic Materials (2016)vol. 400p. 248–252.


  • D. Vodenicarevic, N. Locatelli, J. Grollier, D. Querlioz, (IEEE, 2016)p. 2015–2022.
    Résumé : Coupled oscillator-based networks are an attractive approach for implementing hardware neural networks based on emerging nanotechnologies. However, the readout of the state of a coupled oscillator network is a difficult challenge in hardware implementations, as it necessitates complex signal processing to evaluate the degree of synchronization between oscillators, possibly more complicated than the coupled oscillator network itself. In this work, we focus on a coupled oscillator network particularly adapted to emerging technologies, and evaluate two schemes for reading synchronization patterns that can be readily implemented with basic CMOS circuits. Through simulation of a simple generic coupled oscillator network, we compare the operation of these readout techniques with a previously proposed full statistics evaluation scheme. Our approaches provide results nearly identical to the mathematical method, but also show better resilience to moderate noise, which is a major concern for hardware implementations. These results open the door to widespread realization of hardware coupled oscillator-based neural systems.

  • H. Yu, O. d’Allivy Kelly, V. Cros, R. Bernard, P. Bortolotti, A. Anane, F. Brandl, F. Heimbach, D. Grundler, Approaching soft X-ray wavelengths in nanomagnet-based microwave technology. Nature Communications. 7 (2016), doi:10.1038/ncomms11255.
    Résumé : Seven decades after the discovery of collective spin excitations in microwave-irradiated ferromagnets, there has been a rebirth of magnonics. However, magnetic nanodevices will enable smart GHz-to-THz devices at low power consumption only, if such spin waves (magnons) are generated and manipulated on the sub-100 nm scale. Here we show how magnons with a wavelength of a few 10 nm are exploited by combining the functionality of insulating yttrium iron garnet and nanodisks from different ferromagnets. We demonstrate magnonic devices at wavelengths of 88 nm written/read by conventional coplanar waveguides. Our microwave-to-magnon transducers are reconfigurable and thereby provide additional functionalities. The results pave the way for a multi-functional GHz technology with unprecedented miniaturization exploiting nanoscale wavelengths that are otherwise relevant for soft X-rays. Nanomagnonics integrated with broadband microwave circuitry offer applications that are wide ranging, from nanoscale microwave components to nonlinear data processing, image reconstruction and wave-based logic.
    Mots-clés : Magnonics.

  • S. Zhang, A. Fert, Conversion between spin and charge currents with topological insulators. Phys. Rev. B. 94, 184423 (2016).
    Résumé : Injection of a spin current into the surface or interface states of a topological insulator (TI) induces a charge current (inverse Edelstein effect or IEE) and, inversely, a charge current flowing at the surface or interface states of a TI generates a nonzero spin density (Edelstein Effect or EE) from which a spin current can be ejected into an adjacent layer. The parameters characterizing the efficiency of these conversions between spin and charge currents have been derived in recent experiments. By using a spinor distribution function for a momentum-spin locked TI, we determine a number of spin transport properties of TI-based heterostructure and find that the spin to charge conversion in IEE is controlled by the relaxation of an out-of equilibrium distribution in the TI states while the charge to spin conversion in EE depends on the electron transmission rate at the interface of the TI.
    Mots-clés : Spinorbitronics.

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