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Articles

2020


  • F. Abreu Araujo, M. Riou, J. Torrejon, S. Tsunegi, D. Querlioz, K. Yakushiji, A. Fukushima, H. Kubota, S. Yuasa, M. - D. Stiles, J. Grollier, Role of non-linear data processing on speech recognition task in the framework of reservoir computing. Scientific Reports. 10, 328 (2020).
    Résumé : The reservoir computing neural network architecture is widely used to test hardware systems for neuromorphic computing. One of the preferred tasks for bench-marking such devices is automatic speech recognition. This task requires acoustic transformations from sound waveforms with varying amplitudes to frequency domain maps that can be seen as feature extraction techniques. Depending on the conversion method, these transformations sometimes obscure the contribution of the neuromorphic hardware to the overall speech recognition performance. Here, we quantify and separate the contributions of the acoustic transformations and the neuromorphic hardware to the speech recognition success rate. We show that the non-linearity in the acoustic transformation plays a critical role in feature extraction. We compute the gain in word success rate provided by a reservoir computing device compared to the acoustic transformation only, and show that it is an appropriate bench-mark for comparing different hardware. Finally, we experimentally and numerically quantify the impact of the different acoustic transformations for neuromorphic hardware based on magnetic nano-oscillators.

  • C. Back, V. Cros, H. Ebert, K. Everschor-Sitte, A. Fert, M. Garst, T. Ma, S. Mankovsky, T. L. Monchesky, M. Mostovoy, N. Nagaosa, S. S. P. Parkin, C. Pfleiderer, N. Reyren, A. Rosch, Y. Taguchi, Y. Tokura, K. von Bergmann, J. Zang, The 2020 skyrmionics roadmap. Journal of Physics D: Applied Physics. 53, 363001 (2020).

  • L. Baldrati, C. Schmitt, O. Gomonay, R. Lebrun, R. Ramos, E. Saitoh, J. Sinova, M. Kläui, Efficient Spin Torques in Antiferromagnetic CoO/Pt Quantified by Comparing Field- and Current-Induced Switching. Physical Review Letters. 125, 077201 (2020).
    Résumé : We achieve current-induced switching in collinear insulating antiferromagnetic $\mathrm{CoO}/\mathrm{Pt}$, with fourfold in-plane magnetic anisotropy. This is measured electrically by spin Hall magnetoresistance and confirmed by the magnetic field-induced spin-flop transition of the CoO layer. By applying current pulses and magnetic fields, we quantify the efficiency of the acting current-induced torques and estimate a current-field equivalence ratio of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }\mathrm{T}\text{ }{\mathrm{A}}^{\ensuremath{-}1}\text{ }{\mathrm{m}}^{2}$. The N\'eel vector final state $(\mathbit{n}\ensuremath{\perp}\mathbit{j})$ is in line with a thermomagnetoelastic switching mechanism for a negative magnetoelastic constant of the CoO.

  • Y. Bo, P. Zhang, Y. Zhang, J. Song, S. Li, X. Liu, Spiking dynamic behaviors of NbO2 memristive neurons: A model study. Journal of Applied Physics. 127, 245101 (2020).
    Résumé : Neuromorphic computing based on spikes has broad prospects in energy-efficient computation. Memristive neuron in this study is composed of two volatile memristors that have been shown to exhibit rich biological neuronal dynamics. Here, we show spiking dynamic behaviors of NbO2 memristive neurons by a detailed simulation study. With a DC input voltage, the operation windows of both periodic oscillation and neuron-like action potential spikes are recognized in the resistance–voltage phase diagrams of NbO2 memristive neurons. With a voltage pulse as the input, the periodic oscillation region can be classified into three subregions including the spike-OFF, spike-ON, and meta-spike transition regions. When the memristive neuron operates in the meta-spike transition region, it can regulate the “ON” and “OFF” states of the oscillation circuit by changing the ending time of the input pulse. It implies that both the input signal and the output signal determine the state of the circuit. The demonstration of a phase matching method provides a useful way for controlling “ON” and “OFF” states of the periodic oscillation behavior of the memristive neuron. Moreover, the effect of the circuit parameters on the peak-to-valley amplitude of the output spikes with action potential is investigated. A stable and controllable waveform output can be regulated by changing the capacitance, incorporating a series resistor, and customizing the active memristor. All these results provide a reliable reference for implementing memristive neurons in neuromorphic computing.

  • J. Bréhin, F. Trier, L. M. Vicente Arche, P. Hemme, P. Noël, M. Cosset-Chéneau, J. - P. Attané, L. Vila, A. Sander, Y. Gallais, A. Sacuto, B. Dkhil, V. Garcia, S. Fusil, A. Barthélémy, M. Cazayous, M. Bibes, Switchable two-dimensional electron gas based on ferroelectric Ca:SrTiO3. Physical Review Materials. 4, 041002 (2020).
    Résumé : Ferroelectric materials possess electric dipoles adding up to a macroscopic polarization that is switchable by an electric field. Most ferroelectrics are insulators but some are wide bandgap semiconductors that by doping can be turned into metallic conductors. If doping is restricted to a thin slab near the material surface, the conducting region may harbor a two-dimensional electron gas (2DEG). The 2DEG can then be affected by ferroelectric polarization switching, and may even retain ferroelectric properties coexisting with the conducting behavior. The paper by Br\'ehin $e\phantom{\rule{0}{0ex}}t$ $a\phantom{\rule{0}{0ex}}l$ reports indications of this behavior in a 2DEG at the surface of ferroelectric Ca-SrTiO${}_{3}$.

  • J. - Y. Chauleau, T. Chirac, S. Fusil, V. Garcia, W. Akhtar, J. Tranchida, P. Thibaudeau, I. Gross, C. Blouzon, A. Finco, M. Bibes, B. Dkhil, D. D. Khalyavin, P. Manuel, V. Jacques, N. Jaouen, M. Viret, Electric and antiferromagnetic chiral textures at multiferroic domain walls. Nature Materials. 19, 386-390 (2020).
    Résumé : Chiral electric and magnetic structures are observed at domain walls in thin films of the room-temperature multiferroic BiFeO3.

  • M. W. Daniels, A. Madhavan, P. Talatchian, A. Mizrahi, M. - D. Stiles, Energy-Efficient Stochastic Computing with Superparamagnetic Tunnel Junctions. Physical Review Applied. 13, 034016 (2020).
    Résumé : Most computing schemes that employ superparamagnetic tunnel junctions control them with analog currents, leading to substantial Ohmic losses and requiring digital-to-analog converters. Here the authors forego current control and embed these junctions in digital circuits to produce programmable randomness, which leads to a neural network that can recognize handwritten digits at only 150 nJ per inference. This energy efficiency is made possible by the general insight that, while nanodevices provide useful dynamics for innovative computing, understanding their integration with digital logic systems is crucial to developing viable applications.

  • V. E. Demidov, S. Urazhdin, A. Anane, V. Cros, S. O. Demokritov, Spin–orbit-torque magnonics. Journal of Applied Physics. 127, 170901 (2020).
    Résumé : The field of magnonics, which utilizes propagating spin waves for nanoscale transmission and processing of information, has been significantly advanced by the advent of the spin–orbit torque. The l...
    Mots-clés : Magnonics.

  • D. Di Nuzzo, R. Mizuta, K. Nakanishi, M. - B. Martin, A. I. Aria, R. Weatherup, R. H. Friend, S. Hofmann, J. Alexander-Webber, Graphene-passivated nickel as an efficient hole-injecting electrode for large area organic semiconductor devices. Applied Physics Letters. 116, 163301 (2020).
    Résumé : Efficient injection of charge from metal electrodes into semiconductors is of paramount importance to obtain high performance optoelectronic devices. The quality of the interface between the electr...

  • B. Dieny, I. L. Prejbeanu, K. Garello, P. Gambardella, P. Freitas, R. Lehndorff, W. Raberg, U. Ebels, S. O. Demokritov, J. Akerman, A. Deac, P. Pirro, C. Adelmann, A. Anane, A. V. Chumak, A. Hirohata, S. Mangin, S. O. Valenzuela, M. C. Onbaşlı, M. d’Aquino, G. Prenat, G. Finocchio, L. Lopez-Diaz, R. Chantrell, O. Chubykalo-Fesenko, P. Bortolotti, Opportunities and challenges for spintronics in the microelectronics industry. Nature Electronics. 3, 446 (2020).
    Résumé : This Review Article examines the potential of spintronics in four key areas of application —memories, sensors, microwave devices, and logic devices — and discusses the challenges that need be addressed in order to integrate spintronic materials and functionalities into mainstream microelectronic platforms.

  • M. Drong, T. Fördös, H. Jaffrès, J. P. Jr, K. Postava, J. Pištora, H. - J. Drouhin, Local and mean-field approaches for modeling semiconductor spin-lasers. Journal of Optics. 22, 055001 (2020).

  • A. Dyrdał, J. Barnaś, A. Fert, Spin-Momentum-Locking Inhomogeneities as a Source of Bilinear Magnetoresistance in Topological Insulators. Physical Review Letters. 124, 046802 (2020).
    Résumé : A new model for bilinear magnetoresistance, so called because it is linear in both the electric and the magnetic field, proposes another origin mechanism that accounts for recent contradictory experimental observations.

  • D. M. Evans, V. Garcia, D. Meier, M. Bibes, Domains and domain walls in multiferroics. Physical Sciences Reviews. 5, 20190067 (2020).
    Résumé : Multiferroics are materials combining several ferroic orders, such as ferroelectricity, ferro- (or antiferro-) magnetism, ferroelasticity and ferrotoroidicity. They are of interest both from a fundamental perspective, as they have multiple (coupled) non-linear functional responses providing a veritable myriad of correlated phenomena, and because of the opportunity to apply these functionalities for new device applications. One application is, for instance, in non-volatile memory, which has led to special attention being devoted to ferroelectric and magnetic multiferroics. The vision is to combine the low writing power of ferroelectric information with the easy, non-volatile reading of magnetic information to give a “best of both worlds” computer memory. For this to be realised, the two ferroic orders need to be intimately linked via the magnetoelectric effect. The magnetoelectric coupling – the way polarization and magnetization interact – is manifested by the formation and interactions of domains and domain walls, and so to understand how to engineer future devices one must first understand the interactions of domains and domain walls. In this article, we provide a short introduction to the domain formation in ferroelectrics and ferromagnets, as well as different microscopy techniques that enable the visualization of such domains. We then review the recent research on multiferroic domains and domain walls, including their manipulation and intriguing properties, such as enhanced conductivity and anomalous magnetic order. Finally, we discuss future perspectives concerning the field of multiferroic domain walls and emergent topological structures such as ferroelectric vortices and skyrmions.

  • K. Fallon, S. Hughes, K. Zeissler, W. Legrand, F. Ajejas, D. Maccariello, S. McFadzean, W. Smith, D. McGrouther, S. Collin, N. Reyren, V. Cros, C. H. Marrows, S. McVitie, Controlled Individual Skyrmion Nucleation at Artificial Defects Formed by Ion Irradiation. Small. 16, 1907450 (2020).
    Résumé : Nanoscale artificial defects in magnetic multilayer films with perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii–Moriya interaction (DMI), made in a controllable manner using a focused ion b...

  • A. E. Giba, X. Gao, M. Stoffel, X. Devaux, B. Xu, X. Marie, P. Renucci, H. Jaffrès, J. - M. George, G. Cong, Z. Wang, H. Rinnert, Y. Lu, Spin Injection and Relaxation in (In,Ga)As/GaAs Quantum-Dot Spin Light-Emitting Diodes at Zero Magnetic Field. Physical Review Applied. 14, 034017 (2020).
    Résumé : We report on efficient spin injection in p-doped $(\mathrm{In},\mathrm{Ga})\mathrm{As}/\mathrm{Ga}\mathrm{As}$ quantum-dot (QD) spin light-emitting diodes (spin LEDs) under zero applied magnetic field. A high degree of electroluminescence circular polarization (${P}_{c}$) \ensuremath{\sim}19% is measured in remanence up to 100 K. This result is obtained thanks to the combination of a perpendicularly magnetized $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}/\mathrm{Mg}\mathrm{O}$ spin injector allowing efficient spin injection and an appropriate p-doped $(\mathrm{In},\mathrm{Ga})\mathrm{As}/\mathrm{Ga}\mathrm{As}$ QD layer in the active region. By analyzing the bias and temperature dependence of the electroluminescence circular polarization, we evidence a two-step spin-relaxation process. The first step occurs when electrons tunnel through the $\mathrm{Mg}\mathrm{O}$ barrier and travel across the $\mathrm{Ga}\mathrm{As}$ depletion layer. The spin relaxation is dominated by the Dyakonov-Perel mechanism related to the kinetic energy of electrons, which is characterized by a bias-dependent ${P}_{c}$. The second step occurs when electrons are captured into QDs prior to their radiative recombination with holes. The temperature dependence of ${P}_{c}$ reflects the temperature-induced modification of the QD doping, together with the variation of the ratio between the charge-carrier lifetime and the spin-relaxation time inside the QDs. The understanding of these spin-relaxation mechanisms is essential to improve the performance of spin LEDs for future spin optoelectronic applications at room temperature under zero applied magnetic field.

  • O. Gladii, M. Collet, Y. Henry, J. - V. Kim, A. Anane, M. Bailleul, Determining Key Spin-Orbitronic Parameters via Propagating Spin Waves. Physical Review Applied. 13, 014016 (2020).
    Résumé : We characterize spin-wave propagation and its modification by an electrical current in permalloy($\mathrm{Py}$)/$\mathrm{Pt}$ bilayers with $\mathrm{Py}$ thickness between 4 and 20 nm. First, we analyze the frequency nonreciprocity of surface spin waves and extract from it the interfacial Dzyaloshinskii-Moriya interaction constant ${D}_{s}$ accounting for an additional contribution due to asymmetric surface anisotropies. Second, we measure the spin-wave relaxation rate and deduce from it the $\mathrm{Py}$/$\mathrm{Pt}$ spin-mixing conductance ${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$. Last, applying a dc electrical current, we extract the spin Hall conductivity ${\ensuremath{\sigma}}_{\mathrm{SH}}$ from the change of spin-wave relaxation rate due to the spin Hall spin-transfer torque. We obtain a consistent picture of the spin-wave propagation data for different film thicknesses using a single set of parameters ${D}_{s}=0.25$ pJ/m, ${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}=3.2\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.2em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$ and ${\ensuremath{\sigma}}_{\mathrm{SH}}=4\ifmmode\times\else\texttimes\fi{}{10}^{5}$ S/m.
    Mots-clés : Magnonics.

  • F. Godel, V. Zatko, C. Carrétéro, A. Sander, M. Galbiati, A. Vecchiola, P. Brus, O. Bezencenet, B. Servet, M. - B. Martin, B. Dlubak, P. Seneor, WS2 2D Semiconductor Down to Monolayers by Pulsed-Laser Deposition for Large-Scale Integration in Electronics and Spintronics Circuits. ACS Applied Nano Materials. 3, 7908 (2020).
    Résumé : We report on the achievement of a large-scale tungsten disulfide (WS2) 2D semiconducting platform derived by pulsed-laser deposition (PLD) on both insulating substrates (SrTiO3), as required for in-plane semiconductor circuit definition, and ferromagnetic spin sources (Ni), as required for spintronics applications. We show thickness and phase control, with highly homogeneous wafer-scale monolayers observed under certain conditions, as demonstrated by X-ray photoelectron spectroscopy and Raman spectroscopy mappings. Interestingly, growth appears to be dependent on the substrate selection, with a dramatically increased growth rate on Ni substrates. We show that this 2D-semiconductor integration protocol preserves the interface integrity. Illustratively, the WS2/Ni electrode is shown to be resistant to oxidation (even after extended exposure to ambient conditions) and to present tunneling characteristics once integrated into a complete vertical device. Overall, these experiments show that the presented PLD approach used here for WS2 growth is versatile and has a strong potential to accelerate the integration and evaluation of large-scale 2D-semiconductor platforms in electronics and spintronics circuits.

  • J. Grollier, D. Querlioz, K. Y. Camsari, K. Everschor-Sitte, S. Fukami, M. D. Stiles, Neuromorphic spintronics. Nature Electronics. 3, 360 (2020).
    Résumé : This Review Article examines the development of spintronic devices for neuromorphic computing, exploring how magnetic tunnel junctions and magnetic textures can act as artificial neurons and synapses, as well as considering the challenges that exist in scaling up current systems.

  • T. Guillet, C. Zucchetti, Q. Barbedienne, A. Marty, G. Isella, L. Cagnon, C. Vergnaud, H. Jaffrès, N. Reyren, J. - M. George, A. Fert, M. Jamet, Observation of Large Unidirectional Rashba Magnetoresistance in Ge(111). Physical Review Letters. 124, 027201 (2020).
    Résumé : A new experiment shows that the semiconductor germanium exhibits unidirectional magnetoresistance, which had previously only been seen in more exotic materials

  • A. Haykal, J. Fischer, W. Akhtar, J. - Y. Chauleau, D. Sando, A. Finco, F. Godel, Y. A. Birkhölzer, C. Carrétéro, N. Jaouen, M. Bibes, M. Viret, S. Fusil, V. Jacques, V. Garcia, Antiferromagnetic textures in BiFeO3 controlled by strain and electric field. Nature Communications. 11, 1704 (2020).
    Résumé : Tailoring antiferromagnetic domains is critical for the development of low-dissipative spintronic and magnonic devices. Here the authors demonstrate the control of antiferromagnetic spin textures in multiferroic bismuth ferrite thin films using strain and electric fields.

  • M. - A. Husanu, L. Vistoli, C. Verdi, A. Sander, V. Garcia, J. Rault, F. Bisti, L. L. Lev, T. Schmitt, F. Giustino, A. S. Mishchenko, M. Bibes, V. N. Strocov, Electron-polaron dichotomy of charge carriers in perovskite oxides. Communications Physics. 3, 62 (2020).
    Résumé : The underlying mechanisms of the metal-insulator transition in correlated oxides are a rich source of interesting physics and a topic of long-standing investigation. Here, the authors use angle-resolved photoelectron spectroscopy to investigate changes in charge carrier properties and electron-phonon interactions as a function of Ce-doping across the metal-insulator transition in CaMnO3.

  • A. Jouan, G. Singh, E. Lesne, D. C. Vaz, M. Bibes, A. Barthélémy, C. Ulysse, D. Stornaiuolo, M. Salluzzo, S. Hurand, J. Lesueur, C. Feuillet-Palma, N. Bergeal, Quantized conductance in a one-dimensional ballistic oxide nanodevice. Nature Electronics. 3, 201 (2020).
    Résumé : A quantum point contact formed in the two-dimensional electron gas of a LaAlO3/SrTiO3 interface exhibits quantized conductance due to ballistic transport in a controllable number of one-dimensional conducting channels.

  • W. Legrand, D. Maccariello, F. Ajejas, S. Collin, A. Vecchiola, K. Bouzehouane, N. Reyren, V. Cros, A. Fert, Room-temperature stabilization of antiferromagnetic skyrmions in synthetic antiferromagnets. Nature Materials. 19, 34-42 (2020).
    Résumé : Antiferromagnetic skyrmions—which have distinct advantages over skyrmions found in other magnetic systems—are observed at room temperature in synthetic antiferromagnets. These results hold promise for low-power spintronic devices.

  • X. Li, Q. Zhu, L. Vistoli, A. Barthélémy, M. Bibes, S. Fusil, V. Garcia, A. Gloter, In‐Depth Atomic Mapping of Polarization Switching in a Ferroelectric Field‐Effect Transistor. Advanced Materials Interfaces. 7, 2000601 (2020).
    Résumé : Mott transistor combining ferroelectric BiFeO3 and charge‐transfer insulator (Ca,Ce)MnO3 are investigated. Scanning probe microscopy, transport measurements, and scanning transmission electron micros...

  • J. Liang, W. Wang, H. Du, A. Hallal, K. Garcia, M. Chshiev, A. Fert, H. Yang, Very large Dzyaloshinskii-Moriya interaction in two-dimensional Janus manganese dichalcogenides and its application to realize skyrmion states. Physical Review B. 101, 184401 (2020).
    Résumé : The intense recent research on skyrmions has focused on multilayers of classical magnetic materials, such as Co, CoFeB, or Fe. Here, the authors explore skyrmions in van der Waals bi-dimensional magnets, a new type of magnetic material in the broad family of 2D materials that contains graphene. Using $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ and Monte Carlo calculations, they demonstrate that skyrmions should exist in the so-called Janus transition metal dichalcogenides. The temperature and magnetic field range for observation of skyrmions are also predicted, as a guide for experimental exploration.

  • D. Marković, J. Grollier, Quantum neuromorphic computing. Applied Physics Letters. 117, 150501 (2020).
    Résumé : Quantum neuromorphic computing physically implements neural networks in brain-inspired quantum hardware to speed up their computation. In this perspective article, we show that this emerging paradigm could make the best use of the existing and near future intermediate size quantum computers. Some approaches are based on parametrized quantum circuits and use neural network-inspired algorithms to train them. Other approaches, closer to classical neuromorphic computing, take advantage of the physical properties of quantum oscillator assemblies to mimic neurons and synapses to compute. We discuss the different implementations of quantum neuromorphic networks with digital and analog circuits, highlight their respective advantages, and review exciting recent experimental results.

  • D. Marković, N. Leroux, A. Mizrahi, J. Trastoy, V. Cros, P. Bortolotti, L. Martins, A. Jenkins, R. Ferreira, J. Grollier, Detection of the Microwave Emission from a Spin-Torque Oscillator by a Spin Diode. Physical Review Applied. 13, 044050 (2020).
    Résumé : Magnetic tunnel junctions are nanoscale spintronic devices with microwave-generation and -detection capabilities. Here, we use the rectification effect called a ``spin diode'' in a magnetic tunnel junction to wirelessly detect the microwave emission of another junction in the auto-oscillatory regime. We show that the rectified spin-diode voltage measured at the receiving junction end can be reconstructed from the independently measured auto-oscillation and spin-diode spectra in each junction. Finally, we adapt the auto-oscillator model to the case of the spin-torque oscillator and the spin diode and we show that it accurately reproduces the experimentally observed features. These results will be useful for the design of circuits and chips based on spintronic nanodevices communicating through microwaves.

  • H. Naganuma, V. Zatko, M. Galbiati, F. Godel, A. Sander, C. Carrétéro, O. Bezencenet, N. Reyren, M. - B. Martin, B. Dlubak, P. Seneor, A perpendicular graphene/ferromagnet electrode for spintronics. Applied Physics Letters. 116, 173101 (2020).
    Résumé : We report on the large-scale integration of graphene layers over a FePd perpendicular magnetic anisotropy (PMA) platform, targeting further downscaling of spin circuits. An L10 FePd ordered alloy s...

  • Y. Nahas, S. Prokhorenko, J. Fischer, B. Xu, C. Carrétéro, S. Prosandeev, M. Bibes, S. Fusil, B. Dkhil, V. Garcia, L. Bellaiche, Inverse transition of labyrinthine domain patterns in ferroelectric thin films. Nature. 577, 47-51 (2020).
    Résumé : The labyrinthine domain patterns formed in ultrathin films of ferroelectric oxides by subcritical quenching undergo an inverse phase transition to the less-symmetric parallel-stripe domain structure upon increasing temperature.

  • S. K. Nandi, S. K. Nath, A. E. El-Helou, S. Li, T. Ratcliff, M. Uenuma, P. E. Raad, R. G. Elliman, Electric Field- and Current-Induced Electroforming Modes in NbOx. ACS Applied Materials & Interfaces. 12, 8422 (2020).
    Résumé : Electroforming is used to initiate the memristive response in metal/oxide/metal devices by creating a filamentary conduction path in the oxide film. Here, we use a simple photoresist-based detection technique to map the spatial distribution of conductive filaments formed in Nb/NbOx/Pt devices, and correlate these with current–voltage characteristics and in situ thermoreflectance measurements to identify distinct modes of electroforming in low- and high-conductivity NbOx films. In low-conductivity films, the filaments are randomly distributed within the oxide film, consistent with a field-induced weakest-link mechanism, while in high-conductivity films they are concentrated in the center of the film. In the latter case, the current–voltage characteristics and in situ thermoreflectance imaging show that electroforming is associated with current bifurcation into regions of low and high current density. This is supported by finite element modeling of the current distribution and shown to be consistent with predictions of a simple core–shell model of the current distribution. These results clearly demonstrate two distinct modes of electroforming in the same material system and show that the dominant mode depends on the conductivity of the film, with field-induced electroforming dominant in low-conductivity films and current bifurcation-induced electroforming dominant in high-conductivity films.

  • S. K. Nath, S. K. Nandi, A. El-Helou, X. Liu, S. Li, T. Ratcliff, P. E. Raad, R. G. Elliman, Schottky-Barrier-Induced Asymmetry in the Negative-Differential-Resistance Response of Nb/NbOx/Pt Cross-Point Devices. Physical Review Applied. 13, 064024 (2020).
    Résumé : The negative-differential-resistance (NDR) response of $\mathrm{Nb}$/${\mathrm{Nb}\mathrm{O}}_{x}$/$\mathrm{Pt}$ cross-point devices is shown to have a polarity dependence due to the effect of the metal-oxide Schottky barriers on the contact resistance. Three distinct responses are observed under opposite polarity testing: bipolar S-type NDR, bipolar snapback NDR, and combined S-type and snapback NDR, depending on the stoichiometry of the oxide film and device area. In situ thermoreflectance imaging is used to show that these NDR responses are associated with strong current localization, thereby justifying the use of a previously developed two-zone, core-shell thermal model of the device. The observed polarity-dependent NDR responses, and their dependence on stoichiometry and area, are then explained by extending this model to include the effect of the polarity-dependent contact resistance. This study provides an improved understanding of the NDR response of metal-oxide-metal structures and informs the engineering of devices for neuromorphic computing and nonvolatile memory applications.

  • P. Noël, M. Cosset-Cheneau, V. Haspot, V. Maurel, C. Lombard, M. Bibes, A. Barthelemy, L. Vila, J. - P. Attané, Negligible thermal contributions to the spin pumping signal in ferromagnetic metal–platinum bilayers. Journal of Applied Physics. 127, 163907 (2020).
    Résumé : Spin pumping by ferromagnetic resonance is one of the most common techniques to determine spin Hall angles, Edelstein lengths, or spin diffusion lengths of a large variety of materials. In recent y...

  • P. Noël, F. Trier, L. M. Vicente Arche, J. Bréhin, D. - C. Vaz, V. Garcia, S. Fusil, A. Barthélémy, L. Vila, M. Bibes, J. - P. Attané, Non-volatile electric control of spin–charge conversion in a SrTiO3 Rashba system. Nature. 580, 483 (2020).
    Résumé : The polarization direction of a ferroelectric-like state can be used to control the conversion of spin currents into charge currents at the surface of strontium titanate, a non-magnetic oxide.

  • X. Palermo, N. Reyren, S. Mesoraca, A. V. Samokhvalov, S. Collin, F. Godel, A. Sander, K. Bouzehouane, J. Santamaria, V. Cros, A. I. Buzdin, J. - E. Villegas, Tailored Flux Pinning in Superconductor-Ferromagnet Multilayers with Engineered Magnetic Domain Morphology From Stripes to Skyrmions. Physical Review Applied. 13, 014043 (2020).
    Résumé : Superconductor-ferromagnet (S/F) hybrid systems show interesting magnetotransport behaviors that result from the transfer of properties between both constituents. For instance, magnetic memory can be transferred from the F into the S through the pinning of superconducting vortices by the ferromagnetic textures. The ability to tailor this type of induced behavior is important to broaden its range of application. Here we show that engineering the F magnetization reversal allows the tuning of the strength of the vortex pinning (and memory) effects, as well as the field range in which they appear. This is done by using magnetic multilayers in which Co thin films are combined with different heavy metals ($\mathrm{Ru}$, $\mathrm{Ir}$, $\mathrm{Pt}$). By choosing the materials, thicknesses, and stacking order of the layers, we can design the characteristic domain size and morphology, from out-of-plane magnetized stripe domains to much smaller magnetic skyrmions. These changes strongly affect the magnetotransport properties. The underlying mechanisms are identified by comparing the experimental results to a magnetic pinning model.

  • D. Perconte, K. Seurre, V. Humbert, C. Ulysse, A. Sander, J. Trastoy, V. Zatko, F. Godel, P. R. Kidambi, S. Hofmann, X. P. Zhang, D. Bercioux, F. S. Bergeret, B. Dlubak, P. Seneor, J. - E. Villegas, Long-Range Propagation and Interference of d-Wave Superconducting Pairs in Graphene. Physical Review Letters. 125, 087002 (2020).
    Résumé : Recent experiments have shown that proximity with high-temperature superconductors induces unconventional superconducting correlations in graphene. Here, we demonstrate that those correlations propagate hundreds of nanometers, allowing for the unique observation of $d$-wave Andreev-pair interferences in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$-graphene devices that behave as a Fabry-Perot cavity. The interferences show as a series of pronounced conductance oscillations analogous to those originally predicted by de Gennes--Saint-James for conventional metal-superconductor junctions. The present demonstration is pivotal to the study of exotic directional effects expected for nodal superconductivity in Dirac materials.

  • T. Peronnin, D. Marković, Q. Ficheux, B. Huard, Sequential Dispersive Measurement of a Superconducting Qubit. Physical Review Letters. 124, 180502 (2020).
    Résumé : We present a superconducting device that realizes the sequential measurement of a transmon qubit. The device disables common limitations of dispersive readout such as Purcell effect or transients in the cavity mode by turning on and off the coupling to the measurement channel on demand. The qubit measurement begins by loading a readout resonator that is coupled to the qubit. After an optimal interaction time with negligible loss, a microwave pump releases the content of the readout mode by upconversion into a measurement line in a characteristic time as low as 10 ns, which is 400 times shorter than the lifetime of the readout resonator. A direct measurement of the released field quadratures demonstrates a readout fidelity of 97.5% in a total measurement time of 220 ns. The Wigner tomography of the readout mode allows us to characterize the non-Gaussian nature of the readout mode and its dynamics.

  • V. Rouco, R. El Hage, A. Sander, J. Grandal, K. Seurre, X. Palermo, J. Briatico, S. Collin, J. Trastoy, K. Bouzehouane, A. I. Buzdin, G. Singh, N. Bergeal, C. Feuillet-Palma, J. Lesueur, C. Leon, M. Varela, J. Santamaría, J. - E. Villegas, Quasiparticle tunnel electroresistance in superconducting junctions. Nature Communications. 11, 658 (2020).
    Résumé : The non-volatile switching of tunnel electroresistance in ferroelectric junctions provides the basis for memory and neuromorphic computing devices. Rouco et al. show tunnel electroresistance in superconductor-based junctions that arises from a redox rather than ferroelectric mechanism and is enhanced by superconductivity.

  • I. V. Rozhansky, D. Q. To, H. Jaffrès, H. - J. Drouhin, Chirality-induced tunneling asymmetry at a semiconductor interface. Physical Review B. 102, 045428 (2020).
    Résumé : The chiral tunneling asymmetry for the transmission through a semiconductor interface with spin-orbit coupling (SOC) and antiparallel magnetization is presented from the perspective of the transfer Hamiltonian approach. We explicitly show that the expression for the tunneling rate contains terms which are odd in the momentum along the interface thus contributing to the skew tunneling and the transverse current along the interface. These terms contain the scalar chirality that is the mixed vector product of the effective SOC and exchange fields acting on the tunneling electron or hole. The crucial role of the evanescent states in the tunneling asymmetry is revealed. It is shown that the chiral Hall effect also results in a spin injection across the interface. We finally come up with a more general view on the chiral tunneling as a universal phenomenon expected in various systems.

  • D. Sando, M. Han, V. Govinden, O. Paull, F. Appert, C. Carrétéro, J. Fischer, A. Barthélémy, M. Bibes, V. Garcia, S. Fusil, B. Dkhil, J. Juraszek, Y. Zhu, X. Ma, V. Nagarajan, Interfacial Strain Gradients Control Nanoscale Domain Morphology in Epitaxial BiFeO3 Multiferroic Films. Advanced Functional Materials. 30, 2000343 (2020).
    Résumé : The domain morphology in multiferroic BiFeO3 films is shown to depend on the strain profile across the film–substrate interface. Strong interfacial strain gradients induce mosaic domains, while unifo...

  • I. Saniour, M. Geahel, J. Briatico, C. - J. van der Beek, G. Willoquet, L. Jourdain, B. Baudouy, G. Authelet, J. C. Ginefri, L. Darrasse, M. Poirier-Quinot, Versatile cryogen-free cryostat for the electromagnetic characterization of superconducting radiofrequency coils. EPJ Techniques and Instrumentation. 7, 3 (2020).
    Résumé : The use of high temperature superconducting (HTS) radio frequency (RF) coils in Magnetic Resonance Imaging (MRI) greatly improves the signal-to-noise ratio (SNR) in many biomedical applications and particularly in micro-MRI. However, a detailed understanding of the electrical behavior of HTS coils is important in order to optimize their performance through MR experiments. This paper presents a simple and versatile cryogen-free cryostat designed to characterize the RF properties of HTS coils prior to their use in MRI. The cryostat can be used at temperatures from 50 K to 300 K, with a control precision of approximately 3 mK at 70 K, and can measure the RF electrical power transmitted to an HTS coil over a range from 1 μW to 10 W. The quality factor and resonance frequency of the tested HTS coil are determined as a function of the temperature and the power it dissipates. This cryostat also permits the dynamic adjustment of the coil resonance frequency via temperature control. Finally, this study demonstrates that the HTS coil takes less than 12 μs to transit from the superconducting to the dissipative state, which is compatible with MRI requirements.

  • D. Sanz-Hernández, A. Hierro-Rodriguez, C. Donnelly, J. Pablo-Navarro, A. Sorrentino, E. Pereiro, C. Magén, S. McVitie, J. - M. De Teresa, S. Ferrer, P. Fischer, A. Fernández-Pacheco, Artificial Double-Helix for Geometrical Control of Magnetic Chirality. ACS Nano. 14, 8084 (2020).
    Résumé : Chirality plays a major role in nature, from particle physics to DNA, and its control is much sought-after due to the scientific and technological opportunities it unlocks. For magnetic materials, chiral interactions between spins promote the formation of sophisticated swirling magnetic states such as skyrmions, with rich topological properties and great potential for future technologies. Currently, chiral magnetism requires either a restricted group of natural materials or synthetic thin-film systems that exploit interfacial effects. Here, using state-of-the-art nanofabrication and magnetic X-ray microscopy, we demonstrate the imprinting of complex chiral spin states via three-dimensional geometric effects at the nanoscale. By balancing dipolar and exchange interactions in an artificial ferromagnetic double-helix nanostructure, we create magnetic domains and domain walls with a well-defined spin chirality, determined solely by the chiral geometry. We further demonstrate the ability to create confined 3D spin textures and topological defects by locally interfacing geometries of opposite chirality. The ability to create chiral spin textures via 3D nanopatterning alone enables exquisite control over the properties and location of complex topological magnetic states, of great importance for the development of future metamaterials and devices in which chirality provides enhanced functionality.

  • F. Schreiber, L. Baldrati, C. Schmitt, R. Ramos, E. Saitoh, R. Lebrun, M. Kläui, Concurrent magneto-optical imaging and magneto-transport readout of electrical switching of insulating antiferromagnetic thin films. Applied Physics Letters. 117, 082401 (2020).
    Résumé : We demonstrate stable and reversible current induced switching of large-area (>100 μm2) antiferromagnetic domains in NiO/Pt by performing concurrent transport and magneto-optical imaging measurements in an adapted Kerr microscope. By correlating the magnetic images of the antiferromagnetic domain changes and magneto-transport signal response in these current-induced switching experiments, we disentangle magnetic and non-magnetic contributions to the transport signal. Our table-top approach establishes a robust procedure to subtract the non-magnetic contributions in the transport signal and extract the spin-Hall magnetoresistance response associated with the switching of the antiferromagnetic domains, enabling one to deduce details of the antiferromagnetic switching from simple transport measurements.

  • P. Talatchian, M. Romera, F. Abreu Araujo, P. Bortolotti, V. Cros, D. Vodenicarevic, N. Locatelli, D. Querlioz, J. Grollier, Designing Large Arrays of Interacting Spin-Torque Nano-Oscillators for Microwave Information Processing. Physical Review Applied. 13, 024073 (2020).
    Résumé : Arrays of spin-torque nano-oscillators are promising for broadband microwave signal detection and processing, as well as for neuromorphic computing. In many of these applications, the oscillators should be engineered to have equally spaced frequencies and equal sensitivity to microwave inputs. Here we design spin-torque nano-oscillator arrays with these rules and estimate their optimum size for a given sensitivity, as well as the frequency range that they cover. For this purpose, we explore analytically and numerically conditions to obtain vortex spin-torque nano-oscillators with equally spaced gyrotropic oscillation frequencies and having all similar synchronization bandwidths to input microwave signals. We show that arrays of hundreds of oscillators covering ranges of several hundred MHz can be built taking into account nanofabrication constraints.

  • L. Thomas, D. Guérin, B. Quinard, E. Jacquet, R. Mattana, P. Seneor, D. Vuillaume, T. Mélin, S. Lenfant, Conductance switching at the nanoscale of diarylethene derivative self-assembled monolayers on La0.7Sr0.3MnO3. Nanoscale. 12, 8268 (2020).
    Résumé : We report on the phosphonic acid route for the grafting of functional molecules, optical switch (dithienylethene diphosphonic acid, DDA), on La0.7Sr0.3MnO3 (LSMO). Compact self-assembled monolayers (SAMs) of DDA are formed on LSMO as studied by topographic atomic force microscopy (AFM), ellipsometry, water contact angle measurements and X-ray photoemission spectroscopy (XPS). The conducting AFM measurements show that the electrical conductance of LSMO/DDA is about 3 decades below that of a bare LSMO substrate. Moreover, the presence of the DDA SAM suppresses the known conductance switching of the LSMO substrate that is induced by mechanical and/or bias constraints during C-AFM measurements. A partial light-induced conductance switching between the open and closed forms of the DDA is observed for the LSMO/DDA/C-AFM tip molecular junctions (closed/open conductance ratio of about 8). We show that, in the case of long-time exposure to UV light, this feature can be masked by a non-reversible decrease (a factor of about 15) of the conductance of the LSMO electrode.

  • J. Trastoy, A. Camjayi, J. del Valle, Y. Kalcheim, J. - P. Crocombette, D. A. Gilbert, J. A. Borchers, J. E. Villegas, D. Ravelosona, M. J. Rozenberg, I. K. Schuller, Magnetic field frustration of the metal-insulator transition in V2O3. Physical Review B. 101, 245109 (2020).
    Résumé : Despite decades of efforts, the origin of metal-insulator transitions (MITs) in strongly correlated materials remains one of the main long-standing problems in condensed-matter physics. An archetypal example is ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$, which undergoes simultaneous electronic, structural, and magnetic phase transitions. This remarkable feature highlights the many degrees of freedom at play in this material. In this work, acting solely on the magnetic degree of freedom, we reveal an anomalous feature in the electronic transport of ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$: On cooling, the magnetoresistance changes from positive to negative values well above the MIT temperature, and shows divergent behavior at the transition. The effects are attributed to the magnetic field quenching antiferromagnetic fluctuations above the N\'eel temperature ${T}_{N}$, and preventing long-range antiferromagnetic ordering below ${T}_{N}$. In both cases, suppressing the antiferromagnetic ordering prevents the opening of the incipient electronic gap. This interpretation is supported by Hubbard model calculations which fully reproduce the experimental behavior. Our study sheds light on this classic problem providing a clear and physical interpretation of the nature of the metal-insulator transition.

  • F. Trier, D. - C. Vaz, P. Bruneel, P. Noël, A. Fert, L. Vila, J. - P. Attané, A. Barthélémy, M. Gabay, H. Jaffrès, M. Bibes, Electric-Field Control of Spin Current Generation and Detection in Ferromagnet-Free SrTiO3-Based Nanodevices. Nano Letters. 20, 395 (2020).
    Résumé : Spintronics entails the generation, transport, manipulation and detection of spin currents, usually in hybrid architectures comprising interfaces whose impact on performance is detrimental. In addition, how spins are generated and detected is generally material specific and determined by the electronic structure. Here, we demonstrate spin current generation, transport and electrical detection, all within a single non-magnetic material system: a SrTiO3 two-dimensional electron gas (2DEG) with Rashba spin-orbit coupling. We show that the spin current is generated from a charge current by the 2D spin Hall effect, transported through a channel and reconverted into a charge current by the inverse 2D spin Hall effect. Furthermore, by adjusting the Fermi energy with a gate voltage we tune the generated and detected spin polarization and relate it to the complex multiorbital band structure of the 2DEG. We discuss the leading mechanisms of the spin-charge interconversion processes and argue for the potential of quantum oxide materials for future all-electrical low-power spin-based logic.

  • D. C. Vaz, F. Trier, A. Dyrdał, A. Johansson, K. Garcia, A. Barthélémy, I. Mertig, J. Barnaś, A. Fert, M. Bibes, Determining the Rashba parameter from the bilinear magnetoresistance response in a two-dimensional electron gas. Physical Review Materials. 4, 071001 (2020).
    Résumé : The Rashba spin-orbit coupling is a relativistic interaction appearing in systems lacking inversion symmetry such as surfaces or interfaces. It locks the electrons' spin and angular momentum and enables efficient means to interconvert spin currents and charge currents through the direct and inverse Edelstein effects. The Rashba coefficient sets the amplitude of the Rashba spin-orbit coupling but its quantification, using for instance angle-resolved photoemission spectroscopy, is very challenging. The authors demonstrate that the Rashba coefficient can be reliably extracted from a simple magnetotransport experiment and apply it to LaAlO${}_{3}$/SrTiO${}_{3}$ interfaces for a broad range of Fermi energies, tuned by an electrostatic gate.

  • E. Y. Vedmedenko, R. K. Kawakami, D. D. Sheka, P. Gambardella, A. Kirilyuk, A. Hirohata, C. Binek, O. Chubykalo-Fesenko, S. Sanvito, B. J. Kirby, J. Grollier, K. Everschor-Sitte, T. Kampfrath, C. - Y. You, A. Berger, The 2020 magnetism roadmap. Journal of Physics D: Applied Physics. 53, 453001 (2020).

  • Z. Wang, H. Cheng, K. Shi, Y. Liu, J. Qiao, D. Zhu, W. Cai, X. Zhang, S. Eimer, D. Zhu, J. Zhang, A. Fert, W. Zhao, Modulation of field-like spin orbit torque in heavy metal/ferromagnet heterostructures. Nanoscale. 12, 15246 (2020).
    Résumé : Spin orbit torque (SOT) has drawn widespread attention in the emerging field of magnetic memory devices, such as magnetic random access memory (MRAM). To promote the performance of SOT-MRAM, most efforts have been devoted to enhance the SOT switching efficiency by improving the damping-like torque. Recently, some studies noted that the field-like torque also plays a crucial role in the nanosecond-timescale SOT dynamics. However, there is not yet an effective way to tune its relative amplitude. Here, we experimentally modulate the field-like SOT in W/CoFeB/MgO trilayers through tuning the interfacial spin accumulation. By performing spin Hall magnetoresistance measurement, we find that the CoFeB with enhanced spin dephasing, either generated from larger layer thickness or from proper annealing, can distinctly boost the spin absorption and enhance the interfacial spin mixing conductance Gr. While the damping-like torque efficiency increases with Gr, the field-like torque efficiency is found to decrease with it. The results suggest that the interfacial spin accumulation, which largely contributes to the field-like torque, is reduced by higher interfacial spin transparency. Our work shows a new path to further improve the performance of SOT-based ultrafast magnetic devices.

  • M. - W. Yoo, D. Rontani, J. Létang, S. Petit-Watelot, T. Devolder, M. Sciamanna, K. Bouzehouane, V. Cros, J. - V. Kim, Pattern generation and symbolic dynamics in a nanocontact vortex oscillator. Nature Communications. 11, 601 (2020).
    Résumé : Controlling chaotic behavior in spintronic devices is promising for signal-processing applications. Here, the authors unveil the symbolic patterns hidden in the magnetization dynamics of a nanocontact vortex oscillator and detail how to control chaos complexity with a single experimental parameter.

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