Recent works provide evidence for a large orbital Rashba-Edelstein effect at the interface between Cu and its oxide. Here, we experimentally demonstrate that a very large enhancement of both the net torque and the spin-pumping voltage (up to a factor of two) can be obtained with the insertion of a Pt layer whose large spin-orbit coupling helps to convert a pure orbital current into a spin current. These two reciprocal phenomena, observed simultaneously for the first time in the same Co/Pt/Cu/CuOx samples, are in agreement and their orbital angular momentum nature associated to a charge-to-orbit (orbit-to-charge) conversion at the Cu/CuOx interface constitutes a robust interpretation. To disentangle spin and orbital currents in these systems, we also measure the ferromagnet thickness dependence of the net torques and observe a clear increase of the corresponding dephasing length, indicating the contribution of pure orbital currents acting on the magnetization. From the Cu thickness dependence, we also verify that the conversion occurs at the Cu/CuOx interface through the orbital Rashba effect as observed in both torque and spin-pumping measurements.
Very recently, a dramatic increase in magnetic torque has been observed in ferrimagnetic insulators interfaced with CuOx and the results are explained in the framework of orbital-current generation and emission from CuOx. The conversion of charge-current into the orbital current is governed via the orbital Hall or orbital Rashba effect, a similar analogy to the spin Hall and Rashba effect. In this talk, we will present our latest results on the observation of damping-like torque in Co/CuOx without any heavy-metal layer due to orbital Rashba effect at Co/CuOx interface. Furthermore, we observe a two-fold increase in damping-like torques in Co(2)/Pt(4)/Cu/CuOx(3) that further validates our hypothesis.
Non-collinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fueled by possible fine engineering of several magnetic interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This allows the stabilization of complex chiral spin textures such as chiral magnetic domain walls (DWs), spin spirals, and magnetic skyrmions among others. The presentation will focus on the behavior of chiral DWs at ultrashort timescale after optical pumping in perpendicularly magnetized asymmetric multilayers. The magnetization dynamics is probed using time-resolved circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). In the first 2 picosecond, a transient reduction of the CD-XRMS asymmetry ratio is attributed to the spin current-induced coherent and incoherent torques within the continuously dependent spin texture of the DWs. On the one hand, this time-varying change of the DW texture shortly after the laser pulse is identified as a distortion of the homochiral Néel shape toward a transient mixed Bloch-Néel-Bloch texture along a direction transverse to the DW due to the coherent torque. On the other hand, the overall effect of the spin current incoherent torque results in an average loss of angular momentum that induces an increase of the spin relaxation processes within the DW at the ps timescale. It leads to a faster remagnetization inside the DWs compared to domains.
THz emission spectroscopy reveals to be a very powerful experimental method to investigate the properties of Rashba or topological insulator surface states. The THz emission can be also used in heavy metallic or in more general Rashba systems. We prove here the ability of the present method. In 3d/5d transient metal bilayers and beyond heavy metal structures, Rashba states and Topological insulators are expected candidates for spintronic-terahertz domains due to their high spin to charge conversion properties. In this scheme, we are interested in the samples based on 2D electron gas, topological insulators and Heusler alloys with strong spin-orbit coupling.
Spin-orbit torques (SOTs) allow controlling the magnetization of diverse classes of magnetic multilayers and devices. The mechanism utilizes spin-orbit interactions such as spin Hall effect in heavy metals and/or Rashba effect at ferromagnetic/heavy-metal interface with broken inversion symmetry. The SOTs have damping-like (HDL) and field-like (HFL) effective field components. In this talk, we will present the mechanism of spin-transport in ultrathin magnetic multilayer whose thicknesses span across the characteristic spin-dephasing length, and how it results in HDL and HFL nearby the crossing point of this specific length. To this aim, we have quantified SOTs in a series of samples Pt 8|Co x|Al 1.4|Pt 3 with x = 0.55, 0.7, 0.9, 1.2, 1.4 nm. Our experiments demonstrate the presence of very large field-like torque arising from Co|Al interface for Co thickness smaller than spin-dephasing length. The results suggest the contribution of additional mechanisms of spin-current generation.
Generation and detection of pure spin currents circulating in magnetic insulator materials have proven its worth by enabling transport of spin information across large distances, much further than in metals, thanks to the abscence of decay mechanism through the delocalized electrons. Spin currents here propagate over localized magnetic moments via spin-waves (SW), or their quanta the magnons, with characteristic frequencies ranging from GHz to THz and associated wavelengths from μm to nm. Most of the work so far has focus on Yttrium Iron garnets (YIG), which is a ferrimagnetic insulator with the lowest known amount of magnetic damping. In this talk, I will present recent result of spin waves transport in high quality ultra-thin films exploiting spin-orbit tools to interconvert the spin signal into an electrical signal. By injecting a high current density in Pt electrodes deposited few microns apart on top of a YIG film, a pure magnon current propagating in the YIG can be induced/detected via the direct/inverse spin Hall effect. In this work, I will mainly focus on investigating the behavior at large energies. We have found that the magnon tranresistance in this system can cross several regimes that involve strong change in the magnon distribution. Throughout various techniques such as Brillouin light scattering spectroscopy, spin Seebeck and spin Hall magnetoresistance measurements, we provide a complete analysis of the different phenomena surrounding the spin transport in thin YIG films and we will show that our experimental findings do not support yet the emergence of new collective behaviors, such as Bose-Einstein condensation at room temperature.
Magnetic skyrmions are magnetic textures, topologically different from the uniform ferromagnetic state, holding a lot of promise for applications as well as of fundamental interest. They have been observed in magnetic multilayers at room temperature only a couple of years ago [1]. In magnetic multilayers, a key to stabilize magnetic skyrmions is the Dzyaloshinskii-Moriya interaction, obtained at the interfaces between ferromagnetic layers and heavy-metal/oxides spacers, which promotes a unique chirality of the skyrmionic spin textures. Combined with spin-orbit torques generated in heavy-metal layers, this unique chirality allows very efficient current-induced motion at speeds reaching 100m/s [2].
In this presentation, we report about our predictions and observations of hybrid chirality in skyrmionic systems, arising from a competition between the Dzyaloshinskii-Moriya interaction and the other magnetic interactions. After having demonstrated a direct evidence of such hybrid chirality [3] by probing the surface spin ordering of multilayers with circular dichroism in X-ray resonant magnetic scattering [4], we will discuss the impact of hybrid chirality in technologically relevant multilayers depending on different parameters such as the number of stacked layers, interfacial anisotropy or interlayer exchange coupling. In the perspective of technological applications of skyrmions, controlling their chirality to match the spin-orbit torques injection geometry of the multilayers is required to achieve efficient current-induced motion.
[1] C. Moreau-Luchaire et al, Nat. Nano. 11, 444 (2016).
[2] A. Hrabec et al, Nat. Comm. 8, 15765 (2017).
[3] W. Legrand et al, arXiv:1712.05978v2 (2017).
[4] J.-Y. Chaleau et al, Phys. Rev. Lett. 120, 037202 (2018).
Lucile Soumah, Lilia Qassym, Cécile Carretero, Eric Jacquet, Jamal Ben-Youssef, Richard Lebourgeois, Nathan Beaulieu, Vincent Cros, Paolo Bortolotti, Abdelmadjid Anane
Pulsed laser deposition (PLD) of high quality nanometers thick (YIG) films have recently allowed to open the field of spintronics and magnonics nanostructured magnetic insulators[1,2]. YIG is a versatile material in term of anisotropy or magnetization as doping and growth induced strain can significantly change those properties. We present here the effect of Bi substitution on Bi:YIG PLD grown films with thicknesses ranging from 10 nm to 40 nm. By using lattice matched substituted GGG substrates (sGGG) it is possible to stabilize for specific growth conditions an out-of plane easy magnetization axis. We present comprehensive structural charcterisation using X-ray diffraction and squid magnetometry that shows the transition from an easy in-plane magnetization direction to an out-of plane magnetization. The effect of Bi doping is to significantly increase the Faraday rotation of the films. Using magneto-optical Kerr microscopy, it is therefore possible to observ the shape and the sizes of the magnetic domains for films thicknesses down to 10 nm. Using the Kooy and Enz model it is possible to extract a domain wall energy of 0.49 erg/cm2.
[1] O. D. Kelly et al., Applied Physics Letters 103, 4, 082408 (2013).
[2] M. Collet et al., Nature communications 7, 10377 (2016).
[3] C. Kooy and U. Enz, Philips Res. Rep., vol. 15, pp. 7-29(1960)
Martin Collet, Paolo Bortolotti, Vincent Cros, Abdelmadjid Anane, José Luis Prieto, Manuel Muñoz, Jamal Ben Youssef, Vladimir Naletov, Grégoire de Loubens, Olivier Klein, Michael Evelt, Vladimir Bessonov, Sergej Demokritov, Vladislav Demidov
In recent years, Spin Orbit interaction as a source of spin current has been widely used through the physics of Spin Hall Effect (SHE). The peculiar symmetry of SHE allows creating a spin accumulation at the interface between a spin-orbit metal and a magnetic insulator that could lead to a net pure spin current flowing from the metal into the insulator. This spin current will induce a torque on the magnetization and eventually could drive it into steady motion. As a ferromagnetic insulator with a very low Gilbert damping, Yttrium Iron Garnet (YIG) is a very promising candidate to investigate pure spin current phenomena. Only very recently, with the developments in preparation of high-quality nanometer-thick YIG films, the implementation of insulator-based spin-torque devices became practically feasible.
Here, we report on the excitation of auto-oscillations in microstructures of YIG(20nm)\Pt(8nm) driven by Spin Orbit torque (SOT). By injection of a dc current in the adjacent Pt layer, we have been able to prove that the SOT due to SHE is sufficiently strong and efficient to drive the YIG magnetization dynamics at frequencies that closely follow the ferromagnetic resonance mode. These auto-oscillations have been detected either inductively using a spectrum analyzer or directly observed using micro-focus Brillouin Light Scattering. Furthermore, we achieved an efficient control of spin waves attenuation length in a YIG waveguide using the SHE in the sub-critical regime i.e. below the auto-oscillations threshold. We believe that our finding pave the path to active magnonics devices made out of YIG films.
Sub-100-nm skyrmions are stabilized in magnetic metallic multilayers and observed using transmission electron microscopy, magnetic force microscopy, scanning transmission X-ray microscopy and X-ray resonant magnetic scattering. All these advanced imaging techniques demonstrate the presence of 'pure' Neel skyrmion textures with a determined chirality. Combining these observations with electrical measurements allows us to demonstrate reproducible skyrmion nucleation using current pulses, and measure their contribution to the transverse resistivity to detect them electrically. Once nucleated, skyrmions can be moved using charge currents. We find predominantly a creep-like regime, characterized by disordered skyrmion motion, as observed by atomic force microscopy and scanning transmission X-ray microscopy. These observations are explained qualitatively and to some extent quantitatively by the presence of crystalline grains of about 20nm lateral size with a distribution of magnetic properties.
Nicolas Reyren, Davide Maccariello, Juan-Carlos Rojas-Sánchez, Constance Moreau-Luchaire, Simon Oyarzun, Yu Fu, Alain Marty, Céline Vergnaud, Serge Gambarelli, Laurent Vila, Matthieu Jamet, Yoshiyuki Ohtsubo, Amina Taleb-Ibrahimi, Patrick Le Fèvre, François Bertran, Christoforos Moutafis, João Sampaio, Carlos A. Vaz, Noah Van Horne, Karim Bouzehouane, Karin Garcia, Cyrile Deranlot, Peter Warnicke, Phillip Wohlhüter, Markus Weigand, Jörg Raabe, Vincent Cros, Jean-Marie George, Albert Fert
Spintronics evolves along new paths involving non-magnetic materials having large spin-obit coupling, typically 5d metals, allowing for example large spin-to-charge current conversion (spin Hall and Rashba-Edelstein effects). These heavy metals have other effects: in proximity of magnetic thin films they can burst out the Dzyaloshinskii-Moriya interaction leading to the stabilization of chiral magnetic structures. Another source of recent interest relies on “non-trivial topologies”, either of the band structure of the topological insulators, or of the spin textures in magnetic thin films.
We will discuss our recent progress to control the topological textures known as skyrmions in multilayers made of heavy metals and magnetic layers. Aiming at using skyrmions as magnetic bits in “racetrack memory” structures, one of the present challenges is to efficiently move skyrmions with dimensions of a few tens of nanometers. The topology of these magnetic structures imposes peculiar dynamics, interesting both in fundamental and applied perspectives. Simulations indicate that spin-orbit torques, through the absorption of the spin current generated by a nearby layer, should be the most efficient method. The conducting surfaces of topological insulators at which the carriers’ spin and momentum are locked, can display better spin-to-charge conversion than what is found using heavy metals. However, the control of the interfaces is crucial to conserve the Dirac cone and the associated spin-momentum locking. We demonstrate by ARPES and spin pumping experiments how the properties of the α-Sn thin film topological insulator are preserved and can be used for spintronics, maybe to move skyrmions!
The rich physics of spin transfer nano-oscillators (STNO) has provoked a huge interest to create a new generation of multi-functional microwave spintronic devices [1]. It has been often emphasized that their nonlinear behavior gives a unique opportunity to tune their radiofrequency (rf) properties but at the cost of large phase noise, not compatible with practical applications. To tackle this issue as well as to open the opportunities to new developments for non-boolean computations [1], one strategy is to use electrical synchronization of STOs through the rf current. Thereby, it is crucial to understand how the synchronization forces transmitted through the electric current. In this talk, we will first present the results of an experimental study showing the self-synchronization of STNO by re-injecting its rf current after a certain delay time [2]. In the second part, we demonstrate that the synchronization of two vortex-STNOs connected in parallel can be tuned either by an artificial delay or by the spin transfer torques [3]. The synchronization of spin-torque oscillators, combined with the drastic improvement of the rf-features (linewidth decreases by a factor of 2 and power increases by a factor of 4) in the synchronized state, marks an important milestone towards a new generation of rf-devices based on STNO.
The authors acknowledge the financial support from ANR agency (SPINNOVA: ANR-11-NANO-0016) and EU grant (MOSAIC: ICT-FP7-317950).
[1] N. Locatelli, V. Cros, and J. Grollier, Nat Mater 13, 11 (2014).
[2] S. Tsunegi et al., arXiv:1509.05583 (2015)
[3] R. Lebrun et al., arXiv:1601.01247 (2016)
The brain displays many features typical of non-linear dynamical networks, such as synchronization or chaotic behaviour. These observations have inspired a whole class of models that harness the power of complex non-linear dynamical networks for computing. In this framework, neurons are modeled as non-linear oscillators, and synapses as the coupling between oscillators. These abstract models are very good at processing waveforms for pattern recognition or at generating precise time sequences useful for robotic motion. However there are very few hardware implementations of these systems, because large numbers of interacting non-linear oscillators are indeed. In this talk, I will show that coupled spin-torque nano-oscillators are very promising for realizing cognitive computing at the nanometer and nanosecond scale, and will present our first results in this direction.
We measured transverse magneto-thermoelectric voltage on devices made of a Permalloy (Py) line and a transverse
electrode made of platinum (Pt), copper (Cu) or bismuth (Bi). We show that the angular dependence of the voltage is the
same for Pt and Cu but different with a Bi electrode. We interpret the angular dependence with Pt and Cu electrode as
anomalous and planar Nernst and Righi-Leduc effect on Py. The results obtained with a Bi electrode can be explained as
the Nernst effect of the electrode itself which overwhelms the signal coming from the Py.
We report on microwave oscillations induced by spin-transfer-torque in metallic spin-valves obtained by electrodeposition
of Co-Cu-Co trilayer structures in nanoporous alumina templates. Using micromagnetic calculations
performed on similar spin-valve structures it was possible to identify the magnetization dynamics associated
with the experimentally determined microwave emission. Furthermore it appears that in our particular geometry
the microwave emission is generated by the vortex gyrotropic motion which occurs in, at least, one of the two
magnetic layers of our spin-valve structures. Microwave emission was obtained in the absence of any external
magnetic field with the appropriate magnetization configuration.
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