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.
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.
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).
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.
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