In bulk transition metals the orbital degree of freedom is normally quenched due to band structure and crystal field effects. However, recent theoretical work indicates that an applied electric field can induce a net flow of orbital angular momentum in transition metals such as Ti, Cr, and Mn. The generation of the orbital current relies on the variable orbital character of the electronic bands that cross the Fermi level and does not require spin-orbit coupling (SOC). This makes it possible to use light metals in spin-orbitronic devices to actively generate orbital and spin currents. In this talk, we will present experimental results evidencing strong orbital torques and orbital Rashba-Edelstein magnetoresistance in light metal/ferromagnet heterostructures. The observation of an orbital torque requires the conversion of the orbital current into a spin current, which occurs due to SOC in the ferromagnetic itself or in nonmagnetic metallic and insulating spacers placed between the light metal and the ferromagnet. Our work shows that the orbital torque can be tuned by choosing orbital-to-spin conversion layers with different SOC, and the magnitude of the orbital torque can be larger than the spin torque. Finally, we will show that orbital currents are also responsible for magnetoresistive effects akin to the spin Hall magnetoresistance.
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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