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.
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.
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