X-ray spectroscopy of current-induced spin-orbit torques and spin accumulation in Pt / 3 d -transition-metal bilayers

An electric current flowing in Pt, a material with strong spin-orbit coupling, leads to spins accumulating at the interfaces by virtue of the spin Hall effect and interfacial charge-spin conversion. We measure the influence of these interfacial magnetic moments onto adjacent $3d$ transition-metal layers by x-ray absorption spectroscopy and x-ray magnetic circular dichroism in a quantitative and element-selective way, with sensitivity below ${10}^{\ensuremath{-}5}{\ensuremath{\mu}}_{B}$ per atom. In Pt(6 nm)/Co(2.5 nm), the accumulated spins cause a deviation of the Co magnetization direction, which corresponds to an effective spin Hall angle of 0.08. The spin and orbital magnetic moments of Co are affected in equal proportion by the absorption of the spin current, showing that the transfer of orbital momentum from the recently predicted orbital Hall effect is either below our detection limit, or not directed to the $3d$ states of Co. For Pt/NM (NM = Ti, Cr, Cu), we find upper limits for the amount of injected spins corresponding to about $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}{\ensuremath{\mu}}_{B}$ per atom.