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

Abstract

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 −5 µB 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 × 10 −6 µB per atom.

Spin accumulation in metallic thin films induced by electronic impurity scattering

“…In contrast, by doping the full 3d series of impurities into the Pt thin film, we find that all values and profiles of normalized spin accumulation remain similar, which highlights the importance of strong intrinsic spin-orbit coupling of the host. Furthermore, our results of normalized spin accumulation for the Pt thin film are in good agreement with experimental observations [51,52], while our results of Cu thin film are at first glance incompatible with experimental findings [53,54]. This discrepancy can be attributed to the fact that in experiments ferromagnetic materials at the interface to Cu might alter the actual spin accumulation significantly, which remains an open topic for further investigations.…”

“…[51]. The same group also reported measurements for a Pt(10 nm)/Cu(10 nm) bilayer system and found the magnetic moments of a y ≈ 1.5 × 10 −6 μ B in Pt with an injected current density of 2.6 × 10 6 A cm −2 [52]. This would result in a normalized spin accumulation of α yx = 5.7 × 10 −13 (μ B cm 2 A −1 ), again comparable to our prediction.…”

Electronic Impurity Scattering Induced Spin Accumulation in Metallic Thin Films

“…[51]. The same group also reported measurements for a Pt(10 nm)/Cu(10 nm) bilayer system and found the magnetic moments of a y ≈ 1.5 × 10 −6 µ B in Pt with an injected current density of 2.6 × 10 6 Acm −2 [52]. This would result in a normalized spin accumulation of α yx = 5.7 × 10 −13 (µ B cm 2 A −1 ) again comparable to our prediction.…”

First-principles calculations of steady-state voltage-controlled magnetism: Application to x-ray absorption spectroscopy experiments

“…For the resulting out-of-equilibrium situation one may have pure charge rearrangements with a continuous flow of electric charge prevented by applying the electric field across an interface to vacuum or an insulating layer, i.e., having a capacitorlike experimental setup [14,15]. For the combination of conducting subsystems, on the other hand, a steady-state out-of-equilibrium situation will be created with a constant electric current flowing [16,17]. In this case one may focus on the electric-field-induced change of the magnetization longitudinal [16] or transverse [17] with respect to the electric field.…”

“…For the combination of conducting subsystems, on the other hand, a steady-state out-of-equilibrium situation will be created with a constant electric current flowing [16,17]. In this case one may focus on the electric-field-induced change of the magnetization longitudinal [16] or transverse [17] with respect to the electric field. The electric-field-induced electric current will in general be accompanied by a spin current that might be used, for example, for switching the magnetization via the spin transfer torque (STT), spin-orbit torque (SOT), or the spin Hall effect (SHE) [18].…”