X-ray spectroscopy of current-induced spin-orbit torques and spin accumulation in $\mathrm{Pt}/3d$-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 ${10}^{-5}{\mu }_B$ 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 $3\times {10}^{-6}{\mu }_B$ per atom.

Figure 1

Schematic drawing of the x-ray transmission setup: it comprises the sample, an in-vacuum photodiode, amplifier, low/high pass filter, ADC, and lock-in amplifier. A sine wave drives the current source, which generates the current that is passed through the sample, monitored by a second lock-in and oscilloscope.

Figure 2

Sample and measurement geometries for (a) close to perpendicular incidence $\left({10}^{\circ }\right)$ mostly sensitive to the perpendicular magnetization component and (b) tilted $\left(\approx {60}^{\circ }\right)$ x-ray incidence for detecting in-plane magnetization along $y$. (c) Optical micrograph (above) and x-ray transmitted signal recorded by sample scanning (below) of a sample chip with four current strips. Five wires are attached to the metallic contact pads; in between are the ${\mathrm{Si}}_3{\mathrm{N}}_4$ membranes with the deposited samples. (d) Close-up sketch of one current strip of 200 nm width (green), on top of the 500-nm-wide membrane that extends below the sample (orange). The current flow between the contact pads is indicated by the red arrow.

Figure 3

Co spectra of the Pt/Co sample under the influence of current flow, for given current densities: (a) average XAS, (b) AC-XMCD, and (c) AC-XMCD from above rescaled to static XMCD (thick black line).

Figure 4

AC-XMCD versus injected current. (a) Integrated around the Co $L_2$ and $L_3$ edges, lines denote fits proportional to $j$. (b) Magnetic spin and orbital moment per atom obtained in a sum rules analysis, with line fits.

Figure 5

Pt/Ti bilayer under current injection: (a) XAS (black line) and current-induced XMCD (green dots, standard error marked as light-green band) magnified by the given factor; (b) current-induced change of the absorption, for circular positive and circular negative x-ray helicity.

Figure 6

Pt/Cr bilayer under current injection. (a) and (b) as in Fig. 5.

Figure 7

Pt/Cr measurement under current injection, focused around the Cr $L_3$ edge. (a) and (b) as in Fig. 5.

Figure 8

XAS and XMCD of Pt/Cu, measured at the Cu $L_3$ edge. (a) Pt(10 nm)/Cu(10 nm) subjected to a current of 100 mA: XAS (black line) and current-induced XMCD (green dots) magnified by ${10}^4$, with XMCD standard error as a light-green band. (b) Current-induced change of the absorption, for circular positive and circular negative x-ray helicity. (c), (d) Same graphs as (a) and (b), for Pt(20 nm)/Cu(10 nm) subjected to a current of 80 mA.