Interplay of spin-orbit torque and thermoelectric effects in ferromagnet/normal-metal bilayers

TLDR

The study measures harmonic transverse voltages in ferromagnet/normal‑metal bilayers to investigate the simultaneous influence of current‑induced thermoelectric gradients and spin‑orbit torques. Using symmetry and field‑dependence of the transverse resistance, the authors separate thermoelectric and spin‑orbit torque contributions in both light‑metal and heavy‑metal bilayers, revealing that second‑harmonic thermoelectric signals can inflate antidamping SOT estimates. Vertical thermal gradients in FM/NM bilayers depend on resistivity contrast and layer order, and thermoelectric contributions can significantly bias antidamping SOT measurements—strong in Ta/Co, negligible in Pt/Co—so that, after correction, the antidamping SOTs are comparable to those in thinner perpendicular‑magnet Co layers while field‑like SOTs are an order of magnitude smaller.

Abstract

We present harmonic transverse voltage measurements of current-induced thermoelectric and spin-orbit torque (SOT) effects in ferromagnet/normal metal bilayers, in which thermal gradients produced by Joule heating and SOT coexist and give rise to ac transverse signals with comparable symmetry and magnitude. Based on the symmetry and field-dependence of the transverse resistance, we develop a consistent method to separate thermoelectric and SOT measurements. By addressing first ferromagnet/light metal bilayers with negligible spin-orbit coupling, we show that in-plane current injection induces a vertical thermal gradient whose sign and magnitude are determined by the resistivity difference and stacking order of the magnetic and nonmagnetic layers. We then study ferromagnet/heavy metal bilayers with strong spin-orbit coupling, showing that second harmonic thermoelectric contributions to the transverse voltage may lead to a significant overestimation of the antidamping SOT. We find that thermoelectric effects are very strong in Ta(6nm)/Co(2.5nm) and negligible in Pt(6nm)/Co(2.5nm) bilayers. After including these effects in the analysis of the transverse voltage, we find that the antidamping SOTs in these bilayers, after normalization to the magnetization volume, are comparable to those found in thinner Co layers with perpendicular magnetization, whereas the field-like SOTs are about an order of magnitude smaller.