Spin-orbit torque in Cr/CoFeAl/MgO and Ru/CoFeAl/MgO epitaxial magnetic heterostructures

TL;DR

This study investigates spin-orbit torque effective fields in Cr/CoFeAl/MgO and Ru/CoFeAl/MgO heterostructures, revealing material-dependent variations and confirming opposite spin Hall angles in Cr and Ru.

Contribution

It provides experimental evidence of SOT effects in Cr and Ru underlayers with different thicknesses, highlighting the influence of material properties on SOT behavior.

Findings

SOT effective fields depend on underlayer material and thickness.

Opposite signs of damping-like fields indicate opposite spin Hall angles.

Sizable SOT contributions observed in elements with small spin orbit coupling.

Abstract

We study the spin-orbit torque (SOT) effective fields in Cr/CoFeAl/MgO and Ru/CoFeAl/MgO magnetic heterostructures using the adiabatic harmonic Hall measurement. High-quality perpendicular-magnetic-anisotropy CoFeAl layers were grown on Cr and Ru layers. The magnitudes of the SOT effective fields were found to significantly depend on the underlayer material (Cr or Ru) as well as their thicknesses. The damping-like longitudinal effective field ({\Delta}H_L) increases with increasing underlayer thickness for all heterostructures. In contrast, the field-like transverse effective field ({\Delta}H_T) increases with increasing Ru thickness while it is almost constant or slightly decreases with increasing Cr thickness. The sign of {\Delta}H_L observed in the Cr-underlayer devices is opposite from that in the Ru-underlayer devices while {\Delta}H_T shows the same sign with a small magnitude. The opposite directions of {\Delta}HL indicate that the signs of spin Hall angle in Cr and Ru are opposite, which are in good agreement with theoretical predictions. These results show sizable contribution from SOT even for elements with small spin orbit coupling such as 3d Cr and 4d Ru.