Inductive detection of field-like and damping-like AC inverse spin-orbit torques in ferromagnet/normal metal bilayers

TL;DR

This paper demonstrates a microwave spectroscopy technique to detect and quantify AC charge currents driven by inverse spin-charge conversion in ferromagnet/normal metal bilayers, revealing both damping-like and field-like inverse spin-orbit torques.

Contribution

It introduces a VNA-FMR method to inductively measure all AC charge currents, including those from spin pumping and spin-charge conversion, providing new insights into inverse spin-orbit torques.

Findings

NiFe/Pt bilayers exhibit both damping-like and field-like inverse spin-orbit torques.

Magnitudes of inverse spin-orbit torques are comparable to prior reports.

Damping-like torque magnitude depends on deposition order, indicating interface effects.

Abstract

Functional spintronic devices rely on spin-charge interconversion effects, such as the reciprocal processes of electric field-driven spin torque and magnetization dynamics-driven spin and charge flow. Both damping-like and field-like spin-orbit torques have been observed in the forward process of current-driven spin torque and damping-like inverse spin-orbit torque has been well-studied via spin pumping into heavy metal layers. Here we demonstrate that established microwave transmission spectroscopy of ferromagnet/normal metal bilayers under ferromagnetic resonance can be used to inductively detect the AC charge currents driven by the inverse spin-charge conversion processes. This technique relies on vector network analyzer ferromagnetic resonance (VNA-FMR) measurements. We show that in addition to the commonly-extracted spectroscopic information, VNA-FMR measurements can be used to quantify the magnitude and phase of all AC charge currents in the sample, including those due to spin pumping and spin-charge conversion. Our findings reveal that Ni$_{80}$Fe$_{20}$/Pt bilayers exhibit both damping-like and field-like inverse spin-orbit torques. While the magnitudes of both the damping-like and field-like inverse spin-orbit torque are of comparable scale to prior reported values for similar material systems, we observed a significant dependence of the damping-like magnitude on the order of deposition. This suggests interface quality plays an important role in the overall strength of the damping-like spin-to-charge conversion.