High accuracy Spin Hall Effect Induced Spin Accumulation detection in MOKE Measurements

TL;DR

This paper introduces a new, highly sensitive MOKE measurement methodology to accurately detect spin accumulation due to the Spin Hall Effect, addressing previous discrepancies and improving quantification accuracy.

Contribution

The authors develop a novel MOKE measurement protocol with enhanced sensitivity, enabling precise quantification of spin accumulation in platinum and revising previous experimental results.

Findings

Achieved a sensitivity of 354 nV/nrad in MOKE measurements.

Measured spin accumulation per current density significantly lower than prior reports.

Revealed the need to revise experimental procedures and models for accurate spin detection.

Abstract

Charge to spin (orbital) momentum conversion phenomena enclose great potential for advancing applications in spin/orbitronics. Although current-induced magnetic moment accumulation is crucial both for fundamental understanding and practical applications, direct quantifications are scarce. Optical polarization measurements, namely magneto-optical Kerr rotation (MOKE) ($\theta_K$), have been used to get direct evidence of magnetic accumulation perpendicular to a current flow density ($J$) in late transition metals (Pt) as well as in light transition elements (Ti, Cr), and used to conclude evidence of spin or orbital momentum accumulation. However, discrepancies of the reported $\theta_K/J$ values, exceeding one order of magnitude, together with early claims that conventional MOKE experiments were not a suitable tool, is prompting revisions of methods and results. Here, we report on a new methodology for MOKE measurements that solves known bottlenecks. We obtain a sensitivity of $(354 \pm 27)$ nV/nrad and use the designed protocol to measure $|\theta_K^S/J| = (7.92 \pm 1.94)$ nrad/$(10^7$ A/cm$^2)$ and $|\theta_K^P/J| = (6.89 \pm 1.74)$ nrad/$(10^7$ A/cm$^2)$ in a 50 nm thick Pt bar for S and P polarized incident light, respectively.The extracted value of $|\theta_K^S/J|$ is significantly smaller, about a 7-fold reduction, than previous results on a nominally identical device. Given that differences in the microstructure of Pt films cannot account for such large discrepancy, this implies that experimental procedures and models should be revised accordingly.