Sensing Spin Polarized Currents in Atomically Thin Material Systems – Improving the Sensitivity of the Optical Ferromagnetic Resonance Method

The conversion of charge to spin current is of considerable current interest from both fundamental and technological perspectives. It can be used to manipulate the magnetization order parameter in spintronic devices such as the three-terminal magnetic tunnel junction device and the racetrack memory.

The spin momentum locking in topological insulators arises from the quantum spin Hall effect (QSHE). Conventional materials can also possess an intrinsic spin Hall effect (SHE) but in both sets of materials there can also be an extrinsic SHE that arises from scattering from impurities, surface states, and other defects.

Despite the intense efforts that have been invested in recent years since its discovery, resolving the nature of the SHE in the various material systems still awaits experimental verification and is left to much discussion and controversy.

Measurement of the SHE especially for atomically thin systems is not straight forward. While several techniques have been used, the vast majority are limited to metallic ferromagnets that display magnetoresistance and/or have limited sensitivity. To that end we have recently developed ultra-sensitive time domain optical-electrical method for the measurement of the SHE which we name the optical-ferromagnetic resonance (OFMR). We have experimentally demonstrated that the OFMR is well fit to measure material systems as thin as 2-3 atomic layers.

In this work we explore novel ways to further increase the sensitivity of the OFMR for the measurement of spin polarized currents. We identify the geometrical arrangements that exhibit superior sensitivity. Our analysis is based on first principles and generality is achieved by introducing spin currents directly to the Euler-Lagrange set of equations.

This study is key to resolving and further exploring the nature of the SHE.