Tuning spin-orbit torques across the phase transition in VO$_2$/NiFe heterostructure

TL;DR

This study investigates how spin-orbit torques in VO$_2$/NiFe heterostructures can be tuned across the VO$_2$ phase transition, revealing mechanisms and potential for device applications.

Contribution

It demonstrates the modulation and sign change of spin-orbit torques across the VO$_2$ phase transition, identifying bulk spin Hall effect and anomalous torque as key mechanisms.

Findings

Spin-orbit torque magnitude modulates by ±100% across phase transition.

Sign change of spin-orbit torque observed at the transition.

Bulk spin Hall effect in VO$_2$ confirmed as main torque source.

Abstract

The emergence of spin-orbit torques as a promising approach to energy-efficient magnetic switching has generated large interest in material systems with easily and fully tunable spin-orbit torques. Here, current-induced spin-orbit torques in VO$_2$/NiFe heterostructures were investigated using spin-torque ferromagnetic resonance, where the VO$_2$ layer undergoes a prominent insulator-metal transition. A roughly two-fold increase in the Gilbert damping parameter, $\alpha$, with temperature was attributed to the change in the VO$_2$/NiFe interface spin absorption across the VO$_2$ phase transition. More remarkably, a large modulation ($\pm$100%) and a sign change of the current-induced spin-orbit torque across the VO$_2$ phase transition suggest two competing spin-orbit torque generating mechanisms. The bulk spin Hall effect in metallic VO$_2$, corroborated by our first-principles calculation of spin Hall conductivity, $\sigma_{SH} \approx 10^4 \frac{\hbar}{e} \Omega^{-1} m^{-1}$, is verified as the main source of the spin-orbit torque in the metallic phase. The self-induced/anomalous torque in NiFe, of the opposite sign and a similar magnitude to the bulk spin Hall effect in metallic VO$_2$, could be the other competing mechanism that dominates as temperature decreases. For applications, the strong tunability of the torque strength and direction opens a new route to tailor spin-orbit torques of materials which undergo phase transitions for new device functionalities.