Large Exotic Spin Torques in Antiferromagnetic Iron Rhodium

Abstract

Precise control of magnetic dynamics can enable many novel computing applications, including magnetic random access memory and spin torque oscillator-based neuromorphic computing. Spin torques stand out as an excellent tool for efficiently manipulating magnetic states, and can be easily produced via spin-orbit effects in a spin source layer. Unfortunately, underlying crystal symmetries in the spin source material often restrict the geometry of the spin torques, making them poorly suited to practical applications. Magnetic ordering can break the symmetries of a spin source material and allows the generation of versatile spin torques with controllable geometries tailored to applications. Antiferromagnetic materials in particular have robust magnetic ordering that makes them ideal as spin sources.We present spin torque ferromagnetic resonance measurements (Fig. 1) and second harmonic Hall measurements characterizing the geometry and efficiency of spin torques generated in antiferromagnetic iron rhodium alloy. We find highly temperature-dependent spin torques with spin torque efficiencies above 90% at room temperature and above 300% at 170K, much higher than most competing materials. Further, these torques do not show the angular dependence expected for ordinary spin Hall torques. Instead, it appears that the torque derives from a spin current with spin polarization parallel to the FeRh magnetic order, which makes these torques useful for exciting dynamics in a variety of samples. This makes iron rhodium a compelling option for the development of spin torque technology.This work was supported as part of Quantum Materials for Energy Efficient Neuromorphic Computing, an Energy Frontier Research Center funded by the U.S. DOE, Office of Science.  Fig. 1 DC Biased Spin Torque Ferromagnetic Resonance. (a) Diagram of the measurement setup. (b) Characteristic resonance scans. (c) Linewidth over a range of DC currents. Increasing current increases the spin current, modifying the effective damping linearly. Shown at 300K, 275K, 260K, and 170K. (d) Spin torque efficiency is huge and grows by a factor of 3.5 between 300K and 170K.