Nonlinear Optical Imaging of Current Induced Spin Switching of Antiferromagnetic NiO: Spin Torque or Magnetoelasticity?

Electrical switching of the Neel order in antiferromagnetic (AF) materials via spin orbit torque (SOT) has drawn considerable interest [1]. Recent reports have suggested that magnetoelastic (ME) effects could be responsible for the measured spin Hall magnetoresistance (SHMR) changes that are used to detect switching [2]. Table-top techniques that can directly image all magnetic domains in AF materials are needed to better understand the physics associated with spin switching. We show that optical second harmonic generation (SHG) can be used to image and identify all spin and twin domains in thin AF NiO. Using this technique, we image spin domains, and their current-induced switching, in epitaxially-grown NiO/Pt devices. Our results show that AF switching is spatially non-uniform. We observe different regions that individually exhibit AF switching in agreement with SOT or ME predictions. We also observe regions that do not exhibit any switching. We also demonstrate that SHMR changes can be induced by optically heating the devices in patterns similar to Joule heating induced by an applied current thereby demonstrating the importance of ME effects. [1] Phys. Rev. Lett. 120, 207204 (2018). [2] Phys. Rev. Lett. 123, 227203 (2019).