Spin-Orbit Torque in Magnetic Heterostructures: Exchange Interactions and Ultrasensitive Sagnac Magneto-Optic Interferometry

In this two-part talk, we will first present our recent progress in advancing spin-orbit-torque metrology by adapting an ultrasensitive Sagnac magneto-optic interferometry originally developed for measuring time-reversal-symmetry breaking in exotic superconductors, and applying it for the first time to measuring spin-orbit torques. We will introduce the concept of Sagnac interferometry with an ultrahigh Kerr sensitivity of ~ 5μRad/√Hz and describe how such high sensitivity allows quantitative measurements of spin-orbit torque by sensing current-induced small-angle magnetization tilting. This method is especially advantageous for insulating magnets for which conventional transport spin-orbit torque metrology can be disrupted by magneto-thermal artifacts, but it can also be applied widely to broad classes of magnets regardless of conductivity and small net magnetization.<br/>Secondly, we will present our recent work in identifying the gigahertz antiferromagnetic resonances within the easy-axis van der Waals (vdW) magnet CrSBr and understanding how their frequencies and intermode coupling can be controlled by the combination of external magnetic fields and magnetic anisotropy [3]. We will show that exchange bias from CrSBr acting on the van der Waals ferromagnet Fe3GeTe2 induces a spatially non-uniform spin configuration through the thickness of the Fe3GeTe2 that is not readily achievable with conventional magnetic materials, and can provide the symmetry breaking needed to enable field-free spin-orbit-torque switching in Pt/Fe3GeTe2/CrSBr heterostructures [4].<br/><br/>[1] S. Karimeddiny*, T. M. Cham, D. C. Ralph, and Y. K. Luo*, Sagnac interferometry for high-sensitivity optical measurements of spin-orbit torque. arXiv: 2109.13759 (2021)<br/>[2] S. Karimeddiny*, T. M. J. Cham*, O. Smedley, D. C. Ralph, and Y. K. Luo*, Sagnac interferometry for high-sensitivity optical measurements of spin-orbit torque. Science Advances, 9 eadi9039 (2023)<br/>[3] T. M. J. Cham*, S. Karimeddiny, A. H. Dismukes, X. Roy, D. C. Ralph, and Y. K. Luo*, Anisotropic gigahertz frequency antiferromagnetic resonance in layered van der Waals semiconductor. Nano Letters, 22, 6716-6723 (2022)<br/>[4] T. M. J. Cham*, R. Dorrian, X. S. Zhang, A. H. Dismukes, D. G. Chica, X. Roy, A. F. May, D. A. Muller, D. C. Ralph, and Y. K. Luo*, Exchange bias between van der Waals materials: tilted magnetic states and field-free spin-orbit-torque switching. Advanced Materials, 2305739 (2023)