In this two-part talk, we will first present our recent progress in advancing spin-orbit-torque metrology by developing an ultrasensitive fiber-based Sagnac magneto-optic interferometer [1]. We adapt the interferometer design concept originally developed for measuring time-reversal-symmetry breaking in exotic superconductors, and apply it to measure spin-orbit torques. We achieve a DC Kerr sensitivity of < 5μRad/√Hz, which allows quantitative optical measurements of spin-orbit torques for samples with either perpendicular or in-plane magnetic anisotropy. The Sagnac 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.

In the second part, 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. We will also describe direct electrical detection of antiferromagnetic resonance in bilayer CrSBr samples. Using a three-terminal device geometry, we measure dynamics via the tunnel magnetoresistance along the c-axis of CrSBr, sensitive to the relative orientation of the magnetic sublattices. We find that the spin-orbit torque is highly local - it acts only on the spin sublattice immediately adjacent to the source electrode, selectively addressing just one spin sublattice within the antiferromagnet. This localized nature allows for manipulation of individual sublattice dynamics, offering a new degree of control for layered antiferromagnets.

[1] Sagnac interferometry for high-sensitivity optical measurements of spin-orbit torque, S. Karimeddiny, T. M. J. Cham, O. Smedley, D. C. Ralph, and Y. K. Luo, Sci. Adv. 9, eadi9039 (2023).

[2] Spin-filter tunneling detection of antiferromagnetic resonance with electrically-tunable damping, T. M. J. Cham*, D. G. Chica, K. Watanabe, T. Taniguchi, X. Roy, A. F. May, Y. K. Luo*, and D. C. Ralph, arXiv: 2407.09462

[3] Exchange bias between van der Waals materials: tilted magnetic states and field-free spin-orbit-torque switching, T. M. J. Cham*, R. J. 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*, Advanced Materials, 2305739 (2024)

[4] Anisotropic gigahertz frequency antiferromagnetic resonance in layered van der Waals semiconductor, T. M. J. Cham*, S. Karimeddiny, A. H. Dismukes, X. Roy, D. C. Ralph, and Y. K. Luo*, Nano Letters, 22, 6716-6723 (2022)