Ultrafast optical microscopy integrated with Sagnac interferometry and microwave electronics provide unparalleled Kerr sensitivity, temporal, and spatial resolutions to study interactions between various quantum degrees of freedom. We will build such a microscope to image ultrafast dynamics in atomically-thin magnets driven by spin-orbit torque generated by vdW TI, Weyl, and Dirac semimetals.
Previous work highlights:
Sagnac interferometry for high-sensitivity optical measurements of spin-orbit torque
We develop an ultrasensitive magneto-optic method to quantify spin-orbit torques (SOT) based on a modified Sagnac magneto-optic Kerr effect (MOKE) interferometry (5 μRad/√Hz). The high sensitivity of Sagnac interferometry permits for the first time optical quantification of spin-orbit torque from small-angle magnetic tilting of samples with perpendicular magnetic anisotropy (PMA). We find significant disagreement between Sagnac measurements and simultaneously-performed harmonic Hall (HH) measurements of spin-orbit torque on Pt/Co/MgO and Pd/Co/MgO samples with PMA. The Sagnac results for PMA samples are consistent with both HH and Sagnac measurements for the in-plane geometry, so we conclude that the conventional analysis framework for PMA HH measurements is flawed. We suggest that the explanation for this discrepancy is that although magnetic-field induced magnetic tilting in PMA samples can produce a strong planar Hall effect, when tilting is instead generated by spin-orbit torque it produces negligible change in the planar Hall signal. This very surprising result demonstrates a flaw in the most-popular method for measuring spin-orbit torques in PMA samples, and represents an unsolved puzzle in understanding the planar Hall effect in magnetic thin films.
Future directions:
High-resolution, accurate space-time spin-torque readout using magneto-optical techniques
Optical readout using the magneto-optical Kerr (MOKE) effect provides a unique advantage as it can avoid artifacts from transport techniques such as thermoelectric voltages, nonlinear-in-current Hall resistances, and modification of transport coefficients by magnons or heating. We will continue to advance the resolutions of magneto-optical microscopy in the Kerr sensitivity, temporal, and spatial domains by developing near-field MOKE microscopy and ultrafast Sagnac interferometry. These pioneering techniques will position our group uniquely to image the atomic-scale domains and disorders in low dimensional magnetic systems. In addition, ultrafast optics such as TRKR with THz resolution is a natural and powerful platform to investigate antiferromagnetic dynamics, where much faster magnetic resonance and switching rates are expected that are beyond the capacity of any transport measurement.