  We adapt Sagnac interferometry for magneto-optic Kerr effect measurements of
spin-orbit-torque-induced magnetic tilting in thin-film magnetic samples. 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 an error 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.
