Hydrogen-Sensitive Antisymmetric Magnetoresistance in Co/Pd Multilayers Driven by Anomalous Hall Effect and Domain Wall Motion

We report tunable antisymmetric magnetoresistance (MR) in Co/Pd multilayers, governed by the interplay among current direction, magnetization orientation, domain wall dynamics, and hydrogen absorption. Under ambient conditions, the presence of perpendicular magnetic anisotropy (PMA) and domain wall motion gives rise to a pronounced antisymmetric MR. Through a combination of magneto-optical Kerr microscopy and magnetotransport measurements, we attribute this behavior to the anomalous Hall effect (AHE) inherent in the PMA state. Upon hydrogen exposure ranging from a vacuum of 1 × 10^-3 mbar to 1 bar H₂, the magnetization of the Co/Pd multilayers undergoes a spin reorientation transition, progressively tilting from the out-of-plane direction toward the in-plane orientation, as evidenced by a reduction in remanence from 100% to nearly 20%. This reorientation is accompanied by a pronounced shift of approximately 50 Oe in the MR spike, observable even under low H₂ pressures up to 40 mbar. At higher hydrogen pressures approaching 1 bar, the AHE signal decreases by more than 70%, while the asymmetric MR spikes under a perpendicular magnetic field diminish from 0.1% to nearly 0.0%. In contrast, symmetric MR spikes of about 0.05% appear under in-plane magnetic fields, confirming the emergence of in-plane anisotropy. These findings demonstrate the pronounced hydrogen sensitivity of the MR spike shift, amplitude, and symmetry in Co/Pd multilayers, establishing a controllable platform for tuning spin-dependent transport with promising potential for multifunctional spintronic sensing applications.