Magnetoresistance governs apparent magnetic-field enhancements in water electrolysis

The origin of the recently discovered magnetic-field-induced enhancement of the oxygen evolution reaction (OER) remains controversial, with partly contradictory experimental observations and several competing mechanistic interpretations. Using epitaxial multilayers as well-defined model systems, we disentangle the various effects induced by the applied magnetic field, and establish a quantitative correlation between the observed OER current increments and magnetoresistance (MR) effects. In parallel, operando magneto-optical Kerr effect (MOKE) microscopy was performed to evaluate the frequently proposed role of domain-wall removal. Surprisingly, the measured OER enhancements under magnetic field do not correlate with the appearance or disappearance of domain walls. We find no catalytic current enhancements when MR effects are eliminated. Our work thus reveals the strictly magnetoresistive origin of the observed catalytic current increase in transition metal oxides under applied magnetic field and the absence of effects related to changes in surface long-range spin order. Moreover, we highlight the need for advanced methodology to separate between effects arising from MR and the long range order at the surface.