Observation of a magneto-nonlinear Hall effect in an altermagnet candidate

Nonlinear Hall effects have emerged as defining probes of quantum geometry. Here we observe a Hall response that is nonlinear in the magnetic field rather than the electric field, in Mn5Si3 thin films, an altermagnet candidate, in the canonical Hall geometry: a high-field Hall conductivity component that grows quadratically with an out-of-plane field. Opposite-sign Berry curvatures associated with opposite-spin bands nearly cancel at low fields, but Zeeman band shifts at high fields lift this cancellation and generate increasing net total Berry curvature. This magnetic-field-tuned Berry-curvature imbalance can generally produce a nonlinear Hall conductivity, and the observed quadratic dependence in Mn5Si3 is consistent with chiral next-nearest-neighbor hopping involving magnetic exchange and Haldane-like flux phases. Moreover, reversal of magnetic order under opposite fields reverses the hopping chirality, making this quadratic contribution odd in field. Beyond fundamental advances, the nonlinear response remains unsaturated up to 57 T, suggesting possible opportunities for high-field Hall sensing.