Gyrotropic Fingerprints of Magnetic Topological Insulator-Unconventional Magnet Interfaces

Abstract

Unambiguously identifying unconventional magnetic orders requires probes that are directly sensitive to their momentum-dependent spin-split band structures. Here, we employ a framework based on Zeeman quantum geometry to study magnetotransport at the interface between a magnetic topological insulator and an unconventional magnetic insulator. By choosing the magnetic layer to be insulating, we ensure that the transport response originates solely from the proximity-induced magnetic exchange field, eliminating contributions from itinerant magnetic carriers. We focus on the linear intrinsic gyrotropic magnetic (IGM) response, which naturally decomposes into conduction and displacement current components governed by the Zeeman Berry curvature and the Zeeman quantum metric, respectively. We uncover a universal hierarchy in which the transverse displacement IGM response exhibits characteristic even-fold angular harmonics for magnetic orders ranging from $p$- to $i$-wave, while the longitudinal IGM response distinguishes the parity of the magnetic order through robust sign-reversal patterns. In contrast, the conduction IGM component remains largely insensitive to the underlying magnetic symmetry. Consequently, the displacement IGM current emerges as a high-fidelity symmetry fingerprint of unconventional magnetic order. Using realistic parameter estimates for experimentally accessible heterostructures, we demonstrate that these signatures are well within measurable ranges, establishing Zeeman quantum geometry as a powerful and general framework for characterizing unconventional magnetic insulators via their gyrotropic transport responses.