LAUSR

A very large negative magnetoresistance (LNMR) is observed in the insulating regime of the antiferromagnet ${\mathrm{BaMn}}_{2}{\mathrm{Bi}}_{2}$ when a magnetic field is applied perpendicular to the direction of the sublattice magnetization. A high perpendicular magnetic field eventually suppresses the insulating behavior and allows ${\mathrm{BaMn}}_{2}{\mathrm{Bi}}_{2}$ to reenter a metallic state. This effect is seemingly unrelated to any field-induced magnetic phase transition, as measurements of magnetic susceptibility and specific heat did not find any anomaly as a function of magnetic fields at temperatures above $2\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The LNMR appears in both current-in-plane and current-out-of-plane settings, and Hall effects suggest that its origin lies in an extreme sensitivity of conduction processes of holelike carriers to the infinitesimal field-induced canting of the sublattice magnetization. The LNMR-induced metallic state may thus be associated with the breaking of the antiferromagnetic parity-time symmetry by perpendicular magnetic fields and/or the intricate multiorbital electronic structure of ${\mathrm{BaMn}}_{2}{\mathrm{Bi}}_{2}$.