Probing Site-Specific Magnetism in Time-Reversal-Odd Antiferromagnet via Electric Field-Induced Nonreciprocal Directional Dichroism

We investigated the optical absorption spectra of the time-reversal-odd antiferromagnet ErCrO_{3} under an applied electric field (E). This material, with its two distinct magnetic sites (Er^{3+} and Cr^{3+}), exhibits successive antiferromagnetic (AFM) transitions at T_{N}≈133 K and T_{SR}≈10 K. We observed nonreciprocal directional dichroism induced by E (ENDD) in the AFM phases, spanning the near-infrared to visible light regions. The phenomenon is explained by the electrotoroidic effect, where a magnetic toroidal moment is induced by the applied electric field. We also found that the spatial distributions of ENDD-reflecting magnetic domain structures-obtained at photon energies corresponding to Er^{3+} f-f and Cr^{3+} d-d transitions, were in good agreement. The result indicates a strong interplay between Er^{3+} 4f and Cr^{3+} 3d moments in the AFM phases of this rare-earth orthochromite. The combination of ENDD spectroscopy and microscopy is a powerful tool for elucidating site-specific magnetic information in time-reversal-odd antiferromagnets such as altermagnets.