Electric Field-Induced Nonreciprocal Directional Dichroism in a Time-Reversal-Odd Antiferromagnet

Antiferromagnets with broken time-reversal ( $T$ ) symmetry ( $T$ -odd antiferromagnets) have gained extensive attention, mainly due to their ferromagnet-like behavior despite the absence of net magnetization. However, certain types of $T$ -odd antiferromagnets remain inaccessible by the typical ferromagnet-like phenomena (e.g., anomalous Hall effect). One such system is characterized by a $T$ -odd scalar quantity, the magnetic toroidal monopole. To access the broken $T$ symmetry in such a system, a unique nonreciprocal optical phenomenon, electric field-induced nonreciprocal directional dichroism (E-induced NDD), is employed. Signals of E-induced NDD are successfully detected in a $T$ -odd antiferromagnet, Co₂SiO₄, whose magnetic structure is characterized by the magnetic toroidal monopole. Furthermore, by spatially resolving the E-induced NDD, spatial distributions of a pair of domain states related to one another by the $T$ operation are visualized. The domain imaging revealed the inversion of the domain pattern by applying a magnetic field, which is explained by trilinear coupling attributed to the piezomagnetic effect. The observation of E-induced NDD highlights unique functionalities of $T$ -odd antiferromagnets.