Rare-earth orthoferrites (RFeO3, R: rare-earth ions) have long been studied for their unique antiferromagnetic properties arising from the interplay between two magnetic sublattices, formed by Fe³⁺ and R³⁺ ions. From a modern perspective, the antiferromagnetic order in RFeO₃ can be classified as altermagnetic, characterized by macroscopic time-reversal symmetry breaking without net magnetization. In this study, we focus on the altermagnetic Γ₁ phase of DyFeO₃, which exhibits time-reversal symmetry breaking while spontaneous magnetization is forbidden. We successfully visualize altermagnetic domains in this phase using electric field-induced nonreciprocal directional dichroism (NDD), in which the optical absorption changes with the applied electric field. We further demonstrate that the domain structure is influenced by an external magnetic field, mediated by piezomagnetic coupling through internal strain. Our results establish electric field-induced NDD as a powerful tool for probing altermagnetic domain structures, where conventional magneto-optical techniques such as the Faraday or Kerr effect are ineffective.
Kobayashi et al. (Fri,) studied this question.