Takeshi Hayashida

Hematite ( α − F e 2 O 3 ) is a prototypical room-temperature antiferromagnet whose time-reversal-odd magnetic structure has recently attracted renewed attention. While such magnetic symmetry can be characterized in terms of higher-order multipoles beyond the magnetic dipole, their manifestation in measurable physical phenomena has remained largely elusive. In this work, we investigate x-ray absorption near the Fe K -edge of hematite under an applied electric field, which explicitly breaks space-inversion symmetry. We observe an electric-field-induced x-ray nonreciprocal linear dichroism that reflects the time-reversal-odd nature of the magnetic order. A symmetry-resolved multipole analysis attributes this response to the electric-field-induced magnetic quadrupole and the magnetic toroidal octupole induced by the applied electric field. Furthermore, numerical simulations based on density functional theory reproduce the observed spectra, including their dependence on the antiferromagnetic domain and x-ray polarization. These results indicate that electric-field-modulated x-ray absorption measurements, combined with simulations, provide a framework for identifying the higher-order multipoles contributing to measurable physical responses in time-reversal-odd antiferromagnets, thereby facilitating the investigation of hidden symmetry properties in magnetic materials.

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.

Raman optical activity (ROA), the dependence of Raman intensity on the circular polarization of incident and scattered light, has traditionally been observed in chiral molecules and magnetic materials, where inversion or time-reversal symmetry is broken. Here we demonstrate that ROA can also arise in a centrosymmetric and nonmagnetic ferroaxial crystal. Using circularly polarized Raman spectroscopy on single-crystalline NiTiO_{3}, we observed a pronounced ROA signal in the cross-circular polarization configurations, which correlates with the ferroaxial domain structure. Our symmetry analysis, first-principles calculations of phonons, and tight-binding model calculations reveal that the natural ROA originates from the ferroaxial order and persists even within the electric dipole approximation. These results establish ROA as a powerful probe of ferroaxial order in centrosymmetric systems.

The linear electrogyration effect refers to optical rotation induced in proportion to an applied electric field. In comparison with other optical phenomena related to optical rotation, such as natural optical activity and the Faraday effect, the linear electrogyration effect is much less investigated because of the small magnitude of the electric field‐induced rotation. In this study, a microscopic mechanism of the linear electrogyration effect is discussed and it is demonstrated that the effect in a ferroaxial crystal NiTiO ₃ can be significantly pronounced at near infrared wavelengths where the magnetic dipole transitions are strong. Furthermore, it is shown that the effect is amplified in an artificial laminated structure in which the front and back sides of the ferroaxial crystal are laminated alternately with electrodes in between. It is expected this work stimulates further studies on electrogyration and its potential applications.

Antiferromagnets with broken time‐reversal () symmetry (‐odd antiferromagnets) have gained extensive attention, mainly due to their ferromagnet‐like behavior despite the absence of net magnetization. However, certain types of ‐odd antiferromagnets remain inaccessible by the typical ferromagnet‐like phenomena (e.g., anomalous Hall effect). One such system is characterized by a ‐odd scalar quantity, the magnetic toroidal monopole. To access the broken 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 ‐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 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 ‐odd antiferromagnets.