arXiv subjects

Koei Matsumoto

Publications and source records attributed to Koei Matsumoto.

3 recordsLinked to original sources

arXiv2026

Electric-field-induced X-ray Nonreciprocal Dichroism in Hematite

Hematite (alpha-Fe2O3) 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 (E-induced XNLD) that reflects the time-reversal-odd nature of the magnetic order. Numerical simulations based on ab-initio density functional theory reproduce the observed spectra, including their dependence on the antiferromagnetic domain and x-ray polarization. Furthermore, a symmetry-resolved multipole analysis reveals that this response originates from the magnetic quadrupole and the magnetic toroidal octupole induced by the applied electric field. These results demonstrate that electric-field-modulated x-ray absorption provides direct access to the antiferroic order of higher-order multipoles in time-reversal-odd antiferromagnets, thereby establishing a general framework to uncover hidden symmetry properties in magnetic materials.

cond-mat.mtrl-sci↗

arXiv2025

Electric-field-induced magnetic toroidal moment and nonlinear magnetoelectric effect in antiferromagnetic olivines

Beyond conventional electric and magnetic monopoles, electric and magnetic toroidal monopoles, which are rank-0 multipoles distinguished by opposite parities under spatial inversion and time reversal, can exist in nature. The recent observation of electric-field-induced directional dichroism in antiferromagnetic olivine Co$_2$SiO$_4$ has provided the first concrete example of a magnetic toroidal monopole; however, its microscopic origin remains elusive. Here, we propose a minimal spin model that incorporates magnetoelectric coupling via the $d$-$p$ hybridization mechanism and analyze it within the mean-field approximation. The model qualitatively reproduces the experimentally observed temperature dependence of the dielectric constant and its pronounced sensitivity to the direction of the applied electric field. Furthermore, it elucidates the temperature evolution of the magnetic toroidal monopole and the strong electric-field-direction dependence of the magnetic toroidal moment. Our calculations also predict a second-order nonlinear magnetoelectric response, consistent with the symmetry classification of Co$_2$SiO$_4$ as an altermagnet. Additionally, we demonstrate that the same framework is applicable to other antiferromagnetic olivines with analogous magnetic order, indicating the robustness and generality of the toroidal-type magnetoelectric response in this material family.

cond-mat.str-el↗

arXiv2024

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, we employ a unique nonreciprocal optical phenomenon, electric field-induced nonreciprocal directional dichroism (E-induced NDD). We successfully detected signals of E-induced NDD in a T-odd antiferromagnet, Co2SiO4, whose magnetic structure is characterized by the magnetic toroidal monopole. Furthermore, by spatially resolving the E-induced NDD, we visualized spatial distributions of a pair of domain states related to one another by the T operation. 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. Our observation of E-induced NDD provides a unique approach to accessing the order parameter in T-odd antiferromagnets.

cond-mat.mtrl-sci↗