A research team from School of Engineering, the University of Tokyo and RIKEN SPring-8 Center, in collaboration with researchers in France, has uncovered the hidden magnetism of hematite (α-Fe₂O₃), the main component of common rust, by combining x-ray absorption spectroscopy under an electric field with the first-principles calculations.
Hematite (α-Fe₂O₃), a form of iron oxide and the primary component of rust, is a well-known antiferromagnet in which magnetic dipole moments, called spins, align in an antiparallel fashion. It has recently drawn renewed attention as a material exhibiting "altermagnetism" that is a newly recognized magnetic class promising for next-generation spintronic devices. While theoretical calculations have predicted even more complex forms of magnetic order, known as higher-order multipoles, their experimental detection has remained elusive.
To clarify hidden magnetic states in hematite, the research team has developed a new measurement technique: x-ray absorption spectroscopy under an applied electric field. Applying an electric field breaks space-inversion symmetry, enabling the extraction of minute magnetic signals that were previously hidden by mutual cancellation. Specifically, the team conducted x-ray absorption measurements under an applied electric field at SPring-8 beamline BL19LXU in Japan, using x-rays tuned near the iron K-absorption edge. As a result, they successfully detected x-ray absorption signals responding to the applied electric field—a phenomenon termed as electric-field-induced x-ray nonreciprocal linear dichroism. By combining these experimental findings with first-principles calculations, they revealed that the electric field induces femtometer-scale displacements of iron ions relative to oxygen ions, a distance comparable to the size of an atomic nucleus. This structural shift broke the symmetry and rendered the hidden multipoles observable in the x-ray signals through their nonreciprocal nature.
The methodology established in this study, combining x-ray absorption measurements under an applied electric field with theoretical simulations, is expected to become a powerful tool for elucidating the hidden magnetic properties of a wide range of materials beyond hematite. Furthermore, this approach holds promise as a novel analytical technique capable of quantitatively capturing minute, electric-field-induced ionic displacements on the femtometer scale—phenomena that have been remained elusive with conventional methods.
Figure. (Top) Experimental setup used in this study. (Bottom) Electric-field-induced ionic displacements responsible for the observed electric-field-induced x-ray nonreciprocal linear dichroism. Note that the magnitude of the displacements is exaggerated for clarity.
Papers
Journal: Physical Review X
Title: Electric-field-induced X-ray Nonreciprocal Dichroism in Hematite
Authors: Takeshi Hayashida, Koei Matsumoto, Keito Arakawa, Yves Joly, Sergio Di Matteo, Kenji Tamasaku, Yoshikazu Tanaka, and Tsuyoshi Kimura*
DOI: 10.1103/p5ry-x2wr