Untangling magnetism

Kyoto, Japan -- Magnetostriction and spin dynamics are fundamental properties of magnetic materials.  Despite having been studied for decades, finding a decisive link between the two in bulk single crystals had remained elusive. That is until a research team from several institutions, including Kyoto University, sought to examine these properties in the compound CoFe₂O₄, a spinel oxide (chemical formula AB₂O₄) widely used in numerous medical and industrial applications.

Spin dynamics describe how the tiny magnetic moments of atoms in a magnetic material interact and change orientation with time, while magnetostriction describes how a material changes shape or dimensions in response to a change in magnetization. These properties are central to the operation of sensors and actuators that employ magnetoelastic materials that change their magnetization under mechanical stress.

Rare-earth compounds often exhibit large magnetoelastic effects, explaining their desirability, but they also face constraints in resources, cost, and operating temperature. The spinel oxide CoFe₂O₄ combines non‑rare‑earth chemistry with strong room‑temperature magnetostriction, and has a high Curie temperature, at which materials start to lose their magnetism, which is essential to many devices that operate at or above ambient temperature.

"Spinel compounds can exhibit A/B site mixing (in AB₂O₄) that impacts magnetic anisotropy and dynamics," explains Yusuke Nambu of Kyoto University. "Furthermore, subtle tetragonal distortion accompanies magnetic ordering, yet the strain can be below the detection limit for powder diffraction."

"By combining neutron spectroscopy with diffraction, advanced analysis, and modeling," adds Nambu, "we were able to quantify that link between spin dynamics and magnetostriction."

By measuring the spin dynamics over a broad energy range with neutron spectroscopy on a single crystal, the team identified a large band splitting of about 60 millielectronvolts (meV) between two magnon branches, a 3 meV anisotropy gap in the lower branch, and an avoided crossing near 75 meV in the upper branch. The researchers were then able to reproduce these important features quantitatively using theoretical calculations based on spin-wave theory.

This led the team to an elucidation of the mechanism of the strong magnetostriction in CoFe₂O₄. The A/B site mixing generates enormous internal magnetic fields that effectively lock each atomic magnetic moment in place within a given structural (tetragonal) domain.  Thus, the application of an external magnetic field leads to domain switching, rather than a global spin rotation, thereby amplifying magnetostriction.

This study lays the groundwork for a number of design metrics, most notably the band-splitting magnitude for engineering magnetostrictive functionality. In particular, the spin-locking / domain-switching mechanism provides a blueprint for low‑field, high‑sensitivity magnetostrictive sensors and actuators.

The opposite chiralities of the split magnon branches open possibilities in spin‑caloritronics. This approach can be generalized and applied to other spinels or ferrimagnets, offering band-splitting magnitude and related quantities as practical design metrics. Additionally, fine control of tetragonal strain and domain engineering support compact, low‑power devices at room temperature.

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The paper "Anisotropic Band‑Split Magnetism in Magnetostrictive CoFe₂O₄." appeared on 12 November 2025 in Advanced Functional Materials, with doi: 10.1002/adfm.202516830

About Kyoto University

Kyoto University is one of Japan and Asia's premier research institutions, founded in 1897 and responsible for producing numerous Nobel laureates and winners of other prestigious international prizes. A broad curriculum across the arts and sciences at undergraduate and graduate levels complements several research centers, facilities, and offices around Japan and the world. For more information, please see: http://www.kyoto-u.ac.jp/en