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| Spin-wave spectrum of CoFe₂O₄ measured on the MAPS spectrometer (left) and the corresponding spin-wave calculation (right). The large ~60 meV splitting between the two magnon branches originates from the strong imbalance of molecular fields on the A and B cation sites, as illustrated in the inset crystal structure. Image Credit: KyotoU / Yusuke Nambu |
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.
Title: Anisotropic Band-Split Magnetism in Magnetostrictive CoFe2O4
Authors: Harry Lane, Guratinder Kaur, Masahiro Kawamata, Yusuke Nambu, Lukas Keller, Russell A. Ewings, David J. Voneshen, Travis J. Williams, Helen C. Walker, Dwight Viehland, Peter M. Gehring, and Chris Stock
Source/Credit: Kyoto University
Reference Number: ms120125_02
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