Magnetostriction is the change in the dimensions of a material in response to an applied magnetic field. It's a property that is extremely useful for a wide variety of technical applications such as sensors, actuators and transducers.
This study focussed on the material CoFe2O4. Although one of many known magnetostrictive materials, CoFe2O4 is of particular interest because of its stability and because it is made of abundant transition metal ions.
Previous studies have focussed on nanoparticle and thin film samples, used in devices. However, to understand the physics that underpins their performance these researchers instead studied bulk CoFe2O4. They used X-ray diffraction and neutron diffraction at the Paul Scherrer Institut to study the structure, and neutron spectroscopy on Merlin and MAPS at ISIS for the dynamics. Their aim was to find out what influence these structural and magnetic properties had on the material's magnetostrictive behaviour.
“Our structural measurements show a kink in the lattice parameters at the onset of long-range magnetic order, confirming strong coupling between the magnetism and structure," explains Harry Lane, from The University of Manchester, the study's lead author. “But our neutron spectroscopy results were not what we initially expected."
Their inelastic neutron scattering measurements showed the presence of two strongly dispersive spin wave modes with a large inter-mode gap. Rather than this being caused by the two drastically different ground states of the two magnetic ions, Fe3+ and Co2+, their analysis of the inelastic scattering suggests that this is not the case.
Instead, this large gap between the modes is more likely to be due to the strong magnetic interactions in the spinel structure. The fine details of the spectra also reveal a significant anisotropy, where the magnetic moments favour pointing along particular directions, which is driven by the complicated competition between strong interactions and local effects. From these results, the group were able to model the system, leading to a comprehensive characterisation of the material.
“The strong interactions demonstrated in this study are important for two reasons," explains ISIS beamline scientist David Voneshen. “Firstly, they give rise to a high ordering temperature, which means the devices work at room temperature and above. Then the anisotropy means that, rather than the system evolving gradually in response to an externally applied field, it jumps to a completely different direction, greatly amplifying the magnetostrictive response."
The work was supported by the Royal Commission for the Exhibition of 1851, EPSRC, STFC and the Royal Society of Edinburgh.
DOI:10.1002/adfm.202516830
