Noncubic crystallographic symmetry of a cubic ferromagnet: Simultaneous structural change at the ferromagnetic transition

Conventional diffractometry over the past decades has revealed that the ferromagnetic transition, an ordering of the magnetic moment, involves no crystal structure change in general; thus a cubic paramagnet has been considered to transform into a cubic ferromagnet upon a ferromagnetic transition. However, with high-resolution synchrotron x-ray diffractometry (XRD), we show direct evidence for (i) the noncubic symmetry of typical cubic ferromagnets CoFe₂O₄, Tb0.3Dy0.7Fe₂ (Terfenol-D), and DyCo₂ and (ii) a simultaneous structural change at ferromagnetic transition temperature (TC) in DyCo₂. These results suggest that ferromagnetic transition is also a structural transition, yielding a low crystallographic symmetry that conforms to the spontaneous magnetization (MS) direction. In situ XRD observation further revealed that the switching of magnetic domains is also a switching of the noncubic crystallographic domains, in the same way as the ferroelectric domain switching. By a phenomenological approach based on magnetoelastic coupling, we proved theoretically that structure change upon a ferromagnetic transition is a general effect for all cubic ferromagnets. Our work leads to a simple and unified mesoscopic explanation for both magnetostriction in ferromagnets and electrostrain effect in ferroelectrics. It may also provide insight for developing highly magnetoresponsive materials.