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 (T-C) in DyCo2. These results suggest that ferromagnetic transition is also a structural transition, yielding a low crystallographic symmetry that conforms to the spontaneous magnetization (M-S) 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.
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