Origin of heterogeneous magnetostriction: A case study of nanoprecipitates-bearing Fe-Mo alloys

Ferromagnets with structural nanoheterogeneities have recently been found to exhibit large magnetostriction, as reported in a considerable number of Fe-based alloys soluted with non-magnetic elements. Unlike homogeneous ferromagnets, where their large magnetostriction stems from strong spin-orbit coupling, however origin of the large heterogeneous magnetostriction is still in debate. In this work, we performed a comparative study between the cubic supersaturated solid solution Fe₉₆Mo₄ and its aged counterpart with low-symmetry nanoprecipitates. The results showed that the saturation magnetostriction can be enhanced from 19 ppm to 40 ppm by 110% through introducing coherent low-symmetry nanoprecipitates, despite of reduced saturation magnetization and weakened net magnetocrystalline anisotropy. Further structural characterizations revealed that the coherent nanoprecipitates result in an apparent tetragonal lattice distortion of the cubic matrix, i.e., the spontaneous magnetostriction constant (lattice strain) is enlarged. These results indicate that the heterogeneous magnetostriction cannot merely be attributed to the spin-orbit coupling, but also is closely related to the lattice distortion induced by the coherent low-symmetry nano-heterogeneities.