Magnetoelastic equilibrium and super-magnetostriction in highly defected pre-transitional materials

Abstract Large hysteresis or irreversibility of a magnetic field-induced displacive transformation is a major obstacle for its magnetostrictive applications. The problem is the difficulty of nucleating the product phase because the lattice misfit between the parent and product phases is usually large, which is unavoidable because this is a very reason for the expectation of large magnetostriction. This work reported a nano-embryonic mechanism of giant and anhysteretic magnetostrictions in pre-transitional materials with stress-generating defects like dislocations and coherent nano-precipitates. We showed that the stress concentration regions generated by these defects could locally eliminate the nucleation barrier and thus induce equilibrium nano-sized embryos of orientation variants of the product phase. The corresponding athermal process, in fact, excludes a necessity of thermal nucleation that is a main source of hysteresis. The induced embryos are in a magnetoelastic equilibrium if the parent and product phases have different magnetic properties. Shifting this equilibrium by applying a magnetic field changes the equilibrium volume fraction of embryos, resulting in a giant and anhysteretic strain response that is comparable or even greater than magnetostriction of the expensive rare earth containing magnetostrictors. This paper also demonstrated that the magnetocrystalline anisotropy plays a significantly role on the values of embryonic magnetostriction.