Rearrangement of martensite variants in iron-based ferromagnetic shape memory alloys under magnetic field

We have investigated magnetic field-induced strain (MFIS) associated with rearrangement of martensite variants and its corresponding magnetization process in a disordered Fe-31.2Pd(at%) single crystal and an ordered Fe3Pt single crystal, exhibiting a cubic to tetragonal martensitic transformation at 230 K and 85 K, respectively. When magnetic field is applied along [001] direction to a specimen with a multi-variant state, it expands along the field direction for Fe-31.2Pd and contracts for Fe3Pt, because the variants whose easy axis of magnetization (a axis for Fe-31.2Pd and c axis for Fe3Pt) lies along the field direction is selected to grow. The fraction of such variants reaches 100% for Fe-31.2Pd but does not for Fe3Pt. In the field removing process, a part of the MFIS recovers for Fe3Pt but does not for Fe-31.2Pd. From the magnetization curve, the energy dissipated due to the rearrangement of variants by magnetic field is obtained to be about 260 kJ/m(3) for Fe-31.2Pd and about 180 kJ/m(3) for Fe3Pt. Concerning Fe-31.2Pd, this value is roughly the same as that evaluated by stress-strain curves, suggesting that the rearrangement of variants by magnetic field takes essentially the same path as that by external stress. Based on these results and magnetocrystalline anisotropy constants of martensite phases, the mechanism of rearrangement of variants under magnetic field is discussed from a macroscopic point of view.