Magnetization process associated with rearrangement of martensite variants in iron-based ferromagnetic shape memory alloys

Magnetization processes of Fe-31.2Pd(at.%) and Fe3Pt (Sapproximate to0.8) single crystals in martensite state have been examined in order to confirm the propriety of the condition for the rearrangement of variants under magnetic field: tau(mag)>tau(req), where tau(mag) is the magnetic shear stress and tau(req) is the shear stress required for the rearrangement. When the magnetic field is applied along the [001] direction of each specimen, the magnetization curve shows a large hysteresis due to the rearrangement of variants. Its area, i.e., energy dissipation, is nearly the same as that obtained by stress-strain curves, suggesting the path of the rearrangement of variants by magnetic field is essentially the same as that by external stress. From the magnetization curve, the uniaxial magnetocrystalline anisotropy constant K-u is estimated: it is about 350 kJ/m(3) for Fe-31.2Pd at 77 K, and is about 500 kJ/m(3) for Fe3Pt at 4.2 K. The maximum of tau(mag), being evaluated from K-u and twinning shear, is about 2.8 MPa for Fe-31.2Pd at 77K and is about 4.3 MPa for Fe(3)Rt at 4.2K. For Fe-31.2Pd, the value Of tau(req) is obtained by tensile tests at 80 K to be 0.5-2.5MPa, and the above condition is satisfied. The above condition is also confirmed to be adequate by examining the influence of field direction on the magnetic filed-induced strain.