Magnetocrystalline anisotropy and magnetic field-induced strain of three martensites in Fe3Pt ferromagnetic shape memory alloys

Magnetocrystalline anisotropy and magnetic field-induced strain were investigated for three tetragonal martensite phases formed in Fe3Pt ferromagnetic shape memory alloys. The phases examined are the body-centered tetragonal (bct, c/a < 1) martensite with a degree of order S = 0.57, the face-centered tetragonal (fct, c/a < 1) martensite with S = 0.75 and the fct (c/a > 1) martensite with S = 0.88. It was found that the c-axis is the easy axis of magnetization for the two fct martensites and the hard axis of magnetization for the bct martensite. The uniaxial magnetocrystalline anisotropy constant, Ku, is nearly proportional to |1 − c/a| for the two fct martensites, but the relation is not satisfied for the bct phase. When a magnetic field is applied in the [0 0 1] direction, a large magnetic field-induced strain appears in the two fct martensites but not in the bct martensite. The strain in the field direction is a contraction in the fct martensite with c/a < 1, whereas it is an expansion in the fct martensite with c/a > 1. These behaviors are explained by evaluating the magnetic shear stress, which is given by Ku divided by the twinning shear.