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 bet martensite. The uniaxial magnetocrystalline anisotropy constant, K-u, is nearly proportional to vertical bar 1 - c/a vertical bar for the two fct martensites, but the relation is not satisfied for the bet 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 bet 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 K-u divided by the twinning shear. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.