Magnetically induced reorientation of martensite variants in constrained epitaxial Ni-Mn-Ga films grown on MgO(001)

Abstract

Magnetically induced reorientation (MIR) is observed in epitaxial orthorhombic Ni-Mn-Ga films. Ni-Mn-Ga films have been grown epitaxially on heated MgO(001) substrates in the cubic austenite state. The unit cell is rotated by 45° relative to the MgO cell. The growth, structure texture and anisotropic magnetic properties of these films are described. The crystallographic analysis of the martensitic transition reveals variant selection dominated by the substrate constraint. The austenite state has low magnetocrystalline anisotropy. In the martensitic state, the magnetization curves reveal an orthorhombic symmetry having three magnetically non-equivalent axes. The existence of MIR is deduced from the typical hysteresis within the first quadrant in magnetization curves and independently by texture measurement without and in the presence of a magnetic field probing micro structural changes. An analytical model is presented, which describes MIR in films with constrained overall extension by the additional degree of freedom of an orthorhombic structure compared to the tetragonal structure used in the standard model.