Magnetoelastic behavior of Ni-Mn-Ga martensitic alloys

The magnetostriction measurements of almost stoichiometric single crystalline Ni2MnGa compound with T-m < T-c (T-m and T-c are the martensitic and ferromagnetic transformation temperatures respectively) at constant temperatures have been performed in a temperature range of 100-400 K. The measurements resulted in a temperature evolution of magnetostrain, <epsilon>, with two extromums caused by soft mode condensation and martensitic transformation. Magnetic field induced strain of 0.4% along [ 100] his been observed near T-m. In turn, the dilatometric mode of measurements along [ 100] under magnetic field (either constant or cyclic) led to 1.5% of permanent strain in martensite, Using a phenomenological approach both the anisotropic and volume magnetoelastic constants have been evaluated in cubic phase to be equal to - 1.2 MPa and -0.4 GPa respectively. The experimentally observed smearing of Curie temperature in single crystalline Ni51.2Mn31.1Ga17.7 alloy with T-m > T-c is quantitatively explained by the spatially inhomogeneous mass density of martensitic phase and volume magnetostriction.