Investigation of the shape-memory properties in the ferromagnetic Ni2MnGa system

Transition metal compounds with compositions close to Ni2MnGa are studied. The materials investigated Ni2.19Mn0.81Ga, Ni1.9Co0.1MnGa, Ni1.9CU0.1MnGa, Ni2MnGa0.95In0.05, and Ni2MnGa0.95Sn0.05, as well as Ni2MnGa show the shape memory effect. The dependence of magnetic properties and the properties of the structural phase transition on composition is investigated. X-ray structure determination, magnetisation measurements, thermal expansion and magnetostriction measurements are performed. The specimens are prepared and characterised at room temperature using X-ray powder diffraction. The structure of Ni2.19Mn0.81Ga is found to be orthorhombic, the other materials crystallise in the cubic Heusler L21 structure. Measurements of the lattice properties are important due to the intrinsic coupling of magnetic degrees of freedom with the lattice. The response of the lattice to temperature changes and applied magnetic fields have been probed by the use of the aforementioned techniques. The experimental results are presented and discussed. Various theoretical approaches are considered and compared with the obtained data. These data support the hypothesis the dominant factor influencing the martensitic transition is the conduction electron density.