Magnetic Field, Stress and Temperature Control of Phase Transitions in Ni$_{55}$ Mn$_{20}$ Ga$_{25}$ Shape Memory Alloy

The phase transitions of off-stoichiometry single crystals of Ni
₂
MnGa, observed by varying the sample temperature and the applied stress, are shown to be strongly influenced by the presence of a magnetic field. The single crystals present a first order phase transition from a paramagnetic-austenite to a ferromagnetic-martensite: stress, temperature and field may cooperate to induce the phase transition when a proper choice of experimental procedures is made. Strains of the order of several percent are observed and connected with super-plastic and super-elastic phenomena. Such phenomena are strongly affected by a unidirectional magnetic field, which produces effects similar to an applied stress. Entropy changes due to the application of magnetic field and stress are analyzed in the framework of the Clausius-Clapeyron equation.