Simultaneous magnetooptic observation and thermomagnetic analysis of phase transitions in shape-memory Ni-Mn-Ga alloys

Direct observations of the austenite and martensite twin and magnetic domain structures are combined with thermomagnetic analysis (low-field AC susceptibility temperature measurements) of phase transitions in ferromagnetic shape memory alloys Ni2+xMn1-xGa. Thermal hysteresis and magnetization jumps are clearly resolved by both techniques. Large Barkhausen-like jumps associated with discontinuous phase boundary and martensite twin motion are observed during the austenite-martensite transformation in contrast to the smooth ferromagnetic-paramagnetic transition near the Curie temperature