Study of austenite-martensite transformation in Ni-Mn-Ga magnetic shape memory alloy

Ni 49.7 Mn 29.1 Ga 21.2 magnetic shape memory sample showing magnetic-field-induced strain of 5.7 % in the magnetic field 0.5 T was studied at different conditions during austenite-martensite and martensite-austenite transformations. Transformation temperatures T A = 317 K, T M = 308 K were determined by AC susceptibility measurement Structure parameters obtained from X-ray measurement are a A = 0.584 nm for austenite and a M = bM = 0.595 nm, c M = 0.561 nm for martensite which has five-layered modulated structure. Mechanical stress up to 4 MPa and magnetic field up to 1 T were applied in order to study their influence on the transformations. Totally 39 transformation cycles were measured. The thermoelastic strains during transformations vary from -3.6 % to +1.8 %. Simultaneous change of magnetization and strain has been observed. Thermomechanical and magnetic history of the sample seems to have no or only very weak influence on the character of austenite-martensite transformation and no clear trend emerges in our experiments.