Field and temperature induced giant strain in single crystal Ni-Mn-Ga

We analyze the effect of temperature coupled to dc or ac magnetic fields on the strain properties of an oriented single crystal of Ni₂MnGa. The sample has an austenite-martensite phase transformation temperature T_M=160 K and reverse transformation at T_A=180 K. No external stress was applied before or during the tests. It is shown that the shape memory features may be deeply influenced by the presence of external magnetic fields: during heating or cooling through the transformation an ac field applied along the [100] direction is able to trigger an irreversible strain of 2.5% [100] at T_A. This value can be compared with a 1.2% [100] strain with a dc field and about 0.17% in absence of field (both at T_A). The peak cyclic strain at T_A is 0.45% when a cyclic field is applied at constant temperature. It is shown that, in this single crystal, twin rearrangements leading to large values of strain only occur in limited temperature ranges, in correspondence with the phase transformations at T_M and T_A