Temperature dependence of rearrangement of martensite variants by magnetic field in 10M, 14M and 2M martensites of Ni–Mn–Ga alloys

We have examined the condition for the rearrangement of martensite variants (RMV) by magnetic field in a wide temperature range for three types of Ni–Mn–Ga ferromagnetic shape memory alloys: a Ni2MnGa with the 10M martensite, a Ni2.02Mn1.09Ga0.89 (at.%) with the 14M martensite and a Ni2.14Mn0.92Ga0.94 with the 2M martensite structure. The RMV by magnetic field occurs at any temperature for the 10M martensite, while it occurs only in a limited temperature range for the 14M martensite and does not occur at any temperature for the 2M martensite. The reason of such difference is quantitatively explained by evaluating two shear stresses: the magnetic shear stress acting across a twinning plane τmag and the stress required for the twinning plane movement τreq. That is, by comparing the values of τmag and τreq, we have confirmed that the following condition is universal regardless of temperature, field direction and structure of the martenstie phase: the value of τmag is larger than τreq when the RMV occurs by magnetic field and vice versa.