Role of magnetism on the martensitic transformation in Ni–Mn-based magnetic shape memory alloys

The effect of magnetism on the martensitic structural transformation has been analyzed through the evolution of the transformation temperatures of several Ni–Mn–Ga and Ni–Mn–In alloys subjected to high-temperature quenching and post-quench annealing thermal treatments. It is found that the atomic order variations associated with the thermal treatments affect the structural transformation in different ways depending on the character of the magnetic ordering in the austenitic and the martensitic phases. In particular, regardless of composition, the variation in the atomic order affects the martensitic transformation temperature only in those alloys in which at least one of the structural phases show magnetic order at the transformation temperature, whereas those transformations taking place between paramagnetic phases remain unaffected. The observed behaviors are explained in terms of the effect of the magnetic exchange coupling variations on the free energy difference between austenite and martensite. The results confirm the key role of magnetism in the martensitic transformation.