The martensitic transformation (MT) in metamagnetic shape memory alloys takes place between a ferromagnetic austenite and a weak magnetic martensitic phase, so the large magnetization drop taking place at the MT allows the induction of the reverse MT from martensite to austenite by an applied magnetic field. Interesting properties related to this effect are being actively investigated because of their potential applications (sensors, actuators, damping, energy harvesting, etc.). Furthermore, since the different magnetic exchange interactions in each of the structural phases result in a discontinuity in the magnetic entropy at the MT, the magnetic induction of the MT may lead to giant magnetocaloric effects, which may produce adiabatic cooling by application of a magnetic field. These large magnetocaloric effects make these materials especially interesting for magnetic refrigeration applications.
As a consequence of the counterbalance between the vibrational and magnetic contributions to the entropy change at the transformation, the forward MT can be inhibited (arrested) on cooling by the application of a strong magnetic field, and hence the austenitic phase can be stabilized at low temperatures, far below the MT temperature. On heating, this metastable austenite transforms to martensite through an ‘anomalous’ forward MT. The characteristics of the retained austenite and the entropy change linked to such a peculiar MT has been evaluated in a metamagnetic alloy. The low temperature range where the anomalous MT occurs, favours the predominance of the magnetic contribution to the total entropy change (at the expense of the vibrational term), and this leads to the observation of a direct magnetocaloric effect (DS<0), contrary to the inverse effect (DS>0) observed in the conventional forward MT. A large adiabatic temperature change of 9 K under a moderate applied field of 10 kOe is reported for the ‘anomalous’ forward MT.