Hysteresis of the martensitic phase transition in magnetocaloric-effect Ni-Mn-Sn alloys

We have investigated the hysteresis of the temperature- and magnetic field-induced martensitic phase transition in Ni(50.4)Mn(34.0)Sn(15.6). Specifically, we have characterized the fraction of martensite and cubic Heusler phase present along a variety of temperature and magnetic field paths using magnetization as a proxy for the fraction of phases present. We present these results and discuss (1) the thermodynamics of the thermal- and magnetic field-induced hysteresis and (2) the impact of this hysteresis on the magnetocaloric effect of the alloy. We demonstrate that both temperature and magnetic field are equivalent driving forces for the phase transition and result in equivalent hysteresis behavior linked through the magnetic Gibbs free energy. The hysteresis reduces the useful magnetocaloric effect of the alloy by allowing only a limited fraction of the alloy to transform cyclicly between the martensite and the austenite phase under application and removal of magnetic fields up to 9 T, and by dissipating work through irreversible energy loss. While the importance of a large thermodynamically reversible magnetic entropy change is generally accepted, our results suggest that the hysteresis behavior is equally critical in evaluating the effective magnetocaloric effect of a material.