Direct measurements of the entropy change and its history dependence in Ni-Mn-Ga alloys

The role of hysteresis on materials with giant magnetocaloric effect (GMCE) was pointed out by comparing isothermal entropy change measured on a Ni(55)Mn(20)Ga(25) single crystal presenting a first order magnetostructural phase transition. The entropy change is determined by a laboratory Peltier calorimeter with magnetic field of up to 2 T. The measurements performed have allowed to compare the entropy change in partial or complete phase transitions induced by temperature or magnetic field, respectively. The sequence in the temperature and field change affects dramatically the observed MCE demonstrating that a control on both quantities must be accurately performed in order to extract the whole available entropy change. This kind of behavior can be explained if hysteresis in the first order phase transition is taken into account and a correct field-temperature sequence is proposed in order to maximize the MCE. (C) 2008 American Institute of Physics.