Simultaneous direct measurements of conventional and inverse magnetocaloric effects in Ni–Mn-based Heusler alloys using lock-in thermography technique

The second-order conventional and first-order inverse magnetocaloric effects (MCEs) in Ni–Mn-based quaternary Heusler alloys have been systematically investigated by means of the lock-in thermography technique, which enables the direct measurement of the MCE-induced temperature change in a periodic magnetic field. Through systematic measurements of the temperature dependence of the MCE signals, the tuning of the conventional and inverse MCEs with temperature for the same Heusler alloys has been demonstrated, where the phase transitions responsible for the MCEs are clearly distinguished. The lock-in thermography measurements show that some Ni–Mn-based Heusler alloys exhibit much smaller temperature changes due to the inverse MCEs in the periodic field as compared to the conventional MCEs, even though they exhibit a larger magnetic entropy change across the first-order transition responsible for the inverse MCEs. We discuss the origin of this behavior in terms of the field-induced entropy change and thermal hysteresis of the alloys. These findings will be useful not only in accelerating the optimization of inverse MCE materials but also in understanding the mechanism of the MCEs.