Hysteresis and Kinetics of the First-Order Phase Transition: Losses in Magnetocaloric Effect Illustrated on the Ni-Mn-Ga-Cu

The hysteresis that usually accompanies the first-order phase transition and its kinetics causes efficiency losses in any cyclical refrigeration processes. Establishing the correlation between time and hysteresis could provide insight into this issue. This study presents experimental results on the evolution of magneto-induced structures under different thermodynamic conditions and cycling in a magnetic field, and the subsequent effect on the magnetocaloric effect for the NhMno.74Cuo.26Ga Heusler alloy. The study reveals the impact of the evolution of the magneto-induced structural subsystem on the irreversibility of the magnetocaloric effect near the first-order magnetostructural transition. The presence of residual martensite, formed during the initial magnetization of the sample, is identified as the cause of the irreversibility of the magnetocaloric effect. The results demonstrate the difference in the course of the martensitic transition in adiabatic and isothermal conditions. It also highlights the dependence of the nucleation of the martensitic phase and the movement of the boundaries of the new phase on the magnetic field sweep rate.