According to Clausius–Clapeyron relation ΔT/ΔH = ΔM/ΔS, a large field-induced transition temperature modulation ΔT can be expected for a phase transformation involving substantial change of the saturation magnetization ΔM 1. Fig. 1(a) describes a strategy for obtaining large ΔM by exploiting the transition between a ferromagnetic martensite and a paramagnetic austenite 2. The Curie temperature of the austenite T^A_C is far below the martensitic and reverse martensitic transition temperatures, denoted as T_M and T_A, respectively. Applying a magnetic field stabilizes the ferromagnetic phase, giving rise to increased T_M and T_A, without changing the thermal hysteresis T_M - T_A.
We next consider a particular case where T_M < T^A_C < T_A (Fig. 1(b)). Since T_M < T^A_C, the martensitic transition now involves two ferromagnetic phases with much smaller ΔM. In contrast, the ΔM of the reverse martensitic transition is unaffected. Such an asymmetric thermomagnetic coupling controlled by the position of T^A_C provides a unique opportunity to tune the thermal hysteresis by external field.
As a proof of concept, off-stoichiometric Fe₄₈Mn₂₄Ga₂₈ Heulser alloy with T^A_C ~ 180 K satisfying the above conditions was chosen. The ingots were prepared by arc melting followed by annealing at 1273 K and then quenched in cold water. We used differential scanning calorimetry (DSC), thermomagnetic (M-T) curves, isothermal magnetization (M-H) loops and M-T minor loops 3 to characterize the dissimilar magnetic field effects on the martensitic and the reverse phase transformations. Figure 1(c) shows the experimental M-T curves with applied fields up to 8 T. T_M clearly shows weaker field dependence than T_A, leading to large field-controlled tunability of the thermal hysteresis. In the talk, we will also discuss the role of interface friction and magnetic entropy change in the phase transformation of this alloy.