Large magnetic entropy change and magnetic phase transitions in rapidly quenched bulk Mn-Fe-P-Si alloys

Mn1.15Fe0.85P0.50Si0.45B0.05 alloys were prepared by copper mold suction casting method. Subsequent annealing processes were employed to tune the microstructures and magnetocaloric effects. Results show that Mn1.15Fe0.85P0.50Si0.45B0.05 alloys crystallize in a hexagonal phase with the Fe2P structure type. With increasing annealing temperature, the lattice parameters of a and c of Fe2P-type phase increase from 5.7988(1)Å to 6.0785(2)Å and from 3.3218(1)Å to 3.4727(2)Å for the Mn1.15Fe0.85P0.50Si0.45B0.05 alloys according to XRD. At the same time the Curie temperature and magnetic entropy change increase from 233 K to 244 K and from 8.8 J kg−1K−1 to 17.0 J kg−1K−1 (ΔH = 2 T), respectively, while the thermal hysteresis decreases. The entropy changes of these alloys are also estimated from the Clausius-Clapeyron equation, Landau theory and heat capacity, respectively. The nature of the phase transitions were studied through the Arrott plots, which indicate that the samples undergo first-order ferro-paramagnetic phase transitions. However, the universal curve of scaled magnetic entropy change reveals that during the magnetic phase transition, there exists a critical magnetic field, about 1.25 T, below which the sample undergoes a second-order phase transition and above which it displays a first-order phase transition. Furthermore, mechanisms for the large magnetic entropy change and possibilities for the room-temperature magnetic refrigeration are discussed.