Microstructure and transition behavior of Fe 2 P-based first-order magnetic phase-transition alloys

Fe2P-based first-order magnetic phase-transition alloys have been considered as one of the best candidates for room-temperature magnetic refrigeration applications due to their low cost, tunable working temperature range and superior magnetocaloric performance. Previous studies mainly focused on optimizing the magnetocaloric properties as well as uncovering the spin-lattice-electronic multi-coupling in the Fe2P-based alloys, while the structure and phase transition behavior at the micrometer scale have rarely been reported. In the present study, we performed intensive studies on the microstructure and phase transition behavior in the Fe2P-based alloys by means of transmission electron microscopy and X-ray powder diffraction (XRD). SiO2 and Fe3Si-type secondary phases with micrometer or sub-micrometer sizes are distributed along the grain boundary. In-situ XRD measurements indicate that the paramagnetic-ferromagnetic (PM-FM) transition is accompanied with significant lattice distortion in the hexagonal structure. The hexagonal unit cell is expanded by approximately 0.8% along the a axis, while it is contracted by about 1.8% along the c axis. The serve lattice distortion will induce large elastic energy, raise the energy barrier and thus bring about thermal hysteresis for the first-order magnetic transition. The magnetic transition-induced lattice distortion is more easily accommodated in the regions close to secondary phases and grain boundaries, where the atomic arrangement is more disordered. As a result, the nucleation of the PM-FM transition is found to start from the defect areas. However, the secondary phases and grain boundaries pin the domain walls, which hinders the growth of the ferromagnetic phase. The pinning effect of the defects will increase the energy barrier, causing an increase in the thermal hysteresis and the critical driven field for the first-order PM-FM transition. As a consequence, in order to further optimize the magnetocaloric performance, more efforts should made on tailoring both the intrinsic (e.g., lattice discontinuity) and extrinsic (e.g., size and distribution of the secondary phases) factors dominating the first-order magnetic transition of Fe2P-based alloys.