Fe_(2)P基一级磁相变合金的微观结构及相变行为

Microstructure and transition behavior of Fe_(2)P-based first-order magnetic phase-transition alloys

Fe_(2)P基一级磁相变合金因其价格低廉、工作温域易于调控、磁热性能优异等显著特性,在室温磁制冷领域具有良好应用前景.本文利用透射电子显微镜和X射线衍射仪深入研究了Fe_(2)P基合金的微观结构,并原位观察了其微观尺度下的磁相变过程.研究发现,该合金晶界处分布着微米或亚微米尺寸的非晶SiO_(2)和立方Fe_(3)Si型杂相,这些杂相显著影响着Fe_(2)P基合金一级磁相变的形核和长大过程. Fe_(2)P基合金的顺磁-铁磁相变伴随着较大晶格畸变,晶胞沿着a轴和c轴方向分别伸长0.8%和收缩1.8%左右.该磁相变诱导的晶格畸变易于在杂相和晶界等晶体缺陷处被容纳,因此铁磁相率先在杂相和晶界处形核.然而,这些微米或亚微米尺寸晶体缺陷会对磁畴壁产生钉扎作用,不利于铁磁相的长大过程,增加相变能垒,导致热滞增大或者相变驱动场上升.由此可见,要获得低热滞、低驱动场和高磁热性能的Fe_(2)P基一级磁相变合金,有必要同时优化影响其磁相变过程的本征因素(如两相晶格失配度)和非本征因素(如缺陷尺寸、种类).

Fe_(2)P-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 Fe_(2)P-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 Fe_(2)P-based alloys by means of transmission electron microscopy and X-ray powder diffraction(XRD). SiO_(2) and Fe_(3)Si-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 firstorder 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 firstorder 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 Fe_(2)P-based alloys.