基于MnFe(P,Si)单晶的磁弹耦合相变机理研究

Magnetoelastic coupling materials have become the best candidate for magnetic refrigeration/heat pump and energy conversion technology due to their remarkable response to external physical conditions (such as temperature, magnetic field, pressure, etc.). In order to fundamentally realize the performance control and the exploration of new materials, it is an important scientific problem to understand the mechanism of first-order magnetic phase transition. This project intends to select (Mn,Fe)2(P,Si) material system with magneto-elastic coupling phase transition (crystal symmetry is not destroyed when the first-order magnetic phase transition occurs) to study the potential electronic, atomic and other micro-interactions that lead to a strong coupling among magnetic, electronic and crystal, when ferromagnetic-paramagnetic phase transition occurs. High quality (Mn,Fe)2(P,Si) single crystals will be grown by solvent method. The electrical resistivity, thermal conductivity and thermo power as a function of temperature and magnetic field. The evolution of electronic structure in reciprocal space of (Mn,Fe)2(P,Si) single crystals during first-order magnetic phase transition is observed directly by synchrotron radiation ARPES technique. The microscopic mechanism of magnetoelastic coupling phase transition of the compound will be revealed. The relationship between its chemical composition, intrinsic properties, microstructures and magnetocaloric effect will be sorted out, which will provide us new idea and scientific guidance for the development of new high-performance magneto-thermal energy conversion materials.

由于对外界物理条件（如温度、磁场、压力等）有着极其显著的响应，磁弹耦合材料成为磁制冷/热和能源转换技术的最佳候选工质。为从根本上实现性能调控和新材料的探索，深入理解一级磁相变机理成为亟待解决的重要科学问题。本申请项目拟选取具有磁弹耦合相变（发生一级磁相变时晶体的对称性不被破坏）的 (Mn,Fe)2(P,Si)体系，研究其发生铁磁-顺磁相变时导致磁、电子、晶体之间强耦合的潜在电子、原子等微观相互作用。采用溶剂法生长出高质量(Mn,Fe)2(P,Si)单晶样品，全面分析其电阻率、热导率、热电势等物理内秉性能随温度和磁场的变化规律。用同步辐射ARPES技术直接观测(Mn,Fe)2(P,Si)单晶在发生一级磁相变时的倒空间电子结构演化过程。揭示该化合物的磁弹耦合相变的微观机理，并梳理其化学成分-内禀性能-微结构-磁热性能之间的关系链条，为研发新型高性能磁-热能量高效转换材料提供新的思路和科学指导。

一级磁相变材料由于其丰富的物理特性，在固态制冷和能源转换技术具有广阔的应用前景，是凝聚态物理和材料科学领域的研究热点。而深入理解磁弹相变的微观机理是研发新材料的前提。本项目系统研究了Fe2P型材料的磁性、比热、电子结构、磁相变和磁热效应，揭示了磁弹耦合相变的机理和基本规律。本项目取得的主要成果包括：（1）成功利用助溶剂法，生长了一系列高质量Fe2P型单晶，并利用改进的生长工艺，获得了具有一级磁相变和较小热滞的(Mn,Fe)2(P,Si)单晶，为其磁热效应和基础物性研究提供了材料基础。（2）首次利用低温半绝热弛豫量热法、自制搭建的高磁场Peltier cell DSC、电阻测量等实验手段，结合第一性原理计算和德拜理论，揭示了Fe2P型材料发生一级相变时晶格、电子和磁自由度对总比热的贡献。（3）通过同步辐射高分辨X射线吸收谱和发射谱（HERFD-XAS 和XES）实验，研究了(Mn,Fe)2(P,Si)单晶发生铁磁至顺磁一级磁相变时的电子结构和局域磁矩随温度的演化规律，发现在顺磁态下3f晶位的Fe原子仍保持其局域磁矩，未发生文献中理论计算预测的淬灭现象。（4）在Fe2P型材料体系中，揭示了其发生一级磁相变时晶格畸变与总熵变之间的密切关联。本项目研究创新性地提出了晶格对磁弹耦合相变的重要性，对于深入理解磁弹相变机理和探索新材料有指导意义。