磁性准同型相界对Heusler合金磁热效应的影响及机理研究

For decades, magnetic refrigeration, as an environment-friendly and high efficiency refrigeration technologies, have recently gained strong scientific interests. To realize this technology, it is vital to develop the magnetocaloric materials exhibiting large magnetic entropy change over a wide temperature range. Although the giant magnetocaloric effect materials, such as Gd5Si4-xGex, LaFe13-xSix, Ni-Mn-Ga and so on, have a large magnetic entropy change, the crystal structure of materials will change also during the magnetic phase transition at the same time, which is accompanied by a considerable thermal hysteresis. This makes the materials unsuitable in a real refrigerator due to the cycle operation and energy loss. Recently, we proposed a new magnetocaloric effect (MCE) in Heusler alloys with nearly zero thermal hysteresis and large refrigerant capacity using the theory of morphotropic phase boundary (MPB). Indeed ，an enhancement performance is obtained, some key questions have not been resolved yet. As demonstrated by Landau free energy equation, the slope of MPB will determine the energy barrier, as well as the properties of phase transition. Thus, in this project, we will firstly study the relationship between the slope of MPB and the performance of MCE. At the other hand, the microstructure of magnetic domain at MPB is never been reported, which is significant to understanding the behavior of MCE. We then will observe the morphology and structure of magnetic domain around MPB, as well as the evolution of domain in the presence of external magnetic field. Finally, based on the experiment results and phase field simulation, a general law and physical mechanism for the design of high-performance magnetocaloric materials using MPB principle can be expected.

高性能磁制冷材料一般要求具有较大的磁熵变、较小的热滞及较大的制冷能力。但是具有较大等温磁熵变的巨磁热材料通常在磁有序转变的同时还伴随晶体结构的变化，因此相变过程存在着较大的热滞现象。针对此问题，申请人首次提出利用磁性准同型相界（MPB）这一概念，设计并制备了具有窄热滞和大制冷能力的铁磁Heusler合金材料。本项目拟在前期研究成果基础上，主要研究：1）磁性相图中MPB斜率对磁热效应的影响规律；2）通过观测 MPB 附近的磁畴微观结构，建立起其与磁制冷过程中磁熵变、制冷温区和热滞等信息的关联；3）基于相场模拟方法建立朗道自由能理论模型。最终揭示磁性MPB对Heusler合金磁热效应的影响机理，总结出基于磁性MPB开发高性能Heusler合金磁热材料的设计准则。本项目对于磁热材料的研究具有重要的理论意义和应用价值。

磁热效应是指绝热过程中铁磁体或顺磁体的温度随磁场强度的改变而变化的现象，基于具有较大磁熵变、较小热滞以及较大制冷能力的磁热材料在新型制冷机中有着较大的应用前景。然而目前具有较大等温磁熵变的巨磁热材料普遍存在较大的热滞，进而使得循环制冷效率极低。针对以上问题，本项目在执行期内围绕Heusler合金的磁性准同型相界的构筑以及其磁热效应展开了多项具体的工作，包括系统地研究了四元NiMnYZ(Y、Z=In、Sb、Ga、Sn)的磁性准同型相界的构筑方法及其相界处斜率对 Heusler 合金磁性能的影响，同时阐明 MPB 附近磁畴的微观结构与磁热效应宏观性能之间的内在关联，顺利的完成本项目的研究内容和关键问题。此外，在项目执行过程中，我们通过对Heusler合金MPB附近磁性相变的研究、探究了自旋玻璃的产生及演化机理，解释了其产生交换偏置现象的物理机理及变化规律。