MnxGa (1≤x≤2)合金结构和磁性调控研究

Tetragonal L10-MnGa alloy is predicted to be a promising novel permanent magnetic material without rare earth elements according to the theoretical maximum magnetic energy product of 28 MGOe. However, several outstanding problems that restrict its development must be addressed: The first and most urgent is to prepare single phase L10-MnGa since Mn-Ga alloys have multiple crystal structures; the second is to determine the relationship between intrinsic magnetic properties and crystal structure. However, for practical applications, the most challenging task is how to achieve anisotropic L10-MnGa magnets with high coercivity. In this project, we first try to prepare a series of L10-MnxGa (1≤x≤2) alloys, and study the phase transition process using in-situ Neutron and X-ray diffraction analysis. The intrinsic magnetic dependence on chemical composition (Mn content) and crystal structure will be elucidated, and the L10-MnxGa alloy with high magnetic moment and large anisotropy field will be identified based on the results of the above analyses and confirmed by first-principles-density-functional-theory calculations. Furthermore, combining multiple novel preparation techniques, such as fast hot deformation, magnetic heat treatment, magnetic hardening and magnetic anisotropy, we hope to fabricate anisotropic L10-MnxGa magnets by optimizing its microstructure such as grain size, texture and interfacial structure. Finally, we will study the coercivity mechanism for L10-MnxGa magnets in order to provide theoretical and scientific guidance for the realization of new Mn-Ga permanent magnetic materials.

L10-MnGa合金的理论最大磁能积可达28MGOe，有望发展为新型高性能非稀土永磁材料。然而其成相条件苛刻，而且内禀磁性高度依赖于尚不完全明确的晶体结构和磁结构。此外，材料获得高矫顽力和磁各向异性也极具挑战。本项目首先制备系列L10-MnxGa(1≤x≤2)合金，采用变温中子衍射和X射线技术阐明合金的相变过程及其晶体结构、磁结构，并结合第一性原理计算揭示合金的结构与内其禀磁性关联关系，制备出高磁矩、高各向异性的L10-MnxGa合金。在此基础上，综合采用放电等离子快速热压变形、磁场热处理等材料合成新技术，探索磁体实现磁硬化和磁各向异性化的有效途径，通过对合金显微组织（晶粒尺寸、晶体织构、界面结构等）的有效调控，研制出高性能各向异性L10-MnxGa磁体，并阐明矫顽力机理，为实现新型Mn-Ga永磁材料的实用化提供理论依据和科学指导。

L10-MnGa合金的理论最大磁能积可达28MGOe，有望发展为新型高性能非稀土永磁材料，然而其成相、结构、高外禀磁性等研究都面临挑战。本项目首先探索了单相L10-MnxGa合金的熔炼-退火制备工艺，并获得了基于该工艺的合金相图。其室温饱和磁化强度随Mn含量的降低逐渐增加，拟合本实验结构得到的L10-MnGa的饱和磁化强度为101 emu/g，略低于理论值116 emu/g，该差异来源于多晶样品中存在的孪晶、物相的不均匀以及微量杂相。. 基于实验和密度泛函理论，计算得到四方MnGa合金晶胞的生成焓为-4.85 eV，合金体具有净有效磁矩，呈弱的亚铁磁性。四方MnGa合金存在着明显的自旋极化，靠近费米能级两侧的s电子和靠近费米能级下方的p电子具有较弱的自旋极化，形成浅能级价带和导带的d电子产生高强度的自旋极化，对磁性质贡献较大。应力下MnGa能够产生较明显的磁相变。. 采用液氮低温液氮冷却低温球磨法实现了L10-MnxGa粉体的磁硬化，发现了MnxGa体系中大微观应变诱发粉末磁硬化及矫顽力提高的现象，微观应变对矫顽力的贡献均在50%以上。球磨时间为20h时，Mn1.33Ga球磨粉的微观应变为0.93%，此时矫顽力提高到6.7kOe，并表现出钉扎机制。对于不同组分的合金粉末，热处理会降低应变并发生矫顽力机制的转变。 . 首次采用放电等离子烧结热变形技术 (SPS-HD) 实现了L10-MnxGa脆性合金的磁硬化，获得了HD88%的Mn1.33Ga热变形磁体，其平均晶粒大小约1.5μm，矫顽力达到5.65kOe。小回线分析和洛伦兹磁畴观察表明热变形磁体的矫顽力机制趋于弱钉扎模型，高矫顽力主要源于再结晶引发的超细晶普通晶界（而非孪晶界）的钉扎作用。1wt.%Sr元素掺杂显著降低了变形磁体的晶粒尺寸（约556nm），经400℃热处理1 h后，晶粒生长到约1.6μm，矫顽力提高到8.01kOe，相比于未掺杂的提高了39%，是目前非稀土锰镓磁性合金的最高性能。热处理后无孪晶缺陷的超细晶是Mn1.32GaSr0.01变形磁体获得高性能的重要原因。