“TiNiSi-Ni2In型”磁结构相变的调控机理及相关物性的研究

The MnNiGe-based and MnCoGe-based intermetallic compounds which undergo the TiNiSi-to-Ni2In-type magnetostructural transformations have attracted great interests recently for their potential applications in magnetic refrigeration and magnetic-field-induced-strain (MFIS). However, the features of the magnetostructural transformations depend on cell volumes, Mn-Mn distances and electron concentrations variously. From the view points of the couplings and competitions between two magnetic and crystallographic phases, this project will investigate the relationships among crystallographic symmetries, magnetic orderings and stabilities of the two types of structural phases for the TiNiSi-to-Ni2In-type magnetostructural transformation systems. By tuning the magnetostructural transformations with multiple operations which could induce compensative and cooperative effects, the compositions of the MnNiGe-based and MnCoGe-based systems would be optimized. We intend to reduce the concentrations of Ge element largely and realize the magnetostructural transformations around room temperature, which could give rise to giant magnetocaloric effects and MFIS. In addition, magnetostructural material systems which are based on the MnCoSi and MnNiSi alloys could be expected. The implementation of this project would not only shed lights on manipulating-mechanisms for the present system but also benefit to potential applications for materials which experience similar magnetostructural transformations.

具备"TiNiSi-Ni2In型"磁结构相变的MnNiGe基和MnCoGe基合金在磁制冷、磁致应变等领域有着潜在的应用价值，但其相变特征随晶胞体积、Mn-Mn原子间距、价电子浓度等调控参数之间的变化关系较为复杂。本项目拟从两种磁有序状态和两种晶格对称度之间的竞争与耦合的角度出发，研究晶格对称度和磁有序类型的变化对磁结构相变平衡条件的影响，探讨此类磁结构相变的物理机制和调控规律。在此基础上，利用多重调控手段之间的配合和补偿作用，力求在室温附近实现磁结构相变的同时，大幅降低MnNiGe和MnCoGe基合金中昂贵金属元素Ge的含量，并研究低磁场条件下的磁热和磁致应变效应；此外还将探索基于MnCoSi和MnNiSi合金的新型廉价磁结构相变材料体系。本项目的实施不仅将有助于阐明"TiNiSi-Ni2In型"磁结构相变的物理机制和调控规律，也将有力地拓展此类磁结构相变功能材料的应用前景。

本项目系统研究了TiNiSi-Ni2In 型磁结构相变的调控规律和作用机制及其相关物理效应。从价电子数相关的原子占位规律出发，深入分析了等结构合金法的原理. 综合利用了多元素多晶位共掺杂、热循环锻炼、分步热处理等手段，调控了两类晶格的相对稳定度、磁有序结构、残余内应力和晶粒尺寸等参数，从实验上阐明了它们对相变温度、相变势垒、热滞/磁滞、温度驱动能力等特征参数的影响。在此基础上，通过优化几类多元素多晶位共掺杂体系的元素成分，我们在大幅降低了贵重元素Ge的含量的同时，提高了磁化强度的变化量和磁场驱动能力；报道了一类不含Ge的合金材料体系的铁磁/顺磁型磁结构相变；利用等结构合金法调控磁结构相变温度和磁有序态，在一类MnCoSi基合金中实现了铁磁/顺磁型磁结构相变。上述在合金材料中的获得的磁结构相变均可以调节至室温附近，可被温度/磁场驱动，并伴随巨磁热效应和晶格参数的跃变。此外，我们开发了一种制取MnCoSi巨磁致伸缩块体材料的有效方法。与现有的高磁场凝固法不同，我们采用 的环氧树脂真空浸渍法，在保留MnCoSi合金铸锭的天然织构的同时，大幅强化了力学性能和可加工性。这种方法不需要使用超导磁体和多次退火，具有高效、低成本和低能耗的巨大优势。通过进一步改变合金成分调节相变类型、相变温度和临界磁场，我们在低磁场和室温下获得了一系列高度可逆的巨磁致伸缩效应。