反钙钛矿结构反常热膨胀锰氮化物的高场电子磁共振研究

Chemically doped or nanocrystalline antiperovskite compounds ANMn3 (A means main group elements or transition metals) exhibit excellent anomalous thermal expansion (ATE) (negative thermal expansion and zero thermal expansion), which have potential applications in the fields of high-precision optics, aerospace, cryogenics and so on. Previous research results indicate the ATE is related with the magnetovolume effect (MVE) of some particular antiferromagnetic order. However a uniformed mechanism is far away from being reached. The fact that the existence of micro-phase magnetic separation is closely related with the ATE in those materials provides a new strategy for clarifying the ATE’s mechanism. High-field electron magnetic resonance spectrometer (EMR) of the steady high magnetic field facility is a high-resolution and high-sensitivity micro-probe of magnetism and magnetic excitations. By employing this instrument, we will deeply study the micro-magnetism of ANMn3 compounds. Based on the field-frequency dispersion relation, the evolutions of magnetic exchange field and anisotropy field with temperature, chemical composition, and grain size will be revealed. Furthermore, how the coexisting magnetic order affects the ordering process of antiferromagnetic phase will be investigated and the physical picture of ATE will be then constructed. This research will provide some new references for exploring new ATE materials based on MVE.

反钙钛矿结构ANMn3(A为主族或过渡族金属元素)的化学掺杂和纳米晶材料表现出优异的反常热膨胀性能(负热膨胀、零膨胀)，在精密仪器、低温工程、航空航天等领域有着重要的应用前景。一般认为其反常热膨胀与特定反铁磁序(如Γ5g型)的磁体积效应相关，而对其微观机制尚未形成统一认识。微观磁相分离与反常热膨胀之间的紧密联系为澄清后者的物理机制提供了新视角。稳态强磁场装置的高场电子磁共振谱仪是一种高灵敏、高分辨的微观磁性和磁激发态研究手段。本项目拟采用该谱仪深入研究反常热膨胀ANMn3材料的微观磁相分离，揭示各类磁序随温度、化学组分及晶粒尺寸的演化规律；通过频率-磁场色散关系，获得共存磁序的交换场、各向异性场等微观磁参数以及它们随温度的演变过程；弄清伴生磁序对反铁磁序有序化进程的干预机制，建立反常热膨胀的微观物理图像。该研究将为基于磁体积效应探索新型反常热膨胀材料提供新思路。

反钙钛矿结构锰氮化物等体系在磁相变处往往呈现陡峭的晶格收缩。通过适当元素掺杂或颗粒尺寸调控可以展宽相变温窗，获得宽温区负膨胀（NTE）材料，具有重要应用价值。一般认为NTE起源于特定磁序的自发磁致伸缩效应（即磁体积效应）。我们通过高/低场电子磁共振（EMR）谱研究了反钙钛矿结构锰氮化物的微观磁性、磁相变及NTE之间的关联，揭示了伴生的竞争磁序对母磁序相变过程的阻碍机理，进而提出了基于竞争磁序的NTE性能调控新策略。发现了Hf1-xTaxFe2合金的宽温区大NTE效应及其与Fe-2a和Fe-6h次晶格的非协同磁相变的密切关联，利用高场EMR阐明了非协同磁相变的物理根源：隐藏于Fe-6h的弱反铁磁序导致Fe-2a位置受到一定程度的磁阻挫，相应的磁相变受到了阻碍，滞后于Fe-6h；针对NTE材料在磁场环境下的应用，提出了低磁性NTE材料概念，并通过元素调控和同构合金化方法获得了MnNiGe基低磁性、巨NTE新材料；提出了基于低熔点金属粘接方法的低膨胀复合材料设计新途径。截至目前，在Acta Materialia、Scripta Materialia、Applied Physics Letters等期刊发表论文19篇。应邀在国内外学术会议上做交流6次。培养毕业研究生3名。