Mn基 Heusler合金磁结构相变的调控和磁特性的关联研究

Ni-Mn-X (X =Ga, In, Sn, Sb) ferromagnetic shape memory alloys (FSMAs), which show shape memory effect and magnetism simultaneously, have attracted considerable interest for their potential applications in recent years, due to their ability to undergo a reversible first-order martensitic transformation (MT) from a high-temperature cubic austenite phase to a structurally modulated martensitic phase.The first-order transformation is strongly coupled with a magnetic transition that can be induced by changes in temperature, stress, as well as magnetic field and therefore some unique and promising physical properties, such as magnetic-field induced strain (MFIS), giant magnetocaloric effect (GMCE), giant magnetoresistance (GMR) and exchange bias (EB), have been observed in these alloys. The MT temperature and the Curie temperature of the martensite phase are highly sensitive to composition (corrsponding to valence electron concentration) and the Mn-Mn interatomic distance (corrsponding to atomic order) in Ni-Mn-X alloys. So far the previous work has been mostly focused on bulk polycrystalline materials produced by arc-melting with subsequent heat treatments at high temperatures for compositional homogenization. .Melt spinning has been proven to be an effective single-step processing route to synthesize ribbons with homogeneous chemical composition combining with shorter annealing time. Due to the high cooling rate, rapid solidification from the liquid phase can result in atoms being located in a non-equilibrium state. This allows for a modification of the atomic order, and thus it is of great interest to investigate melt-spun ribbon materials. Meanwhile, thin films attached to substrates have promoted their integration into magnetostrictive and magneto-optical devices designed for different microsystem technologies..In this work, the alloy ribbons with different MT temperature regions (TM＜TC, TM≈TC, TM ＞TC) are prepared by melt-spun process.A series of films with different thicknesses are deposited by DC or RF magnetron sputtering using some targets of different composition on different substrates. The effect of preparation conditions (involving alloy composition, rapid quenching process, growth conditions, elements doping, annealing temperature and time) on structure, phase components, phase transition temperature, magnetic properties will be investigated systemically. From atomic-order-disorder viewpoint, the influence of MT temperature, magnetic state, magnetic exchange interaction on magnetic entropy changes, electrical properties and exchange bias behavior will be studied in these alloys through controlling of magneto-structural phase transformation. The forming mechanism of magnetic characteristics will be described. The correlation between MT, magnetic state and magnetic characteristics will be reviewed uniformly in order to extend application fields in magnetic refrigeration, magnetic memory and microsystem technologies.

Ni-Mn-X(X=Ga,In,Sn,Sb) Heusler合金由于结构相变和磁性相变之间共同的耦合作用，在马氏体相变过程及马氏体状态下表现出奇异的磁弹、磁热、磁输运和交换偏置等物理特性。价电子浓度和晶格参数是影响马氏体相变温度的主要因素。本项目采用快淬和磁控溅射技术制备出马氏体相变温度在不同温区(TM＜TC, TM≈TC, TM ＞TC)的合金薄带及薄膜。通过磁结构相变的调控，研究合金成分、快淬工艺、生长条件及退火等制备条件对合金薄带和薄膜的相组成、相稳定性、马氏体相变特征及温度的影响规律。从原子有序-无序角度出发，系统分析马氏体转变、不同温区的磁性状态和磁交互作用对Ni-Mn-X 合金薄带及薄膜的磁热、磁电阻和交换偏置行为的影响机理。从一个统一的角度揭示Ni-Mn-X 合金薄带及薄膜的磁特性与马氏体相变的关联关系及物理本质，为拓展Ni-Mn基铁磁形状记忆合金的应用领域提供科学依据。

Mn基Heusler合金由于其结构和磁相变耦合，在其马氏体相变附近表现出丰富的物理特性，诸如磁卡效应、巨磁电阻、磁场诱导的应变和交换偏置现象。.本项目采用快淬单辊甩带、磁控溅射技术，制备了一系列Mn基合金：NiMnIn (Sn, Al)，Fe-Mn-Ga, MnxGa。利用结构分析和磁性测量等手段研究了合金成分、生长条件、后续处理工艺等制备条件对合金薄带和薄膜的相组成、微观形貌、马氏体相变特征及磁性相变温度的影响规律。研究了MnxGa合金的永磁特性及矫顽力机制。.实验发现Ni-Mn基薄带晶化成部分有序的B2-type、有序的L21-type 四方相及调制的马氏体相, 而在薄带断裂面上观察到有序的柱状晶结构，这些微观组织的形成源于快速凝固过程中高的冷却速率。后续退火处理（Mn-Mn原子间的距离）和化学成分（合金的价电子浓度）是影响马氏体相变的关键因素，进而影响到合金的磁热、磁电阻、交换偏置行为。在马氏体转变温度TM和居里温度TC附近，最大的正磁熵变 (+ΔSM)和负磁熵变(-ΔSM)分别约为 30J/kg K和6.5 J/kg K。大的磁熵变源于低温相和高温相之间较大的磁化强度差。在整个循环中总的最大制冷率 RCs (RC-1s+RC-2s) 是477J/kg。而在Fe-Mn-Ga体系中，在低温磁性态，交换偏置场HE可以通过冷却场的大小（AFM/FM相的分数变化）来调节。.MnGa快淬薄带中同时存在高温亚稳相（Mn8Ga5和D019-Mn3Ga相）和室温平衡相（L10-Mn1.86Ga和D022-Mn3Ga相）。L10和D022型MnGa相的存在使薄带表现出硬磁特性，L10-Mn1.86Ga薄带的矫顽力被形核机制控制，而四方D022-Mn3Ga薄带由钉扎机制控制。反铁磁性的D019-Mn3Ga与亚铁磁性的D022-Mn3Ga之间存在交换耦合作用使得快淬薄带中出现交换偏置效应。电输运性质测量结果显示MnGa快淬薄带为典型金属性。在Si/SiO2基片上生长的各向同性MnGa薄膜也形成了D022和L10型结构，其最大矫顽力为9.7kOe, 矫顽力机制由磁化翻转的形核机制控制。.本项目的研究为拓展Mn基Heusler合金在室温磁制冷、自旋电子学器件、永磁应用领域提供了依据。