基于反铁磁-铁磁翻转机制的MnNiGe基合金条带磁结构相变调控及相关物理性质研究

The MnNiGe-based alloys have been paid phenomenal attention due to the excellent magneto-caloric properties. After the magneto-thermal cycles and large voluminous expansion and contraction through the structural phase transformation, the alloys are so brittle that they are hard to be fabricated into ribbons. Therefore, the bulk samples have been used mostly to study the magnetocaloric effect solely so far. In this project, MnNiGe-based alloys are selectively made into ribbon samples by studying carefully the composition chemistry such as the element substitution, the transitional element-vacancy, doping of interstitial atoms, and exerting hydrostatic pressure or chemical pressure (doping atoms with large radius) as well as manufacturing parameters and heat treatment (annealing temperarure and time, quenching methods and cooling speed) to eventually tailor the magnetic and structural phase transformation based on antiferromagnetic-ferromagneitic conversion and in wide temperature range covering room temperature and finally to improve their magneto-thermal stability. MnNiGe-based alloys and MnNiGe/polymer composites are also obtained by high pressure alloying methods and polymer combining techniques. The investigations will be extended into the scope of the general magnetic functional properties including not only magneto-caloric capacity but also magnetoresistance, magnetoelectric effect, magnetic stress-strain behavior, Hall effect and so on. This study will shed light on the new direction and development of the MnNiGe-based alloys and lay a solid and wider exprimental foundation of their applications.

MnNiGe基合金以其优异的磁热性能备受关注。由于结构相变前后体积变化大，经历热、磁循环后合金很容易碎裂，给后续的条带制备造成困难，因此目前对该类合金的研究大都集中在块体，对合金磁功能性质的研究也还仅限于磁热效应。本项目拟以MnNiGe基合金为研究对象，通过仔细摸索成分、合理控制制备参数制备出合金条带样品。通过元素取代、过渡元素缺位、间隙位原子掺杂、施加外压力或化学压力（大半径原子掺杂）等方法实现基于反铁磁-铁磁翻转机制和包括室温在内的大温度范围覆盖的合金磁结构相变调控；通过控制制备参数和热处理条件（退火温度、时间和淬火方式、冷却速率等）、利用高压合成和聚合物复合等方法制备合金和合金/聚合物复合材料等提高合金的热-磁稳定性。在此基础上对制备出的合金（条带）样品进行磁热、磁电阻、磁致应变、磁电效应、霍尔效应等磁功能性质研究，为MnNiGe基合金的发展和拓宽其实际应用范围提供实验和理论基础。

Ni2In类型六角结构的MnMX (M=Co、Ni, X=Ge、Si)合金具有非常丰富的磁-结构特性，近年来正成为一种潜在的磁性多功能材料。作为MnMX系列合金的成员，Mn(Co,Ni)Ge基合金的磁-结构相变在很窄的温区内就能完成，导致合金的负热膨胀温区很窄，且合金易碎，不利于实际应用。同时，该类合金的马氏体相变对磁场非常不敏感，磁场很难驱动变磁性相变，导致合金磁致功能性质很弱，极大地限制了合金的研究和应用领域。在国家自然科学基金资助下，我们研究了合金磁结构相变与磁热效应，有效拓宽了合金负热膨胀温区，提高了体系合金的力学性能。主要研究结果如下：（1）在Mn1-xFexNiGe中获得铁磁马氏体到顺磁奥氏体的一级磁-结构耦合相变，通过对合金进行二次退火处理，在合金中实现了磁场驱动的变磁性马氏体相变，获得了优异的磁热性能；（2）通过双向调控Mn/Ge比例，在两个方向上均实现了磁结构耦合相变，相变前后磁化强度突变增大，在Mn0.98CoGe1.02合金中观察到磁场诱导的变磁性马氏体相变，由此导致了巨室温磁热效应；（3）在MnCoGe基合金中通过双向调控Co/Ge比例，在两个合金体系中均实现了一级磁结构耦合相变，获得巨大室温磁热效应；（4）制备出了致密的Mn0.84Fe0.16NiGe/Sn块状复合材料，表面具有金属光泽，表现出良好的加工性能和耐腐蚀性，且获得了优异的负膨胀性能；（5）在Mn0.965Co1.035Ge合金中引入缺陷和残余应力，稳定了高温六角Ni2In相，导致较高比例的奥氏体相不再发生马氏体相变，扩大了奥氏体相与马氏体相共存的温度范围，一级磁-结构相变温区变宽，使得合金负热膨胀温区变宽；（6）采用Co取代Mn，显著提高了TbMn2基合金的居里温度，提升了合金的磁热效应，有望应用于磁致冷领域。至结题时，我们已经在上述几个方面取得了很好的进展，共发表SCI 论文13篇。我们的成果为MnNiGe 基合金相变调控及相关物理性质研究提供了理论和实验依据。