MM’X合金和全d-metal Heusler合金中的磁相变设计

发布时间：2019-06-10 12:25

【摘要】：铁磁马氏体相变作为凝聚态物理和磁性功能材料领域的研究热点之一,可以带来很多物理效应,比如磁性形状记忆效应、磁诱发应变输出、磁熵变、交换偏置等。本论文以构建六角MM’X合金中的具有强磁-结构耦合的宽居里温度窗口和开发新型Heusler铁磁马氏体相变材料为目的,利用磁性测量、X射线衍射、透射电子显微镜、扫描差热分析、应变、输运以及第一性原理计算等实验方法进行了如下两个方面的研究。利用等结构合金化的原理,选取Mn_(1-y)Fe_yNiGe体系的等结构体MnNiSi构成Mn_(1-y) Fe_yNiGe1-xSix体系,借助于高马氏体相变温度Tt和高马氏体居里温度TC M的MnNiSi能同时提高新体系Tt和TC M的行为特征,不仅将居里温度窗口宽度扩展为400 K(从40 K到450 K),而且也在整个温区内获得了具有强铁磁耦合、大饱和磁化强度的马氏体相,从而在整个窗口内都获得了具有大的饱和磁化强度差ΔM的铁磁马氏体相变。实验给出了从120 K到445 K的宽温域内的大磁熵变和低场大磁熵变。与其他典型的磁熵变材料相比,本材料体系覆盖的宽温区范围,是从来未报道过的。此外,窗口内铁磁马氏体相变表现出良好的功能热稳定性,为实际应用提供了基础。该体系中所得的一系列居里温度窗口为材料应用设计提供了一个宽阔的平台,使得该类材料不仅在磁制冷,而且可以在大应变输出、多卡效应、梯度材料、能源转化、负膨胀等方面成为极具应用潜力的智能材料。在对Heusler合金的结构特点和成相规律的深入认识的基础上,指出占据D晶位的主族元素同其最近邻A/C位的过渡族元素之间的p-d共价键对于合金的成相和相稳定性具有决定性的作用,提出满壳层的d-d共价也可能促进Heusler相的形成并稳定母相。基于此,本论文在NiMn合金中引入Ti开发了Ni50Mn50-yTiy和Mn50Ni50-y Tiy两不含主族元素的、全新的“全d-metal Heusler合金”,并且增加Ti含量能够有效地降低马氏体相变温度Tt。其母相为B2(L21)有序的Heusler结构。实验发现Ni50Mn50-yTiy的母相为反铁磁耦合。通过在Ni位引入Co研究了Ni50-x CoxMn25Ti25母相的磁性演化,Co可以有效建立母相的铁磁耦合。并且在铁磁与反铁磁竞争的临界成分x=17.5样品中观察到了交换偏置。我们也利用Co激活效应在Ni50-xCoxMn35Ti15和Mn50Ni40-x CoxTi10的母相中均建立了强铁磁耦合,获得了具有大ΔM的铁磁马氏体相变,开发出新一类d-metal Heusler铁磁形状记忆合金。结合第一性原理计算,解释了Co的引入建立母相铁磁耦合的机制,能够形成Mn(B)-Co(A/C)-(MnD)的局部铁磁构型。基于d-metal Heusler合金中铁磁马氏体相变,获得了一系列磁多功能性包括磁驱形状记忆效应、磁场诱发大应变、磁熵变、磁电阻等。特别地,Mn50Ni44.5Co9.5Ti10表现出多晶各项同性磁场诱发大应变高达6950 ppm,这完全来自于马氏体相变引起的巨大的体积变化-2.54%,而且在相变过程中样品不碎裂也无裂纹出现。d-metal Heusler的发现打破了人们对于Heusler合金必须包含主族元素的认识,极大地拓展了Heusler多功能合金的成相空间,为Heusler合金的成相机理、马氏体相变、铁磁形状记忆合金中的磁耦合以及新材料的开发设计提供了重要启示。
[Abstract]:The ferromagnetic martensitic transformation, as one of the hot spots in the field of condensed matter physics and magnetic functional materials, can bring many physical effects, such as magnetic shape memory effect, magnetic induced strain output, magnetic entropy change, exchange bias, etc. In order to build a wide Curie temperature window with strong magnetic-structure coupling in the hexagonal MM 'X alloy and the development of a new Heusler ferromagnetic martensitic transformation material, the magnetic measurement, the X-ray diffraction, the transmission electron microscope, the scanning differential thermal analysis, the strain, The experimental methods, such as the transport and the calculation of the first principle, are studied in the following two aspects. The Mn _ (1-y) Fe _ yNiGe1-xSix system of Mn _ (1-y) Fe _ yNiGe system is selected by the principle of equal structure alloying, and the behavior characteristics of the new system Tt and the TC M can be simultaneously improved by means of the MnNiSi of the high-martensitic transformation temperature Tt and the high-martensitic Curie temperature TC M, the Curie temperature window width is not only extended to 400 K (from 40K to 450K), but also a martensite phase with strong ferromagnetic coupling and large saturation magnetization is obtained in the whole temperature range, So as to obtain a ferromagnetic martensitic transformation with a large saturation magnetization difference (m) throughout the window. In this paper, the large-magnetic entropy change and the low-field large-magnetic entropy change in a wide temperature range from 120K to 445K are given. Compared with other typical magnetic entropy-changing materials, the wide temperature range covered by the material system is not reported. In addition, the ferromagnetic martensitic transformation in the window shows good functional thermal stability and provides the basis for practical application. The series of Curie temperature windows obtained in the system provide a wide platform for the material application design, so that the material not only is in magnetic refrigeration, but also can be converted into a large-strain output, a multi-card effect, a gradient material and an energy source, Negative expansion and so on become an intelligent material with great application potential. On the basis of the in-depth understanding of the structural characteristics and the phase-forming rule of the Heusler alloy, it is pointed out that the p-d covalent bond between the main family element occupying the D-crystal position and the transition group element of the nearest neighbor A/ C position has a decisive effect on the phase and phase stability of the alloy, It is also possible to promote the formation of the Heusler phase and to stabilize the mother phase. Based on this, the new "all-d-metal Heusler alloy" of Ni50Mn50-yTy and Mn50Ni50-y Tiy, which does not contain the main group elements, is introduced in the NiMn alloy, and the increase of the Ti content can effectively reduce the martensitic transformation temperature Tt. The parent phase of the Ni50Mn50-yTy and Mn50Ni50-y Tiy is the ordered Heusler structure of B2 (L21). The results show that the mother phase of Ni50Mn50-yTiy is anti-ferromagnetic coupling. The magnetic evolution of the Ni50-x CoxMn25Ti25 mother phase is studied by the introduction of Co in the Ni site, and the ferromagnetic coupling of the mother phase can be effectively established by Co. And the exchange bias was observed in the ferromagnetic and anti-ferromagnetic competing critical components x = 17.5. A new type of d-metal Heusler ferromagnetic shape memory alloy was developed by using Co-activating effect in the mother phase of Ni50-xCoxMn35Ti15 and Mn50Ni40-x CoxTi10, and a new type of d-metal Heusler ferromagnetic shape memory alloy was developed. Combined with the first principle, the mechanism of Co is introduced to establish the ferromagnetic coupling of the mother phase, and the local ferromagnetic configuration of Mn (B)-Co (A/ C)-(MnD) can be formed. Based on the phase transition of the ferromagnetic martensite in the d-metal Heusler alloy, a series of magnetic properties are obtained, including the magnetic drive shape memory effect, the magnetic field induced large strain, the magnetic entropy change, the magnetic resistance, and the like. In particular, that Mn50Ni42.5 Co9. 5Ti10 show that the polycrystalline various same-sex magnetic field induce a large strain of up to 6950 ppm, which is entirely from the great volume change due to the martensitic transformation-2.54%, and the sample does not break or crack in the phase change process. The discovery of d-metal Heusler broke people's understanding that the Heusler alloy must contain the main group elements, and greatly expanded the phase space of the Heusler multi-function alloy, and it is the phase-forming mechanism and the martensitic transformation of the Heusler alloy. The magnetic coupling in the ferromagnetic shape memory alloy and the development and design of the new material provide important enlightenment.
【学位授予单位】：中国科学院大学(中国科学院物理研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O469

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