摘要
Heusler合金中蕴藏着丰富的物理特性和应用功能,是开发功能材料的巨大宝库。特别是近年发现的铁磁性形状记忆合金和半金属磁性材料,在航空、航天、医学以及当前最为热门的计算机信息产业方面都展现出了巨大的开发潜力和应用前景,成为目前国际凝聚态物理研究的热点之一。本论文工作以此为契机,主要探索合成新的Heusler高有序合金,寻找其中具有特殊功能性质的材料,深入研究它们展现出的特殊性质、形成机制和其它相关物性;并尝试开发其可能存在的应用特性。其主要研究工作和成果概括如下:
首次发现并报道了一种新的具有铁磁性形状记忆效应的Heusler合金Mn2NiGa,这种合金具有发生在室温附近的马氏体相变、50K的相变温度滞后、高达588K的居里温度和21.3%的晶格扭曲。就应用功能而言,材料展现出极好的双向形状记忆效应,其自由相变应变和磁增强应变分别高达1.7%和4.0%。这些都表明Mn2NiGa合金是一种具有大应变开发潜力,适合在室温应用的新型功能材料。
开发出一些列新的偏分合金:Mn50-xNi25+xGa25(0 结合实验和理论两方面工作研究了Mn2NiGa合金的晶体结构,电子结构和磁性。明确提出合金形成一种不同于传统L21结构的新型高有序结构—Hg2CuTi型结构。基于这一结构,通过第一性原理方法计算了合金奥氏体和马氏体的电子结构和磁性。发现在奥氏体状态下,合金中的两个Mn原子磁矩大小不同,且反平行排列,即,合金是亚铁磁体。当马氏体相变发生时,Mn原子的磁矩发生巨大变化,最终导致合金的马氏体状态表现出铁磁性。实验方面,研究了合金马氏体和奥氏体的基本磁性,并结合上述理论研究,成功解释了合金相变前后磁性变化的奇特行为和特殊的热磁曲线,这是本工作的又一创新之处。
研究了Mn50-xNi25+xGa25系列合金的磁性随成分的变化关系,分析了它们的变化机理。发现随着Mn含量的增加,材料的饱和磁化强度成线性下降。进一步分析发现,在偏分合金中,掺杂Mn原子所起的作用与正分Mn2NiGa合金中Mn原子的作用相同,并没有随着Mn含量的增加而发生质的变化。从而也进一步证明无论哪种成分的合金,掺入的Mn原子总是趋向于占相同的晶位。
为了改善和提高Mn2NiGa合金多方面的性能,我们对Mn2NiGa合金进行了掺杂实验。即:分别用Fe和Co替代合金中的Ni元素,制备了Mn50Ni25-xFe(Co)xGa25系列合金。研究了Fe和Co元素对Mn2NiGa合金的结构、马氏体相变行为、磁性和机械性能等方面的影响,发现了一些有意义的结果。
从理论上探索了一系列新的具有Hg2CuTi型结构的Heusler合金:Mn2CoZ (Z=Al, Ga, In, Si, Ge, Sn, Sb)。利用第一性原理方法计算了它们的电子结构和磁性,首次发现并报道了一系列新的半金属合金:Mn2CoZ (Z=Al, Si, Ge, Sn, Sb)。
深入研究了材料的半金属带隙形成机理和磁性的变化行为。首次发现并提出合金中半金属带隙的形成来源于两种机制:共价作用机制和d-d带隙机制,而d-d带隙机制最终决定了合金半金属带隙的宽度。Mn2CoZ合金遵守Slater-Pauling规则:MH =NV ?24。但我们发现,实现这一规则的内在机制是合金中Mn(C)原子磁矩的减少,而其它原子的贡献非常微小。这在前人的工作中还未见报道。
进一步,我们在实验上首次成功合成了Mn2CoZ (Z=Al, Ga, In, Ge, Sn, Sb)六种合金,并对其结构和磁性进行了测量和分析,与我们的理论计算非常一致。
另外,我们尝试合成了一批新的Mn2基Heusler合金,初步研究了它们的磁性和一些基本物性,进一步的工作还有待继续开展。
Heusler alloys exhibit rich physical properties and many applicable functions. Two of the most important functions are ferromagnetic shape memory effect and half metallic properties. In this dissertation, we will try to develop new Heusler alloys with these two properties and investigate their structure and basic physical properties. We abstract the main content of this dissertation as following:
Ferromagnetic Heusler alloy Mn2NiGa has been discovered, which exhibits a martensitic transformation around the room temperature with a large thermal hysteresis up to 50K and a lattice distortion as large as 21.3%. It was also found that Mn2NiGa has a Curie temperature up 588 K being much higher than that of studied Ni2MnGa, whose Curie temperature is about 370 K. The excellent two-way shape memory behavior with a strain of 1.7% was observed in the single crystal Mn2NiGa. The magnetic-field-controlled effect created a total strain up to 4.0% and changed the sign of the shape deformation effectively.
Both experimental and theoretical studies have been carried out to investigate the structural and magnetic properties of Mn2NiGa alloys. We have found, instead of forming L21 structure where both A and C sites are occupied by Mn atoms, the alloy favor a structure where C site is occupied by Ni atoms and Mn atoms at A and B sites. The electronic structures of both cubic austenite and tetragonal martensite Mn2NiGa were calculated by self-consistent full-potential linearized-augmented plane-wave (F-LAPW) method. Austenite Mn2NiGa materials show ferrimagnetism due to antiparallel but unbalanced magnetic moments of Mn atoms at A and B sublattice. The magnetic moment of Mn atoms decrease greatly upon martensitic transformation to a tetragonal structure with a 50% reduction in Mn moments at A site and almost completely suppressed Mn moments at B sites. Consequently, martensite Mn2NiGa alloys show ferromagnetic coupling. Different magnetic orderings in martensite and austenite also lead to very different temperature dependence, with which the abnormal behavior of magnetization upon martensitic transformation can be understood. In the off-stoichiometric samples with composition between Ni2MnGa and Mn2NiGa, We show that additional Mn atoms that substitute for Ni atoms in Ni2MnGa have the same magnetic behaviors as Mn in Mn2NiGa phase, which successfully explains the dependence of the magnetization on Mn content.
In order to improve the properties of this material and develop new ferromagnetic shape memory alloys, an experiment of partially substituting Ni in Mn2NiGa alloys with Fe or Co was carried out to examine the effect of Fe or Co in quaternary Mn50Ni25-xFe(Co)xGa25 alloys on their structure, magnetic properties, martensitic transformation, and mechanical properties. Some interesting results were found.
First-principle FLAPW calculations have been carried out to study the electronic structure and magnetic properties of compounds with Hg2CuTi-type structure. It is found that the compounds with Z=Al, Si, Ge, Sn and Sb are half-metallic ferrimagnet. Experimentally, we successfully synthesized the Mn2CoZ (Z=Al, Ga, In, Ge, Sn, Sb) compounds. Using the XRD measurements and Rietveld refinements, we have confirmed that these new compounds form Hg2CuTi-type structure instead of conventional L21 structure. Based on the analysis on the electronic structures, we find that there are two mechanisms to induce the minority-spin band gap near the Fermi level, but only d-d band gap determines the final width of the band gap. The magnetic interaction is quite complex in these alloys. The competition between the intra-atomic exchange splitting and covalency mechanism dominate the formation and coupling of the magnetic moments. The Mn2CoZ alloys follow the Slater-Pauling rule: MH = NV ? 24 with the varying Z atom. It was further elucidated that the molecular magnetic moment, MH, increases with increasing valence concentration only by decreasing the antiparallel magnetic moment of Mn(C) while the magnetic moments of Mn(B) and Co are unaffected .
We successfully synthesized some of Mn2-based Heusler alloys using the melt-spun method. Some basic physical properties of these alloys were also investigated primarily.
引文
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