Langmuir薄膜及其应用前景

Langmuir薄膜近年来引起人们越来越大的兴趣。由于这种薄膜制备方式上的特点,使之具有其他薄膜技术无法实现的独到之处。本文对这一薄膜的制备方法、结构,电学性质、光电导和能量传递以及应用前景等几方面给予综述。     

Magnetic hysteresis and refrigeration capacity of Ni-Mn-Ga alloys near Martensitic transformation

This paper studies the magnetic hysteresis and refrigeration capacity of Ni-Mn-Ga alloys in detail during heating and cooling isothermal magnetisation processes. The Ni-Mn-Ga alloys show larger magnetic hysteresis when they transform from austenite to martensite, but smaller magnetic hysteresis when they transform from martensite to austenite. This behaviour is independent of either the pure Ni-Mn-Ga alloys or the alloys doped with other elements. Because of the existence of the magnetic hysteresis, the relation between the magnetic entropy change and refrigeration capacity is not simply linear. For practical consideration, magnetocaloric effect of Ni-Mn-Ga alloys should be investigated both on cooling and heating processes.     

亚铁磁Heusler合金Mn2CoGa和Mn2CoAl掺杂Cr,Fe和Co的局域铁磁结构

通过实验和计算的方法研究了Mn₂CoM_xGa_1-x 和Mn₂CoM_xAl_1-x (M=Cr, Fe, Co)掺杂系列合金样品. 研究发现, 在共价作用的影响下, Fe和Co原子占A位, 使被取代的MnA (-2.1 μ_B)变成MnD (3.2 μ_B), 在最近邻的强交换作用下亚铁磁基体中形成了MnB-CoC-MnD局域铁磁性结构, 使分子磁矩的增量最高可达6.18 μ_B. Fe, Co 掺杂后建立同样的局域铁磁结构, 居里温度的变化趋势却不同. 实验观察到Mn₂Co_1+xAl_1-x中掺杂容忍度高达x=0.64, 远高于在Mn₂CoGa中(x=0.36)的结果; 以及随着Al的减少, 合金由B2有序向A2混乱转变等现象, 为共价作用对合金结构稳定的影响提供了证据. 磁测量中发现Cr掺杂后磁矩增量高达3.65 μ_B以及居里温度快速上升的反常现象, 意味着对占位规则的违背.     

Possible Martensitic Transformation in Heusler Alloy Pt_2MnSn from First Principles

Using density functional theory calculations, we investigate the tetragonal distortion, electronic structure and magnetic property of Pt_2 Mn Sn. The results indicate that, when the volume-conserving tetragonal distortion occurs, the energy minimum appears at c/a(6 = 0.84, and the energy difference between the minimum and cubic phase is as high as 107 me V/f.u. Thus from the point of view of thermodynamics, martensitic transformation may occur in Pt_2 Mn Sn with decreasing the temperature. The electronic structure of its cubic and martensitic phases also approves this. Moreover, both the cubic and tetragonal phases of Pt_2 Mn Sn are ferromagnetic structures and their total magnetic moments are 4.26 μ_B and 4.12 μ_B, respectively.     

La-doped BiFe03： Synthesis and multiferroic property study

A new sol-foam-gel method was developed to fabricate La-doped BiFeO3 muttiferroic materials. It was demonstrated that a gradual increase in the content of La-doped into BiFeO3 results in its structure changing from rhombohedral to orthorhombic. A study of other property changes indicates that La-doping in BiFeO3 enhances its ferromagnetism and ferroelectricity. A temperature-dependent magnetization study suggests that the magnetic property of La-doped BiFeO3 samples varied from antiferromagnetic to ferromagnetic as the content of La-doped into BiFeO3 increased from 0 to 20%. Unique temperature-dependent zero field cooling （ZFC） and field cooling （FC） magnetization behaviors were observed in 15% La-doped BiFeO3 -- its ZFC temperature-dependent magnetization being ferromagnetic and its FC temperature- dependent magnetization being antiferromagnetic. A possible mechanism of such an interesting M-T behavior is discussed.     

Heusler型X_2RuPb(X=Lu,Y)合金的反带结构和拓扑绝缘性

采用第一性原理的计算方法,在不同条件下对Heusler型X2RuPb(X=Lu,Y)体系的电子结构展开研究.计算结果表明,这些合金在适当晶格变形或掺杂条件下,能够具有真正的拓扑绝缘体特性.杂化作用和自旋-轨道耦合作用都对材料产生"反带"结构发挥作用.但是针对不同成分所构成的材料,它们各自所起作用的程度有所不同,二者可以相辅相成.利用替换掺杂和四角变形双重调控方式可以更理想地进行"反带"结构调控进而获得理想的拓扑绝缘体,这对于材料的实际制备具有重要意义.     

内应力对Mn2NiGa铁磁形状记忆合金的结构、相变和磁性能的影响

通过施加压应力的方法,在铁磁形状记忆合金Mn₂NiGa中引入残留内应力,研究了内应力对 Mn₂NiGa材料的结构、相变和磁性能的影响.研究发现,加压过程使材料发生了塑性形变,在材料内部引入了大量的位错缺陷.卸载后保留的位错缺陷在材料中造成了残留的内应力,导致了马氏体相变温度大幅度提高, 使原本室温下的母相转变成了马氏体相.测量到导致样品转变成马氏体的阈值压应力为1.0 GPa.加压形成的马氏体中的残留内应力将矫顽力从低于50 Oe提高到350 Oe.残留内应力在730 K的热处理中由于位错缺陷的消失而得以消除,样品实现了马氏体逆相变.如此高的逆相变温度使得 Mn₂NiGa马氏体的居里温度测量成为可能,获得了530K的数值.     

Heusler合金Fe2CrGa的磁性与结构

采用KKR-CPA-LDA方法研究了不同混乱占位时Fe₂CrGa合金基态的电子结构和磁结构. 基态能量表明Fe₂CrGa合金更倾向于形成Hg₂CuTi型有序结构,而不是L2₁结构. 能态密度(DOS)分析进一步揭示受晶体场影响的磁性原子内部交换作用是使Fe₂CrGa合金形成 Hg₂CuTi型有序结构的主要原因.测量了不同热处理所得Fe₂CrGa合金的居里温度和分子磁矩, 发现原子占位有序化可以在137K温度范围内调控合金的居里温度.分子磁矩随有序化占位也有相应变化, 分布在2.28μ_B/f.u.—2.48μ_B/f.u.之间.理论计算和实验对比可证明Fe₂CrGa合金是Hg₂CuTi型Heusler合金.     

铁基Heusler合金Fe2Co1-xCrxSi的结构、磁性和输运性质的研究

基于多种实验手段和能带计算的方法, 对四元合金Fe₂Co_1-xCr_xSi的晶体结构、 磁性、输运性质及能带结构进行了研究. 研究发现, 随着Cr的增加, 合金Fe₂Co_1-xCr_xSi保持了高度有序结构, 逐渐从Hg₂CuTi结构的Fe₂CoSi 过渡到L2₁结构的Fe₂CrSi; 由于次晶格网络的破坏, 居里温度逐渐下降; 系列合金的分子磁矩呈现线性下降, 符合半金属特性; 剩余电阻比率与原子占位有序程度密切相关, 呈现两端大、 中间小的特点. 在Cr替代Co的过程中, 材料半金属能隙逐渐打开, 表现半金属特征. 同时费米能级从Fe₂CoSi半金属能隙的价带顶上移至Fe₂CrSi能隙的导带底. 最大的能隙宽度出现在x= 0.75处, 这表明四元合金有可能成为具有更高自旋极化率和更强抗干扰能力的自旋电子学材料.     

Ce1-xYxFe2的电子结构和磁性质

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