Co50Fe25-xMnxSi25系列合金的结构、磁性和半金属性研究

利用实验和能带计算相结合的方法,对介于两种预期的半金属Heusler合金Co₂FeSi和Co₂MnSi间的四元合金Co₅₀Fe_25-xMn_xSi₂₅的晶体结构、磁性、能带结构和半金属性进行了研究.采用考虑库仑相互作用的的广义梯度近似方法计算了系列合金的能带结构,通过与实验结果进行对比,揭示了成分变化过程中合金分子磁矩及原子磁矩的变化规律.研究发现, 关键词： 磁性 半金属 Heusler合金     

Heusler合金Co2MnAl(100)表面电子结构、磁性和自旋极化的第一性原理研究

基于第一性原理计算方法系统地研究了L2₁和B2结构下的Heusler合金Co₂MnAl(100)表面原子的原子弛豫、电子结构、磁性和自旋极化行为. L2₁和B2结构的Co₂MnAl(100)表面由于Co–Mn和Co–Al的成键差异, 使得不同原子分别发生不同程度的伸缩. 与块体相比, Co和Mn原子的自旋磁矩由于表面效应而明显增大, 电子结构计算显示L2₁结构块体中的带隙被表面态破坏, 表面效应使得两种结构的CoCo端面自旋极化率降低, 但MnAl端面并未受到显著影响, 呈现了较大的自旋极化, 预测其在隧道结中可能具有很好的应用潜力.     

Co50Fe50-xSix合金的结构相变和磁性

利用实验测量和理论计算相结合的方法,研究了介于B2结构CoFe低有序合金和L2₁结构Co₂FeSi高有序合金之间的Co₅₀Fe_50-xSi_x合金的结构相变、磁相变、分子磁矩和居里温度.采用考虑Coulomb相互作用的广义梯度近似(GGA+U)方法计算了合金的能带结构.研究发现,合金出现较强的原子有序倾向,表现出较强的共价成相作用.合金的晶格常数、磁矩、居里温度随Si含量的增加而线性地降低,极限成分Co₂FeSi合金的分子磁矩和居里温度分别达到5.92μ_B和777 ℃.原子尺寸效应导致合金晶格发生变化,但并未成为居里温度和分子磁矩变化的主导因素.分子磁矩的变化符合Slater-Pauling原理,但发现原子磁矩的变化并非线性,据此提出了共价成相对磁性影响的观点.采用Stearns理论解释了居里温度的变化趋势,排除了原子间距对居里温度的主导影响作用.能带计算的结果还表明,Co₂FeSi作为半金属材料并非十分完美,可能在实际应用中会出现自旋极化率降低的问题.发现该系列合金的结构相变和磁相变随着成分的变化聚集在窄小的成分和温度范围内. 关键词： 磁性 Heusler合金 结构相变     

B元素添加对Co-Zr-Mo合金薄带的磁性能及结构的影响

利用X射线和磁性测量研究了Co_77Zr（18-x）Mo₅B_x合金薄带的结构和磁性.实验发现,在Co-Zr-Mo合金中添加适当含量的B,可以使其矫顽力显著提高,当x=2.0时,制备出具有迄今为止Co-Zr基永磁合金最大矫顽力H_c=7.0 kOe（1 Oe=79.5775 A/m）的快淬薄带.随着B元素添加,Co₇₇Zr_18-xMo₅B_x合金薄带的晶粒逐渐细化,并根据Henkel plot模型计算得出软磁相fcc-Co与硬磁相Co₅Zr相之间的交换耦合作用逐渐增强.合金薄带的矫顽力主要受硬磁相Co₅Zr相的晶粒尺寸控制,并随着晶粒尺寸的减小先升高后降低.另一方面,Co₇₇Zr₁₈Mo₅合金薄带的矫顽力机理为反磁化核形核模型,添加B元素之后矫顽力机理变为畴壁钉扎模型.通过X射线衍射和热磁分析发现,B元素并没有进入到Co₅Zr相的晶格中,而是存在于非晶相中.     

KKR-CPA-LDA计算方法对CuMnAl和CoMnAl以及CuCoMnAl合金的磁性研究

采用KKR-CPA-LDA方法研究了CuMnAl,CoMnAl和CuCoMnAl四元合金中磁性原子磁矩和Co-Mn间的交换作用.通过与实验结果对比,揭示了Mn的磁矩和Co的磁矩以及它们的相互作用随成分变化的规律.研究发现,在Cu₅₀Mn_25+xAl25-x合金中超过化学配比并占据Al位的Mn原子是反铁磁的,而且由于近邻环境的不同,其磁矩大于原有Mn原子的磁矩.在Co₅₀Mn_{25+< 关键词： KKR-CPA-LDA计算 Co-Mn间交换作用}     

非晶态合金Co81.5B18.5结构的研究

本文叙述了用X射线衍射方法及偏振中子衍射方法对Co_81.5B_18.5非晶合金的结构研究。利用两种数据处理方法,从实验结果中计算了Co_81.5B_18.5的偏干涉函数(PSF),S_ij(Q),和偏简约径向分布函数G_ij(r)。并对这两种结果作了比较。最后计算了一种模型简单单元的中子衍射散射强度,并讨论了这一非晶合金的结构模型。 关键词：     

Pr2Fe14(C,B)/α-(Fe,Co)型纳米晶复合磁体的结构与磁性

研究了Pr_xFe_82-x-yTi_yCo₁₀B₄C₄ (x=9—10.5;y=0, 2)纳米晶薄带的结构与磁性. 结果表明,所有薄带皆主要由2∶14∶1, 2∶17和α-(Fe, Co)三相组成. 对于y=0的合金,其内禀矫顽力随Pr含量x的增加而增加,剩磁随Pr含量x的增加而减小. 以Ti置换部分Fe (y=2),合金的磁性能得到显著提高,表现为:添加Ti后,合金的剩磁B_r基本不降低,x=10.5时合金的B_r值甚至有较明显的提高;同时添加Ti后,合金的内禀矫顽力及退磁曲线的方形度都明显改善. 当x=10.5,y=2时,合金薄带的磁性能达到最佳值为: B_r=9.6 kGs(1 Gs=10^-4 T),_iH_c =10.2 kOe(1 Oe=79.5775 A/m)和(BH)_max＝17.4 MGOe. 随着Pr含量的提高,合金中的硬磁相2 ∶14 ∶1的含量相对增加,内禀矫顽力提高;而Ti置换Fe抑制了软磁相α-(Fe, Co)在快淬和热处理过程中的优先长大,使合金中软磁相和硬磁相的晶粒尺寸及比例趋向最佳组合,交换耦合作用明显增强. 关键词： 纳米晶永磁材料 2Fe₁₄(C')" href="#">Pr₂Fe₁₄(C B) Ti添加 交换耦合     

Mn(SiO2)3(M=Fe,Co,Ni;n=1-3)团簇的几何结构、光电性质及磁性

基于密度泛函理论中的广义梯度近似系统研究M_n(SiO₂)₃(M=Fe,Co,Ni;n=1-3)团簇的几何结构、光电性质和磁学性质.结果表明:Fe,Co原子相对于Ni原子更易于在(SiO₂)₃团簇上聚集;通过分析团簇的分裂途径及其产物,发现稳定性较好的氧化硅是一种很好的用于负载过渡金属"岛膜"的载体材料;M_n(SiO₂)₃团簇的能隙恰好位于近红外光谱范围内.通过磁性分析发现,该复合团簇的磁矩主要局域在过渡金属原子周围,而且,Fe₂(SiO₂)₃和Co₃(SiO₂)₃具有相对较大的磁矩,这主要源于过渡金属原子的d轨道间相互耦合.能隙和磁性两方面性质进一步肯定了二氧化硅磁性复合材料在医学界被用作光动力靶向治疗的可观前景.     

双合成反铁磁结构及其对自旋阀巨磁电阻效应的影响

用直流磁控溅射方法制备了双合成反铁磁结构Co₉₀Fe₁₀(5 nm)/Ru(x nm)/Co₉₀Fe₁₀(3 nm)/Ru(y nm)/Co₉₀Fe₁₀(5 nm)(x=0.45,0.45,1.00; y=0.45,1.00,1.00)的系列样品,并对样品的性能及其作为钉扎层对自旋阀巨磁电阻(GMR)效应的影响进行了研究 关键词： 双合成反铁磁 自旋阀 巨磁电阻     

哈斯勒合金Ni-Co-Mn-Sn的马氏体相变及其磁热效应研究

通过结构和磁性测量,研究了四元哈斯勒合金Ni_50-xCo_xMn₃₈Sn₁₂（x=1, 2, 4, 6, 8）的晶体结构和相变特征.结果表明,Co原子的掺杂不但没有影响三元哈斯勒合金Ni-Mn-Sn的原有结构,而且还增强了样品在奥氏体相的铁磁交换作用.此外,通过Maxwell方程计算了其中三种成分样品（x= 2, 4, 6）的磁熵变ΔS_{M关键词： 哈斯勒合金 Ni-Co-Mn-Sn 马氏体相变 磁热效应}     

Band structure calculations for Heusler phase Co2YBi and half-Heusler phase CoYBi (Y=Mn, Cr)

We have studied the electron structure and magnetic properties of Heusler phase Co₂YBi and half-Heusler phase CoYBi (Y=Mn, Cr) by using the full-potential linearized-augmented plane-wave (FLAPW) method. Co₂MnBi and Co₂CrBi are predicted to be half-metallic magnetism with a total magnetic moment of 6 and 5 μ_B, respectively, well consistent with the Slater-Pauling rule. We also predict CoMnBi to be half-metallic magnetism with a slight compression. The gap origin for Co₂MnBi and Co₂CrBi is due to the 3d electron splitting of Mn (Cr) and Co atoms, and the gap width depends on Co electron splitting. The atom coordination surroundings have a great influence on the electron structure, and consequently the Y site in the X₂YZ structure has a more remarkable electron splitting than the X site due to the more symmetric surroundings. The investigation regarding the lattice constant dependence of magnetic moment shows that the Co magnetic moment exhibits an opposite behavior with the change of the lattice constant for Heusler and half-Heusler alloys, consequently leading to the different variation trends for total magnetic moment. The variation of total and atom magnetic moment versus lattice constant can be explained by the extent of 3d electron splitting and localization of Mn (Cr) and Co atoms for both the series of alloys.     

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.     

On the interpretation of NMR experiments in CoGa alloys

We show that in Co_xGa_1?x alloys NMR data reported by Grover et al. in combination with existing magnetization data can be interpreted as follows. The group of atoms consisting of a Co antistructure atom and its eight nearest neighbours behaves as a magnetic entity. The (saturation) magnetic moment of the anti-structure atom (?1.6μ_B) is much larger than the corresponding moment on a neighbouring Co atom (?0.16 μ_B).     

XMCD studies of magnetic polarization at Mo atoms in CoMo alloy and magnetically coupled Co/Mo multilayers

Magnetic polarization of Mo atoms in Co₉₆Mo₄ alloy film and Co/Mo multilayered structures has been studied by X‐ray magnetic circular dichroism. Samples with Mo spacers of two different thicknesses (0.9 nm and 1.8 nm) were investigated. Mo atoms receive a magnetic moment of ?0.21μ_B in the alloy. In the multilayer with the thinner Mo spacer (d_Mo = 0.9 nm) the magnetic moment is much smaller (?0.03μ_B). In both cases the measured induced moment at the Mo site is oriented antiparallel to the moment at the Co atoms. The presence of the induced moment in the Mo spacer coincides with antiferromagnetic coupling between the Co component slabs. In contrast, neither measurable induced moment at the Mo site nor interlayer coupling between the Co layers has been found for the multilayer with the thicker Mo spacer. Possible mechanisms of the coupling associated with the induced moment are discussed in detail.     

First-principles investigations of Cr doping effects on electronic structure and magnetic properties in Sr2FeReO6

The electronic structures and magnetic properties of double perovskites Sr₂Fe_1−xCr_xReO₆ (x=0.0, 0.25, 0.5, 0.75, 1.0) have been studied within the local spin density approximation (LSDA) and LSDA+U schemes. The calculated results reveal that with increasing Cr content the cell volume shrinks 2.61%; the Fe/Cr site magnetic moment decreases while the Re-site moment increases. The total spin magnetic moment linearly decreases with the Cr doping from 3.00μB for x=0.00 down to 1.00μB for x=1.00 per formula unit. The magnetic coupling constants increase with increasing x. The electronic structure calculations indicate that the electronic concentration in the Re spin-down subband slightly increases resulting from the increase of bonding-antibonding interaction between the localised and the delocalised states in spin-down band; the coupling of O-2p_↑ and transition-metal-3d_↑ is substantially enhanced with the Cr doping. We discuss the origin of the anomalously high TC of Cr-doped Sr₂FeReO₆ compounds in terms of band hybridization effects.     

化合物SmCo5的电子结构、自旋和轨道磁矩及其交换作用分析

用自旋极化的MS-Xα方法研究了稀土-过渡族化合物SmCo5₅的电子态密度、自 旋能级劈裂及原子磁矩.研究结果显示，由于化合物中Sm-Co间的轨道杂化效应，使Sm原子原来的5d00空轨道上占据了少量5d电子.由于Co(3d)-Sm(5d)电子间的直接交换作用，导致了Sm-Co间的磁性交换耦合，这是化合物中形成Sm-Co铁磁性长程序的一个重要原因.在SmCo5_{5化合物中存在6个能级呈现负交换耦合，导致了SmCo55化 关键词： 电子结构 自旋极化 原子磁矩 交换耦合}     

Tb1-xGdxFe2的磁矩和矫顽力

测量了赝二元立方Laves相化合物TbGdFe₂(0≤x≤1)的磁化曲线、居里温度和矫顽力随温度的变化。发现化合物由TbFe₂向GdFe₂过渡时,样品的饱和磁化强度几乎直线下降。在所有成分下,Fe原子的磁矩都是常数(μ_Fe=1.60±0.04μ_B),但Tb原子的磁矩,由于受晶场影响,却小于自由离子的值(gJ=9.Oμ_B)。矫顽力由两部分组成:一为畴壁受Peierls势垒和稀土 关键词：     

A first-principle study of half-metallic ferrimagnetism in the Ti2CoGa Heusler compound

Electronic structure calculations based on density functional (DFT) theory within the generalized gradient approximation (GGA) for the Ti₂CoGa Heusler compound have been performed using the self-consistent full-potential linearized augmented plane wave (FPLAPW) method. The electronic band structures and density of states of the Ti₂CoGa compound show that the spin-up electrons are metallic, but the spin-down bands have a gap of 0.5 eV, resulting in stable half-metallic ferrimagnetic behavior with a magnetic moment of 2μ_B.     

A study of the magnetic moments and Fe site hyperfine fields in iron-chromium alloys

The magnetization of Fe-Cr alloys ranging from 1 to 15 atomic % of Cr has been measured at room temperature in order to study the relationship between the Fe site hyperfine fields and the magnetic moment. The average moment decreases linearly, at a rate of -2.36 μ_B per Cr atom, up to 10% Cr concentration. The Fe site hyperfine fields were measured in a previous study¹ using the same samples. It is found that the hyperfine fields measured are not proportional to the corresponding magnetic moments. The results are interpreted using a model previously developed for other binary alloys of iron².