Ni4NdB电子结构和磁性能第一性原理研究

采用第一性原理,在局域自旋密度近似LSDA及LSDA+U近似,对Ni₄NdB化合物进行结构优化,计算体系晶格常数,电子结构和磁性能.结果表明,Ni₄NdB为带隙很小的金属导体,存在Nd-Ni铁磁耦合,体系总磁矩由Nd原子局域磁矩提供.体系原子成键较为复杂,Nd原子与近邻Ni原子成金属键,Nd原子与近邻B原子成较强离子键,Ni原子与近邻Ni原子间存在间接交换相互作用.在U作用下,体系磁矩与Nd原子磁矩变化一致,Ni原子磁矩在2.75 eV呈现磁有序-磁有序崩溃转变 关键词： 密度泛函理论 电子结构 磁性能 稀土过渡金属间化合物     

多种有序钙钛矿结构的高压制备与特殊物性

具有ABO_3钙钛矿或类似结构的强关联电子体系是凝聚态物理研究的重要前沿领域,而高压是制备新型钙钛矿特别是A位与/或B位有序钙钛矿材料的有效手段.在这些有序钙钛矿中,因A,B位可同时容纳过渡金属离子,因而可导致A-A,B-B,A-B等多种磁电相互作用的出现,进而诱导系列新颖有趣的物理现象.本文介绍高压下制备的几种化学式为AA'_3B_4O_(12)的新型A位有序钙钛矿以及化学式为AA'_3B_2B'_2O_(12)的A,B位同时有序的钙钛矿体系.在LaMn_3Cr_4O_(12)中发现了具有立方钙钛矿结构的磁电多铁性,为多铁新材料探索与新机理研究提供范例;在CaCu_3Fe_2Os_2O_(12)中发现了远高于室温的亚铁磁半导体行为,并指出A位磁性离子的引入可大大增加磁相互作用强度从而大幅度提高磁有序温度;在LaMn_3Ni_2Mn_2O_(12)中观察到A位磁性离子调控的B位Ni~(2+)/Mn~(4+)子晶格正交自旋有序结构.以上研究结果为探索新型磁电多功能钙钛矿材料提供了重要参考.     

复合多铁链的磁电耦合行为与外场调控

对含有界面磁电耦合的有限长铁电-铁磁多铁链体系进行了研究,基于矢量离散化思想,构建了描述其磁电性质的微观海森伯模型.利用传递矩阵方法获得了磁化强度、电极化强度、磁电化率等关键热力学量的解析表达式,重点探讨了界面磁电耦合、外场以及单离子各向异性对体系磁电耦合行为的影响和调控.研究结果表明,界面磁电耦合对体系的磁化强度和电极化强度均起促进作用.电场驱动下的电致磁电化率具有更强的磁电关联效应,预示着外电场能够有效地调控体系的磁性行为.而在磁致磁电化率中观察到的低温峰主要源于外磁场的诱导.此外,在高电场作用下体系比热容还呈现出有趣的三峰结构,这种三峰结构是自旋态的热激发以及电偶极矩的电场和温度共同激发导致的.     

Pr掺杂对DyMnO3体系结构、磁性与电极化特性的影响

本文报告了Pr掺杂对Dy_(1-x)Pr_xMnO_3多铁性化合物结构与磁电性能的影响.研究表明Pr的掺杂增强了A位离子Dy与B为离子Mn之间的相互作用,提高了DyMnO_3的磁化与电极化强度.其机理可归结为Pr~(3+)掺杂使得Dy~(3+)在低温条件下的磁矩排列发生变化,使得Dy与Mn的螺旋耦合作用增强,从而提高了的体系的宏观磁性能和电极化强度.     

氧空位对Co掺杂TiO2稀磁半导体中杂质 分布和磁交换的影响

本文使用基于密度泛函理论的第一性原理方法研究了Co掺杂TiO₂稀磁半导体中氧空位对体系能量和磁性的影响. 通过对总能量的计算发现当引入氧空位后近邻杂质体系能量高于均匀掺杂体系, 同时氧空位易在Co近邻位置富集. 进而发现氧空位的存在及其占位可以影响Co离子间的磁交换, 近邻Co离子体系下氧空位的引入使Co离子间的铁磁耦合减弱; 非近邻Co离子体系下, 底面氧空位使Co离子间呈反铁磁耦合而顶点氧空位使Co离子间呈铁磁耦合. 总之, 氧空位的存在对Co掺杂TiO₂材料的能量及磁性都有较大影响.     

结构相变对Fe掺杂TiO2薄膜室温铁磁性的影响

采用直流磁控溅射方法在玻璃基底上制备了不同Fe掺杂浓度的TiO₂薄膜, 并对其晶体结构和磁特性进行了研究.在所有掺杂样品中,均观察到了室温铁磁性, 磁性源于Fe离子与其近邻空间分布的空穴相互作用. 在掺杂量为7%的锐钛矿相薄膜中观察到了最大的磁化强度. 随着Fe掺杂浓度的进一步增加, TiO₂的晶体结构逐渐由锐钛矿相向金红石相转变,并且磁性减弱. 不同结构的TiO₂中Ti–O键长不同,导致替代的磁性Fe离子与空穴的作用强度发生改变, 进而使其磁性发生变化. 关键词： 稀磁半导体 结构相变 铁磁性     

轨道序和巡游性对尖晶石结构钒氧化物AV2O4 (A = Mn,Fe, Co) 物性影响的研究

在尖晶石钒基氧化物AV_2O_4中,因为自旋阻挫、轨道序、巡游性等独特的内禀属性间存在着相互合作和竞争,所以该体系常常表现出复杂而有趣的物理现象。通过对A位磁性离子的调控,我们详细研究该体系中不同物性的起源,比如磁相变与结构相变的不同起因,局域和巡游电子的交叉行为等等。我们以Mn_(1-x)Co_x V_2O_4和Fe_(1-x)Co_x V_2O_4体系为研究对象,通过变温X射线衍射、磁化率、比热和中子散射等测试手段,结合第一性原理计算,对其物性起源进行了详细的研究。我们发现:(i)在低Co2+离子掺杂时,体系受到局域V~(3+)离子以及A位Fe~(2+)离子的轨道序作用,往往会在磁有序附近伴随着结构相变的出现,以至于弱化体系中钒离子独立形成的四面体造成的几何阻挫。这也说明Co~(2+)离子低掺杂下,该体系有着强的自旋–晶格耦合;(ii)在高Co~(2+)离子掺杂时,由于巡游性的增强,轨道序的弱化,JAB交换相互作用增强,体系也表现出明显的磁各向同性。因此磁相变温度向更高的温度移动,而结构相变温度向低温移动甚至消失。     

自旋玻璃的重入行为及CoxZn1-x(FeyCr1-y)2O4尖晶石系统的磁相图

本文研究了Co_xZn_1-x(Fe_yCr_1-x2O₄尖晶石系统的磁性,测量了不同成份样品的低频弱场交流磁化率与低场直流磁化强度的温度关系。根据实验结果,给出了该系统可能的磁相图。发现在该系统中相当宽的成份范围内,都存在自旋玻璃的重入现象。同时发现,自旋玻璃的重入温度随磁性离子浓度增加而增加。这些行为是磁性离子浓度含量较高和多种磁性离子共存系统的共同特性。还讨论了自旋玻璃重入行为 关键词：     

Ni13Pr3B2电子结构和磁性能第一性原理研

考虑电子自旋极化作用,本研究基于第一性原理的全电子投影缀加平面波赝势法理论,采用局域自旋密度近似LSDA,对Ni13Pr3B2金属间化合物进行结构优化,计算体系晶格常数,电子结构和磁性能。结果表明,Ni13 Pr 3B2为带隙很小的金属导体。LSDA近似下体系原子间存在复杂作用类型,Pr原子与近邻Ni、B原子以离子键作用为主,Ni原子与近邻Ni原子间表现共价作用情形。体系存在Pr -Ni铁磁耦合,总磁矩约2.80212μB,主要由Pr原子磁矩提供,自旋极化引起体系Pr-4f、Ni-3p、Pr-5p电子自旋劈裂为体系表现磁性的根本原因。     

尖晶石铁氧体TixNi1-xFe2O4中阳离子分布和Ti离子磁矩的实验研究

采用固相反应法制备了系列样品Ti_xNi_1-xFe₂O₄ (x=0.0, 0.1, 0.2, 0.3, 0.4). 室温下的X射线衍射谱表明样品全部为(A)[B]₂O₄型单相立方尖晶石结构, 属于空间群Fd3m. 样品的晶格常数随Ti掺杂量的增加而增大. 样品在10 K温度下的比饱和磁化强度σ_S随着Ti掺杂量x的增加逐渐减小. 研究发现, 当Ti掺杂量x≥ 0.2时, 磁化强度σ随温度T的变化曲线出现两个转变温度T_L和T_N. 当温度低于T_N时, 磁化强度明显减小; 当温度达到T_N时, dσ/dT具有最大值. σ-T曲线的这些特征表明, 由于Ti掺杂在样品中出现了附加的反铁磁结构. 这说明样品中的Ti离子不是无磁性的+4价离子, 而是以+2和+3价态存在, 其离子磁矩的方向与Fe和Ni离子的磁矩方向相反. 利用本课题组提出的量子力学方势垒模型拟合样品在10 K温度下的磁矩, 得到了Ti, Fe和Ni三种阳离子在(A)位和[B]位的分布情况, 并发现在所有掺杂样品中, 80%的Ti离子以+2价态占据尖晶石结构的[B]位.     

应变对钴铁氧体电子结构和磁性能影响的第一性原理研究

尖晶石型钴铁氧体(CoFe₂O₄)因具有良好的电磁性质, 广泛应用于计算机技术、航空航天及医学生物等领域. 特别是钴铁氧体薄膜在磁电复合材料中具有良好的应用前景. 本文基于密度泛函理论的第一性原理平面波赝势法, 结合广义梯度近似, 通过采用更接近于实验上外延生长的二维应变模型, 研究了钴铁氧体薄膜的结构稳定性、电子结构和磁性能. 结果表明: 在二维应变作用下, 反尖晶石结构的钴铁氧体比正尖晶石结构的稳定, 但是与平衡基态相比, 两者能量差减小, 这表明在应变作用下, 八面体晶格中的Co²⁺离子与四面体晶格中的Fe³⁺离子更容易进行位置交换, 形成混合型结构的钴铁氧体; 同时随着应变的增大, 钴铁氧体的能带带隙减小, 晶格中的原子磁矩发生变化, 但总磁矩变化不明显. 关键词： 尖晶石型钴铁氧体 第一性原理 电子结构 磁性能     

A-site ordered quadruple perovskite oxides AA'_3B_4O_(12)

The A-site ordered perovskite oxides with chemical formula AA'_3B_4O_(12)display many intriguing physical properties due to the introduction of transition metals at both A and B sites. Here, research on the recently discovered intermetallic charge transfer occurring between A-site Cu and B-site Fe ions in La Cu_3Fe_4O_(12) and its analogues is reviewed, along with work on the magnetoelectric multiferroicity observed in La Mn_3Cr_4O_(12) with cubic perovskite structure. The Cu–Fe intermetallic charge transfer(LaCu_3~(3+)Fe_4~(3+)O_(12)→ LaCu_3~(2+)Fe_4~(3.75+)O_(12)) leads to a first-order isostructural phase transition accompanied by drastic variations in magnetism and electrical transport properties. The La Mn_3Cr_4O_(12) is a novel spindriven multiferroic system with strong magnetoelectric coupling effects. The compound is the first example of cubic perovskite multiferroics to be found. It opens up a new arena for studying unexpected multiferroic mechanisms.     

Magnetization curves of a spin- bilayer system (A2) (ApB1−p)1 (B)2 with disordered interfaces

A calculation is presented for the component magnetizations of an infinite multilayer Ising system, consisting periodically of two layers of spin- A ions, two layers of spin- B ions, and a disordered layer interface in between that is characterized by a random arrangement of A and B ions like a two-dimensional A_pB_1−p alloy. The system is a simple cubic Ising-type structure with a coordination number z = 6. The model is general for ferro- and for antiferromagnetic A-B exchange couplings. The A-A and B-B exchange couplings are regarded as ferromagnetic. An effective field theory that goes beyond mean field, is employed to calculate the bulk-like transition temperature, the different component magnetizations as well as the total bulk-like magnetization. The component magnetizations are calculated for different realistic model values of ferro- and antiferromagnetic A-B exchange constants, as a function of temperature and of the concentration parameter p that characterizes the disorder in the interface. We show that the presence of a disordered interface may significantly affect the component and total magnetizations. In particular, for the case of antiferromagnetic exchange couplings, it is shown that the system can acquire a compensation temperature for certain domains of values of the concentration parameter p in the disordered interface.     

Lithium insertion into Co3O4: A preliminary investigation

Lithium has been inserted into the spinel Co₃O₄ both chemically and electrochemically. During lithiation the tetrahedral A-site Co²⁺ cations are displaced into neighbouring empty octahedral sites: the incoming Li⁺ ions occupy the remaining interstitial octahedra of the spinel structure to produce the partially ordered rocksalt compound LiCo₃O₄. The octahedral B-site cations of the A[B₂]X₄ spinel are unperturbed by this reaction: the oxide lattice expands by 8.6%. Lithium analyses and powder X-ray diffraction spectra indicate that further lithiation is possible. However, it is demonstrated that a fast lithiation in excess of LiCo₃O₄ is followed by a slow extrusion of B-site cobalt at 50°C until all the Li on the 8a sites has moved to the octahedral vacancies thus created.     

Effects of doping site and pre-sintering time on microstructure and magnetic properties of Fe-doped BaTiO3 ceramics

The A-site substituted BaTiO₃ ceramics were prepared by solid-state reaction via partial substitution of Fe for Ba²⁺. By comparison with the B-site substituted sample made under similar conditions, the effect of Fe doping site on microstructure and magnetism was investigated using X-ray diffraction, Mössbauer spectroscopy and vibrating sample magnetometer. It is found that A-site substitution can be realized to a certain extent at 7 at% Fe addition, whereas impurities are observed at higher Fe concentrations. In the nominal (Ba_0.93Fe_0.07)TiO₃ sample, the Fe ions are present as Fe²⁺ and Fe³⁺, respectively, replacing A-site Ba²⁺ and octahedral B-site Ti⁴⁺ in hexagonal perovskite lattice. The double-exchange Fe²⁺-O²⁻-Fe³⁺ interactions produce ferromagnetism well above room temperature, but the saturation magnetization and the Curie temperature are both obviously lower than those for B-site substitution due to different magnetic exchange mechanisms. In the B-site substituted sample Ba(Ti_0.93Fe_0.07)O₃, the super-exchange interactions between Fe³⁺ on pentahedral and octahedral Ti⁴⁺ sites are responsible for ferromagnetism. These results mean that B-site substitution is a better way for Fe-doped BaTiO₃ system to obtain high-Curie-temperature ferromagnetism. Moreover, increasing pre-sintering time can further improve the magnetism of B-site substituted samples, through which the saturation magnetization for Ba(Ti_0.93Fe_0.07)O₃ is enhanced ∼6 times.     

A-site ordered perovskite CaCu_3Cu_2Ir_2O_(12-δ) with square-planar and octahedral coordinated Cu ions

A novel CaCu_3Cu_2Ir_2O_(12-δ) polycrystalline sample was synthesized at 8 GPa and 1373 K.Rietveld structural analysis shows that this compound crystallizes in an AA'_3B_4O_(12)-type A-site ordered perovskite structure with space group Im-3.Xray absorption spectra reveal a +2-charge state for both the square-planar and octahedral coordinated Cu ions,and the valence state of Ir is found to be about +5.Although the A-site Ca and the A'-site Cu~(2+) are 1:3 ordered at fixed atomic positions,the distribution of B-site Cu~(2+) and Ir~(5+) is disorderly.As a result,no long-range magnetic ordering is observed at temperatures down to 2 K.Electrical transport and heat capacity measurements demonstrate itinerant electronic behavior.The crystal structure is stable with pressure up to 35.7 GPa at room temperature.     

Magnetic properties of the mixed spinel Mg1+xMnxFe2-2xO4

The structural and magnetic properties of the mixed spinel Mg_1+xMn_xFe_2-2xO₄ system for 0.1<= x <= 0.9 have been studied by means of X-ray diffraction, magnetization, a.c. susceptibility and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Mn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Mg²⁺ ions substitute for A-site Fe³⁺ ions. All samples are magnetic at 12 K displaying Mössbauer spectra that have magnetic sextets coexisting with a central doublet that increases in population with increasing Mn concentration, indicating the presence of short range ordering (clustering). The Mössbauer intensity data show that Mn possesses a preference for the B-site of the spinel over the whole range of concentration. As expected, the hyperfine field and Curie temperature determined from a.c.susceptibility data decrease with increasing Mn content. Magnetization results indicate that on increasing dilution x, the collinear ferrimagnetic phase breaks down at x = 0.3 before reaching the ferrimagnetic percolation limit (x=0.6), as a result of the presence of competing exchange interactions, which is well supported by Mössbauer results. From all the above results, it is proposed that with increasing Mn content from x=0.6 to 0.9, the frustration and disorder increase in the system suppressing the ferrimagnetic ordering, and the system approaches to a cluster spin glass type of ordering at x=0.8 as reflected in the a.c.susceptibility and Mössbauer spectrum.     

Tuning the multiferroic properties of Pb(Fe1/2Nb1/2)O3 by cationic substitution

A series of substituted lead iron niobate compounds with the general formula Pb²⁺_(1−x)A^Z_x(Fe_{{(1−(2−Z)x)/2}}Nb_{{(1+(2−Z)x)/2}})O₃ (0<x<0.6 and A=La³⁺, K⁺ or Sr²⁺) were prepared by a modified solid-state synthesis. The relative concentrations of Fe³⁺ and Nb⁵⁺ were adjusted to compensate the charge imbalance due to the aliovalent substitution. The dielectric constant and magnetic susceptibilities were studied as a function of temperature. The temperature of the dielectric maximum, T_M, of the substituted compounds decreased linearly with increasing concentration of the substituent ions. The magnetic measurements showed an antiferromagnetic transition at temperatures T_N1 due to the superexchange interactions mediated by Fe–O–Fe and an additional antiferromagnetic-type transition at T_N2. T_N1 linearly increased with the increasing concentration of Fe³⁺ ion at the B-site of ABO₃-type substituted compounds. T_M is shown to be directly dependent on the concentration of the ferroactive Nb⁵⁺ ions at the B-site and Pb²⁺ ions at the A-site.     

Crack formation in composites through a spring model

When two or more different materials are combined to form composites, the property of the composite may be quite different. This depends on relative interaction strengths of cohesion between similar and adhesion between dissimilar particles. We study crack formation in composites through a very general spring network model on a square lattice. The spring model shows the distortion of the lattice, prior to fracture and this affects the pattern of subsequent cracks, unlike the prevalent random fuse models, where no distortion is allowed. Two different types of sites (A and B) are distributed randomly on the lattice, representing two different constituents. There are springs of three types connecting them (A-A, B-B and A-B). We assign two spring parameters for each type of spring. These are a spring constant and a breaking threshold. We show that intermediate compositions may require higher energy to induce the first sample spanning break than either pure A or pure B. So the breaking energy goes through a maximum as the concentration of one component varies from 0% to 100%. The position and height of the peak depend on the spring parameters.     

Structure and magnetic properties of Co–Ni–Li ferrites synthesized by citrate precursor method

Nanoparticles of Co_0.5Ni_0.5−2xLi_xFe_2+xO₄ with x ranging from 0.00 to 0.25 in steps of 0.05 were prepared by using citrate precursor method. The microstructure and the magnetic properties of the as-prepared nanosamples and the effect of increasing Li¹⁺ ions on their physical properties were studied. X-ray diffraction (XRD), transmission electron microscopy (TEM), particle size analyzer (PSA), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometer (VSM) were used to investigate the samples. The XRD analysis confirmed the cubic spinel phase formation of the prepared samples, while TEM images and PSA ensure the nanostructural features of them. The FTIR spectra reveal the presence of two prominent absorption bands v₁ and v₂ in the range of 600 and 400 cm⁻¹ which are usually attributed to the tetrahedral and octahedral complexes of the spinel lattice, respectively. The change of v₂ gradually towards lower frequency and the slightly change of v₁ were explained depending on the effect of increasing Li¹⁺ content on the bond length of B-site metal ions and the spin canting of A-site metal ions, respectively. Saturation magnetization and remnant magnetization were found to increase with adding Li¹⁺ ions up to x=0.15 and then to decrease again, while coercivity decreases monotonically by increasing Li¹⁺ ions. The change in magnetic properties by adding Li¹⁺ ions is explained as to be dependent on many factors such as crystallite size, measured density, porosity, expected cation distribution, A–B exchange interactions, and magnetocrystalline anisotropy.