Structure and the electrical and magnetic properties of perovskite-like Gd x Cu3V4O12 oxide

A new perovskite-like compound Gd _x Cu₃V₄O₁₂ (x = 0.67?0.73) is synthesized under high pressures (P = 4?9 GPa) and at high temperatures (T = 1000°C). Its structure (space group Im3, Z = 2, a = 7.2930(5) ?) is estrablished by X-ray analysis. The bond lengths and bond angles are determined. The temperature dependences of electrical resistivity (10?C300 K) and magnetic susceptibility are studied. The high-pressure Gd _x Cu₃V₄O₁₂ phase is found to have a semi-conductor type of conductivity and paramagnetic properties.     

3d过渡金属掺杂对Cd12O12纳米线电子和磁性能的影响

采用基于密度泛函理论的第一性原理计算方法,系统研究了3d过渡金属元素(Sc、Ti、Cr、Mn、Co、Cu和Zn)掺杂Cd12O12纳米线的几何结构,电子结构和磁性。结果表明：所有掺杂体系均是热力学稳定的;掺杂Ti或Zn时体系保留了原有的非磁半导体特性,掺杂Mn、Co或Cu时能够实现磁性半导体态,而在掺杂Sc（Cr）时体系转变为非磁性金属态（磁性金属态）。研究结果表明,掺杂3d过渡金属元素的Cd12O12纳米线在电子、光电和自旋电子学领域具有潜在的应用价值。     

Effect of thermobaric treatment on the structure and properties of CaCu3Ti4O12

CaCu₃Ti₄O₁₂ is prepared by means of solid-state sintering. The effect of thermobaric treatment (P = 8.0 GPa and T = 1100°C) and partial replacement of titanium by vanadium on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ are investigated.     

Thermobaric synthesis, structure, and properties of Dy x Cu3V4O12

The perovskite-like compound Dy _x Cu₃V₄O₁₂ (x = 0.67–0.75) is synthesized under high pressure (P = 4.0–9.0 GPa) and temperature (T = 1000°C). Its crystal structure is determined (Im-3 space group, Z = 2, a = 7.29348(7) Å) by means of powder X-ray diffraction. The basic lengths and bond angles are defined. It is found that the high-pressure phase of Dy _x Cu₃V₄O₁₂ is characterized by metallic conductivity and paramagnetic properties.     

Zn_3V_2O_8和Zn_2V_2O_7的电子结构与光学性能的第一性原理研究

基于密度泛函理论(DFT)的第一性原理平面波超软赝势方法,在零压下建立了α-Zn_3V_2O_8和α-Zn_2V_2O_7晶体模型,分别对模型进行了电子结构和光学性质的计算.实验结果表明:α-Zn_3V_2O_8和α-Zn_2V_2O_7都属于间接带隙,α-Zn_3V_2O_8的禁带宽度为2. 715 eV,α-Zn_2V_2O_7的禁带宽度为2. 540 eV;同时,α-Zn_3V_2O_8和α-Zn_2V_2O_7都具有很强的紫外-近紫外光吸收能力;反射光谱、吸收光谱及能量损失谱的差异,共同导致Zn_3V_2O_8的光学性能要优异于Zn_2V_2O_7.     

The effect of colossal magnetoresistance in SmCu3Mn4O12 perovskite-like oxide

Journal of Experimental and Theoretical Physics - The magnetoresistance effect in the SmCu3Mn4O12 compound with a perovskite-like structure is investigated for the first time. It is found that an...     

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     

First-principles study of electronic structure and magnetic properties of Sr3Fe2O5 oxide

We investigate the electronic structure and magnetic properties of layered compound Sr₃Fe₂O₅ based on first-principles calculations in the framework of density functional theory with GGA+U method. Under high pressure, the ladder-type layered structure of Sr₃Fe₂O₅ is transformed into the infinite layered structure accompanied by a transition from G-type anti-ferromagnetic (AFM) insulator to ferromagnetic (FM) metal and a spin transition from S=2 to S=1. We reproduce these transformations in our calculations and give a clear physical interpretation.     

Antiferromagnetic phase transition in garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12

Ferrimagnetism has been extensively studied in garnets, whereas it is rare to find the antiferromagnet. Present work will demonstrate antiferromagnetism in the two Mn–V-garnets. Antiferromagnetic phase transition in AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ has been found, where the magnetic Mn²⁺ ions locate only on octahedral A site. The heat capacity shows sharp peak due to antiferromagnetic order with the Néel temperature T_N=23.8 K for AgCa₂Mn₂V₃O₁₂ and T_N=14.2 K for NaPb₂Mn₂V₃O₁₂. The magnetic entropy change over a temperature range 0–50 K is 13.9 J K^?1 mol-Mn²⁺-ions^?1 for AgCa₂Mn₂V₃O₁₂ and 13.6 J K^?1 mol-Mn²⁺-ions^?1 for NaPb₂Mn₂V₃O₁₂, which are in good agreement with calculated value of Mn²⁺ ion with spin S=5/2. The magnetic susceptibility shows the Curie–Weiss behavior over the range 29–350 K. The effective magnetic moment μ_eff and the Weiss constant θ are μ_eff=6.20 μ_B Mn²⁺-ion^?1 and θ=?34.1 K (antiferromagnetic sign) for AgCa₂Mn₂V₃O₁₂ and μ_eff=6.02 μ_B Mn²⁺-ion^?1 and θ=?20.8 K for NaPb₂Mn₂V₃O₁₂.     

The changes in electronic structure and optical properties of magnetic semiconductors at magnetic phase transitions

The influence of magnetic phase transitions on electronic structure and optical properties of magnetic semiconductors is discussed. Europium chalcogenides and chromium chalcogenide spinels are the main subjects of the investigation. It is shown, that many-body effects are responsible for the changes of optical properties and non-rigid band behavior of electronic structure. Magnetic phase transition leads to energy shift of wide bands and change in density of states of “magnetic” d(f)-electrons without any significant shift of their energies. The influence of fluctuations at T ～ T _c and antiferromagnetic semiconductors are also considered.     

Effect of lanthanum ions on magnetic properties of Y3Fe5O12 nanoparticles

Lanthanum ion (La³⁺)-substituted garnet nanoparticles Y_3?x La _x Fe₅O₁₂ (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) were fabricated by a sol–gel method. Their crystalline structures and magnetic properties were investigated by using X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and Mössbauer spectrum. The XRD results show that samples of Y_3?x La _x Fe₅O₁₂ (0.0 ≤ x ≤ 0.8) are all single phase and the sizes of particles range from 32 to 65 nm. Those of Y₂LaFe₅O₁₂ consisted of peaks from garnet and LaFeO₃ structures. Compared to pure YIG, the saturation magnetization is larger when the La concentration x = 0.2. However, with increasing La concentration (x), it decreases obviously. Meanwhile, may be due to the enhancement of the surface spin effects, the saturation magnetization rises as the particle size is increased. Different from the pure YIG, the Mössbauer spectra of Y_2.8La_0.2Fe₅O₁₂ and Y_2.2La_0.8Fe₅O₁₂ are composed of four sets of six-line hyperfine patterns. The results tell us that the substitution of La³⁺ ions with large ionic radius (1.061 Å) will give rise to a microscopic structure distortion of the a- and d-sites to different degrees, and the Zeeman sextets from a- and d-sites begin to split into two sub-sextets, which is helpful to explain the phenomenon observed in the study of the magnetic property.     

First-principles FP-LAPW calculations and X-ray spectroscopy studies of the electronic structure of Zr3V3O and Zr3V3O0.6 oxides

Electronic properties of Zr₃V₃O oxide, a very promising hydrogen-storage material, were studied both from theoretical and experimental points of view employing the full potential linearized augmented plane wave (FP-LAPW) method as well as X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). Total and partial densities of states of the constituting atoms of Zr₃V₃O have been derived from the FP-LAPW calculations. These data indicate that, the O 2p-like states are the dominant contributors in the bottom of the valence band, whilst the top of the valence band and the bottom of the conduction band of Zr₃V₃O are dominated by contributions of the V2 3d-like states, with slightly smaller contributions of the V1 3d-like states as well. Significant contributions of the Zr 4d-like states throughout the whole valence-band region and near the bottom of the conduction band are also characteristic of the electronic structure of Zr₃V₃O. The XPS valence-band spectra and the XES Zr Lβ_2,15, V L_α and O K_α bands have been derived and compared on a common energy scale for Zr₃V₃O and Zr₃V₃O_0.6 oxides. This comparison of the experimental spectra was found to be in excellent agreement with the results of the FP-LAPW calculations. In addition, the XPS Zr 3d, V 2p and O 1s core-level binding energies have been measured for Zr₃V₃O and Zr₃V₃O_0.6 oxides.     

Effects of electron correlation and spin-orbit coupling on the electronic and magnetic properties of TbCu3Mn4O12

Electronic and magnetic properties of the three magnetic-sublattice double perovskite TbCu₃Mn₄O₁₂ (TCMO) are investigated by performing first-principles density-functional theory calculations. Our electronic structure calculations show that TCMO is half-metallic and its half-metallicity can only be correctly described when the electron correlation on Tb³⁺ 4f⁸ electrons are considered. The energies of different magnetic configurations among the three magnetic sublattices are also calculated, revealing that the magnetic configuration with Mn and Cu spins in the antiparallel arrangement and with the Tb magnetic moments ferromagnetically/antiferromagnetically (FM/AFM) coupled to Cu/Mn spins (that is Tb^↓Cu₃^↓Mn₄^↑O₁₂) is the lowest energetic magnetic state, which is consistent with recent experimental results. The magnetic anisotropy is further calculated for the [1 1 1], [1 1 0], and [0 0 1] spin quantization directions. It is found that the [1 1 1]-direction is more stable than the [1 1 0]- and [0 0 1]-directions by 123 and 135 meV per formula unit, respectively, indicating a significant magnetic anisotropy. Our detailed projected partial density of states analysis finally shows that Cu and Mn are antiferromagnetically coupled by superexchange interaction and Tb is expected to interact FM with A-site Cu and AFM with B-site Mn sublattices by way of 4f-2p-3d.     

The influence of oxygen vacancies on the electronic and magnetic properties of perovskite-like SrFeO3-x

The electronic, magnetic properties and lattice relaxations of oxygen-deficient cubic strontium ferrite, SrFeO_2.875, in ferromagnetic configuration are studied by means of the density functional theory using LCAO basis (SIESTA code) calculations. It is shown that Fe and Sr atoms are displaced from oxygen vacancies while oxygen anions are attracted to the vacancies. The DOS distributions, magnetic moments and atomic effective charges are analyzed in comparison with vacancy free SrFeO₃; these parameters are found to change weakly with appearance of oxygen vacancies, in contrast to conventional ionic picture. Some strengthening of Fe-O covalent bonds in the vicinity of the oxygen vacancy is found. The formation energy of oxygen vacancies and divacancies are evaluated.     

Effect of electron correlations on the structure of photoprotein substrates

The electronic structure and total energy of various isomeric forms of coelenterazine and coelenteramide have been calculated by quantum chemistry methods both in the single-electron approximation and taking into account correlation effects. It has been shown that the inclusion of electron correlations makes it possible to obtain the structure of the coelenteramide close to the experimentally determined structure, as well as to choose the structure of the coelenterazine CLZ(1H) as the most probable isomeric form.     

Effect of diamagnetic substitutions in BaFe12O19 on the magnetic properties

The exchange parameters of BaFe₁₂O₁₉ have been calculated from the temperature dependence of the saturation magnetization using the molecular field theory. Under the assumption that the exchange parameters do not change for diamagnetic substitutions of the Fe³⁺ ions, it is shown that the temperature coefficient of the magnetization at room temperature cannot be decreased without decreasing the magnetization. Diamagnetic substitution in the octahedralf ₂ site would solely increase the saturation magnetization.     

Density functional theory study on the magnetic properties of Co3O4 with normal spinel structure

The magnetic properties of Co₃O₄ with a normal spinel structure were investigated via the full potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). The exchange and correlation effects between electrons were treated with a standard generalized gradient approximation (GGA) from Perdew–Burke–Ernzerhof (PBE), as a function of the on-site Coulomb U term, the GGA−PBE+U method, and a B3PW91 hybrid functional with different Hartree–Fock exchange admixtures. Were calculated all of these exchange–correlation (XC) functionals both with and without spin–orbit coupling (SOC). The objective for these calculations was to predict the ground-state magnetic structure of Co₃O₄ crystal using different XC functionals and to investigate the influence that SOC had on these results. All of our calculations confirmed that the collinear antiferromagnetic (AFM) order was energetically more favorable than the ferromagnetic (FM) one, which agrees with experimental findings. This conclusion was not influenced by the XC functional type employed or whether the spin–orbit effect was used. Thus, the present work does not confirm the recent DFT plane wave pseudopotential results that when including spin–orbit effects, the calculations determined that the collinear FM state had lower energy than the AFM one.     

高压下YB6的电子结构与光学性质的第一性原理研究

本文从第一性原理出发,基于密度泛函理论体系下的广义梯度近似（GGA）方法,对不同压力下YB6的电子结构及光学性质方面进行了研究。结果表明：在一定的压力范围内随着压力的增大,费米面以上的能带往高能量处移动,费米面以下的能带往低能量处移动。能量损失谱的第一个峰随着压力的增大往高能量处移动,并且峰强增大。这表明可以通过压力来调节YB6在可见光区的吸收谷的位置及强度,在高压下YB6将展现更好的隔热性能。     

高压下YB_6的电子结构与光学性质的第一性原理研究

本文从第一性原理出发,基于密度泛函理论体系下的广义梯度近似(GGA)方法,对不同压力下YB_6的电子结构及光学性质方面进行了研究.结果表明:在一定的压力范围内随着压力的增大,费米面以上的能带往高能量处移动,费米面以下的能带往低能量处移动.能量损失谱的第一个峰随着压力的增大往高能量处移动,并且峰强增大.这表明可以通过压力来调节YB_6在可见光区的吸收谷的位置及强度,在高压下YB_6将展现更好的隔热性能.     

Effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles

In this work the effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles obtained by the sol-gel method is analyzed. Two sets of samples were prepared: Fe3O4 nanoparticles and Fe3O4@SiO2 core-shell composites. The samples display the characteristic spinel structure associated with the magnetite Fe3O4 phase, with the majority of grain sizes around 5-10 nm. At room temperature the nanoparticles show the characteristic superparamagnetic behavior with mean blocking temperatures around 160 and 120 K for Fe3O4 and Fe3O4@SiO2, respectively. The main effect of the SiO2 coating is reflected in the temperature dependence of the high field magnetization (μ(0)H = 6 T), i.e. deviations from the Bloch law at low temperatures (T < 20 K). Such deviations, enhanced by the introduction of the SiO2 coating, are associated with the occurrence of surface spin disordered effects. The induction heating effects (magnetic hyperthermia) are analyzed under the application of an AC magnetic field. Maximum specific absorption rate (SAR) values around 1.5 W g(-1) were achieved for the Fe3O4 nanoparticles. A significant decrease (around 26%) is found in the SAR values of the SiO2 coated nanocomposite. The different heating response is analyzed in terms of the decrease of the effective nanoparticle magnetization in the Fe3O4@SiO2 core-shell composites at room temperature.