The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA+U studies

The electronic structure and ferromagnetic stability of Co-doped SnO₂ are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U_Co transforms the ground state of Co-doped SnO₂ to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U_Co calculations show that the pure substitutional Co defects in SnO₂ cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U_Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U_Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U_O on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U_Co calculations are not changed by the correction of band gap.     

New beta(fcc)-cobalt to 210 GPa

We report a new phase transition in cobalt from the magnetic varepsilon(hcp) to a beta(fcc) phase, likely nonmagnetic, at 105 GPa. It occurs martensitically in an extended pressure region between 105 and 150 GPa without any apparent volume change. The fcc phase of Co is in systematic accordance with the 4d and 5d counterparts. The pressure-volume isotherm of beta-Co resembles those of alpha(fcc)-Ni and varepsilon(hcp)-Fe within 1%. The phase diagram of cobalt suggests that the fcc stability increases with increasing occupancy of d-band electrons from Fe to Co to Ni.     

On the energetics of transversal and longitudinal fluctuations of atomic magnetic moments

By constrained spin-density functional calculations we estimate the relative role of the longitudinal and transversal fluctuations of the magnetic moments in the series of 3d metals (bcc Fe, hcp and fcc Co, and fcc Ni) for weak excitations from the ferromagnetic ground state. It is shown that the importance of longitudinal fluctuations strongly varies from relatively small in bcc Fe to large in fcc Ni. This means that a consistent adiabatic treatment of the low-energy spin fluctuations should include independent longitudinal fluctuations.     

非静水压条件下铁从α到ε结构相变的第一性原理计算

采用基于密度泛函理论的平面波赝势方法,研究了三轴加载的非静水压力和静水压力对铁从体心立方结构（bcc,α相）到六角密排结构（hcp,ε相）相变压力和磁性的影响,结果发现：在0—18 GPa压力范围内,相对静水压力条件,随着压力的升高,bcc结构的原子磁矩在非静水压力下降低得更快;在非静水压力下,相变更容易发生,相变压力随着非静水压力程度的增加而降低;并且对非静水压力对相变压力影响的物理机理进行了讨论. 关键词： 相变 非静水压力 第一性原理 铁     

Phase transition and magnetic properties of Mg-doped hexagonal close-packed Ni nanoparticles

Mg-doped Ni nanoparticles with the hexagonal close-packed (hcp) and face-centered cubic (fcc) structure have been synthesized by sol-gel method sintered at different temperatures in argon atmosphere. The sintering temperature played an important role in the control of the crystalline phase and the particle size. The pure hcp Mg-doped Ni nanoparticles with average particle size of 6.0 nm were obtained at 320 °C. The results indicated that the transition from the hcp to the fcc phase occurred in the temperature range between 320 °C and 450 °C. Moreover, the VSM results showed that the hcp Mg-doped Ni nanoparticles had unique ferromagnetic and superparamagnetic behavior. The unsaturation even at 5000 Oe is one of the superparamagnetic characteristics due to the small particle size. From the ZFC and FC curves, the blocking temperature T_B of the hcp sample (6.0 nm) was estimated to be 10 K. The blocking temperature was related to the size of the magnetic particles and the magnetocrystalline anisotropy constant. By theoretical calculation, the deduced particle size was 6.59 nm for hcp Mg-doped Ni nanoparticles which was in agreement with the results of XRD and TEM.     

Magnetic anisotropy and X-ray magnetic circular dichroism of MnNi surface alloy: Ab initio studies

The magnetic properties of MnNi surface alloy grown on fcc Co(0 0 1) surface have been explored through the full potential linearized augmented plane wave (FLAPW) method. It has been found that one monolayer (ML) of Mn_0.5Ni_0.5 alloy on Co(0 0 1) surface has a ferromagnetic (FM) ground state. We suggest that the direct exchange coupling between Mn and Ni plays an essential role for the ferromagnetism. The vertical position of Mn is higher by 0.26 Å than that of Ni, i.e., the surface has a buckling geometry. In contrast to pure fcc Co(0 0 1) film which has in-plane magnetization, we have obtained that the magnetocrystalline anisotropy of MnNi/Co(0 0 1) due to spin-orbit interaction is perpendicular to the film surface and the magnetic anisotropy energy is 146 μeV/cell. In addition, the theoretically calculated X-ray magnetic circular dichroism (XMCD) is presented.     

第一性原理对金的高压相变和零温物态方程的计算

在密度泛函理论下，用缀加平面波加局域轨道方法，分别采用广义梯度近似(GGA)和局域密度近似(LDA)对金的面心立方晶格结构(fcc)、体心立方晶格结构(bcc)和六角密堆积结构(hcp)的结构能量进行了计算.在GGA下，计算得出fcc向hcp和hcp向bcc的相变分别发生在380GPa 和1250GPa；而LDA下相变分别发生在490GPa和790GPa.当计算压强达到2TPa时，bcc在这两种近似下仍然保持稳定的结构.根据不同体积下不同结构的电子态密度的特征，对发生相变的物理原因进行了定性的分析，在此基础上得到了金的零温状态方程. 关键词： 缀加平面波方法 固态相变 电子态密度 物态方程     

Importance of shear in the bcc-to-hcp transformation in iron

Iron shows a pressure-induced martensitic phase transformation from the ground state ferromagnetic bcc phase to a nonmagnetic hcp phase at approximately 13 GPa. The exact transformation pressure (TP) and pathway are not known. Here we present a multiscale model containing a quantum-mechanics-based multiwell energy function accounting for the bcc and hcp phases of Fe and a construction of kinematically compatible and equilibrated mixed phases. This model suggests that shear stresses have a significant influence on the bcc<-->hcp transformation. In particular, the presence of modest shear accounts for the scatter in measured TPs. The formation of mixed phases also provides an explanation for the observed hysteresis in TP.     

Electronic and magnetic properties and their response to pressure in Ni2MnB

The electronic structure and magnetic properties of Ni₂MnB upon pressure up to 20 GPa have been studied by using the density functional theory (DFT) method. The results indicate that ferromagnetic ordered Ni₂MnB in L2₁ structure is more stable than the nonmagnetic one. The magnetic moments of Ni and Mn atoms as well as the total magnetic moment of Ni₂MnB are found to decrease weakly with increasing pressure. The pressure derivative of the total magnetic moment is −3.07×10⁻³ GPa⁻¹. The equilibrium bulk modulus and its derivative from the Murnaghan equation of state (EOS) are B₀=247.7 GPa, B′=4.98.     

Elastic properties and phonon dispersions of rhenium in hexagonal-close-packed structure under pressure from first principles

The elastic properties of the hexagonal-close-packed (hcp) structure rhenium (Re) and their behavior under pressure are investigated using the local density approximation (LDA) and the generalized gradient approximation (GGA). The obtained high pressure elastic constants are well consistent with previous theoretical date, while large discrepancies are found between theory and the high pressure experiments. The calculated isothermal bulk modulus B₀ (376 GPa for GGA and 389 GPa for LDA) and its initial pressure derivative (4.52 for LDA and 4.58 for GGA) compare favorably with the experimental values. Moreover, it is found that the value of c/a, B/G, Poisson's ratio, and B_c/B_a are virtually independent of pressure. We also performed calculation for phonon dispersions at high pressure. GGA in our calculation exhibits a same trend as the high pressure experimental curve.     

Structural and magnetic properties of Co50Ni50 powder mixtures

In the present work, morphological, structural, thermal and magnetic properties of nanocrystalline Co₅₀Ni₅₀ alloy prepared by high energy planetary ball milling have been studied by means of scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. The coercivity and the saturation magnetization of alloyed powders were measured at room temperature by a vibration sample magnetization. Morphological observations indicated a narrow distribution in the particle and homogeneous shape form with mean average particle size around 130 μm². The results show that an allotropic Co transformation hcp→fcc occurs within the three first hours of milling and contrary to what expected, the Rietveld refinement method reveals the formation of two fcc solid solutions (SS): fcc Co(Ni) and Ni(Co) beside a small amount of the undissolved Co hcp. Thermal measurement, as a function of milling time was carried out to confirm the existence of the hcp phase and to estimate its amount. Magnetic measurement indicated that the 48 h milled powders with a steady state particles size have the highest saturation (105.3 emu/g) and the lowest coercivity (34.5 Oe).     

Antiferromagnetic and semiconducting material CrNCN: Prediction from first-principles investigation

The structural stability and physical properties of CrNCN were studied using density functional theory with explicit electronic correlation (GGA+U). Calculated results indicate that the title compound, similar to MNCN (M=Mn, Fe, Co, Ni), is thermodynamically stable but mechanically unstable. Analysis of electronic and magnetic structures reveals that CrNCN is an antiferromagnetic semiconductor. However, the exact magnetic structure of CrNCN consists of an antiferromagnetic intralayer and a ferromagnetic interlayer, which differs from that of the type-II antiferromagnetic semiconductor MNCN (M=Mn, Fe, Co, Ni), which consists of a ferromagnetic intralayer and an antiferromagnetic interlayer.     

Band structure and Fermi surfaces of alternate structural phases of Co and Rh

In this work, first-principles DFT scalar-relativistic calculations using the GGA functionals were performed to study the equilibrium properties of alternate structural phases of Co and Rh. The results show that cobalt orders ferromagnetically in the bcc, fcc and hcp phases, where the Co atoms carry magnetic moments of 1.80 μ_B, 1.71 μ_B and 1.69 μ_B, respectively. Rhodium is ferromagnetic only in the bcc phase where the Rh atoms carry a moment of 0.56 μ_B. The results yield evidence for the influence of the crystal symmetry in establishing ferromagnetic order in transition metals.     

Structure and magnetic state of the films deposited by laser ablation of composite nickel and palladium targets

Transmission electron microscopy, electron diffraction, and vibrating-sample magnetometry are used to show that a metastable hcp structure can form in both nickel and Ni-Pd alloy films during alternating sputtering of the Ni and Pd components of composite targets. The hcp lattice parameters increase monotonically when the palladium content in a sputtered target increases in the range 0–75%. The ratio of the hcp lattice parameters c/a is close to the ideal ratio for the hcp lattice (1.63) within the limits of experimental error. In the as-deposited state, nickel and Ni-Pd alloy films with an hcp structure have no magnetic moment. Upon annealing, the films transform into a ferromagnetic state with an fcc structure. The concentration dependence of the lattice parameter of the fcc solid solution a ₀ is found to exhibit a positive deviation from Vegard’s law, which is characteristic of alloys with a concave liquidus line.     

First-principles study of the high pressure phase transition and lattice dynamics of cerium

We present a first-principles study of the phase transition and lattice dynamics of Ce within the framework of the density functional theory using the GGA+U method. Our calculated results denote that under pressure the transition path is α-Ce (fcc)→α″-Ce (monoclinic, with two atoms per unit cell)→bct-Ce (body centered tetragonal), and the transition pressures are located at 5.36 and 14.37 GPa, respectively. The equation of state in a wide range of pressure is consistent with the experimental data. During the γ-α phase transition, the magnetic moment disappears gradually, which is mainly due to the strong interaction between the 4f and 5d electrons. By calculating the free energies from phonon dispersions including electronic contribution, the obtained γ-α transition temperature at zero pressure is 148 K. From the Blackman diagram of dimensionless elastic constant ratios, we can find that both γ- and α-Ce show negative Cauchy pressure—C₄₄>C₁₂.     

过渡金属原子掺杂的锯齿型磷烯纳米带的磁电子学特性

利用基于密度泛函理论的第一性原理方法,研究了掺杂铁、钴和镍原子的锯齿型磷烯纳米带(ZPNR)的磁电子学特性.研究表明,掺杂和未掺杂ZPNR的结构都是稳定的.当处于非磁态时,未掺杂和掺杂钴原子的ZPNR为半导体,而掺杂铁或者镍原子的ZPNR为金属.自旋极化计算表明,未掺杂和掺杂钴原子的ZPNR无磁性,而掺杂铁或者镍原子的ZPNR有磁性,但只能表现出铁磁性.处于铁磁态时,掺杂铁原子的ZPNR为磁性半导体,而掺杂镍原子的ZPNR为磁性半金属.掺杂铁或者镍原子的ZPNR的磁性主要由杂质原子贡献,产生磁性的原因则是在ZPNR中存在未配对电子.掺杂位置对ZPNR的磁电子学特性有一定的影响.该研究对于发展基于磷烯纳米带的纳米电子器件具有重要意义.     

Magnetic configurations of hexagonal iron rhenium multilayers

We present ab initio calculation of the electronic structure and magnetic properties of Fe/Re multilayers assuming hcp crystallographic phase. The system is modeled according to recent structural analysis which shows that Fe/Re has bct(0 0 1) ordering for 0⩽t_Re⩽8 Å after which a structural phase transition occurs to an hcp phase. Antiferromagnetic and ferromagnetic orders appear more stable than the nonmagnetic (NM) state, only when the Fe atoms begin to adopt an expanded volume of about 27% accompanied by an axial distortion (c/a=1.42). The sizeable magnetic moments calculated on Fe atoms contrast with the paramagnetic bulk hcp. These results are discussed and compared with recent experimental data on hcp Fe/Re superlattices and also with magnetic properties of Fe/Ru multilayers.     

室温下Fe62Ni27Mn11（wt%）合金的压致fcc-hcp相变

 本文采用Mao-Bell型金刚石对顶砧（DAC）及高压在位（in situ）粉末X光衍射照相方法研究了Fe₆₂Ni₂₇Mn₁₁（wt%）合金在0~43.2 GPa压力范围内的压致结构相变和等温压缩行为，实验结果表明，该合金在低压时为fcc结构，在19.4 GPa压力附近出现压致fcc→hcp结构相变，直到43.2 GPa一直保持fcc、hcp二相共存；相变过程中，二相的molar体积相同；高压hcp相得晶格参数比值c/a基本上不随压力而变，可以表示为c/a=1.630±0.006；在卸压过程中，hcp相可保持到5.8 GPa，当卸压到常压时，该合金完全恢复到fcc结构；用Murnaghan等温固体状态方程对其压缩数据进行最小二乘法拟合，得到B₀=(166±12) GPa，B₀'=5.2±0.5；本文还给出了该合金的压致fcc→hcp结构相变模型，并对存在很宽的二相共存区间问题进行了初步探讨。     

Density-functional study of the CO adsorption on ferromagnetic Co(0 0 0 1) and Co(1 1 1) surfaces

The regular CO overlayers at coverage θ = 1/3 adsorbed on the (0 0 0 1) surface of hcp Co and (1 1 1) surface of fcc Co are studied by first-principles density-functional theory with the exchange-correlation component in the PBE form. Adsorption in atop, bridge, and three-fold hcp or fcc position are considered. The adsorption energies, CO stretching frequencies, geometry, work function, and local magnetic moments are studied, and, when possible, compared with experimental or theoretical data. Particularly, we show that the recently proposed correction to adsorption energy of CO prefers correctly the atop adsorption site, whereas the remaining sites are almost degenerate in energy. The CO molecule lowers magnetization on neighbouring Co atoms, and the effect decreases with the adsorption site coordination. We show, however, that this trend is not the result of the different C-Co separation at different adsorption sites. A very small magnetic moment appears on CO that couples antiferromagnetically to Co. Most results are very similar for the Co(0 0 0 1) and Co(1 1 1) surfaces.