Temperature dependence of giant magnetoresistance in CoAg granular composite

The temperature dependence of the giant magnetoresistance (GMR) of CoAg composite is studied phenomenologically in this paper. It is considered that the composite contains both large magnetic grains and tiny clusters. The tiny clusters, composing of several atoms, lead to an almost linear increase of the giant magnetoresistance with the temperature reduction. Our calculations are in good agreement with recent experimental data for a nanogranular CoAg films.     

Concentration dependence of giant magnetoresistance in magnetic granular composites

We combine effective medium theory (EMT) with the two-channel conducting model to study the magnetic granular concentration dependence of a giant magnetoresistance (GMR) in magnetic granular composites. The composite is composed of small magnetic granules (such as Co) embedded in an immiscible nonmagnetic metallic matrix (such as Ag). We present a model for the composite in which the magnetic metallic granules are spherical in shape and have a distribution in sizes, and in which there are different contributions of superparamagnetic and ferromagnetic granules to conductance. The calculated result about the concentration dependence of GMR is in agreement with the experimental data.     

温度和电子有效质量对隧穿磁致电阻偏压依赖关系的影响

本文研究温度和势垒中的电子有效质量对铁磁隧道结中隧穿磁致电阻(TMR)的偏压依赖和变号行为的影响。所求得的TMR随温度上升明显减小的结果与实验一致。除了前文中指出的势垒高度(Ф)以外, 还发现垒中电子的有效质量(mB)是物理上控制TMR变号的另一个重要因素。相应于TMR变号的临界电压(VC)将随Ф升高而增大, 但随mB增大而减小。此外, 零偏压的TMR和临界电压VC将因温度升高而减小。作者希望上述理论结果将有助于实验研究。     

Mean-free path effects in magnetoresistance of ferromagnetic nanocontacts

We investigated the mean-free path effects on the magnetoresistance of ferromagnetic nanocontacts. For most combinations of parameters the magnetoresistance monotonously decreases with increasing the contact cross-section. However, for a certain choice of parameters the calculations show non-monotonous behavior of the magnetoresistance in the region in which the diameter of the contact becomes comparable with the mean-free path of electrons. We attribute this effect to different conduction regimes in the vicinity of the nanocontact: ballistic for electrons of one spin projection, and simultaneously diffusive for the other. Furthermore, at certain combinations of spin asymmetries of the bulk mean-free paths in a heterocontact, the magnetoresistance can be almost constant, or may even grow as the contact diameter increases. Thus, our calculations suggest a way to search for combinations of material parameters, for which high magnetoresistances can be achieved not only at the nanometric size of the contact, but also at much larger cross-sections of nanocontacts which can be easier for fabriaction with current technologies. The trial calculations of the magnetoresistance with material parameters close to those for the Mumetal-Ni heterocontacts agree satisfactorily with the available experimental data.     

Structure and analytical potential energy function for the ground state of the BCx (x=0, －1)

In this paper, the electronic states of the ground states and dissociation limits of BC and BC- are correctly determined based on group theory and atomic and molecular reaction statics. The equilibrium geometries, harmonic frequencies and dissociation energies of the ground state of BC and BC- are calculated by using density function theory and quadratic CI method including single and double substitutions. The analytical potential energy functions of these states have been fitted with Murrell-Sorbie potential energy function from our ab initio calculation results. The spectroscopic data （αe, ωe and ωeχe） of each state is calculated via the relation between analytical potential energy function and spectroscopic data. All the calculations are in good agreement with the experimental data.     

Interference commensurate oscillations in quasi-one-dimensional conductors

We suggest an analytical theory to describe angular magnetoresistance oscillations recently discovered in the quasi-one-dimensional conductor (TMTSF)2PF6 [see Phys. Rev. B 57, 7423 (1998)]] and define the positions of the oscillation minima. The origin of these oscillations is related to interference effects resulting from Bragg reflections which occur as electrons move along quasiperiodic and periodic ("commensurate") electron trajectories in the extended Brillouin zone. We reproduce via calculations existing experimental data and predict some novel effects.     

Co2+离子在MgF2和ZnF2晶体中的各向异性g因子的理论研究

利用基于基团模型的3d⁷离子在斜方对称中的高阶微扰公式计算了MgF₂和ZnF₂晶体中Co²⁺杂质中心的各向异性g因子g_x，g_y和g_z. 在计算中，考虑了共价效应， 组态相互作用和斜方晶体场的贡献；而且与此相关的参量可由所研究的晶体的光谱和结构数据得到. 计算结果与实验符合较好.     

Spin-controlled negative magnetoresistance resulting from exchange interactions

We studied conductivity of AlGaAs–GaAs quantum well structures (where centers of the wells were doped by Be) at temperatures higher than 4 K in magnetic fields up 10 T. Throughout all the temperature region considered the conductivity demonstrated activated behavior. At moderate magnetic fields 0.1 T < H < 1 T, we observed negative isotropic magnetoresistance, which was linear in magnetic field while for magnetic field normal with respect to the plane of the wells the magnetoresistance was positive at H > 2T. To the best of our knowledge, it was the first observation of linear negative magnetoresistance, which would be isotropic with respect to the direction of magnetic field. While the isotropic character of magnetoresistance apparently evidences role of spins, the existing theoretical considerations concerning spin effects in conductance fail to explain our experimental results. We believe that such a behavior can be attributed to spin effects supported by exchange interactions between localized states.     

Dy2 AlFe13 Mn3 化合物的本征磁致伸缩

通过X-射线衍射及磁测量手段研究了Dy2AlFe13Mn3化合物的结构及磁性质。研究结果表明Dy2AlFe13Mn3化合物具有六角相的Th2Ni17型结构。通过X-射线热膨胀测定法发现Dy2AlFe13Mn3化合物在245到344K的温度范围内存在负热膨胀现象，其平均热膨胀系数为α＝-1.1×10-4K-1K-1。在105到360K的温度范围内，通过比较磁性状态下的晶胞参数和由高温顺磁状态外延得到的低温顺磁状态下的晶胞参数间的差别计算了Dy2AlFe13Mn3化合物的本征磁致伸缩。结果表明Dy2AlFe13Mn3化合物的本征体磁致伸缩ωS在105到245K的温度范围内随着温度的升高而增大，由105K时的7.0×10-3 增加到245K时的9.1×10-3。随着温度的进一步升高，ωS反而减小。沿c轴方向的本征线磁致伸缩λc随着温度的升高而减小。基面内的本征线磁致伸缩λa在105到270K的温度范围内随着温度的升高而增大，从105K时的0.8×10-3增大到270K时的3.4×10-3，然后随着温度的进一步升高而减小。     

Fusion curves and thermodynamic properties of carbon tetrachloride, chloroform, bromoform and silicon tetrachloride at pressure up to 3500 MPa

The fusion temperature as a function of pressure for carbon tetrachloride, chloroform, bromoform and silicon tetrachloride at pressures up to 3500MPa has been determined. The experimental data were fitted by the equation Tfus=T0（1 ＋ Δp/a1）^a2 exp（-a3Δp） and the changes of the maolar enthalpy and molar internal energy on fusion were calculated using the parameters of the fitted equation. Comparisons with the data from the literature show that the experimental data, parameters of fitted equations, changes of the molar enthalpy and molar internal energy are reliable.     

Enhanced granular magnetoresistance due to ferromagnetic layers

Giant magnetoresistance (GMR) of sequentially evaporated Fe-Ag structures has been investigated. Direct experimental evidence is given, showing that inserting ferromagnetic layers into a granular structure significantly enhances the magnetoresistance. The increase in the GMR effect is attributed to spin polarization effects. The large enhancement (up to more than a fourfold value) and the linear variation of the GMR in low magnetic fields are explained by scattering of the spin polarized conduction electrons on paramagnetic grains.     

Magnetoresistance effect in vertical NiFe/graphene/NiFe junctions

For convenient and efficient verification of the magnetoresistance effect in graphene spintronic devices, vertical magnetic junctions with monolayer graphene sandwiched between two NiFe electrodes are fabricated by a relatively simple way of transferring CVD graphene onto the bottom ferromagnetic stripes. The anisotropic magnetoresistance contribution is excluded by the experimental result of magnetoresistance (MR) ratio dependence on the magnetic field direction. The spin-dependent transport measurement reveals two distinct resistance states switching under an in-plane sweeping magnetic field. A magnetoresistance ratio of about 0.17 % is obtained at room temperature and it shows a typical monotonic downward trend with the bias current increasing. This bias dependence of MR further verifies that the spin transport signal in our device is not from the anisotropic magnetoresistance. Meanwhile, the I—V curve is found to manifest a linear behavior, which demonstrates the Ohmic contacts at the interface and the metallic transport characteristic of vertical graphene junction.     

Tunneling magnetoresistance of phase-separated manganites

A simple model of phase separation is used to study the magnetoresistance of manganites in the nonmetallic state. It is assumed that the phase separation corresponds to the emergence of small ferromagnetic metallic droplets (ferrons) in a nonconducting antiferromagnetic or paramagnetic medium, with the metallic phase concentration being far from the percolation threshold. The charge transfer is accomplished by way of electron jumps between droplets. The magnetoresistance in such a system is defined both by the variation of the volume of the metal phase and by the dependence of the probabilities of electron transitions on the magnitude of the magnetic field. It is demonstrated that, in the region of low magnetic fields, the magnetoresistance is quadratic with respect to the field and decreases with temperature by the T ^?n law, where n takes values from 1 to 5 depending on the correlation between the parameters. In the high-field limit, the magnetoresistance increases abruptly with the volume of the metal phase. The crossover of the field dependence from quadratic to a stronger one may be accompanied by the emergence of a platean in the magnetoresistance. The correlation between the obtained results and the available experimental data is discussed.     

Effect of thermal-annealing on the magnetoresistance of manganite-based junctions

Thermal-annealing has been widely used in modulating the oxygen content of manganites. In this work, we have studied the effect of annealing on the transport properties and magnetoresistance of junctions composed of a La0.9Ca0.1MnO3＋6 film and a Nb-doped SrTiO3 substrate. We have demonstrated that the magnetoresistance of junctions is strongly dependent on the annealing conditions： Prom the junction annealed-in-air to the junction annealedin-vacuum, the magnetoresistance near 0-V bias can vary from ～-60% to N～0. A possible mechanism accounting for this phenomenon is discussed.     

First-principle calculations of the linear and nonlinear optical response for GaX (X = As,Sb, P)

The linear and nonlinear optical properties in non-centro-symmetric cubic semiconductor GaX (X=As, Sb, P) are studied by using the first-principle full potential linear augmented plane wave (FP-LAPW) and the linear muffin-tin orbital (LMTO) methods. We present calculations of the frequency-dependent complex dielectric function and it zero-frequency limit . A simple scissor operator is applied to adjust the band gap from the local-density calculations to match the experimental value. Calculations are reported for the frequency-dependent complex second-order non-linear optical susceptibilities up to 6 eV and it zero-frequency limit . Comparison with available experimental data shows good agreement. Our calculations show excellent agreement between the two methods.     

Effect of power-law nonlinearity on ${ \mathcal P }{ \mathcal T }$-symmetric optical system with fourth-order diffraction

Gaussian-type soliton solutions of the nonlinear Schrödinger (NLS) equation with fourth order dispersion, and power law nonlinearity in the novel parity-time (${ \mathcal P }{ \mathcal T }$)-symmetric quartic Gaussian potential are derived analytically and numerically. The exact analytical expressions of the solutions are obtained in the first two-dimensional (1D and 2D) power law NLS equations. By means of the linear stability analysis, the effect of power law nonlinearity on the stability of Gauss type solitons in different nonlinear media is carried out. Numerical investigations do confirm the stability of our soliton solutions in both focusing and defocusing cases, specially around the propagation parameters.     

Investigation of analytical harmonic frequency and potential energy function,vibrational levels for the X^2∑^＋ and A^2Л states of CN radical

This paper calculates the equilibrium structure and the potential energy functions of the ground state （X^2∑^＋） and the low lying excited electronic state （A^2Л） of CN radical are calculated by using CASSCF method. The potential energy curves are obtained by a least square fitting to the modified Murrell-Sorbie function. On the basis of physical theory of potential energy function, harmonic frequency （ωe） and other spectroscopic constants （ωeχe, βe and αe） are calculated by employing the Rydberg-Klei-Rees method. The theoretical calculation results are in excellent agreement with the experimental and other complicated theoretical calculation data. In addition, the eigenvalues of vibrational levels have been calculated by solving the radial one-dimensional SchrSdinger equation of nuclear motion using the algebraic method based on the analytical potential energy function.     

Spontaneous magnetostriction of Y2Fe16Al compound

The structure and magnetic properties of Y₂Fe₁₆Al compound have been investigated by means of x-ray diffraction and magnetization measurements. The Y₂Fe Fe₁₆Al compound has a hexagonal Th$_{2}$Ni$_{17}$-type structure. Negative thermal expansion was found in Y₂Fe₁₆Al compound in the temperature range from 332 to 438K by x-ray dilatometry. The coefficient of the average thermal expansion is \alpha =-3.4\times 10^-5K^-1. The spontaneous magnetostrictive deformations from 293 to 427K have been calculated based on the differences between the experimental values of the lattice parameters and the corresponding values extrapolated from the paramagnetic range. The result shows that the spontaneous volume magnetostrictive deformation \textit{$\omega $}$_{\rm S}$ decreases from 5.4$\times $10^-3to near zero with temperature increasing from 293 to 427K, the spontaneous linear magnetostrictive deformation \textit{$\lambda $}$_{\rm c}$ along the $c$ axis is much larger than the spontaneous linear magnetostrictive deformation \textit{$\lambda $}$_{\rm a}$ in basal-plane in the same temperature range except near 427K.