铁磁/重金属双层薄膜结构中磁性状态的稳定性分析

本文在理论上研究了铁磁/重金属双层薄膜结构中自旋霍尔效应自旋矩驱动的磁动力学. 通过线性稳定性分析, 获得了以电流和磁场为控制参数的磁性状态相图. 发现通过调节电流密度和外磁场, 可以获得不同的磁性状态, 例如: 平面内的进动态、平面内的稳定态以及双稳态. 当外磁场的方向在一定的范围时, 通过调节电流密度可以实现磁矩的翻转和进动. 同时, 通过数值求解微分方程, 给出了这些磁性状态磁矩的演化轨迹.     

Superconducting fluctuations in a magnetic field

The rounding of the transition curve is measured for superconducting bismuth films in a perpendicular magnetic field. The contribution of the fluctuating superconducting wave function to the conductivity aboveT _c in an applied magnetic field is calculated with a simple model. The allowed states of the fluctuations are cylinders in momentum space. During their life time the fluctuating superconducting electrons can be accelerated by an electrical field and contribute to the conductivity. Experiment and theory are in fair agreement. We obtain some information about the Pauli spin paramagnetism of the electrons.     

Electron spin polarization in field emission

The field emission microscope can be extended to determine the directionally weighted spin-dependent surface density of states of magnetic metals or to obtain information on the electronic structure of metal to magnetic-semiconductor interfaces. We describe the techniques of spin polarization measurements in field emission. The measurement of Kisker et al. on ferromagnetic EuS evaporated on W are discussed along with recent investigations on Ni.     

Low temperature ferromagnetic properties of CdS and CdTe thin films

The magnetic property in a material is induced by the unpaired electrons. This can occur due to defect states which can enhance the magnetic moment and the spin polarization. In this report, CdS and CdTe thin films are grown on FTO glass substrates by chemical bath deposition and close-spaced sublimation, respectively. The magnetic properties, which are introduced from oxygen states, are found in CdS and CdTe thin films. From the hysteresis loop of magnetic moment it is revealed that CdS and CdTe thin films have different kinds of magnetic moments at different temperatures. The M–H curves indicate that from 100 K to 350 K, CdS and CdTe thin films show paramagnetism and diamagnetism, respectively.A superparamagnetic or a weakly ferromagnetic response is found at 5 K. It is also observed from ZFC/FC curves that magnetic moments decrease with temperature increasing. Spin polarized density functional calculation for spin magnetic moment is also carried out.     

Magnetism in iron implanted oxides: a status report

Emission Mössbauer spectroscopy on ⁵⁷Fe fed by ⁵⁷Mn ions implanted in the metal oxides ZnO, MgO and Al₂O₃ has been performed. The implanted ions occupy different lattice sites and charge states. A magnetic part of the spectra in each oxide can be assigned to Fe^3?+? ions in a paramagnetic state with unusually long relaxation time observable to temperatures up to several hundreds Kelvin. Earlier expectations that the magnetic spectra could correspond to an ordered magnetic state could not be confirmed. A clear decision for paramagnetism and against an ordered magnetic state was achieved by applying a strong magnetic field of 0.6 Tesla. The relaxation times deduced were compared to spin–lattice relaxation times from electron paramagnetic resonance (EPR).     

Spin diffusion in CPP spin valves

We present in this paper an analytic model to describe the effect of spin diffusion in CPP (current perpendicular to the plan) spin valves. Two ferromagnetic electrodes are separated by a nonmagnetic metal, organic or inorganic semiconducting spacer. We base our calculations on the evolution of the spin polarized density of states of the saturated ferromagnetic electrodes under an applied rotating magnetic field and the spin diffusion in the spacer. Without treating the mechanism of spin relaxation and dephasing in the spacer, we establish a general model with the cosine evolution of the magnetoresistance modulated by the effect of spin diffusion. Throughout our treatment we consider a tunnel junction at the interface between each ferromagnetic electrode and the spacer.     

Measurement of a pauli and orbital paramagnetic state in bulk gold using x-ray magnetic circular dichroism spectroscopy

We show that bulk gold (Au) exhibits temperature-independent paramagnetism in an external magnetic field by x-ray magnetic circular dichroism spectroscopy at the Au L(2) and L(3) edges. Using the sum-rule analysis, we obtained a magnetic moment of 1.3 × 10(-4) μB/atom in an external magnetic field of 10 T and a paramagnetic susceptibility of 8.9 × 10(-6) for the 5d orbit. The induced paramagnetism in bulk Au is characterized by a large (≈ 30%) orbital contribution. This orbital component was retained even when Au atoms formed nanoparticles, playing an important role in stabilizing the spontaneous spin polarization in the Au nanoparticles.     

Magnetic properties of charged spin-1 Bose gases with ferromagnetic coupling

The magnetic properties of a charged spin-1 Bose gas with ferromagnetic interactions are investigated within mean-field theory. It is shown that a competition between paramagnetism, diamagnetism and ferromagnetism exists in this system. It is shown that diamagnetism, being concerned with spontaneous magnetization, cannot exceed ferromagnetism in a very weak magnetic field. The critical value of reduced ferromagnetic coupling of the paramagnetic phase to ferromagnetic phase transition [Formula: see text] increases with increasing temperature. The Landé-factor g is introduced to describe the strength of the paramagnetic effect which comes from the spin degree of freedom. The magnetization density [Formula: see text] increases monotonically with g for fixed reduced ferromagnetic coupling [Formula: see text] as [Formula: see text]. In a weak magnetic field, ferromagnetism makes an immense contribution to the magnetization density. On the other hand, at a high magnetic field, the diamagnetism tends to saturate. Evidence for condensation can be seen in the magnetization density at a weak magnetic field.     

Electronic structure,field-induced magnetization density and neutron magnetic form factor of palladium

The magnetic field induced magnetization density and neutron magnetic form factor of Pd metal is obtained from an abinitio APW energy band study of its electronic structure and properties. The magnetization consists of the spin density calculated for states on the Fermi surface and a much smaller orbital contribution. The solid state wavefunctions are found to yield a spatial localization of the spin density which is greater than that of the very contracted Hartree-Fock density of the free Pd²⁺ ion. The theretical magnetic form factor, which is dominated by the contribution of the fifth band, is found to be in excellent agreement with the measurements of Cable, Wollan, Felcher, Brun and Hornfeldt.     

Spin-Filter Effect Induced by Magnetic Edge States of Zigzag Carbon Nanotube

Spin-filter effect is predicted in a weak coupled junction composed of a nonmagnetic metal electrode and a zigzag carbon nanotube. This effect is induced by the magnetic edge states of the nanotube, and can produce spin- polarized current in the absence of an external magnetic field. We find that the spin polarization of the current changes its sign at the half-filling point of the nanotube, thus electric field control of spin transport can be realized. Furthermore, we find the coupling strength of the junction may cause a magnetic transition on the edge of the nanotube.     

垂直自由层倾斜极化层自旋阀结构中的磁矩翻转和进动

在理论上研究了垂直自由层和倾斜极化层自旋阀结构中自旋转移矩驱动的磁矩翻转和进动.通过线性展开包括自旋转移矩项的Landau-Lifshitz-Gilbert方程并使用稳定性分析方法,得到了包括准平行稳定态、准反平行稳定态、伸出膜面进动态以及双稳态的磁性状态相图.发现通过调节电流密度和外磁场的大小可以实现磁矩从稳定态到进动态之间的转化以及在两个稳定态之间的翻转.翻转电流随外磁场的增加而增加,并且受自旋极化方向的影响.当自旋极化方向和自由层易磁化轴方向平行时,翻转电流最小;当自旋极化方向和自由层易磁化轴方向垂直时,翻转电流最大.通过数值求解微分方程,给出了不同磁性状态磁矩随时间的演化轨迹并验证了相图的正确性.     

High field superconductivity in thin films

Thin films provide an ideal means for studying the role of spin paramagnetism in the theory of superconductivity. A review is given of the theoretical and experimental work available until now with respect to this problem. It includes a study of the excitation spectrum of thin films in a parallel magnetic field and the experimental evidence of Zeeman splitting of superconducting quasiparticles. The role of spin-orbit interaction is discussed in detail. The application of spin selective tunnelling is shown. Furthermore, it includes a study of the order of phase transition between the normal state and the paramagnetically limited superconducting state. A detailed discussion is given of the generalized Ginzburg-Landau equation including numerical evaluations. The applicability of Tinkham's formula is discussed, which is relevant for the determination of critical fields of arbitrary orientations to the film. Finally, account is given of the work which deals with the influence of spin paramagnetism on superconducting fluctuations. Recent experimental and theoretical work is described and discussed.     

磁场下夹层结构的高能电子屏蔽性能

针对空间高能电子环境可能造成的航天设备故障、宇航员辐照损伤等情况,基于电子束返回效应,提出了一种磁场下金属/真空夹层式高能电子屏蔽结构。采用Geant4软件,模拟空间高能电子连续能谱,研究磁场下夹层式结构的高能电子屏蔽性能。此外,建立体素模型,计算射线在人体中的累积剂量,从而评估磁感应强度、屏蔽体材料对屏蔽性能的影响。结果表明:与传统被动屏蔽方式相比,夹层式结构具有电子屏蔽性能高、生成透射次级X射线少的特点;随着磁感应强度增加,体模中累积剂量下降,证明夹层结构的电子屏蔽性能不断提升;Ti/Ti材料组合的屏蔽方式具有更优越的高能电子屏蔽性能。该结构具备较好的高能电子屏蔽性能,将来有望对空间高能电子辐射环境进行有效防护。     

Magnetic and binding properties of metal-divacancy complexes at MgO (0 0 1) surface: DFT calculations

We have analyzed the magnetic and binding properties of Ni, Cr, Mo, and Pt metals deposited on the defect free and defect containing surfaces of MgO by means of density functional theory calculations and embedded cluster model. Clusters of moderate sizes with no border anions, to avoid artificial polarization effects, were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. Spin quenching occurs for Cr and Mo complexes at the defect free (terrace) surface, and Cr, Mo, and Pt complexes at the defect containing “pit” divacancy surface. The binding energies of the metals are significantly enhanced on the cationic vacancy end of the divacancy. The adsorption energies of the low spin states of spin quenched complexes are always greater than those of the high spin states. The metal-support interactions stabilize the low spin states of the adsorbed metals with respect to the isolated metals, but the effect is not always enough to quench the spin. The encountered variations in magnetic properties of free metals and of metal complexes are correlated with the energy gaps of the frontier orbitals. Spin contamination affect the adsorbate-substrate distances, Mulliken charges, Mulliken spin densities, natural charge, natural orbital population, and provide rationalization for the reported magnetic and binding properties. The electrostatic potential energy curves provide clearer understanding of the nature of magnetic and binding interactions. The magnetic and binding properties of a single metal atom adsorbed on a particular surface result from a competition between Hund's rule for the adsorbed metal, and the formation of a chemical bond at the interface.     

金属表面Rashba自旋轨道耦合作用研究进展

自旋轨道耦合是电子自旋与轨道相互作用的桥梁, 它提供了利用外电场来调控电子的轨道运动、进而调控电子自旋状态的可能. 固体材料中有很多有趣的物理现象, 例如磁晶各向异性、自旋霍尔效应、拓扑绝缘体等, 都与自旋轨道耦合密切相关. 在表面/界面体系中, 由于结构反演不对称导致的自旋轨道耦合称为Rashba自旋轨道耦合, 它最早在半导体材料中获得研究, 并因其强度可由栅电压灵活调控而备受关注, 成为电控磁性的重要物理基础之一. 继半导体材料后, 金属表面成为具有Rashba自旋轨道耦合作用的又一主流体系. 本文以Au(111), Bi(111), Gd(0001)等为例综述了磁性与非磁性金属表面Rashba自旋轨道耦合的研究进展, 讨论了表面电势梯度、原子序数、表面态波函数的对称性, 以及表面态中轨道杂化等因素对金属表面Rashba自旋轨道耦合强度的影响. 在磁性金属表面, 同时存在Rashba自旋轨道耦合作用与磁交换作用, 通过Rashba自旋轨道耦合可能实现电场对磁性的调控. 最后, 阐述了外加电场和表面吸附等方法对金属表面Rashba自旋轨道耦合的调控. 基于密度泛函理论的第一性原理计算和角分辨光电子能谱测量是金属表面Rashba自旋轨道耦合的两大主要研究方法, 本文综述了这两方面的研究结果, 对金属表面Rashba自旋轨道耦合进行了深入全面的总结和分析.     

Development of neutron spin phase contrast imaging

We present an imaging technique utilizing a neutron spin interferometer. Neutron spin phase contrast is achieved in spatial resolved measurements of the phase difference between two superposed neutron spin states introduced by passing through a magnetic sample. Since the phase difference of spin states parallel and anti-parallel to the magnetic field is proportional to the magnetic field integral, it is possible to record images of the internal magnetic field distribution of the sample. Taking advantage of high transmission probabilities, neutron spin phase contrast provides non-destructive images of internal magnetic structures.     

Majorana flipping of quarkonium spin states in transient magnetic field

We demonstrate that spin flipping transitions occur between various quarkonium spin states due to transient magnetic field produced in non central heavy ion collisions (HICs). The inhomogeneous nature of the magnetic field results in non adiabatic evolution of (spin)states of quarkonia moving inside the transient magnetic environment. Our calculations explicitly show that the consideration of azimuthal inhomogeneity gives rise to dynamical mixing between different spin states owing to Majorana spin flipping. Notably, this effect of non-adiabaticity is novel and distinct from previously predicted mixing of the singlet and one of the triplet states of quarkonia in the presence of a static and homogeneous magnetic field.     

Resolution of the spin states of a superconductor by tunneling from a ferromagnet

Conductance measurements of tunnel junctions formed between a superconductor (thin A1) and a ferromagnetic metal (Ni) in a magnetic field are used to resolve the tunneling conductance into conductances for each of the electron spin directions. This resolution can be used to obtain the spin densities of states of high field superconductors for which spin scattering processes are important.     

Third-Order Magnetic Susceptibility of an Ideal Fermi Gas

At low temperature and under weak magnetic field, non-interacting Fermi gases reveal both Pauli paramagnetism and Landau diamagnetism, and the magnitude of the diamagnetic susceptibility is 1/3 of that of the paramagnetic one. When the temperature is finite and the magnetic field is also finite, we demonstrate that the paramagnetism and diamagnetism start to deviate from the ratio 1/3. For understanding the magnetic properties of an ideal Fermi gas at quite low temperature and under quite weak magnetic field, we work out explicitly the third-order magnetic susceptibility in three cases, from intrinsic spin, orbital motion and in total. An interesting property is in third-order magnetic susceptibilities:when viewing individually, they are both diamagnetic, but in total it is paramagnetic.     

Spin torque on magnetic domain walls exerted by supercurrents

We calculate the spin density, spin currents and spin torque due to a spin polarized current on a magnetic domain wall juxtaposed to or inserted in a conventional superconductor. The superconductor is part of a heterostructure of the type NSN or FSF. In general, the spin torque exerted on the domain wall is weaker with respect to a normal metal. However, there are regimes where the torque is enhanced with respect to the normal metal. In these regimes the motion of the domain wall is therefore more efficient. A notable case is the passing of an unpolarized current which leads to a finite torque in the case of the superconductor.