NiMnSb半金属材料的制备、磁性和电输运特性

报道了NiMnSb半金属材料的一种简单方便的制备方法.分析了烧结温度对材料结构特征的影响.结果表明600～800℃的温度烧结4h为较佳的烧结条件.通过研究发现,在300K以下温度区域的磁性行为和Heisenberg铁磁体局域电子模型的经典自旋波理论的预言相符合,遵循温度的T3/2变化规律;在高场磁化过程中,磁硬度系数和磁各向异性随温度的变化反映出材料的内应力和各向异性随温度的变化规律;电阻率随温度的变化关系满足温度的T1.35规律.     

等温退火对Alnico8合金微观结构的影响

利用透射电镜(TEM)和X射线小角衍射技术(SAXS)研究了磁场等温退火工艺参数对Alnico8合金的微观组织的影响,样本的晶格结构清晰可见。结果表明,在等温退火阶段Alnico8合金通过调幅分解形成两相结构,棒状的铁磁相(1α相)粒子高度有序地弥散在弱磁性相(2α相)当中。其中,1α相的平均长度由SAXS测得,并与TEM照片数据进行了验证比对。本文着重研究了1α相粒子平均长度随等温退火处理的温度及时间参数的改变规律,结果显示样本的磁性能与其微观结构有一定相关性。     

Fe、Ni线热膨胀效应的价键理论分析

本文在考虑了磁性电子的影响后,对Pauling—余氏键距公式进行了修正,并由此导出了立方晶体单质线热膨胀一般公式。该公式给出了关于热膨胀的有用知识。对膨胀效应起贡献的有3项:正常膨胀率α_n;电子结构转变引起的膨胀率α_e,以及磁性转变引起的膨胀率α_m。分析了α-Fe和Ni的膨胀效应,它们的总膨胀系数仅包括两项:α_n和α_m。计算结果与实验值相当吻合,并揭示了磁性电子相对于共价电子具有独立的耦合效应。在α-Fe和Ni中,磁性电子的耦合系数A都很小,而符号相反。     

纳米软、硬磁性晶粒间的交换耦合及其有效各向异性

以软磁性相α-Fe和硬磁性相Nd2Fe14B为例,研究了软、硬磁性晶粒间的交换耦合作用和有效各向异性常数〈Ksh〉随晶粒尺寸的变化关系。由于晶粒间的交换耦合作用,晶粒可分为晶粒内部无界面交换耦合作用影响和晶粒表面有界面交换耦合作用影响两部分,其各向异性常数为两部分的统计平均值。计算结果表明：对固定的软磁性晶粒尺寸Ds,〈Ksh〉随硬磁性晶粒尺寸Dh一致增加;对固定的Db,〈Ksj〉随Ds一致减小。为使软、硬磁性晶粒间的有效各向异性常数墨。保持较高的值,应控制硬磁性晶粒大于35nm,软磁性晶粒在10nm左右。     

磁性多层膜系统自旋重取向行为的Monte Carlo模拟

依据Heisenberg模型,利用Monte　Carlo方法模拟了磁性多层膜系统的自旋重取向行为,研究了各向异性、偶极相互作用以及外磁场对系统自旋取向的影响。通过模拟计算,获得了系统组态、磁分量等随偶极相互作用、外加磁场和温度的变化规律,重点研究了磁性多层膜系统在外磁场作用下的磁滞现象。     

Ti-6Al-4V合金中α 相溶解的三维定量相场模型

将三维扩散控制的定量相场方法与Kim-Kim-Suzuki模型相耦合,并以CALPHAD数据库的动力学与热力学数据为依据,研究了Ti-6Al-4V合金中的α相的溶解。将一维溶解动力学与DICTRA结果对比,确定了不同固溶温度下的动力学系数。α相在溶解过程中的Al和V成分分布、扩散路径与DICTRA计算数据吻合较好,从而验证了定量相场模型的正确性。通过单个球状α相的三维溶解模拟研究发现,α相体积分数随时间演化符合指数衰减规律,单个α相的溶解并不简单是生长的逆过程。     

镍锌铁氧体薄膜的显微结构和低温磁性质

通过溶胶凝胶甩膜工艺,在抛光的硅晶片(100)基底上制备出Ni0.5Zn0.5Fe2O4(NZF)薄膜并对其结构和磁性的测量结果进行了分析。薄膜表面平整,具有较好的单相结构,其颗粒平均粒径约30nm。制备的NZF薄膜厚度分别约90、120和180nm。实验结果表明,薄膜在低温下表现出自旋玻璃态行为。当外加磁场为7.96×103A/m时,NZF薄膜自旋冻结温度大约在Tf=140K,自旋冻结程度随薄膜厚度增加而降低。在40~300K之间,薄膜饱和磁化强度和矫顽力都随着温度增加而降低。NZF薄膜在T=40K处存在最大磁化强度。较薄薄膜在40K以下饱和磁化强度的降低是因为磁性颗粒表面自旋被部分冻结而导致的磁性离子相互间自旋耦合作用减弱的结果。     

NiO／NiFeCo／Cu／NiFeCo自旋阀结构的巨磁电阻效应及影响因素的研究

用电子蒸发的方法制备ＮｉＯ／ＮｉＦｅＣ／Ｃｕ／ＮｉＦｅＣｏ自旋阀多层膜,通过磁场中退火得到好的偏置型自旋阀ＧＭＲ效应。通过对制备态以及磁场退火后样品的ＭＲ曲线的研究,讨论了交换耦合作用,单层磁性能以及层间耦合作用对材料ＧＭＲ效应的大小和磁场灵敏度的影响,得出提高交换耦合作用,改善单层磁性能和尽可能减小层间耦合将有得于得到高性能的偏置型自旋阀ＧＭＲ材料的结论。     

三维分形接触热导的建模与多参数影响分析

在传统M-B接触模型的基础上,利用三维分形理论,推导三维分形结合面的接触模型,并建立了三维分形接触热导模型。通过仿真分析揭示了法向载荷、分形维数、分形尺度参数、材料特性参数及各参数的耦合对接触热导的影响。仿真结果表明:接触热导与法向载荷呈正相关,当2.1≤D≤2.4时,两者存在非线性关系,当2.5≤D≤2.9时,两者趋于线性关系;当2.0D≤2.95时,接触热导随分形维数的增大而增大,当2.95D3.0时,接触热导随分形维数的增大而减小;接触热导与分形尺度参数呈负相关,与材料特性参数呈正相关;并得出上述参数两两耦合对接触热导的影响。     

晶粒尺寸对软-硬磁性晶粒间有效各向异性的影响

以软磁性相α—Re和硬磁性相Nd2Fe14B为例，研究了软—硬磁性晶粒间的交换耦合相互作用和有效各向异性随晶粒尺寸和软、硬磁性晶粒尺寸比(Ds：Dh)的关系，软—硬磁性晶粒间的有效各向异性常数可以用软、硬磁性相的平均各向异性常数的统计平均值表示，当晶粒尺寸大于其铁磁交换长度时，晶粒分为有、无交换耦合两部分，无交换耦合部分的各向异性常数为通常的K1，而耦合部分的各向异性常数随到晶粒表面的距离而变化，研究结果表明：软—硬磁性晶粒间的有效各向异性随晶粒尺寸的减小而下降，随着软、硬磁性晶粒尺寸比值(Ds：Dh)的减小而增加，为使软—硬磁性晶粒间的有效各向异性常数Keff保持较高的值，应控制硬磁性晶粒大于35nm，软磁性晶粒尺寸为10nm左右。     

Anisotropic Magneto‐Coulomb Properties of 2D–0D Heterostructure Single Electron Device

Fabrication and spintronics properties of 2D–0D heterostructures are reported. Devices based on graphene (“Gr”)–aluminium nanoclusters heterostructures show robust and reproducible single‐electron transport features, in addition to spin‐dependent functionality when using a top magnetic electrode. The magnetic orientation of this single ferromagnetic electrode enables the modulation of the environmental charge experienced by the aluminium nanoclusters. This anisotropic magneto‐Coulomb effect, originating from spin–orbit coupling within the ferromagnetic electrode, provides tunable spin valve‐like magnetoresistance signatures without the requirement of spin coherent charge tunneling. These results extend the capability of Gr to act both as electrode and as a platform for the growth of 2D–0D mixed‐dimensional van der Waals heterostructures, providing magnetic functionalities in the Coulomb blockade regime on scalable spintronic devices. These heterostructures pave the way towards novel device architectures at the crossroads of 2D material physics and spin electronics.     

Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.     

Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.     

The Effects of 5f Localization on Magnetic Properties of UAl3

The structural, electronic, and magnetic properties of UAl₃ have been calculated using density functional theory by the Wien2k package within LDA, GGA, LDA?+?U, and GGA?+?U approaches. The total energy calculations indicate that at zero pressure the ferromagnetic phase is the most stable phase. The energy band calculation and the density of state curves indicate that the localization of 5f electron and spin orbit coupling have a considerable effect on electronic properties of the UAl₃ compound. The calculations of the electric field gradient, magnetic moment, and optical properties of this compound have been performed under the presence and absence of spin orbit coupling. The contribution of different orbitals to the EFG shows that the strongest anisotropy in the charge distribution is due to the electrons in p orbitals. The electric field gradient and magnetic moment as a function of pressure have been investigated in the presence and absence of spin orbit coupling.     

Size-effect induced short-range magnetic ordering in germanium nanostructures

Formation of ordered magnetic states in germanium nanostructures embedded in SiO2 has been investigated. Samples with the nanostructures were prepared by sputtering deposition on Si(100) substrates, followed by thermal annealing in vacuum. Transmission electron microscopy, energy dispersive X-ray spectrometry, and Raman spectroscopy have been used to characterize the samples. Magnetic measurements were performed using a superconducting quantum interference device. Size-effect induced magnetic orderings in the germanium nanostructures were found to be present at room temperatures and below. Superparamagnetic behavior was observed at temperatures above 230 K, whereas thermal excitation of spin reorientation and magnetic coupling has been revealed at temperatures below 60 K. Inverted hysteresis loops with negative remanences and multiple plateaus revealed the ferri- or antiferromagnetic nature of the coupling. Inter-domain coupling and effect of magnetic anisotropy will be discussed based on the experimental results and simulations with a spin reorientation model.     

Model formalism of paramagnetic solid helium-3

The statistical-thermodynamic formalism of a collection of localized spin-1/2 atoms whose spin Hamiltonian refers to the isotropic antiferromagnetic Heisenberg exchange-interaction scheme is applied here to account for a set of important equilibrium-thermodynamic measurements on paramagnetic solid³He performed some time ago by University of Florida investigators. The measured properties were the temperature-dependent modulations of the pressure, which were proved earlier to arise overwhelmingly from the nuclear spin system. The present formalism of the pressure modulations or of the spin pressures, along specified isochores of the solid, includes the density- or molar-volume-dependent microscopic exchange energy parameter and its derivative. In this paper we have derived directly hitherto unavailable exact values of these parameters from spin pressure data on magnetized solid³He, as well as indirectly through the intermediary of spin pressures in the absence of a magnetic field. The directly derived exact parameters result from a single characteristic equilibrium-thermodynamic state of the solid in the presence of a constant and uniform magnetic field of adequate strength. The indirectly derived but exact parameters, of possibly somewhat lower accuracy, require the knowledge of a single directly derived exchange-energy parameter together with a set of spin pressures of asymptotic high-temperature equilibrium states in the absence of a magnetic field. The indirectly derived microscopic parameters along two of the three experimentally explored magnetized solid isochores yielded calculated spin pressures in fair and acceptable agreement, respectively, with their measured values. The directly derived exact parameters used in calculating the spin pressures along the third experimentally investigated isochore, in the absence of a magnetic field and at three different field strengths, led to complete agreement with the data. These results lend support to the tentative proposition advanced in early work that over a range of temperatures and molar volumes of paramagnetic solid³He, the statistical-thermodynamic formalism based on the antiferromagnetic exchange-interaction scheme may give an acceptable account of the spin pressures as well as of other thermal properties of this quantum solid.     

Proximity-effect correction in electron-beam lithography on metal multi-layers

We report a proximity-effect correction in electron beam patterning when fabricating a spin valve device with a junction size of 100 nm × 100 nm. Since the spin valve device has a stack of magnetic/non-magnetic/magnetic metal multi-layers on oxidized Si substrate, its proximity effect should be appropriately corrected to realize a nano-scale junction. ZEP 520A was chosen as an electron beam resist because its dry-etching resistance is high enough to serve as an etching mask in the post-process. A set of proximity parameters, α, β, and η of ZEP 520A coated metal multi-layers was evaluated by using the doughnut pattern method. A simulation was carried out based on given proximity parameters in order to obtain effective dose factors of each segment of the exposure pattern. The junction with a desired shape and size on a metal multi-layer was successfully fabricated with a help of efficient proximity-effect correction.     

Study of Electronic and Magnetic Properties of MnAu Nanowire

Self-consistent ab initio calculations, based on density functional theory (DFT) approach and using full-potential linear augmented plane wave (FLAPW) method, are performed to investigate both electronic and magnetic properties of the MnAu nanowire. Polarized spin and spin-orbit coupling are included in calculations within the framework of the antiferromagnetic state between two adjacent Mn layers in MnAu nanowire. Magnetic moments considered to lie along the c-axes are computed. Obtained data from ab initio calculations are used as input for the high temperature series expansions (HTSEs) calculations to compute other magnetic parameters. The zero-field high temperature static susceptibility series of the magnetic moment (m) and nearest neighbour Ising model on a MnAu nanowire is thoroughly analyzed by means of a power series coherent anomaly method (CAM) for different layers. The exchanges interactions between the magnetic atoms, the critical exponent associated with the magnetic susceptibility are obtained for MnAu nanowire with different layers.     

A new approach to micromagnetic simulation of thermal magnetic fluctuation noise in magnetoresistive read sensors

Thermal magnetic fluctuation noise forms the ultimate application limit of small magnetoresistive devices for magnetic data storage. The noise analysis of these devices becomes increasingly important for high-density recording. Such noise analyses by micromagnetic simulation are, however, computationally very intensive and require enormous amounts of simulation time. This paper presents a faster micromagnetic method to arrive at the noise and small signal dynamics of these sensors. It uses, for every cell in the simulation, a behavioral analog (i.e., an "analog computer" model) described by the same equations as the basic magnetic simulation cell (the Landau-Lifshitz-Gilbert equation, Slonczewski's spin torque addition and the equipartition principle). The sensor, as a network of exchange and demagnetization coupled cells, is then solved with a network simulator (such as PSpice). This process gives a drastic reduction in computing time and yet leads to high resolution spectra with very little residual uncertainty. The paper further presents a large number of simulation results for uniform sensors as well as for sensors with a nonuniform magnetic bias and a nonuniform electrical bias. It addresses the spatial distribution of the noise (standing spin waves in the noise) and the correlation of the noise in various parts of the sensors. Finally, as a further example of this method, the paper shows the effect of spin torque transfer on the noise and the small signal dynamics of a current perpendicular to plane giant magnetoresistive (CPP GMR) sensor.     

Fulde–Ferrell State in Spin–Orbit-Coupled Superconductor: Application to Dresselhaus SOC

We show that in the presence of magnetic field, two superconducting phases with the center-of-mass momentum of Cooper pair parallel to the magnetic field are induced in Dresselhaus spin–orbit-coupled superconductor. Specifically, at small magnetic field, the center-of-mass momentum is induced due to the energy-spectrum distortion and no unpairing region with vanishing singlet correlation appears. We refer to this superconducting state as the drift-BCS state. By further increasing the magnetic field, the superconducting state falls into the Fulde–Ferrell state with the emergence of the unpairing regions. The observed abrupt enhancement of the center-of-mass momenta and suppression on the order parameters during the transition indicate the occurrence of a first-order phase transition. Enhanced Pauli limit and hence enlarged magnetic-field regime of the Fulde–Ferrell state, due to the spin-flip terms of the spin–orbit coupling, are revealed. We also address the triplet correlations induced by the spin–orbit coupling, and show that the Cooper-pair spin polarizations, generated by the magnetic field and center-of-mass momentum with the triplet correlations exhibit totally different magnetic-field dependences between the drift-BCS and Fulde–Ferrell states.