磁性隧道结中自旋相关的隧穿输运

全面回顾和总结了磁性隧道结中自旋相关的隧穿这一研究领域的理论和实验方面的最新研究进展。讨论了影响磁性隧道结的自旋极化和隧穿磁电阻的各种因素及反映铁磁层和铁磁／绝缘层界面电子结构在隧穿中重要作用的理论模型和近期实验，同时也讨论了绝缘势垒和铁磁／绝缘层界面中的无序性在隧穿过程中对自旋极化与磁电阻效应的影响。     

半导体自旋电子学的研究与应用进展

简单介绍了半导体自旋电子学的研究对象和内容,主要包括磁性半导体、磁性/半导体复合结构、非磁性半导体量子阱和纳米结构中的自旋现象,以及半导体的自旋注入等.综述了半导体自旋电子学目前的研究进展及其在自旋电子器件和量子信息处理中的应用.     

CrTe的半金属性和CrTe/ZnTe异质结的自旋二极管效应研究

基于密度泛函理论的第一性原理方法,对闪锌矿结构CrTe和CrTe/ZnTe(001)异质结的电子结构和自旋极化输运性质进行了计算研究。结果表明,CrTe是一种很好的半金属铁磁体,具有4.00μB/分子的磁矩和很大的半金属隙(0.84eV)。计算得到的电流-电压曲线显示CrTe/ZnTe(001)异质结具有优良的自旋二极管效应,因此,CrTe在自旋电子器件中应该有一定的应用。并从能带理论角度解释了CrTe/ZnTe(001)异质结中产生自旋二极管效应的机理。     

半导体自旋电子学的最新研究进展

自旋电子学起源于巨磁阻效应(GMR),目前已经成为凝聚态物理学领域的研究热点,其中半导体自旋电子学是自旋电子学中人们所关注的一个重要领域。从磁性半导体、自旋电子的注入、检测、输运等方面综述半导体自旋电子学的最新研究进展,并且指出目前半导体自旋电子学研究的重点及难点。     

有机半导体的自旋注入和自旋输运

利用自旋漂移-扩散方程自洽地得到了铁磁/有机半导体自旋注入结构中极化予电导的自旋相关性对自旋注入效率的影响.计算结果表明,自旋注入效率明显依赖于极化子电导的自旋相关性.在自旋注入效率增加30%的情况下,极化子电导的自旋相关性T=5K时将增大4个数量级左右,表明,在电场作用下的有机半导体自旋注入体系中,考虑极化子电导的自旋相关性对自旋注入的影响有着非常重要的意义.另外自旋注入效率还是位置的函数,但当电场大于100mV/μm后,极化子的自旋相关性几乎不再随极化子所在位置发生变化,自旋注入效率此时也几乎与位置无关.     

自旋电子学的研究现状和未来发展趋势

自旋电子学是目前固体物理和电子学中的一个“热点”，其中心议题是利用和控制固体，尤其是半导体中的自旋自由度。本主要内容是：1、MBE生长的Ⅲ-Ⅴ族基铁电薄膜和异质结构，2、具有高铁磁转变温度的Mm6掺杂的GaAs／P-A1GaAs异质结及与自旋相关性质的控制，3、Si基自旋电子学。     

自旋在有机半导体中的输运

当前的微电子器件主要是通过控制半导体中载流子的电荷来存储、处理和传输信息,电子的自旋自由度长期在这些应用中被忽略。自旋电子学主要是研究与载流子的自旋相关的各种现象和效应,为在微电子器件中集成这些自旋效应,产生各种更新更强的功能的自旋器件奠定基础。有机半导体材料由于自旋轨道作用和超精细作用很弱,自旋扩散长度被认为很长,因而是一种具有很强应用潜力的自旋电子材料。最近,我们研究组首次成功的在低温下,以有机材料Alq_3作为中间层,La_(2/3)Sr_(1/3)MnO_3(LSMO)和Co分别作为两个电极的三明治结构中,低温下获得了高达40%的巨磁电阻效应。磁电阻随有机材料厚度的增加指数衰减,根据修正后的Jullierre模型,自旋扩散长度估计为约45nm。另外,巨磁电阻效应随温度的升高而降低,在温度较高时,样品除了有巨磁电阻效应之外,还明显表现出一种物理机理完全不同的高场磁电阻效应,电阻随磁场的增加而持续降低。为了进一步研究这种高场磁电阻效应的物理机理,我们制备了仅有一个铁磁性电极的有机发光二极管LSMO/有机/Al器件,实验结果表明高场磁阻效应与有机材料和制备方法无关,同时,发光器件的电致发光效应随磁场的增大而...     

新型磁性隧道结材料及其隧穿磁电阻效应

典型的磁性隧道结是“三明治”结构,即由上下两个铁磁电极以及中间厚度为1 nm量级的绝缘势垒层构成.当外加磁场使两铁磁电极的磁矩由平行态向反平行态翻转时,隧穿电阻会发生低电阻态向高电阻态的转变.自从1995年发现室温隧穿磁电阻(TMR)以来,非晶势垒的AlOx磁性隧道结在磁性随机存储器(MRAM)和磁硬盘磁读头(Read Head)中得到了广泛的应用,2007年室温下其磁电阻比值可达到80％.下一代高速、低功耗、高性能的自旋电子学器件的发展,迫切需要更高的室温TMR比值和新型的调制结构.2001年通过第一性原理计算发现:由于MgO(001)势垒对不同对称性的自旋极化电子具有自旋过滤(Spin Filter)效应,单晶外延的Fe(001)/MgO(001)/Fe(001)磁性隧道结的TMR比值可超过1000％,随后2004年在单晶或多晶的MgO磁性隧道结中获得室温约200％的TMR比值,2008年更是在赝自旋阀结构CoFeB/MgO/CoFeB磁性隧道结中获得高达604％的室温TMR比值.伴随着新势垒材料的不断发现和各种磁性隧道结结构的优化,共振隧穿和自旋依赖的库仑阻塞磁电阻等新效应以及磁性传感器、磁性随机存储器和自旋纳米振荡器及微波检测器等新器件逐渐成为科学和工业界所关注的研究与应用热点.对磁性隧道结(MTJ)材料及其器件应用研究和进展进行了简要介绍.     

自旋注入对有机半导体极化子电导的影响

在有机半导体自旋电子器件中,自旋从铁磁极注入到有机半导体后,自旋相上的极化子和自旋向下的极化子有不同的态密度,从而产生不同的电导.利用自旋漂移一扩散方程通过自洽计算得到了铁磁/有机半导体自旋注入结构中极化子自旋相关的电导和电流的自旋极化率.计算结果表明,极化子电导的自旋相关性是自旋注入引起的,和电流的自旋极化率密切相关;在自旋注入发生后,有机半导体内不同位置上极化子自旋态密度不同,由此产生的极化子电导也不相同,极化予电导是位置的函数.另外还发现,外电场会增强有机半导体电流的自旋极化率.     

自旋电子学功能材料

巨磁电阻效应的发现开拓了磁电子学的新领域,20世纪90年代,磁电子学得到迅速的发展,并在应用上取得显著的经济效益与巨大的社会效应,本世纪初,研究的重点已转移到半导体自旋电子学的新方向,并已取得重要的进展.本文将结合我们科研组的研究工作,概述从磁电子学到半导体自旋电子学材料的发展,重点介绍稀磁半导体材料研究的进展.     

Ferromagnetic Nanowires with Superconducting Electrodes

The proximity effect in mesoscopic ferromagnet/superconductor(FS) Ni/Al structures of various geometrieswas studied experimentally on both F- and S-sides of thestructures. Samples with a wide range of interfacetransparency were fabricated. The dependence of the effect onFS interface transparency was investigated. The amplitude ofthis effect was found to be larger than expected fromclassical theory of proximity effect. Preliminary experimentsshowed no phase-sensitive oscillations in Andreevinterferometer geometry. Various theoretical models arediscussed.     

Spin-Polarized Transport in F/S Nanojunctions

We study spin-dependent electronic transport across ferromagnet/superconductor ballistic junctions modeled using tight-binding Hamiltonians with s, p and d orbitals and material-specific parameters. The first result of this paper is that, by accurately modeling the band structure of the bulk materials, one can reproduce the measured differential conductance of Cu/Pb nanocontacts^1,2. In contrast the differential conductance of CO/Pb contacts can only be reproduced if an enhanced magnetic moment is present at the interface. The second result concerns the reliability of a method proposed in Refs. 1–3 for determining the degree of polarization of a ferromagnet. By fitting the material-specific differential conductance curves to curves calculated using a single-band model we show that this method does not yield reliable values for polarization and spin-dependent transmission.     

Thermopower Properties of Ferromagnetic/Insulator/Ferromagnetic Tunnel Junction

The thermal conductance, thermopower, and figure of merit in the ferromagnetic/insulator/ferromagnetic tunnel junction are investigated by using the nonequilibrium Green’s function technique. The magnetic regions are treated as band ferromagnetic and are described by using the single-band Hubbard model; the insulator region and the hybridization between neighboring regions are also described, respectively, by using a quantum microscopic model. It was found that the strength of the thermal conductance, the thermopower, and the figure of merit increased with the increase of the level of the left ferromagnet $\varepsilon _{k_\mathrm{L}}$ ε k L and the temperature $T.$ T . Finally, the calculated results of thermal properties are explained theoretically.     

Large n-Expansion Limit of the Three-Dimensional Ferromagnetic Quantum Phase Transition

We use the large n-expansion method to study the role of the long-range interaction, topological and dissipation effects for the case of an itinerant quantum ferromagnet in the limit of the Landau–Ginzburg–Wilson theory. In the one-loop approximation, we calculate the explicit form of the electronic self-energy as a result of electron–fluctuation interaction and extract the temperature dependence of the scattering time. The temperature dependence of the relative resistivity shows that both the dissipative and topological terms of the action determine the non-Fermi behavior of the system in the critical region around the quantum phase transition.      

Magnetic and electrical characterization of “ferromagnet/insulator/silicon” diodes

This work is focused on the study of magnetic and electrical properties of ferromagnet/semiconductor heterostructures that can be used for spin injection into silicon. Three different studies are conducted whose principal results will be presented. In all these studies, a simple diode-like ferromagnet/insulator/semiconductor (FM/I/S) structure is used. The first study aimed to investigate whether a magnetic “dead” layer is obtained at the ferromagnet/oxide barrier that could lead to spin depolarization of the injected electrons. The results show the absence of such layer even after annealing at temperatures up to 723 K (450 °C). The second study focused on the mechanisms of electrical transport through the insulator barrier. Capacitance–voltage as well as current–voltage characteristics have been measured. The results underline the importance of controlling the ferromagnet deposition process in obtaining defect-free silicon–insulator interface, a prerequisite to spin conservative direct-tunnel transport process. In the third study, magnetic characterization of diodes that may be used for spin injection and collection were performed.     

Hysteresis Properties of Magnetic/Non-Magnetic (M/NM) Multilayers: Monte Carlo Investigation

In this work, Monte Carlo simulation was used to model the dynamic hysteresis behavior of magnetic/non-magnetic multilayers in the framework of Heisenberg model using the spin-flip algorithm. The purpose is to investigate the hysteresis properties while varying the direction of external applied magnetic field and temperature taking account interface composition of magnetic/non-magnetic atoms. From the results, with thermal variation and interface composition, the simulated hysteresis loops change significantly due to the competition between magnetic anisotropy in magnetic films and the presence of isolated clusters at the interface.     

Tuning the magnetic coupling across ultrathin antiferromagnetic films by controlling atomic-scale roughness

Characterization and control of the interface structure and morphology at the atomic level is an important issue in understanding the magnetic interaction between an antiferromagnetic material and an adjacent ferromagnet in detail, because the atomic spins in an antiferromagnet change direction on the length scale of nearest atomic distances. Despite its technological importance for the development of advanced magnetic data-storage devices and extensive studies, the details of the magnetic interface coupling between antiferromagnets and ferromagnets have remained concealed. Here we present the results of magneto-optical Kerr-effect measurements and layer-resolved spectro-microscopic magnetic domain imaging of single-crystalline ferromagnet-antiferromagnet- ferromagnet trilayers. Atomic-level control of the interface morphology is achieved by systematically varying the thicknesses of the bottom ferromagnetic and the antiferromagnetic layer. We find that the magnetic coupling across the interface is mediated by step edges of single-atom height, whereas atomically flat areas do not contribute.     

Graphene/Half-Metallic Heusler Alloy: A Novel Heterostructure toward High-Performance Graphene Spintronic Devices

Graphene-based vertical spin valves (SVs) are expected to offer a large magnetoresistance effect without impairing the electrical conductivity, which can pave the way for the next generation of high-speed and low-power-consumption storage and memory technologies. However, the graphene-based vertical SV has failed to prove its competence due to the lack of a graphene/ferromagnet heterostructure, which can provide highly efficient spin transport. Herein, the synthesis and spin-dependent electronic properties of a novel heterostructure consisting of single-layer graphene (SLG) and a half-metallic Co₂Fe(Ge_0.5Ga_0.5) (CFGG) Heusler alloy ferromagnet are reported. The growth of high-quality SLG with complete coverage by ultrahigh-vacuum chemical vapor deposition on a magnetron-sputtered single-crystalline CFGG thin film is demonstrated. The quasi-free-standing nature of SLG and robust magnetism of CFGG at the SLG/CFGG interface are revealed through depth-resolved X-ray magnetic circular dichroism spectroscopy. Density functional theory (DFT) calculation results indicate that the inherent electronic properties of SLG and CFGG such as the linear Dirac band and half-metallic band structure are preserved in the vicinity of the interface. These exciting findings suggest that the SLG/CFGG heterostructure possesses distinctive advantages over other reported graphene/ferromagnet heterostructures, for realizing effective transport of highly spin-polarized electrons in graphene-based vertical SV and other advanced spintronic devices.     

Negative Magnetoresistance in Dual Spin Valve Structures With a Synthetic Antiferromagnetic Free Layer

Giant magnetoresistance (GMR) in spin valves is due to spin-dependent scattering occurring at ferromagnet/normal metal (F/N) interfaces and/or in the ferromagnetic layers. In a spin valve with a typical F/N/F structure where the spin scattering asymmetry factor $(alpha)$ of both F/N interfaces is the same (more or less than 1), the GMR is expected to be positive. If $alpha$ is greater than one at one F/N interface and less than one at the other F/N interface, however, the GMR is expected to be negative. Here, we show that the F1/Cu/SAF/Cu/F2/IrMn dual spin valve structure exhibits negative GMR, where F1 and F2 are CoFe and ${rm SAF} = {rm CoFe}/{rm Ru} t/{rm CoFe}$, due to both opposite electron spin scattering asymmetry factor at the CoFe/Ru/CoFe interfaces as well as the electrical separation of the overall structure into two GMR spin valves connected in parallel. A GMR of 6% is observed in the structure without the Ru spacer layer, insertion of a 0.6 nm thick Ru in the SAF results in a negative GMR ratio of ${-}3hbox{%}$ , which becomes positive again at the Ru thickness of 0.8 nm, the oscillation from positive to negative MR is consistent with interlayer exchange coupling period across the Ru spacer.      

Shot noise in resonant tunneling through an interacting quantum dot with intradot spin-flip scattering

We present theoretical investigation of the zero-frequency shot-noise spectra in electron tunneling through an interacting quantum dot connected to two ferromagnetic leads with the possibility of spin-flip scattering between the two spin states by means of the recently developed bias-voltage and temperature-dependent quantum rate equations. For this purpose, a generalization of the traditional generation-recombination approach is made for properly taking into account the coherent superposition of electronic states, i.e., the nondiagonal density matrix elements. Our numerical calculations find that the Fano factor increases with increasing the polarization of the two leads, but decreases with increasing the intradot spin-flip scattering.