NM/FI/NI/FI/NM新型双自旋过滤隧道结的隧穿电导和隧穿磁电阻

在NM/FI/FI/NM型双自旋过滤隧道结(此处NM为非磁金属层，FI为铁磁绝缘体或半导体层)的基础上，我们提出一种NM/FI/NI/FI/NM新型双自旋过滤隧道结(此处NI表示非磁绝缘体或半导体层). 插入NI层的目的是为了避免原双自旋过滤隧道结中相邻FI层界面处磁的耦合作用所导致的对隧穿磁电阻的不利影响. 在自由电子近似的基础上，利用转移矩阵方法，对NM/FI/NI/FI/NM新型双自旋过滤隧道结的隧穿电导、隧穿磁电阻与FI层及NI层厚度的变化关系以及随偏压的变化关系进行了理论研究.计算结果表明，在NM/FI/NI/FI/NM新型双自旋过滤隧道结中仍可以得到很大的TMR值. 关键词： 双自旋过滤隧道结 隧穿磁电阻 非磁绝缘(半导)体间隔层     

The effect of the ferromagnetic metal layer on tunnelling conductance and magnetoresistance in double magnetic planar junctions

Based on the free-electron approximation,we investigate the effect of the ferromagnetic metal layer on the tunnelling magnetoresistance(TMR) and tunnelling conductance(TC)in the double magnetic tunnel junctions(DMTJs) of the structure NM/FM/I(S)/NM/I(S)/FM/NM,where FM,NM and I(S) represent the ferromagnetic metal,nonmagetic metal and insulator(Semiconductor),respectively,The FM,I(S)and inner NM layers are of finite thickness,while the thickness of the outer NM layer is infinite.The calculated results show that,due to the spin-dependent interfacial potential barriers caused by electronic band mismatch between the various magnetic and nonmagnetic layers,the dependences of the TMR and TC on the thicknesses of the FM layers exhibit oscillations,and a much higher TMR can be obtained for suitable thicknesses of FM layers.     

TUNNEL MAGNETORESISTANCE IN THE FERROMAGNETIC TUNNEL JUNCTION WITH FERROMAGNETIC LAYERS OF FINITE THICKNESS SUBJECTED TO AN ELECTRIC FIELD

Based on the two-band model, we investigate the tunnel magnetoresistance(TMR) in ferromagnet/insulator (semiconductor)/ferromagnet(FM/I(S)/FM) tunnel junction covered on both sides by nonmagnetic metal layers subjected to an electric field. Our results show that TMR oscillates with the thickness of ferromagnetic layers owing to the quantum-size effect and can reach very large value under suitable conditions, which may in general not be reached in FM/I(S)/FM with infinitely thick ferromagnetic layer. Although the electric field causes the change of the oscillation period, phase and amplitude of the TMR, a large TMR is still obtained in some situations with the electric field. Furthermore, the electric field does not change the feature that TMR varies monotonously with the change of magnetization angle of the middle ferromagnetic layer.     

多层结构双自旋过滤隧道结中的电子输运特性

在转移矩阵方法及Mireles和Kirczenow的量子相干输运理论的基础上,研究了正常金属层/磁性半导体层/非磁绝缘层/磁性半导体层/正常金属层型双自旋过滤隧道结中Rashba自旋轨道耦合效应和自旋过滤效应对自旋相关输运的影响.讨论了隧穿磁电阻（TMR）、隧穿电导与各材料层厚度、Rashba自旋轨道耦合强度以及两磁性半导体中磁矩的相对夹角θ之间的关系.研究表明：含磁性半导体层的双自旋过滤隧道结由于磁性半导体层的自旋过滤效应和Rashba自旋轨道耦合作用可获得极大的TMR值.另外TMR和隧穿电导随着Rashba自旋轨道耦合强度的变化而振荡,振荡周期随Rashba自旋轨道耦合强度的增大逐渐减小. 关键词： 双自旋过滤隧道结 Rashba自旋轨道耦合 隧穿磁电阻 隧穿电导     

Spin-filtering junctions with double ferroelectric barriers

An FS/FE/NS/FE/FS double tunnel junction is suggested to have the ability to inject, modulate and detect the spin-polarized current electrically in a single device, where FS is the ferromagnetic semiconductor electrode, NS is the nonmagnetic semiconductor, and FE the ferroelectric barrier. The spin polarization of the current injected into the NS region can be switched between a highly spin-polarized state and a spin unpolarized state. The high spin polarization may be detected by measuring the tunneling magnetoresistance ratio of the double tunnel junction.     

Bias Dependence in Spin-Polarized Tunneling of Ferromagnet/Ferromagnetic Insulator (Semiconductor)/Ferromagnet Junctions

Based on the nearly-free-electron approximation, the bias dependencies of electron transport properties of ferromagnet/ferromagnetic insulator (semiconductor)/ferromagnet junctions have been studied. Resonances appear in electron transmission probability. These resonances cause oscillations in the zero-temperature tunnel current and the resonances occur in tunnel conductance. Tunnel magnetoresistance (TMR) is an oscillatory function of bias. The TMR can reach a value as high as 100%. The bins dependencies of electron transport properties relate to the magnetic configurations of the junctions.     

Nonequilibrium spin accumulation and tunneling magnetoresistance in a ferromagnet/semiconductor/ferromagnet double tunnel junction

Taking into account the nonequilibrium spin accumulation, we apply a quantum-statistical approach to study the spin-polarized transport in a two-dimensional ferromagnet/semiconductor/ferromagnet (FM/SM/FM) double tunnel junction. It is found that the effective spin polarization is raised by increasing the barrier strength, resulting in an enhancement of the tunneling magnetoresistance (TMR). The nonequilibrium spin accumulation in SM may appear in both antiparallel and parallel alignments of magnetizations in two FMs, in particular for high bias voltages. The effects of spin accumulation and TMR on the bias voltage are discussed.     

Magnetoresistance in the ferromagnet/insulator/ferromagnet tunnel junction

Recently experiments and theories show that the tunnel magnetoresistance （TMR） does not only depend on the ferromagnetic metal electrodes but also on the insulator. Considering the rough-scattering effect and spin-flip effect in the insulator, this paper investigates the TMR ratio in a ferromagnet/insulator/ferromagnet （FM/I/FM） tunnelling junction by using Slonczewsik＇s model. A more general expression of TMR ratio as a function of barrier height, interface roughness and spin-flip effect is obtained. In lower barrier case, it shows that the TMR ratio depends on the roughscattering effect and spin-flip effect.     

Tunneling Conductance and Magnetoresistance in Ferromagnet/Ferromagnet/d-Wave Superconductor Double Tunnel Junctions

The tunneling conductance and tunneling magnetoresistance (TMR) are investigated in ferromagnet/insulator/ferromagnet/insulator/d-wave superconductor (FM/I/FM/I/d-wave SC) structures by applying an extended Blonder-Tinkham-Klapwijk (BTK) approach. We study the effects of the exchange splitting in the FM, the magnetic impurity scattering in the thin insulator interface of FM/I/FM, and noncollinear magnetizations in adjacent magnetic layers on the TMR. It is shown (1) that the tunneling conductance and TMR exhibit amplitude-varying oscillating behavior with exchange splitting, (2) that with the presence of spin-flip scattering in insulator interface of FM/I/FM, the TMR can be dramatically enhanced, and (3) that the TMR depends strongly on the angle between the magnetization of two FMs.     

Modulated replica symmetry breaking schemes for antiferrimagnetic spin glasses

The temperature and voltage dependence of spin transport is theoretically investigated in a new type of magnetic tunnel junction, which consists of two ferromagnetic outer electrodes separated by a ferromagnetic barrier and a nonmagnetic (NM) metallic spacer. The effect of spin fluctuation in magnetic barrier, which plays an important role at finite temperature, is included by taking the mean-field approximation. It is found that, the tunnel magnetoresistance (TMR) and the electron-spin polarization depend strongly on the temperature and the applied voltage. The TMR and spin polarization at different temperatures show an oscillatory behavior as a function of the NM spacer thickness. Also, the amplitude of these oscillations is regularly reduced when the temperature increases. The maximum TMR value, varies approximately from 270 in reverse bias (at T = 0 K) to 25 in forward bias (at ).Received: 25 June 2004, Published online: 14 December 2004PACS: 72.25.Hg Electrical injection of spin polarized carriers - 73.23.Ad Ballistic transport - 73.40.Gk Tunneling     

Intrinsic asymmetries in spin-valves: a tight binding model study

We have calculated the I–V curves, dynamical conductance, and tunneling magnetoresistance (TMR) of 1D magnetic tunneling junction through singleband tight binding model calculations based on the non-equilibrium Green's function approach. The difference in density of state of two ferromagnetic leads and the bias dependence of the propagator cause intrinsic asymmetries in TMR and dynamical conductance at finite bias. Besides, we have displayed that large TMR can be obtained even at high bias for half metallic leads.     

Electrical conductance properties for magnetic tunnel junctions with MgO barriers

We measured inelastic electron tunneling (IET) spectra and conductance for MgO tunneling magnetoresistance (TMR) films to obtain information on the ferromagnetic/barrier layer interface. The IET spectra showed the difference between amorphous and crystalline structures in the barrier. In the magnetic tunnel junction (MTJ) with a crystalline barrier the IET spectra indicated an Mg-O phonon peak at a low bias voltage by measurement with a parallel magnetization configuration. On the other hand, no peak was observed in the MTJ with an amorphous barrier.     

Magnetization reversal and spin accumulation in nonmagnetic metals by spin injection from lateral Co/Cu,Co/Al heterojunctions

We have fabricated ferromagnetic/nonmagnetic (FM/NM) metal heterojunctions for the detection of the spin accumulation effect in different nonmagnetic metals. To understand the effect of spin accumulation in more detail, the switching behavior of the ferromagnetic wires was studied by means of magnetoresistance (MR) measurements and Monte Carlo simulations (MC). The polycrystalline heterojunctions were prepared by high-resolution electron beam lithography (HR-EBL) and a special oblique evaporation technique. The ferromagnetic (FM) and the nonmagnetic (NM) metal were evaporated on top of each other in a single-evaporation step to achieve an interface between the two metals of high quality. To verify the quality of the interface, we measured the spin accumulation effect in nonmagnetic copper (Cu) and aluminum (Al) and determined the spin polarization of the current at the interface between the ferromagnetic and nonmagnetic metals.     

Electrical control of valley and spin polarized current and tunneling magnetoresistance in a silicene-based magnetic tunnel junction

We investigate the electronic transport in a silicene-based ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal tunnel junction. The results show that the valley and spin transports are strongly dependent on local application of a vertical electric field and effective magnetization configurations of the ferromagnetic layers. In particular, it is found that the fully valley and spin polarized currents can be realized by tuning the external electric field. Furthermore, we also demonstrate that the tunneling magnetoresistance ratio in such a full magnetic junction of silicene is very sensitive to the electric field modulation.     

Giant magnetoresistance effect in hybrid ferromagnetic-Schottky-metal and semiconductor nanosystem

We report on a theoretical investigation of the giant magnetoresistance (GMR) effect in hybrid ferromagnetic-Schottky-metal and semiconductor nanosystem. Experimentally, this GMR device can be realized by the deposition of two ferromagnetic (FM) stripes and one Schottky normal metal (NM) in parallel way on the top of a semiconductor GaAs heterostructure. The GMR effect emanates from the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations of the device, and its magnetoresistance ratio (MR) can reach the order of ¹⁰⁶%. Furthermore, it is also shown that the MR of the device depends strongly on the relative location of the Schottky NM stripe between two FM stripes.     

Electrically controllable conductance and tunneling magnetoresistance through a topological insulator quantum well coupled to ferromagnetic electrodes

We theoretically investigate the electrically controllable conductance and tunneling magnetoresistance (TMR) through a two-dimensional topological insulator (TI) quantum well sandwiched between ferromagnetic (FM) electrodes in the method of nonequilibrium Green’s function (GF). It is demonstrated that the inter-edge tunnelings modulated conductance for spin-up and spin-down carriers presents an opposite tend with the polarization of the FM electrodes. The system TMR from the spin-valve effect is observed to be up to 65,000 %, as can be significantly suppressed and enhanced by the backscattering and spin-dephasing effect of the inter-edge spin-conserving and spin-flipping tunneling, respectively, other than the quite different energy-dependent oscillation behavior. The obtained results may provide a deeper understanding of the TI edge states and be used to design a dissipationless spintronic device based on TIs.     

Room-temperature tunnel magnetoresistance and spin-polarized tunneling through an organic semiconductor barrier

Electron spin-polarized tunneling is observed through an ultrathin layer of the molecular organic semiconductor tris(8-hydroxyquinolinato)aluminum (Alq3). Significant tunnel magnetoresistance (TMR) was measured in a Co/Al2O3/Alq3/NiFe magnetic tunnel junction at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the current-voltage behavior and temperature and bias dependence of the TMR, show the good quality of the organic tunnel barrier. Spin polarization (P) of the tunnel current through the Alq3 layer, directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly improves spin transport.     

Observation of minority spin character of the new electron doped manganite La0.7Ce0.3MnO3 from tunneling magnetoresistance

We report the magnetotransport characteristics of a trilayer ferromagnetic tunnel junction built of an electron doped manganite (La0.7Ce0.3MnO3) and a hole doped manganite (La0.7Ca0.3MnO3). At low temperatures the junction exhibits a large positive tunneling magnetoresistance (TMR), irrespective of the bias voltage. At intermediate temperatures below T(C) the sign of the TMR is dependent on the bias voltage across the junction. The magnetoresistive characteristics of the junction strongly suggest that La0.7Ce0.3MnO3 is a minority spin carrier ferromagnet with a high degree of spin polarization, i.e., a transport half-metal.     

磁性多层膜的巨磁电阻随铁磁和非磁层厚度变化的唯象理论计算

根据唯象理论，并采用以铁磁─非磁混合层代替铁磁／非磁层界面的理论方法，计算了Ｆｅ／Ｃｒ多层膜的巨磁电阻随铁磁和非磁层厚度的变化关系与实验结果做了比较，发现它们符合得较好．还绘出了巨磁电阻随铁磁和非磁层厚度变化的二元函数图 关键词：     

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。