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.     

Vertical WS_2 spin valve with Ohmic property based on Fe_3GeTe_2 electrodes

The two-dimensional(2 D) transition-metal dichalcogenides(TMDCs) have been recently proposed as a promising class of materials for spintronic applications. Here, we report on the all-2 D van der Waals(vd W) heterostructure spin valve device comprising of an exfoliated ultra-thin WS_2 semiconductor acting as the spacer layer and two exfoliated ferromagnetic Fe_3 GeTe_2(FGT) metals acting as ferromagnetic electrodes. The metallic interface rather than Schottky barrier is formed despite the semiconducting nature of WS_2, which could be originated from the strong interface hybridization. The spin valve effect persists up to the Curie temperature of FGT. Moreover, our metallic spin valve devices exhibit robust spin valve effect where the magnetoresistance magnitude does not vary with the applied bias in the measured range up to 50 μA due to the Ohmic property, which is a highly desirable feature for practical application that requires stable device performance. Our work reveals that WS_2-based all-2 D magnetic vd W heterostructure, facilitated by combining 2 D magnets, is expected to be an attractive candidate for the TMDCs-based spintronic applications.     

Detection of a spin accumulation in nondegenerate semiconductors

Electrical detection of a spin accumulation in a nondegenerate semiconductor using a tunnel barrier and ferromagnetic contact is shown to be fundamentally affected by the energy barrier associated with the depletion region. This prevents the ferromagnet from probing the spin accumulation directly, strongly suppresses the magnetoresistance in current or potentiometric detection, and introduces nonmonotonic variation of spin signals with voltage and temperature. Having no analogue in metallic systems, we identify energy mismatch as an obstacle for spin detection, necessitating control of the energy landscape of spin-tunnel contacts to semiconductors.     

Tunneling Anisotropic Magnetoresistance in L1_0-MnGa Based Antiferromagnetic Perpendicular Tunnel Junction

We report on the tunneling anisotropic magnetoresistance in antiferromagnetic perpendicular tunnel junction consisting of L1_0-MnGa/FeMn/AlO_x/Pt grown on GaAs(001) substrates by molecular-beam epitaxy. The temperature-dependent perpendicular exchange bias effect reveals an exchange coupling between ferromagnetic L1_0-MnGa and antiferromagnetic FeMn. The rotation of antiferromagnetic spins in FeMn can be driven by perpendicularly magnetized L1_0-MnGa due to the exchange-spring effect at the interface and leads to roomtemperature tunneling anisotropic magnetoresistance ratio of 0.86%. We also find that the tunneling anisotropic magnetoresistance strongly depends on temperature and angle. These results have broadened the material selection range for high performance antiferromagnetic spintronic devices.     

The effects of electric and magnetic fields on the current spin polarization and magnetoresistance in a ferromagnetic/organic semiconductor/ferromagnetic（FM/OSC/FM） system

From experimental results of spin polarized injection and transport in organic semiconductors(OSCs),we theoretically study the current spin polarization and magnetoresistance under an electric and a magnetic field in a ferromagnetic/organic semiconductor/ferromagnetic(FM/OSC/FM) sandwich structure according to the spin drift-diffusion theory and Ohm’s law.From the calculations,it is found that the interfacial current spin polarization is enhanced by several orders of magnitude through tuning the magnetic and electric fields by taking into account the specific characteristics of OSC.Furthermore,the effects of the electric and magnetic fields on the magnetoresistance are also discussed in the sandwich structure.     

Hybrid ferromagnetic/semiconductor heterostructures for spintronics

Heterostructures that integrate conventional semiconductors with ferromagnetic semiconductors and ferromagnetic metals are important for developing a framework for semiconductor spintronics. We describe recent efforts to study ‘hybrid’ ferromagnetic/semiconductor heterostructures that combine conventional III-V and II-VI semiconductors with the ferromagnetic semiconductor (Ga,Mn)As and the ferromagnetic metal MnAs. We focus on the characteristics of two novel classes of heterostructures: (a) (Ga,Mn)As/AlAs/MnAs magnetic tunnel junctions (MTJs) that provide an all-electrical scheme for probing spin injection from metals into GaAs and (b) n-ZnSe/(Ga,Mn)As heterojunction diodes that surprisingly exhibit a magnetically-driven photoconductivity.     

Characteristic features of the extrinsic electric resistance in ferromagnets with low carrier density

Switching from simple semiconductors to more complicated chemical compositions, we encounter mainly nonstoichiometric or undoped compounds. Combined with other characteristic features of d(f) compounds, this can lead, together with the ordinary scattering by spin disorder in magnetic semiconductors, to an unusual impurity contribution to the total scattering of carriers even in intrinsic semiconductors. A unique scheme for calculating the energy structure of the conduction-band bottom of a ferromagnetic semiconductor and the temperature and field dependences of the impurity contribution to the resistivity is proposed on the basis of a model Hamiltonian. The computed magnetoresistance ratio is negative and has a maximum near T _c . A qualitative comparison is made between the results and the experimental temperature dependences of the Hall mobility and magnetoresistance ratio in the ternary semiconductor n-HgCr₂Se₄, which is nonstoichiometric with respect to the chalcogen. To identify previously unobserved temperature oscillations of the resistance, a careful analysis is made of the low-temperature part of the resistance using the relations obtained. Fiz. Tverd. Tela (St. Petersburg) 41, 68–76 (January 1999)     

Observation of magnetoresistance in CrI3/graphene van der Waals heterostructures

Two-dimensional ferromagnetic van der Waals (2D vdW) heterostructures have opened new avenues for creating artificial materials with unprecedented electrical and optical functions beyond the reach of isolated 2D atomic layered materials, and for manipulating spin degree of freedom at the limit of few atomic layers, which empower next-generation spintronic and memory devices. However, to date, the electronic properties of 2D ferromagnetic heterostructures still remain elusive. Here, we report an unambiguous magnetoresistance behavior in CrI₃/graphene heterostructures, with a maximum magnetoresistance ratio of 2.8%. The magnetoresistance increases with increasing magnetic field, which leads to decreasing carrier densities through Lorentz force, and decreases with the increase of the bias voltage. This work highlights the feasibilities of applying two-dimensional ferromagnetic vdW heterostructures in spintronic and memory devices.     

有机自旋阀的磁电阻性质研究

考虑到有机半导体中极化子和双极化子特殊的电荷-自旋关系,从自旋扩散方程和欧姆定律出发,理论研究了"铁磁/有机半导体/铁磁"有机自旋阀结构中的磁电阻性质.计算发现,磁电阻在数值上随有机半导体层中极化子比率的增加而增大,随有机半导体层厚度的增加而迅速减小.同时发现自旋相关界面电阻能在很大程度上提高系统的磁电阻.讨论了铁磁层和有机半导体电导率比率、铁磁层极化率等对系统磁电阻性质的影响. 关键词： 磁电阻 有机自旋电子学 极化子     

极化子比率变化对自旋注入有机半导体性质的影响

从自旋扩散方程和欧姆定律出发研究了铁磁层到有机半导体的自旋注入,得到了系统的电流自旋极化率。有机半导体中的载流子为自旋极化子和不带自旋的双极化子,极化子比率在有机半导体内随输运距离变化。通过计算发现匹配的铁磁和有机半导体电导率有利于自旋注入;通过调节界面电阻自旋相关性,电流自旋极化率可获得很大程度提高;极化子比率衰减速率对有机半导体电流自旋极化率具有非常重要的影响。     

肖特基势垒对铁磁/有机半导体结构自旋注入性质的影响

理论研究了铁磁/有机半导体肖特基接触时的电流自旋极化注入,并讨论了电流自旋极化率随界面处肖特基势垒高度、有机半导体层中特殊载流子及其迁移率、界面附近掺杂浓度的变化关系.通过计算发现,寻找在势垒区中载流子迁移率比较大的有机半导体材料对实现有效的自旋注入是必要的;同时还发现,由于铁磁/有机半导体接触而形成的肖特基势垒不利于自旋注入.因此要想实现有效的自旋注入,界面附近必须采用重掺杂来有效减少势垒区的宽度,且势垒的高度要限制在一定的范围内.     

Resonant tunneling in magnetic semiconductor tunnel junctions with arbitrary magnetic alignments

Combining an extended Julliere model with transfer matrix method, we study the spin-polarized resonant tunneling in GaMnAs/AlAs/GaAs/AlAs/GaMnAs double barrier ferromagnetic semiconductor (FS) tunnel junctions with the arbitrary angle θ between the magnetic directions of two FS's. It is shown that tunneling magnetoresistance (TMR) ratio linearly varies with sin²(θ/2). We also demonstrate that for the heavy and light holes, the properties of the spin-polarized resonant tunneling are obviously different. The present results are expected to be instructive for manufacturing the relevant semiconductor spintronic devices.     

Tunneling magnetoresistance of the double spin-filter junctions at nonzero bias

A recent theoretical estimation indicated that the NM/FI/FI/NM double spin-filter junction (DSFJ, here the NM and FI represent the nonmagnetic electrode and the ferromagnetic insulator (semiconductor) spacer, respectively) could have very high tunneling magnetoresistance (TMR) at zero bias. To meet the requirement in research and application of the magnetoresistance devices, we have calculated the dependences of tunneling magnetoresistance of DSFJ on the bias (voltage), the thicknesses of ferromagnetic insulators (semiconductors) and the average barrier height. Our results show that except its very high value, the TMR of DSFJ does not decrease monotonously and rapidly with rising bias, but increase slowly at first and decrease then after having reached a maximum value. This feature is in distinct contrast to the ordinary magnetic tunnel junction FM/NI/FM (FM and NI denote the ferromagnetic electrode and the nonmagnetic insulator (semiconductor) spacer, respectively), and is of benefit to the use of DSFJ as a magnetoresistance device.     

Diffusion thermopower of (Ga,Mn)As/GaAs tunnel junctions

We report the observation of tunneling anisotropic magnetothermopower, a voltage response to a temperature difference across an interface between a normal and a magnetic semiconductor. The resulting voltage is related to the energy derivative of the density of states in the magnetic material, and thus has a strongly anisotropic response to the direction of magnetization in the material. The effect will have relevance to the operation of semiconductor spintronic devices, and may indeed already play a role in correctly interpreting the details of some earlier spin injection studies.     

Organic magnetoresistance and spin diffusion in organic semiconductor thin film devices

We review recent work in the field of organic spintronics, focusing on our own contributions to this field. There are two principle magnetoresistance effects that occur in organic devices. (i) Organic magnetoresistance (OMAR), which occurs in nonmagnetic organic semiconductor devices. For example, in devices made from the prototypical small molecule Alq₃ OMAR reaches values of 10% or more at room temperature. (ii) Organic spin‐valve effects that occur in devices that employ ferromagnetic electrodes for spin‐polarized current injection and detection. We undertake an analysis of these two types of magnetoresistance with the goal of identifying the dominant spin‐scattering mechanism. Analysis of OMAR reveals that hyperfine coupling is the dominant spin‐coupling mechanism. Spin–orbit coupling, on the other hand, is important only in organic semiconductor materials containing heavy atoms. We explore the reasons why spin–orbit coupling is relatively unimportant in hydrocarbon materials. Next, we present a theory for spin diffusion in disordered organic semiconductors based on hyperfine coupling, taking into account a combination of incoherent carrier hopping and coherent spin precession in the random hyperfine magnetic fields. We compare our findings with experimental values for the spin‐diffusion length. Finally, we demonstrate a criterion that allows the determination whether the organic spin‐valves operate in the tunneling or injection regimes. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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.     

Spin injection into semiconductors using dilute magnetic semiconductors

So far, attempts at realizing spin-polarized current injection into a semiconductor using metallic ferromagnetic contacts have yielded unsatisfying results. In this paper, we present a simple model of diffusive transport, which shows that the principle reason for these negative results is a conductivity mismatch between the ferromagnetic contacts and the semiconductor. Moreover, we demonstrate that this problem can be addressed by using dilute magnetic semiconductor (DMS) contacts instead of metallic contacts. We present experimental results of optical measurements on a GaAs/AlGaAs diode fitted with a DMS spin injector contact. These measurements show a spin polarization of around 90% in the semiconductor. Furthermore, we discuss a novel magnetoresistance effect based on the suppression of one of the spin channels in the semiconductor which should allow the detection of a spin-polarized current by magnetoresistance measurements.     

Spin dynamics in magnetic semiconductor nanostructures

The studies of the magnetic and electrical transport properties of ordered magnetic semiconductor nanostructures have been generalized. This new area lies at the intersection of nanotechnologies and fundamental problems of magnetism. The prospects for application of ferromagnetic semiconductors in spintronics have been discussed. A comparative analysis of the magnetic and electrical transport properties of nanowires, thin films, and bulk elemental semiconductors doped with transition metals has been performed. The influence of size effects on the spin dynamics, magnetization, and magnetoresistance of nanostructures has been considered.     

铁磁/有机半导体/铁磁系统的电流自旋极化性质研究

根据有机半导体中的电流自旋极化注入和输运实验现象，理论上研究了铁磁/有机半导体/铁磁系统的电流自旋极化性质.考虑到有机半导体的具体特性，从自旋扩散理论和欧姆定律出发，得到了系统的电流自旋极化率.假设自旋极化子和不带自旋的双极化子为有机半导体中的载流子.通过计算发现，极化子为实现有机半导体中电流极化注入和输运的有效自旋载流子，即使它只占总载流子很少一部分.还进一步研究了自旋相关界面电阻和电导率匹配以及有机半导体长度等因素对系统电流自旋极化的影响. 关键词： 自旋电子学 自旋注入 有机半导体 极化子