Observation of a tunneling magnetoresistance effect in magnetic tunneling junctions with a high resistance ferromagnetic oxide Fe2⋅5Mn0⋅5O4 electrode

A high resistance ferromagnetic oxide Fe_2⋅5Mn_0⋅5O₄ (FMO) property as a novel spin injector was investigated with a structure of a magnetic tunneling junction (MTJ) composed of FMO/Al-O/Ni₈₀Fe₂₀, in order to reduce an impedance mismatch problem on molecular spintronics. A tunneling magnetoresistance (TMR) effect in the MTJs was observed. The maximum TMR ratio observed in the MTJs was approximately 0.85% at room temperature (RT), and the spin-polarization of FMO was estimated to be at least 0.94% at RT.     

Magnetotransport and interdiffusion characteristics of magnetic tunnel junctions comprising nano-oxide layers upon exposure to postdeposition annealing

Magnetic tunnel junction (MTJ) structures based on underlayer (CoNbZr)/bufferlayer (CoFe)/antiferromagnet (IrMn)/pinned layer (CoFe)/tunnel barrier (AlO_x)/free layer (CoFe)/capping (CoNbZr) have been prepared to investigate thermal degradation of magnetoresistive responses. Some junctions possess a nano-oxide layer (NOL) inside either in the underlayer or bufferlayer. The main purpose of the NOL inclusion was to control interdiffusion path of Mn from the antiferromagnet so that improved thermal stability could be achieved. The MTJs with NOLs were found to have reduced interfacial roughness, resulting in improved tunneling magnetoresistance (TMR) and reduced interlayer coupling field. We also confirmed that the NOL effectively suppressed the Mn interdiffusion toward the tunnel barrier by dragging Mn atoms toward NOL during annealing.     

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.     

Co基磁性隧道结势垒结构的电子全息研究

利用高分辨电子显微术和电子全息方法研究了Co基磁性隧道结退火热处理前后的微观结构及相应势垒层结构的变化. 研究结果表明，退火处理可以明显地改善势垒层和顶电极、底电极之间的界面质量，改进势垒层本身的结构. 这与该磁性隧道结经过280℃退火处理后，隧道磁电阻值大大增加是一致的. 关键词： 磁性隧道结 隧道磁电阻 高分辨电子显微学 电子全息     

Negative spin polarization and large tunneling magnetoresistance in epitaxial Co/SrTiO(3)/Co magnetic tunnel junctions

We perform an ab initio study of spin-polarized tunneling in epitaxial Co/SrTiO(3)/Co magnetic tunnel junctions with bcc Co(001) electrodes. We predict a large tunneling magnetoresistance in these junctions, originating from a mismatch in the majority- and minority-spin bands both in bulk bcc Co and at the Co/SrTiO(3)/Co interface. The intricate complex band structure of SrTiO(3) enables efficient tunneling of the minority d electrons which causes the spin polarization of the Co/SrTiO(3)/Co interface to be negative in agreement with experimental data. Our results indicate that epitaxial Co/SrTiO(3)/Co magnetic tunnel junctions with bcc Co(001) electrodes are a viable alternative for device applications.     

Temperature and bias voltage dependence of Co/Pd multilayer-based magnetic tunnel junctions with perpendicular magnetic anisotropy

Temperature- and bias voltage-dependent transport measurements of magnetic tunnel junctions (MTJs) with perpendicularly magnetized Co/Pd electrodes are presented. Magnetization measurements of the Co/Pd multilayers are performed to characterize the electrodes. The effects of the Co layer thickness in the Co/Pd bilayers, the annealing temperature, the Co thickness at the MgO barrier interface, and the number of bilayers on the tunneling magneto resistance (TMR) effect are investigated. TMR-ratios of about 11% at room temperature and 18.5% at 13 K are measured and two well-defined switching fields are observed. The results are compared to measurements of MTJs with Co-Fe-B electrodes and in-plane anisotropy.     

Effect of an interface boundary on the magnetooptical and magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> heterostructures

The optical, magnetooptical (Kerr effect and magnetotransmission), and magnetotransport properties of La_2/3Ca_1/3MnO₃/La_2/3Sr_1/3MnO₃ and La_2/3Ca_1/3MnO₃/SrTiO₃/La_2/3Sr_1/3MnO₃ heterostructures on SrTiO₃ substrates are studied. The contribution of the interface boundary to the magnetotransmission is typical of a material with a transitional composition. It is found that a 2-nm-thick SrTiO₃ spacer does not influence the shape and position of the magnetotransmission peak in a field normal to the surface of the heterostructure but increases the contribution of the upper layer to the magnetotransmission in the Voigt geometry and also enhances the magnetoresistance that is due to the tunneling of spin-polarized carriers through the spacer. The Kerr spectra taken of the heterostructures are typical of single-layer single-crystal films.     

Hard axis magnetic field dependence on current-induced magnetization switching in MgO-based magnetic tunnel junctions

We conducted a detailed study of hard axis magnetic field (H_hard) dependence on current-induced magnetization switching (CIMS) in MgO-based magnetic tunnel junctions (MTJs) with various junction sizes and various uniaxial anisotropy fields. The decreases in critical current density (J_c) and the intrinsic critical current density (J_c0) estimated from the pulse duration dependence on J_c in CIMS are observed when applying H_hard for all MTJs. The decrease in energy barrier of CIMS is also observed except for the largest sample. These results indicate that the reduction of J_c is attributable to both the increase of spin-transfer efficiency and the decrease in energy barrier in the case of applying H_hard. The J_c0 decreases with increase in the mutual angle between the direction of magnetization and the easy axis (θ_f), which is consistent with the theoretical prediction proposed by Slonczewski. The degree of the reduction of J_c0 for the same value of H_hard decreases with decreasing size of MTJs. This behavior is considered to be related to not only decrease in θ_f due to the increase in anisotropy field in MTJs, but also to the increase in the variance of the initial angle of magnetization due to the thermally activated magnon excitation. The stable switching endurance related to CIMS was observed in a wide range of MTJ sizes when applying H_hard. Moreover, we proposed a new architecture and a new switching method considering write disturbance. These results would be useful for application to spin memory and other spin-electronic devices.     

Tunneling magnetoresistance based on a Cr/graphene/Cr magnetotunnel junction

Using the density functional theory and the nonequilibrium Green's function method, we studied the finite-bias quantum transport in a Cr/graphene/Cr magnetotunnel junction(MTJ) constructed by a single graphene layer sandwiched between two semi-infinite Cr(111) electrodes. We found that the tunneling magnetoresistance(TMR) ratio in this MTJ reached108%, which is close to that of a perfect spin filter. Under an external positive bias, we found that the TMR ratio remained constant at 65%, in contrast to Mg O-based MTJs, the TMR ratios of which decrease with increasing bias. These results indicate that the Cr/graphene/Cr MTJ is a promising candidate for spintronics applications.     

Multi-channel magnetotransport in Co2FeSi/(Al,Ga)As spin-LEDs

We have performed Hall effect measurements on Co₂FeSi/(Al,Ga)As spin light emitting diodes and have found unique field dependencies that differ strongly from the expected behaviors for both the ferromagnetic Co₂FeSi layer and the underlying semiconductor structure. To understand such unique field dependencies, we have developed a multi-channel transport model for parallel transport through a ferromagnet and a semiconductor. By applying this model to our data for the Hall and sheet resistance, we extract values for the carrier density and mobility in the semiconductor layer. We find that these values decrease with increasing growth temperature of the Co₂FeSi layer, presumably due to stronger in-diffusion of Co and Fe impurities, which compensate the n-type dopants in the underlying n-(Al, Ga) As layer. Despite such compensation, spin-LEDs with the Co₂FeSi layer grown at the relatively high temperature of 280 °C exhibit the highest spin injection efficiencies of more than 50%, hence calling into question the requirement of electron tunneling through the ferromagnet/semiconductor Schottky barrier for efficient spin injection.     

Spin-dependent current in resonant tunneling diode with ferromagnetic GaMnN layers

The spin-polarized tunneling current through a double barrier resonant tunneling diode (RTD) with ferromagnetic GaMnN emitter/collector is investigated theoretically. Two distinct spin splitting peaks can be observed at current-voltage (I-V) characteristics at low temperature. The spin polarization decreases with the temperature due to the thermal effect of electron density of states. When charge polarization effect is considered at the heterostructure, the spin polarization is enhanced significantly. A highly spin-polarized current can be obtained depending on the polarization charge density.     

Effects of Rashba spin–orbit coupling interface on the orientation-dependent conductance in ferromagnet/spin-triplet superconductor junction

We study theoretically the tunneling charge conductance in ferromagnet/spin-triplet superconductor junction with the spin–orbit coupling interface. It is shown the symmetry of the conductance about the relative angle between the magnetization in ferromagnet and the d-vector in superconductor is broken due to the presence of the interfacial Rashba spin–orbit coupling. We present the conductance for various cases of the angle. For each angle, the spin-active mechanism provided by the interface is investigated. The interface effects for different spin polarization in the ferromagnet is also considered.     

Fe/Al2O3/Fe隧道结特性分析

用离子束溅射方法制备磁性隧道结(MTJ). 研究MTJ样品的隧道结磁电阻(TMR)效应.用X射线 光电子能谱分析了MTJ的软、硬磁层和非磁层及其界面的化学组成与微结构.研究了MTJ的微 结构对氧化铝势垒高度与有效宽度和TMR效应的影响. 关键词： 磁性隧道结 X射线光电子能谱 隧道结磁电阻     

Bias voltage dependence of inverted magnetoresistance on the annealing temperature in MgO-based magnetic tunnel junctions

MgO-based magnetic tunnel junctions (MTJs) with a layer sequence Ir₂₂Mn₇₈ or Fe₅₀Mn₅₀ (10 nm)/CoFe (2 nm)/Ru (0.85 nm)/CoFeB (0.5?t<2 nm)/MgO (2.5 nm)/CoFeB (3 nm) have been fabricated. The bias voltage dependence of tunneling magnetoresistance (TMR) is given as a function of the annealing temperature for these MTJs, which shows the TMR ratio changes its sign from inverted to normal at a critical bias voltage (V_C) when an unbalanced synthetic antiferromagnetic stack CoFe/Ru/CoFeB is used. V_Cs change with the thickness of the pinned CoFeB and annealing temperature, which implies one can achieve different V_Cs by artificial control. The asymmetric V_C values suggest that a strong density-of-states modification occurs at bottom oxide/ferromagnet interface.     

Sr/Si(100)表面TiSi2纳米岛的扫描隧道显微镜研究

为了解半导体衬底与氧化物之间存在的相互作用,以及量子尺寸效应对不同再构体的影响,制备了1—2个原子层厚的TiSi₂/Si(100)纳米岛,并使用扫描隧道显微镜(STM)表征手段详细地研究了TiSi₂ /Si(100)纳米岛的电子和几何特性. 结果发现:这些纳米岛表面显示出明显的金属性;其空态STM图像具有典型的偏压依赖性:在高偏压下STM 图像由三聚物形成的单胞构成,并在低偏压下STM 图像显示为密堆积的图案,这些不同的图案反映出不同能量位的态密度有明显差异. 关键词： 2纳米岛')" href="#">TiSi₂纳米岛 Sr/Si(100)表面 扫描隧道显微镜     

基于赫斯勒合金Co2MnSi电极的磁性隧道结呈现巨磁阻效应

本文基于电子密度泛函理论计算和非平衡态格林函数技术研究了具有三明治结构的磁性隧道结构(非极化SrTiO₂薄层被夹在两个赫斯勒合金Co₂MnSi电极之间)的自旋极化输运特性. 理论计算结果清楚地表明磁平行组态的磁性隧道结呈现出几乎完美的自旋过滤效应. 磁反平行组态的隧穿系数比磁平行组态的隧穿系数小几个数量级,导致体系的磁阻比高达10⁶. 电子结构计算分析表明该磁性隧道结的巨磁阻效应源自赫斯勒合金Co₂MnSi电极内在的半金属性、以及阻挡层和电极之间界面处过渡金属原子3d电子的显著自旋极化.     

Tunneling magnetoresistance in exchange-biased CoFeB/AlOx/Co/IrMn junctions

A series of exchange-biased magnetic tunneling junctions (MTJs) were made in an in-plane deposition field (h) = 500 Oe. The deposition sequence was Si(1 0 0)/Ta(30 Å)/CoFeB(75 Å)/AlO_x(d Å)/Co(75 Å)/IrMn(90 Å)/Ta(100 Å), where d was varied from 12 Å to 30 Å. The MTJ was formed by the cross-strip method with a junction area of 0.0225 mm². The tunneling magnetoresistance (ΔR/R) of each MTJ was measured. The high-resolution cross-sectional transmission electron microscopic (HR X-TEM) image shows the very smooth interface and clear microstructure. X-ray diffraction (XRD) demonstrates that the IrMn layer of the MTJ exhibits a (1 1 1) texture. From the results (ΔR/R) increases from 17% to 50%, as d increases from 12 Å to 30 Å. The tunneling resistance (R_o) of these junctions ranges from 150 Ω to 250 Ω. The exchange-biasing field (H_ex) of the MTJ is 50-95 Oe. Finally, the saturation resistance (R_s) was measured as a function of the angle (α) of rotation, where α is the angle between h and the in-plane saturation field (H_s) = 1.1 kOe. The following figure presents the dependence of R_s on α, instead of originally expected independence, the curve actually varies with a period of π.     

Large spin Hall effect and tunneling magnetoresistance in iridium-based magnetic tunnel junctions

Magnetic tunnel junctions(MTJs) switched by spin-orbit torque(SOT) have attracted substantial interest owing to their advantages of ultrahigh speed and prolonged endurance. Both field-free magnetization switching and high tunneling magnetoresistance(TMR) are critical for the practical application of SOT magnetic random access memory(MRAM). In this work, we propose an MTJ structure based on an iridium(Ir) bottom layer. Ir metal is a desirable candidate for field-free SOT switching owing to its strong intrinsic spin Hall conductivity(SHC), which can be enhanced via doping. Herein, we study TMR in Ir-based MTJs with symmetric and asymmetric structures. Ir-based MTJs exhibit large TMR, which can be further enhanced by heavy metal symmetry owing to the resonant tunneling effect. Our comprehensive investigations illustrate that Ir-based MTJs are promising candidates for realizing SOT switching and high TMR.     

In-plane electric current is induced by tunneling of spin-polarized carriers

It has been shown that tunneling of spin-polarized electrons through a semiconductor barrier is accompanied by generation of an electric current in the plane of the interfaces. The direction of this interface current is determined by the spin orientation of the electrons and symmetry properties of the barrier; in particular, the current reverses its direction if the spin orientation changes the sign. Microscopic origin of such a "tunneling spin-galvanic" effect is the spin-orbit coupling-induced dependence of the barrier transparency on the spin orientation and the wave vector of electrons.     

Spin-polarized inter-grain tunneling as the probable mechanism for low-field magnetoresistance in partially crystallized La0.5Sr0.5MnO3 thin films

Amorphous/crystalline mixed La_0.5Sr_0.5MnO₃ (LSMO) thin films on quartz wafers are prepared at different depositing temperatures using laser ablation and their low-field magnetoresistive property is investigated. It is argued that the insulating amorphous layers separating the magnetic microcrystalline grains may act as the barriers for electron tunneling. The rapid decay of magnetoresistance with increasing temperature is explained by the spin-polarized inter-grain tunneling. Given the spin-polarized inter-grain tunneling as the probable mechanism, it is believed that the spin flip during inter-grain tunneling reaches a minimum at the optimized depositing temperature of 600 °C and consequently the maximal low-field magnetoresistance is obtained. Received: 7 September 2000 / Accepted: 19 December 2000 / Published online: 23 March 2001