Spin-polarized current and tunnel magnetoresistance in heterogeneous single-barrier magnetic tunnel junctions

Current in heterogeneous tunnel junctions is studied in the framework of the parabolic conduction-band model. The developed model of the electron tunneling takes explicitly into account the difference of effective masses between ferromagnetic and insulating layers and between conduction subbands. Calculations for Fe/MgO/Fe-like structures have shown the essential impact of effective mass differences in regions (constituents) of the structure on the tunnel magnetoresistance of the junction.     

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.     

Spin torque, tunnel-current spin polarization, and magnetoresistance in MgO magnetic tunnel junctions

We employ the spin-torque response of magnetic tunnel junctions with ultrathin MgO tunnel barrier layers to investigate the relationship between spin transfer and tunnel magnetoresistance (TMR) under finite bias, and find that the spin torque per unit current exerted on the free layer decreases by < 10% over a bias range where the TMR decreases by > 40%. This is inconsistent with free-electron-like spin-polarized tunneling and reduced-surface-magnetism models of the TMR bias dependence, but is consistent with magnetic-state-dependent decay lengths in the tunnel barrier.     

Quantitative analysis of nonequilibrium spin injection into molecular tunnel junctions

We report quantitative analysis of nonequilibrium spin injection from Ni contacts to the octanethiol molecular spintronic system. Our calculation is based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism. The first principles results allow us to establish a clear physical picture on how spins are injected from the Ni contacts through the Ni-molecule linkage to the molecule, why tunnel magnetoresistance is rapidly reduced by the applied bias in an asymmetric manner, and to what extent ab initio transport theory can make quantitative comparisons to the corresponding experimental data. We found that extremely careful sampling of the two-dimensional Brillouin zone of the Ni surface is crucial for accurate results.     

磁性隧道结的隧穿磁电阻效应及其研究进展

文章概括地介绍了磁性隧道结(MTJs)的隧穿磁电阻(TMR)效应的产生机理和特点,主要用途和研究背景以及最近几年的研究进展和现状.对用Al2O3和MgO做绝缘势垒层的MTJs进行了对比,指出用MgO做绝缘势垒层的MTJs的优点.文章还阐明了交换偏置自旋阀(EB-SV)型MTJs的问题和不足,以及新兴的赝自旋阀(PSV)型MTJs的优势.文章最后总结了用于MTJs的各种铁磁层和绝缘势垒层材料,并对TMR材料今后的研究和开发作了展望.     

磁性隧道结的隧穿磁电阻效应及其研究进展

文章概括地介绍了磁性隧道结（MTJs）的隧穿磁电阻（TMR）效应的产生机理和特点,主要用途和研究背景以及最近几年的研究进展和现状.对用Al2O3和MgO做绝缘势垒层的MTJs进行了对比,指出用MgO做绝缘势垒层的MTJs的优点.文章还阐明了交换偏置自旋阀（EB-SV）型MTJs的问题和不足,以及新兴的赝自旋阀（PSV）型MTJs的优势.文章最后总结了用于MTJs的各种铁磁层和绝缘势垒层材料,并对TMR材料今后的研究和开发作了展望.     

Bulk—like contribution of tunnel magnetoresistance in magnetic tunnel junctions

We have demonstrated that the bulk-like contribution to tunnelling magnetoresistance (TMR) exists in the magnetic tunnel junctions, and is determined by the tunnelling characteristic length of the ferromagnetic electrodes. In the experiment, a wedge-shaped CoFe layer is inserted at the interface between the insulating barrier and the reference electrode. It is found that TMR ratio increases from 18% without CoFe layer to a saturation value of 26.5% when the CoFe thickness is about 2.3 nm. The tunnelling characteristic length, l_{tc}, can be obtained to be about 0.8 nm for CoFe materials.     

Interpretation of the magnetoresistance in doped magnetic tunnel junctions

We present a quantum mechanical model of the magnetoresistance in ferromagnetic tunnel junctions artificially doped by the introduction of layers of impurities in the middle of the barrier. The electron transport across the barrier is described by a combination of direct tunneling, tunneling assisted by spin-conserving scattering and tunneling assisted by spin-flip scattering. With this model, we interpret recent experimental results concerning the dependence of the TMR amplitude on the amount of impurities in the barrier and on temperature. Received 1st February 2001 and Received in final form 14 June 2001     

Temperature dependence of the tunneling magnetoresistance for tunnel junctions

The temperature dependence of the tunneling magnetoresistance (TMR) for magnetic tunneling junctions is investigated experimentally before and after the sample is annealed. As grown, the TMR is observed to increase with temperature from 80 to 160 K. A modified Julliere model in conjunction with a spin-dependent two-step tunneling is suggested to describe this temperature dependence.     

Tunnel magnetoresistance in alumina, magnesia and composite tunnel barrier magnetic tunnel junctions

Using magnetron sputtering, we have prepared Co-Fe-B/tunnel barrier/Co-Fe-B magnetic tunnel junctions with tunnel barriers consisting of alumina, magnesia, and magnesia-alumina bilayer systems. The highest tunnel magnetoresistance ratios we found were 73% for alumina and 323% for magnesia-based tunnel junctions. Additionally, tunnel junctions with a unified layer stack were prepared for the three different barriers. In these systems, the tunnel magnetoresistance ratios at optimum annealing temperatures were found to be 65% for alumina, 173% for magnesia, and 78% for the composite tunnel barriers. The similar tunnel magnetoresistance ratios of the tunnel junctions containing alumina provide evidence that coherent tunneling is suppressed by the alumina layer in the composite tunnel barrier.     

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.     

Negative tunnelling magnetoresistance in spin filtering magnetic junctions with spin-orbit coupling

We present theoretical calculations of spin transport in spin filtering magnetic tunnelling junctions based on the Landauer-Buttiker formalism and taking into account the spin-orbit coupling(SOC).It is shown that spin-flip scattering induced by SOC is stronger in parallel alignment of magnetization of the ferromegnet barrier(FB) and the ferromagnetic electrode than that in antiparallel case.The increase of negative tunnelling magnetoresistance with bias is in agreement with recent experimental observation.     

Perpendicular spin torques in magnetic tunnel junctions

We quantitatively determine a perpendicular spin torque in magnetic tunnel junctions by measuring the room-temperature critical switching current at various magnetic fields and current pulse widths. We find that the magnitude of the torque is proportional to the product of the current density and the bias voltage, and the direction of the torque reverses as the polarity of the voltage changes. By taking into account the energy-dependent inelastic scattering of tunnel electrons, we formulate the bias dependence of the perpendicular spin torque which is in qualitative agreement with the experimental results.     

Thermal spin transfer in Fe-MgO-Fe tunnel junctions

We compute thermal spin transfer (TST) torques in Fe-MgO-Fe tunnel junctions using a first principles wave-function-matching method. At room temperature, the TST in a junction with 3 MgO monolayers amounts to 10(-7) J/m(2)/K, which is estimated to cause magnetization reversal for temperature differences over the barrier of the order of 10 K. The large TST can be explained by multiple scattering between interface states through ultrathin barriers. The angular dependence of the TST can be very skewed, possibly leading to thermally induced high-frequency generation.     

高磁电阻磁性隧道结的几种微制备方法研究

利用金属掩模法优化了制备磁性隧道结的实验和工艺条件，金属掩模的狭缝宽度为100 μm. 采用4 nm厚的Co₇₅Fe₂₅为铁磁电极和10或08 nm厚的铝氧化物 为势垒膜， 直接制备出了室温隧穿磁电阻（TMR）为30%—48％的磁性隧道结，其结构为Ta(5 nm)/Cu(25 nm)/Ni₇₉Fe₂₁(5 nm)/Ir₂₂Mn₇₈(10 nm)/ Co₇₅Fe₂₅ (4 nm)/Al(08 nm)-O/Co₇₅Fe₂₅(4 nm)/Ni₇₉Fe ₂₁(20 nm)/Ta(5 nm).同时，利用刻槽打孔法和去胶掀离法两种光刻技术并结合Ar离子束刻蚀及化学反应刻 蚀，制备出面积在4 μm×8 μm—20 μm×40 μm、具有室温高TMR和低电阻的高质量磁性 隧道结.300 ℃ 退火前后其室温TMR可分别达到22% 和50%.研究结果表明，采用光刻中的刻 槽打孔或去胶掀离工艺方法制备的小尺寸磁性隧道结，可用于研制磁动态随机存储器和磁读 出头及其他传感器件的磁敏单元. 关键词： 磁性隧道结 隧穿磁电阻 金属掩模法 光刻法     

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.     

Spin-relaxation and magnetoresistance in FM/SC/FM tunnel junctions

The effect of spin relaxation on tunnel magnetoresistance (TMR) in a ferromagnet/superconductor/ferromagnet (FM/SC/FM) double tunnel junction is theoretically studied. The spin accumulation in SC is determined by balancing of the spin-injection rate and the spin-relaxation rate. In the superconducting state, the spin-relaxation time τ_s becomes longer with decreasing temperature, resulting in a rapid increase of TMR. The TMR of FM/SC/FM junctions provides a useful probe to extract information about spin-relaxation in superconductors.     

Spin-dependent transport in molecular tunnel junctions

We present measurements of magnetic tunnel junctions made using a self-assembled-monolayer molecular barrier. Ni-octanethiol-Ni samples were fabricated in a nanopore geometry. The devices exhibit significant changes in resistance as the angle between the magnetic moments in the two electrodes is varied, demonstrating that low-energy electrons can traverse the molecular barrier while remaining spin polarized. An analysis of the voltage and temperature dependence of the data suggests that the spin-polarized transport signals can be degraded by localized states in the molecular barriers.     

Room-temperature magnetoresistance in CoFeB/STO/CoFeB magnetic tunnel junctions

A series of Co₄₀Fe₄₀B₂₀/SrTiO₃/Co₄₀Fe₄₀B₂₀ magnetic tunnel junctions with a bottom-pinned synthetic antiferromagnet have been prepared by sputtering. Devices optimally annealed at 325 °C exhibit an exchange bias of about 65 mT, and a tunnel magnetoresistance of 2%. The smaller than predicted effect is attributed to the lack of epitaxy between the crystallized CoFeB electrodes and the SrTiO₃ (STO) barrier, due to poor crystal quality of the barrier layer. Unlike MgO, well-crystallized, oriented STO does not grow on amorphous Co₄₀Fe₄₀B₂₀.