Inversion of magnetoresistance in magnetic tunnel junctions: effect of pinhole nanocontacts

Inverse magnetoresistance has been observed in magnetic tunnel junctions with pinhole nanocontacts over a broad temperature range. The tunnel magnetoresistance undergoes a change of sign at higher bias and temperature. This phenomenon is attributed to the competition between the spin conserved ballistic transport through the pinhole contact where the transmission probability is close to unity and spin polarized tunneling across the insulating spacer with weak transmittivity.     

Application of Crystal Elements for Charged Particle Beam Steering and Radiation Beam Generation on the U-70 Synchrotron

Dependences of the tunnel magnetoresistance and in-plane component of the spin transfer torque on the applied voltage in a magnetic tunnel junction have been calculated in the approximation of ballistic transport of conduction electrons through an insulating layer with embedded magnetic or nonmagnetic nanoparticles. A single-barrier magnetic tunnel junction with a nanoparticle embedded in an insulator forms a double-barrier magnetic tunnel junction. It has been shown that the in-plane component of the spin transfer torque in the double-barrier magnetic tunnel junction can be higher than that in the single-barrier one at the same thickness of the insulating layer. The calculations show that nanoparticles embedded in the tunnel junction increase the probability of tunneling of electrons, create resonance conditions, and ensure the quantization of the conductance in contrast to the tunnel junction without nanoparticles. The calculated dependences of the tunnel magnetoresistance correspond to experimental data demonstrating peak anomalies and suppression of the maximum magnetoresistances at low voltages.     

对称抛物势阱磁性隧道结中的自旋输运及磁电阻效应

研究了两端具有铁磁接触的对称抛物势阱磁性隧道结(F/SPW/F)中自旋相关的隧穿概率和隧穿磁电阻,讨论了量子尺寸效应和Rashba 自旋轨道耦合作用对自旋极化输运特性的影响.研究结果表明:隧穿概率和隧穿磁电阻随抛物势阱宽度的增加发生周期性的振荡.抛物势阱深度的增加减小了隧穿概率和隧穿磁电阻的振荡频率.Rashba 自旋轨道耦合强度的增加加大了隧穿概率和隧穿磁电阻的振荡频率.隧穿概率和隧穿磁电阻的振幅和峰谷比强烈依赖于两铁磁电极中磁化方向的夹角. 关键词： 磁性隧道结 Rashba 自旋轨道耦合 隧穿概率 隧穿磁电阻     

Comparison of resonant tunneling in AlGaAs/GaAs parabolic and diffusion modified quantum wells

Double barrier resonant tunneling diode using annealing induced diffusion modified quantum well is proposed as a viable alternative to that using parabolic quantum well which requires complex techniques to fabricate it. The transmission coefficients are calculated using the hybrid incremental airy function plane wave approach. The room temperature current-voltage characteristics have been calculated using transmission coefficients. The current-voltage characteristics are found to be similar in both diodes.     

Formation of tunnel barrier using a pseudo-atomic layer deposition method and its application to spin-dependent tunneling junction

The tunneling barrier is crucial to the overall performance in magnetic tunnel junctions. We have suggested a new formation method for the tunnel barrier, which has utilized pseudo-atomic layer deposition with sputtering. As is well known, all metallic thin films oxidize more or less under atmospheric conditions. Using this phenomenon, an ultra-thin metallic layer was prepared and exposed to the oxygen ambient repeatedly to reach a desired thickness for the tunnel barrier. From transmission electron microscopy, the tunnel barrier has been confirmed to have a clear and smooth interface between magnetic layers and the tunnel barrier. From atomic force microscopy, it has also been confirmed to have a low surface roughness. The fabricated magnetic tunnel junction has been shown to exhibit tunnel resistivities from 60 to 92 kΩ μm² and a maximum tunneling magnetoresistance ratio of 40%. PACS 75.47.-m; 75.70.-i; 72.25.-b     

Spin-valley-dependent transport and giant tunneling magnetoresistance in silicene with periodic electromagnetic modulations

The transport property of electrons tunneling through arrays of magnetic and electric barriers is studied in silicene.In the tunneling transmission spectrum, the spin-valley-dependent filtered states can be achieved in an incident energy range which can be controlled by the electric gate voltage. For the parallel magnetization configuration, the transmission is asymmetric with respect to the incident angle θ, and electrons with a very large negative incident angle can always transmit in propagating modes for one of the spin-valley filtered states under a certain electromagnetic condition. But for the antiparallel configuration, the transmission is symmetric about θ and there is no such transmission channel. The difference of the transmission between the two configurations leads to a giant tunneling magnetoresistance(TMR) effect.The TMR can reach to 100% in a certain Fermi energy interval around the electrostatic potential. This energy interval can be adjusted significantly by the magnetic field and/or electric gate voltage. The results obtained may be useful for future valleytronic and spintronic applications, as well as magnetoresistance device based on silicene.     

一维位势透射系数的计算与谐振隧穿现象的研究

使用具有不同有效质量的一维多阶梯势透射系数的递推计算公式,可以方便地计算任意形状的一维分区位势的透射系数。给出了几种位势的透射系数随入射粒子能量变化的曲线,研究了谐振隧穿现象。对半导体材料GaAlAs/GaAs/GaAlAs的谐振隧穿现象进行了较详细的讨论。计算结果表明,对于两对称方势垒夹一个任意形状势阱的位势,也可能存在谐振隧穿现象。     

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.     

Huge magnetoresistance of structures based on nonferromagnetic wideband polymers

The phenomenon of huge magnetoresistance in structures of the ferromagnetic metal-polymer insulator (FM-PI) type is considered in comparison to other types of magnetoresistance. Similarities and differences between the effects of huge magnetoresistance at an FM-PI interface and on an array of paramagnetic nanoparticles are shown. Conclusions are drawn as to the decisive role of change in the magnetic field of tunneling conditions of a charge through a potential barrier.     

Electromagnetic field penetration through lanthanum manganites

The process of electromagnetic field penetration through lead-doped lanthanum manganites exhibiting colossal magnetoresistance has been studied. The measurements have been performed in a range of radio frequencies from 20 kHz to 10 MHz in the temperature interval containing a magnetic phase transition. Application of a constant external magnetic field leads to an increase in the transmission coefficient. Relative variations of the electromagnetic field transmission coefficient are several times as large as the relative change in the dc magnetoresistance. The temperature dependence of the relative change in the transmission coefficient has been studied. Variations of the transmission coefficient sharply decrease in the vicinity of the phase transition temperature, but they still remain rather large at temperatures above the Curie point.     

恒定电场下双磁垒结构中的电子输运行为

对磁量子结构中电子在外加恒定电场下的输运性质进行了研究.分别计算了电子隧穿相同磁垒磁阱和不同磁垒磁阱构成的两种磁量子结构的传输概率和电流密度.计算结果表明,在相当宽广的非共振电子入射能区,外加电场下电子的传输概率比无电场时增加.对于电子隧穿相同磁垒磁阱构成的双磁垒结构,共振减弱;对于电子隧穿不同磁垒磁阱构成的双磁垒结构,无电场作用时的非完全共振在适当的偏置电压下转化为完全共振,这时的电子可实现理想的共振隧穿.研究同时表明,磁量子结构中存在着显著的量子尺寸效应和负微分电导.     

新型的氧化物磁制冷工质与隧道磁电阻材料--2004年度国家自然科学二等奖获奖项目介绍

文章介绍了2004年度国家自然科学二等奖获奖成果[21].类钙钛矿型材料是一类物理内涵极其丰富的化合物,它是著名的高温超导材料、铁电材料、压电材料,又是庞磁电阻效应材料,目前又显示出具有大磁熵变效应与隧道磁电阻效应.文章作者系统地研究了锰钙钛矿磁性化合物的磁熵变与组成、微结构以及颗粒尺寸的关系,研究结果表明,磁性钙钛矿化合物具有显著的磁熵变,居里温度易调,并且化学稳定性佳,从而成为一类新型的磁制冷工质候选材料.此外,文章作者还研究了钙钛矿化合物纳米颗粒体系的磁电阻效应,发现除人们发现的居里温度附近的本征的庞磁电阻效应外,在很宽的低温区,存在与温度不甚敏感的隧道磁电阻效应.     

Rectangular Waveguide With Metallic Nanostructure Driven by Magnetic Field

The changes in the propagation constant of the TE₁₀ mode of a rectangular waveguide with a metallic nanostructure, occurring under an external magnetic field have been investigated. Expressions for complex reflection and transmission coefficients have been derived. The dependence of the reflection coefficient upon the intensity of the external magnetic field is analyzed. It is shown that the experimental values of the propagation constant or the reflection and transmission coefficients can be used to estimate the value of the microwave magnetoresistance of nanostructures. Experiments on the measurement of the reflection and transmission coefficients in a waveguide with an (Fe/Cr)n nanostructure have been performed in millimeter and centimeter wave bands.     

Enhanced magnetotransport at high bias in quasimagnetic tunnel junctions with EuS spin-filter barriers

In quasimagnetic tunnel junctions with a EuS spin-filter tunnel barrier between Al and Co electrodes, we observed large magnetoresistance (MR). The bias dependence shows an abrupt increase of MR ratio in high bias voltage, which is contrary to conventional magnetic tunnel junctions. This behavior can be understood as due to Fowler-Nordheim tunneling through the fully spin-polarized EuS conduction band. The I-V characteristics and bias dependence of MR calculated using tunneling theory show excellent agreement with experiment.     

多晶La0.7Sr0.3MnO3的低温输运性质和磁电阻效应

详细研究了由纳米晶粒组成的块体多晶La_0.7Sr_0.3MnO₃(LSM)的电阻率和磁电阻效应,以及它们的温度依赖性.随着温度从室温降低,电阻率(ρ)在250K附近存在一最大值,低于该温度后,样品表现为金属导电特性,随后在50K附近存在一极小值.也就是说在低于50K的温度范围内,随着温度降低ρ反而升高,表现为绝缘体性的导电特性.经研究发现,这种随温度降低ρ反而增加的现象与隧穿效应的理论模型(lnρ∝T^-1/2)符合得很好 关键词： 0.7Sr_0.3MnO₃')" href="#">多晶La_0.7Sr_0.3MnO₃ 隧道效应 隧道磁电阻效应     

Giant magnetoresistance effect in a magnetic barrier nanostructure

The giant magnetoresistance (MR) effect is theoretically studied in a magnetically modulated two-dimensional electron gas. We find that the significant transmission difference for electron tunneling through parallel and antiparallel magnetization configurations results in a considerable MR effect. We also find that the MR ratio strongly depends on the magnetic strength and the distance between the left edges of two ferromagnetic strips as well as the temperature.     

Zener tunneling between landau orbits in a high-mobility two-dimensional electron gas

Magnetotransport in a laterally confined two-dimensional electron gas (2DEG) can exhibit modified scattering channels owing to a tilted Hall potential. Transitions of electrons between Landau levels with shifted guiding centers can be accomplished through a Zener tunneling mechanism, and make a significant contribution to the magnetoresistance. A remarkable oscillation effect in weak field magnetoresistance has been observed in high-mobility 2DEGs in GaAs -Al Ga 0.3As (0.7) heterostructures, and can be well explained by the Zener mechanism.     

Tunneling magnetoresistance in Ag/Co nanoparticle composites

Nanocomposite materials, consisting of ensembles of Ag and Co nanoparticles, have been successfully fabricated, with various compositions and packing densities. The transport and magnetic characteristics of the compounds were studied. In particular, a crossover from a positive magnetoresistance (MR) at low applied magnetic fields to a negative magnetoresistance at high applied magnetic fields was observed. The behaviors could be understood by the spin-dependent tunneling mechanism, known as tunneling magnetoresistance.     

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.     

Particle tunneling and scattering in a three-dimensional potential with a barrier

The particle tunneling through a 3-D rectangular potential barrier has been studied. The simplest model for multiple internal reflections has been assumed. The explicit expression for all the transmission and reflection probability amplitudes have been derived, as well as the tunneling and reflection phase times.