Magnetic Switching Dynamics and Tunnel Magnetoresistance Effect Based on Spin-Splitting Noncollinear Antiferromagnet Mn_(3)Pt

In comparison to ferromagnets,antiferromagnets are believed to have superior advantages for applications in next-generation magnetic storage devices,including fast spin dynamics,vanishing stray fields and robust against external magnetic field,etc.However,unlike ferromagnetic orders,which could be detected through tunneling magnetoresistance effect in magnetic tunnel junctions,the antiferromagnetic order(i.e.,Néel vector)cannot be effectively detected by the similar mechanism due to the spin degeneracy of conventional antiferromagnets.Recently discovered spin-splitting noncollinear antiferromagnets,such as Mn_(3)Pt with momentum-dependent spin polarization due to their special magnetic space group,make it possible to achieve remarkable tunneling magnetoresistance effects in noncollinear antiferromagnetic tunnel junctions.Through first-principles calculations,we demonstrate that the tunneling magnetoresistance ratio can reach more than 800% in Mn_(3)Pt/perovskite oxides/Mn_(3)Pt antiferromagnetic tunnel junctions.We also reveal the switching dynamics of Mn_(3)Pt thin film under magnetic fields using atomistic spin dynamic simulation.Our study provides a reliable method for detecting Néel vector of noncollinear antiferromagnets through the tunnel magnetoresistance effect and may pave its way for potential applications in antiferromagnetic memory devices.

基于改进神经网络的TMR磁传感系统温漂校准方法

由于环境以及自身元件的发热等因素的影响,TMR磁传感系统实际工作温度会发生一定程度的波动。随着温度的升高或降低,TMR磁传感系统的实际输出会产生一定的误差,出现输出的温度漂移现象。为了抑制温度漂移对TMR磁传感系统的影响,采用反向传播(BP)神经网络对系统输出的温度漂移现象进行补偿,并利用遗传算法(GA)对BP神经网络进行优化,通过对补偿前后数据的对比,使TMR磁传感系统的输出温漂大幅下降了2个量级,得到了较为理想的效果,提升了TMR磁传感系统的性能和可靠性。

基于最小二乘法的三维TMR数字罗盘系统设计

针对市面上现有的三维数字罗盘在进行地磁场检测时极易受到外界磁场的干扰进而导致测量精度低的问题,设计了一款基于最小二乘法的三维TMR(隧道磁阻效应)数字罗盘系统。对其在现实中的误差特点开展研究,在由椭球拟合法处理校正后,运用了最小二乘法开展误差补偿,补偿前其方位角精度为4.18°,补偿后其方位角精度为0.46°,结果显而易见,在精度方面拔高了一个数量级,降低了三维数字罗盘系统的方位角误差。实验结果显示,最小二乘法可以极大地提高三维数字罗盘系统的精度,该方法具有较高的工程应用价值。

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 quan- tum transport in a Cr/graphene/Cr magnetotunnel junction （MTJ） constructed by a single graphene layer sandwiched be- tween two semi-infinite Cr（111 ） electrodes. We found that the tunneling magnetoresistance （TMR） ratio in this MTJ reached 108%, 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 MgO-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.

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.

Tunneling Negative Magnetoresistance via δ Doping in a Graphene-Based Magnetic Tunnel Junction

We investigate the tunneling magnetoresistance via δ doping in a graphei2e-based magnetic tunnel junction in detail. It is found that the transmission probability and the conductance oscillates with the position and the aptitude of the 8 doping. Also, both the transmission probability and the conductance at the paxallel configuration are suppressed by the magnetic field more obviously than that at the antiparallel configuration, which implies a large negative magnetoresistance for this device. The results show that the negative magnetoresistance of over 300% at B = 1.0 T is observed by choosing suitable doped parameters, and the temperature plays an important role in the magnetoresistance. Thus it is possible to open a way to effectively manipulate the magnetoresistance devices, and to make a type of magnetoresistance device by controlling the structural parameter of the δ doping.

SPIN-DEPENDENT TUNNELING MAGNETORESISTANCE IN Fe-O/AlO_x/Fe-O FILMS

Fe-O/AlO_x/Fe-O tunnel junctions were prepared by reactive magnetronsputtering under mixed working gas Ar+2 percent O_2. The insulating AlO_x layer of 1-2nm thicknesswas sputtered directly from A1_2O_3 target. Electrode layers were made of 80at. percent iron and20at. percent oxygen. Bottom Fe-O electrode deposited on glass substrate annealed at 473K at thepressure of 3 X 10^(-4) Pa for an hour shows disparate crystalline grain structure, lower electricalresistance and coercivity compared to the as-deposited top electrode. Only crystalline structrureof alpha-Fe is observed in both electrodes. Large tunnel magnetoresistance in large Fe-O/AlO_x/Fe-Ojunctions of 1cm^2 is observed at room temperature and the I-V characteristic curve of the junctionshows that the barrier of the junction is of high quality.

Effects of Fe-Oxide and Mg Layer Insertion on Tunneling Magnetoresistance Properties of CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

To study the influence of CoFeB/MgO interface on tunneling magnetoresistance （TMR）, different structures of magnetic tunnel junctions （MTJs） are successfully prepared by the magnetron sputtering technique and characterized by atomic force microscopy, a physical property measurement system, x-ray photoelectron spectroscopy, and transmission electron microscopy. The experimental results show that TMR of the CoFeB/Mg/MgO/CoFeB structure is evidently improved in comparison with the CoFeB/MgO/CoFeB structure because the inserted Mg layer prevents Fe-oxide formation at the CoFeB/MgO interface, which occurs in CoFeB/MgO/CoFeB MTJs. The inherent properties of the CoFeB/MgO/CoFeB, CoFeB/Fe-oxide/MgO/CoFeB and CoFeB/Mg/MgO/CoFeB MTJs are simulated by using the theories of density functions and non-equilibrium Green functions. The simulated results demonstrate that TMR of CoFeB/Fe-oxide/MgO/CoFeB MTJs is severely decreased and is only half the value of the CoFeB/Mg/MgO/CoFeB MTJs. Based on the experimental results and theoretical analysis, it is believed that in CoFeB/MgO/CoFeB MTJs, the interface oxidation of the CoFeB layer is the main reason to cause a remarkable reduction of TMR, and the inserted Mg layer may play an important role in protecting Fe atoms from oxidation, and then increasing TMR.

Tunneling magnetoresistance in ferromagnet/organic-ferromagnet/metal junctions

Spin-dependent transport in ferromagnet/organic-ferromagnet/metal junctions is investigated theoretically.The results reveal a large tunneling magnetoresistance up to 3230%by controlling the relative magnetization orientation between the ferromagnet and the central organic ferromagnet.The mechanism is explained by distinct efficient spin-resolved tunneling states in the ferromagnet between the parallel and antiparallel spin configurations.The key role of the organic ferromagnet in generating the large magnetoresistance is explored,where the spin selection effect is found to enlarge the difference of the tunneling states between the parallel and antiparallel configurations by comparing with the conventional organic spin valves.The effects of intrinsic interactions in the organic ferromagnet including electron–lattice interaction and spin coupling with radicals on the magnetoresistance are discussed.This work demonstrates a promising potential of organic ferromagnets in the design of high-performance organic spin valves.

Strain-mediated magnetoelectric control of tunneling magnetoresistance in magnetic tunneling junction/ferroelectric hybrid structures

Because of the wide selectivity of ferromagnetic and ferroelectric(FE)components,electric-field(E-field)control of magnetism via strain mediation can be easily realized through composite multiferroic heterostructures.Here,an MgO-based magnetic tunnel junction(MTJ)is chosen rationally as the ferromagnetic constitution and a high-activity(001)-Pb(Mg_(1/3)Nb_(2/3))_(0.7)Ti_(0.3)O_(3)(PMN-0.3PT)single crystal is selected as the FE component to create a multiferroic MTJ/FE hybrid structure.The shape of tunneling magnetoresistance(TMR)versus in situ E-fields imprints the butterfly loop of the piezo-strain of the FE without magnetic-field bias.The E-field-controlled change in the TMR ratio is up to-0.27%without magnetic-field bias.Moreover,when a typical magnetic field(~±10 Oe)is applied along the minor axis of the MTJ,the butterfly loop is changed significantly by the E-fields relative to that without magnetic-field bias.This suggests that the E-field-controlled junction resistance is spin-dependent and correlated with magnetization switching in the free layer of the MTJ.In addition,based on such a multiferroic heterostructure,a strain-gauge factor up to approximately 40 is achieved,which decreases further with a sign change from positive to negative with increasing magnetic fields.This multiferroic hybrid structure is a promising avenue to control TMR through E-fields in low-power-consumption spintronic and straintronic devices at room temperature.

基于E-J模型的超导/TMR复合磁传感器设计与实现

超导/隧道磁电阻(TMR)复合磁传感器具有fT级磁场测量能力,在弱磁探测领域展现出巨大应用潜力。本文基于超导材料的E-J幂指数模型,建立了超导磁放大器的COMSOL有限元仿真方法,并利用该方法研究了超导磁放大器的放大倍数随窄区宽度的变化规律。在此基础上,设计并制备了一种双窄区超导/TMR复合磁传感器,经测试结果表明:该复合磁传感器的最大磁场放大倍数达到97,且其双窄区比值与仿真结果相近,本文所提方法可行。

Gate-Voltage-Induced Magnetization Reversal and Tunneling Anisotropic Magnetoresistance in a Single Molecular Magnet with Temperature Gradient

We study the coutrol of gate voltage over the magnetization of a single-molecule magnet （SMM） weakly coupled to a ferromagnetic and a normal metal electrode in the presence of the temperature gradient between two electrodes. It is demonstrated that the SMM＇s magnetization can change periodically with periodic gate voltage due to the driving oI the temperature gradient. Under an appropriate matching of the electrode polarization, the temperature difference and the pulse width of gate voltage, the SMM＇s magnetization can be completely reversed in a period of gate voltage. The corresponding flipping time can be controlled by the system parameters. In addition, we also investigate the tunneling anisotropic magnetoresistance （TAMFt） of the device in the steady state when the ferromagnetic electrode is noncollinear with the easy axis of the SMM, and show the jump characteristic of the TAMR.

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

The tunneling conductance and tunneling magnetoresistance (TMR) are investigated inferromagnet/insulator/ferromagnet/insulator/d-wave superconductor (FM/I/FM/I/d-wave SC) structures by applyingan 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/1/FM, and noncollinear magnetizations in adja-cent magnetic layers on the TMR. It is shown (1) that the tunneling conductance and TMR exhibit amplitude-varyingoscillating 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 themagnetization of two FMs.

基于TMR传感器阵列的微电网过载电流检测方法

过载电流状态检测是微电网安全稳定运行的重要内容,针对单个传感器的测试误差问题以及测试过程中外界磁场的干扰问题,采用高灵敏度的TMR传感器构成传感阵列,减小微电网过载电流检测的测试误差.通过Ansoft Maxwell分析TMR传感器的安装位置,然后对TMR传感器阵列的模型进行了分析;最后通过测试实验,验证了所提测试方案的可行性.实验结果表明:TMR传感器阵列可以完成过载电流的检测,相比于单个TMR传感器,使用TMR传感器阵列可以将过载电流的峰值误差减少到0.12%.这对于微电网运行的状态监测提供了强有力的数据支撑.

Measurement of T wave in magnetocardiography using tunnel magnetoresistance sensor

Several critical clinical applications of magnetocardiography(MCG)involve its T wave.The T wave’s accuracy directly affects the diagnostic accuracy of MCG for ischemic heart disease and arrhythmogenic.Tunnel magnetoresistance(TMR)attracts attention as a new MCG measurement technique.However,the T waves measured by TMR are often drowned in noise.The accuracy of T waves needs to be discussed to determine the clinical value of MCG measured by TMR.This study uses an improved empirical mode decomposition(EMD)algorithm and averaging to eliminate the noise in the MCG measured by TMR.The MCG signals measured by TMR are compared with MCG measured by the optically pumped magnetometer(OPM)to judge its accuracy.Using the MCG measured by OPM as a reference,the relative errors in time and amplitude of the T wave measured by TMR are 3.4%and 1.8%,respectively.This is the first demonstration that TMR can accurately measure the time and amplitude of MCG T waves.The ability to provide reliable T wave data illustrates the significant clinical application value of TMR in MCG measurement.

磁传感器检测磁性纳米粒子方法研究

磁性纳米粒子表面修饰后,可以与核酸、蛋白质、重金属离子等物质结合,通过检测磁粒子的磁响应信号,可以得到待测物的含量。基于电磁学理论与隧道磁阻效应,设计了一种用于检测不同质量磁性纳米粒子磁信号的测试平台。利用COMSOL Multiphysics有限元分析软件优化了磁场产生装置,改进后的线圈在通入直流电流不变的情况下,能将磁场的均匀区域扩大1.5倍,数值提高2 Gs。新型三匝线圈的实验结果与仿真结果相比最大误差为6.25%。选择了适合常规样品的磁场检测装置,分析了各类传感器的最优测量方向,测得了磁粒子质量与输出信号的关系曲线。实验结果表明:随着悬浮液中磁粒子含量的增加,隧道磁阻(TMR)传感器的输出信号也呈上升趋势,两者为线性正比关系。

基于隧道磁电阻效应的宽频带电流无线测量装置

宽频带电气量数据包含大量故障暂态信息,传统互感器带宽受限难以精确全面地测量宽频暂态信号。针对该问题,提出一种含精确时标的非侵入式电流测量方法并设计原理样机。通过隧道磁电阻(TMR)芯片测量线路电流感生磁场,根据被测导体与TMR传感器的相对位置计算变比并通过微处理器进行录波和读数。设计低噪声、可变增益的传感器模组电路和暂态录波无线测量传输模组电路,精确感知宽频信号;根据传感器实际安装位置推导传变关系式,实现对一次电流的精确计算。搭建测试平台对测量装置直流、交流及暂态信号传变能力开展测试,在10 kV配电网进行接地故障电流测量对比实验。分析了影响磁阻传感器测量精度的主要因素,结果表明其直流和工频信号测量误差小于1%,高频信号测量误差小于3%。

基于环路积分型磁阻传感阵列的电流测量方法

电力系统是人类日常活动的重要基础设施,是最复杂的人工系统。为了维持电力系统稳定、可靠地运行,电力系统需要准确、可靠的状态监测,不同的状态监测情景需要有区别的测量方式以获得更精确的测量结果。目前,电力系统中电流的在线监测主要局限于室内场所的导线电流测量,对于电力系统中铜排工作电流的测量手段还较为缺乏。针对电力系统铜排电流监测的需求,以磁场作为耦合方式,需要重点解决两方面关键科学问题:1)矩形截面铜排通流电流后的磁场等效模型;2)基于通流铜排磁场的磁场测量、电流反演方案。本文基于毕奥萨伐尔定律,建立了理想情况下通电铜排磁场模型,研究了矩形截面铜排近场、远场的磁场;基于磁场计算,研究了电流传感器阵列包含的测量单元数量对电流测量的影响,优化了阵列结构;基于安培环路定律、均值算法,计算阵列电流反算结果与给定电流之间的误差,求得基于传感阵列的理论电流测量误差。在此基础上,设计了用于矩形铜排电流测量的矩形阵列、测量信息处理模块、数据通路及相关硬件电路,使用了独特的双层电路板结构以固定并定位多个隧道结磁阻效应传感单元。最后,搭建基于通电铜排的实验平台,利用阵列测量系统实现对实验室条件下铜排电流的测量。结果表明,本文设计的电流测量阵列能够实现1%以内的电流测量精度。

一种基于TMR的地磁场测试平台设计

针对微弱地磁场信号难以精确测量的问题,设计一种三维地磁信号测试平台。以STM32为主要控制芯片,采用隧道磁电阻(TMR)传感器采集微弱地磁信号,MPU6050传感器获取平台姿态角,经两级放大电路放大及失调补偿后进行A/D转换,最终由LCD实时显示电压值与姿态角并将其存入SD存储卡内。实验结果表明该地磁测试平台具有超高的灵敏度和信噪比、良好的低频特性等优点。该设计为微弱地磁信号的检测和获取提供一种可行的方法。

Anisotropic spin transport and photoresponse characteristics detected by tip movement in magnetic single-molecule junction

Orientation-dependent transport properties induced by anisotropic molecules are enticing in single-molecule junctions.Here,using the first-principles method,we theoretically investigate spin transport properties and photoresponse characteristics in trimesic acid magnetic single-molecule junctions with different molecular adsorption orientations and electrode contact sites.The transport calculations indicate that a single-molecule switch and a significant enhancement of spin transport and photoresponse can be achieved when the molecular adsorption orientation changes from planar geometry to upright geometry.The maximum spin polarization of current and photocurrent in upright molecular junctions exceeds 90%.Moreover,as the Ni tip electrode moves,the tunneling magnetoresistance of upright molecular junctions can be increased to 70%.The analysis of the spin-dependent PDOS elucidates that the spinterfaces between organic molecule and ferromagnetic electrodes are modulated by molecular adsorption orientation,where the molecule in upright molecular junctions yields higher spin polarization.Our theoretical work paves the way for designing spintronic devices and optoelectronic devices with anisotropic functionality base on anisotropic molecules.