Spin torque oscillator based on magnetic tunnel junction with MgO cap layer for radio-frequency-oriented neuromorphic computing

Recently,it has been proposed that spin torque oscillators(STOs)and spin torque diodes could be used as artificial neurons and synapses to directly process microwave signals,which could lower latency and power consumption greatly.However,one critical challenge is to make the microwave emission frequency of the STO stay constant with a varying input current.In this work,we study the microwave emission characteristics of STOs based on magnetic tunnel junction with MgO cap layer.By applying a small magnetic field,we realize the invariability of the microwave emission frequency of the STO,making it qualified to act as artificial neuron.Furthermore,we have simulated an artificial neural network using STO neuron to recognize the handwritten digits in the Mixed National Institute of Standards and Technology database,and obtained a high accuracy of 92.28%.Our work paves the way for the development of radio-frequency-oriented neuromorphic computing systems.

A Novel Design and Fabrication of Magnetic Random Access Memory Based on Nano-ring-type Magnetic Tunnel Junctions

Nano-ring-type magnetic tunnel junctions （NR-MTJs） with the layer structure of Ta（5）/Ir22Mn78（10）/ Co75Fe25（2）/Ru（0.75）/CoooFe20B20（3）/Al（0.6）-oxide/Co60Fe20B20（2.5）/Ta（3）/Ru（5） （thickness unit： nm） were nano-fabricated on the Si（100）/SiO2 substrate using magnetron sputtering deposition combined with the optical lithography, electron beam lithography （EBL） and Ar ion-beam etching techniques. The smaller NR-MTJs with the inner- and outer-diameter of around 50 and 100 nm and also their corresponding NR-MTJ arrays were nano-patterned. The tunnelling magnetoresistance （TMR ＆ R） versus driving current （I） loops for a spin-polarized current switching were measured, and the TMR ratio of around 35% at room temperature were observed. The critical values of switching current for the free Co60Fe20B20 layer relative to the reference Co6oFe2oB2o layer between parallel and anti-parallel magnetization states were between 0.50 and 0.75 mA in such NR-MTJs. It is suggested that the applicable MRAM fabrication with the density and capacity higher than 256 Mbit/inch2 even 6 Gbite/inch2 are possible using both I NR-MTJ＋1 transistor structure and current switching mechanism based on based on our fabricated 4×4 MRAM demo devices.

Two-dimensional XSe2（X = Mn, V） based magnetic tunneling junctions with high Curie temperature

Two-dimensional(2D) magnetic crystals have attracted great attention due to their emerging new physical phenomena. They provide ideal platforms to study the fundamental physics of magnetism in low dimensions. In this research,magnetic tunneling junctions(MTJs) based on XSe2(X = Mn, V) with room-temperature ferromagnetism were studied using first-principles calculations. A large tunneling magnetoresistance(TMR) of 725.07% was obtained in the MTJs based on monolayer MnSe2. Several schemes were proposed to improve the TMR of these devices. Moreover, the results of our non-equilibrium transport calculations showed that the large TMR was maintained in these devices under a finite bias.The transmission spectrum was analyzed according to the orbital components and the electronic structure of the monolayer XSe2(X = Mn, V). The results in this paper demonstrated that the MTJs based on a 2D ferromagnet with room-temperature ferromagnetism exhibited reliable performance. Therefore, such devices show the possibility for potential applications in spintronics.

Magnetic Tunnel Junction Based on MgO Barrier Prepared by Natural Oxidation and Direct Sputtering Deposition

Magnetic tunnel junctions(MTJs) based on MgO barrier have been fabricated by sputtering single crystal MgO target and metal Mg target, respectively, using magnetic sputtering system Nordiko 2000. MgO barriers have been formed by a multi-step deposition and natural oxidization of Mg layer. Mg layer thickness,oxygen flow rate and oxidization time were adjusted and the tunnel magnetoresistance(TMR) ratio of optimal MTJs is over 60% at annealing temperature 385. The(001) MgO crystal structure was obtained when the separation distance between MgO target and substrate is less than 6 cm. The TMR ratio of most MgO based MTJs are over 100% at the separation distance of 5 cm and annealing temperature 340. The TMR ratios of MTJs are almost zero when the separation distance ranges from 6 to 10 cm, due to the amorphous nature of the MgO film.

Perpendicular magnetic tunnel junction and its application in magnetic random access memory

Recent progresses in magnetic tunnel junctions with perpendicular magnetic anisotropy （PMA） are reviewed and summarized. At first, the concept and source of perpendicular magnetic anisotropy （PMA） are introduced. Next, a historical overview of PMA materials as magnetic electrodes, such as the RE-TM alloys TbFeCo and GdFeCo, novel tetragonal manganese alloys Mn-Ga, L10-ordered （Co, Fe）/Pt alloy, multilayer film [Co, Fe, CoFe/Pt, Pd, Ni, AU]N, and ultra-thin magnetic metal/oxidized barrier is offered. The other part of the article focuses on the optimization and fabrication of CoFeB/MgO/CoFeB p-MTJs, which is thought to have high potential to meet the main demands for non-volatile magnetic random access memory.

Detection of HIV-1 antigen based on magnetic tunnel junction sensors

We report a p24(HIV disease biomarker)detection assay using an MgO-based magnetic tunnel junction(MTJ)sensor and 20-nm magnetic nanoparticles.The MTJ array sensor with sensing area of 890×890μ2 possessing a sensitivity of 1.39%/Oe was used to detect p24 antigens.It is demonstrated that the p24 antigens could be detected at a concentration of 0.01μg/ml.The development of bio-detection systems based on magnetic tunnel junction sensors with high-sensitivity will greatly benefit the early diagnosis of HIV.

86% TMR at 4.2 K for Amorphous Magnetic-Tunnel-Junctions with Co_(60)Fe_(20)B_(20) as Free and Pinned Layers

Single barrier magnetic-tunnel-junctions (MTJs) with the layer structure of Ta(5)/Cu(30)/Ta(5)/Ni79Fe21(5)/Ir22 Mn78(12)/Co60Fe20B20(4)/Al(0.8)-oxide/Co60Fe20B20(4)/Cu(30)/Ta(5) [thickness unit: nm] using the amorphous Co60Fe20B20 alloy as free and pinned layers were micro-fabricated. The experimental investigations showed that the tunnel magnetoresistance (TMR) ratio and the resistance decrease with increasing dc bias voltage from 0 to 500 mV or with increasing temperature from 4.2 K to RT. A high TMR ratio of 86.2% at 4.2 K, which corresponds to the high spin polarization of Co60Fe20B20, 55%, was observed in the MTJs after annealing at 270℃ for 1 h. High TMR ratio of 53.1%, low junction resistance-area product RS of 3.56 kΩμm2, small coercivity HC of ≤4 Oe, and relatively large bias-voltage-at-half-maximum TMR with the value V1/2 of greater than 570 mV at RT have been achieved in such Co-Fe-B MTJs.

An Explicit Function Expression for dc Bias and Temperature Dependence of Magnetoresistances in Magnetic Tunnel Junctions

An explicit function expression for the bias voltage or/and temperature dependences of tunnel magnetoresistance ratio and resistances were obtained with a unique set of intrinsic parameters. Two of these intrinsic parameters are the Curie temperature TC and the density of state (DOS) for itinerant majority and minority electrons ξ(ρM/ρm), which are the eigen parameters of ferromagnetic electrodes. Others are the spin-dependent matrix-element ratio (i.e., |Td|2/|TJ|2 ) and the anisotropic-wavelength-cutoff energy ECγ of spin-wave spectrum in magnetic tunnel junction (MTJ), which are the structure parameters of an MTJ. These intrinsic parameters can be predetermined using the experimental measurement or, in principle, using the first-principle calculation method for an MTJ with the three key layers of FM/I/FM. Furthermore, a series of experimental data for an MTJ, for example, a spin-valve-type MTJ of Ta (5 nm)/Ni79Fe21(25 nm)/lr22Mn78(12 nm)/Co75Fe25(4 nm)/AI(0.8 nm)-oxide/Co75Fe25(4 nm)/Ni79Fe21(20 nm)/Ta (5 nm) in this work, can be self-consistently evaluated and explained using such concise explicit function formulations.

Low frequency noise in asymmetric double barrier magnetic tunnel junctions with a top thin MgO layer

Low frequency noise has been investigated at room temperature for asymmetric double barrier magnetic tunnel junctions（DBMTJs）, where the coupling between the top and middle CoFeB layers is antiferromagnetic with a 0.8-nm thin top Mg O barrier of the CoFeB/MgO/CoFe/CoFeB/MgO/CoFe B DBMTJ. At enough large bias, 1/f noise dominates the voltage noise power spectra in the low frequency region, and is conventionally characterized by the Hooge parameter αmag.With increasing external field, the top and bottom ferromagnetic layers are aligned by the field, and then the middle free layer rotates from antiparallel state（antiferromagnetic coupling between top and middle ferromagnetic layers） to parallel state. In this rotation process αmag and magnetoresistance-sensitivity-product show a linear dependence, consistent with the fluctuation dissipation relation. With the magnetic field applied at different angles（θ） to the easy axis of the free layer,the linear dependence persists while the intercept of the linear fit satisfies a cos（θ） dependence, similar to that for the magnetoresistance, suggesting intrinsic relation between magnetic losses and magnetoresistance.

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 Biittiker 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.

Thermally activated magnetization reversal in magnetic tunnel junctions

In this paper, the magnetization reversal of the ferromagnetic layers in the IrMn/CoFe/AlOx/CoFe magnetic tunnel junction has been investigated using bulk magnetometry. The films exhibit very complex magnetization processes and reversal mechanism. Thermal activation phenomena such as the training effect, the asymmetry of reversal, the loop broadening and the decrease of exchange field while holding the film at negative saturation have been observed on the hysteresis loops of the pinned ferromagnetic layer while not on those of the free ferromagnetic layer. The thermal activation phenomena observed can be explained by the model of two energy barrier distributions with different time constants.

Tantalum oxide barrier in magnetic tunnel junctions

Tantalum as an insulating barrier can take the place of Al in magnetic tunnel junctions (MTJs). Ta barriers in MTJs were fabricated by natural oxidation. X-ray photoelectron spectroscopy (XPS) was used to characterize the oxidation states of Ta barrier. The experimental results show that the chemical state of tantalum is pure Taand the thickness of the oxide is 1.3 nm. The unoxidized Ta in the barrier may chemically reacted with NiFe layer which is usually used in MTJs to form an intermetallic compound, NiTa. A magnetic "dead layer" could be produced in the NiFe/Ta interface. The "dead layer" is likely to influence the spinning electron transport and the magnetoresistance effect.

Experiments and SPICE simulations of double MgO-based perpendicular magnetic tunnel junction

We investigate properties of perpendicular anisotropy magnetic tunnel junctions(pMTJs) with a stack structure MgO/CoFeB/Ta/CoFeB/MgO as the free layer(or recording layer),and obtain the necessary device parameters from the tunneling magnetoresistance(TMR) vs.field loops and current-driven magnetization switching experiments.Based on the experimental results and device parameters,we further estimate current-driven switching performance of pMTJ including switching time and power,and their dependence on perpendicular magnetic anisotropy and damping constant of the free layer by SPICE-based circuit simulations.Our results show that the pMTJ cells exhibit a less than 1 ns switching time and write energies <1.4 pJ;meanwhile the lower perpendicular magnetic anisotropy(PMA) and damping constant can further reduce the switching time at the studied range of damping constant α <0.1.Additionally,our results demonstrate that the pMTJs with the thermal stability factor■73 can be easily transformed into spin-torque nano-oscillators from magnetic memory as microwave sources or detectors for telecommunication devices.

Tantalum oxide barrier in magnetic tunnel junctions

Tantalum as an insulating barrier can take the place of Al in magnetic tunneljunctions (MTJs). Ta barriers in MTJs were fabricated by natural oxidation. X-ray photoelectronspectroscopy (XPS) was used to characterize the oxidation states of Ta barrier. The experimentalresults show that the chemical state of tantalum is pure Ta^(5+) and the thickness of the oxide is1.3 nm. The unoxidized Ta in the barrier may chemically reacted with NiFe layer which is usuallyused in MTJs to form an intermetallic compound, NiTa_2. A magnetic 'dead layer' could be produced inthe NiFe/Ta interface. The 'dead layer' is likely to influence the spinning electron transport andthe magnetoresistance effect.

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.

A Self-consistent Calculation and an Anisotropic Wavelength Cut off Energy of Spin-wave Spectrumin Magnetic Tunnel Junctions

Temperature dependence of tunnel magnetoresistance (TMR) ratio, resistance, and coercivity from 4.2 K to room temperature (RT), applied de bias voltage dependence of the TMR ratio and resistances at 4.2 K and RT, tunnel current I and dynamic conductance dI/dV as functions of the de bias voltage at 4.2 K, and inelastic electron tunneling (IET) spectroscopy, d(2)I/dV(2) versus V, at 4.2 K for a tunnel junction of Ta(5 nm)/Ni79Fe21(25 nm)/Ir22Mn78(12 nm)/Co75Fe25(4 nm)/Al(0.8 nm)-oxide/Co75Fe25(4 nm)/Ni79Fe21(25 nm)/Ta(5 nm) were systematically investigated. High TMR ratio of 59.2% at 4.2 K and 41.3% at RT were observed for this junction after annealing at 275 degreesC for an hour. The temperature dependence of TMR ratio and resistances from 4.2 to 300 K at 1.0 mV bias and the de bias voltage dependence of TMR ratio at 4.2 K from 0 to 80 mV can be evaluated by a comparison of self-consistent calculations with the experimental data based on the magnon-assisted inelastic excitation model and theory. An anisotropic wavelength cutoff energy of spin-wave spectrum in magnetic tunnel junctions (MTJs) was suggested, which is necessary for self-consistent calculations, based on a series of IET spectra observed in the MTJs.

Dependence of switching process on the perpendicular magnetic anisotropy constant in P-MTJ

We investigate the dependence of the switching process on the perpendicular magnetic anisotropy （PMA） constant in perpendicular spin transfer torque magnetic tunnel junctions （P-MTJs） using micromagnetic simulations. It is found that the final stable states of the magnetization distribution of the free layer after switching can be divided into three different states based on different PMA constants： vortex, uniform, and steady. Different magnetic states can be attributed to a trade-off among demagnetization, exchange, and PMA energies. The generation of the vortex state is also related to the non-uniform stray field from the polarizer, and the final stable magnetization is sensitive to the PMA constant. The vortex and uniform states have different switching processes, and the switching time of the vortex state is longer than that of the uniform state due to hindrance by the vortex.

Two-dimensional magnetic materials for spintronic applications

Spintronic devices are driving new paradigms of bio-inspired,energy efficient computation like neuromorphic stochastic computing and in-memory computing.They have also emerged as key candidates for non-volatile memories for embedded systems as well as alternatives to persistent memories.To meet the growing demands from such diverse applications,there is need for innovation in materials and device designs which can be scaled and adapted according to the application.Two-dimensional(2D)magnetic materials address challenges facing bulk magnet systems by offering scalability while maintaining device integrity and allowing efficient control of magnetism.In this review,we highlight the progress made in experimental studies on 2D magnetic materials towards their integration into spintronic devices.We provide an account of the various relevant material discoveries,demonstrations of current and voltage-based control of magnetism and reported device systems,while also discussing the challenges and opportunities towards integration of 2D magnetic materials in commercial spintronic devices.

CrCl_(3)隧穿磁阻的界面效应与多场效应调控

磁隧道结是研究磁性材料自旋结构、输运特性、磁相变和磁各向异性的重要实验平台.本研究基于干法转移技术制备了以机械剥离的少层范德瓦耳斯反铁磁绝缘体三氯化铬(CrCl_(3))为势垒层、少层石墨烯为电极的磁隧道结原型器件结构,并进行了低温电磁输运测量,除观测到自旋过滤效应引起的隧穿磁阻外,还发现多种由非传统效应引起的磁阻变化.基于对隧道结自旋结构和能带结构的分析,本文将之归因于由磁近邻效应引起的隧穿机制改变,以及石墨烯电极态密度在高磁场下出现的量子振荡行为.本文报道了在二维磁隧道结中与隧穿磁阻相关且此前未被广泛关注的物理现象,加深了对此类二维异质结构中载流子输运特性的理解,为二维磁性材料的物理性质研究及其自旋电子学应用拓展了新的途径.

基于磁性隧道结和双组分多铁纳磁体的超低功耗磁弹模数转换器

本文提出了一种由8个磁性隧道结(magnetic tunnel junction,MTJ)构成的3位磁弹模数转换器(magnetoelastic analog-to-digital converter,MEADC),该转换器中MTJ自由层为双组分多铁纳磁体.通过对多铁纳磁体实施应变介导的电压调控,可以实现零场条件下的确定性磁化翻转.研究发现:对于给定尺寸,给定材料的双组分多铁纳磁体,压电层厚度与双组分多铁纳磁体的临界翻转电压线性相关.基于该原理,通过调整压电层的厚度使得MEADC具有8个不同的电压切换阈值,将模拟信号转换为8个多铁MTJ不同磁化状态组合.同时,设计了锁存比较器和独立的读取电路来检测MTJ的阻态,以此实现了数字信号的输出.Monte Carlo功能模拟表明:该MEADC在室温下写入成功率可达100%;此外,读写电路相互分离,使得压电层厚度与MTJ的输出参考电压无关,因此每个MTJ可设置相同的参考电压,从而具有更高的读取可靠性.微磁仿真和数值模拟分析发现:该MEADC的工作频率可达250 MHz,单次转换能耗仅为20 aJ;与基于Racetr ack技术的磁模数转换器相比,能耗降低了1000倍,采样速率提高了10倍.本文提出的MEADC可为基于自旋电子器件的存算一体电路架构提供重要的技术支撑.