4英寸热氧化硅衬底上磁性隧道结的微制备

就如何在4英寸热氧化硅衬底上沉积高质量的磁性隧道结纳米多层薄膜材料和如何利用光刻方法微加工制备均匀性较好的磁性隧道结方面做了初步研究，并对磁性隧 道结的磁电性质及其工作特性进行了初步测量和讨论.利用现有的光刻设备和工艺条 件在4英寸热氧化硅衬底上直接制备出的磁性隧道结，其结电阻与面积的积 矢的绝对误差在10% 以内，隧穿磁电阻的绝对误差在7% 以内，样品的磁性隧道结性质具有较好的均匀性和一致性，可以满足研制磁随机存储器存储单元演示器件的基本要求. 关键词： 磁性隧道结 隧穿磁电阻 磁随机存储器 4英寸热氧化硅衬底     

自旋电子学材料、物理和器件设计原理的研究进展

文章介绍了作者所在实验室在巨磁电阻(GMR)、隧穿磁电阻(TMR)、庞磁电阻(CMR)和反铁磁钉扎薄膜材料以及单晶金属氧化物、高自旋极化率材料、P-N异质结和纳米环磁随机存储器原理型演示器件设计等研究方面取得的一些重要研究成果和进展.例如:在Al-O势垒磁性隧道结材料体系里,获得室温磁电阻超过80%的国际最好结果;获得两种高性能层状反铁磁钉扎材料体系;发现具有大的电致电阻效应的CMR薄膜材料,并可期望用于电流直接进行磁信息写和读操作的磁存储介质;发现双势垒磁性隧道结中的量子阱态共振隧穿和磁电阻振荡效应,以及纳米器件体系中自旋翻转长度的观测新方法,可用于新型自旋电子学材料及相关器件的人工辅助设计;利用电子自旋共振谱探测和研究了金属氧化物的微观自旋结构和各向异性;在[CoFe/Pt]n磁性金属多层膜中,观测到超高灵敏度的反常霍尔效应;利用纳米环状磁性隧道结作为存储单元,研制出一种新型纳米环磁随机存储器MRAM原理型演示器件.     

CoFeB/AlOx/Ta及AlOx/CoFeB/Ta结构中垂直易磁化效应的研究

本文详细研究了在不同氧化层和铁磁层厚度情况下, 底层CoFeB/AlO_x/Ta结构和 顶层AlO_x/CoFeB/Ta结构中的垂直磁各向异性. 在底层CoFeB/AlO_x/Ta结构中观察到了垂直磁化的磁滞回线, 证明了其垂直易磁化效应的存在; 而在顶层AlO_x/CoFeB/Ta结构中却没有观察到类似的磁滞回线. 对这种对称结构中的非对称现象进行了分析. 研究还发现不同的氧化层和铁磁层厚度均会影响层间界面相互作用的强度, 从而导致结构的垂直磁化曲线矫顽力大小发生改变. 这项研究将对基于AlO_x氧化层垂直磁隧道结的研制具有重要的意义. 关键词： 垂直磁各向异性 磁隧道结 随机存储器     

利用金属掩模法制备钉扎型磁性隧道结

利用金属掩模法和Ir22Mn78合金反铁磁钉扎层，制备了四种钉扎型的Py/Al2O3/Py，Py/Al2O3/Co，Co/Al2O3/Py和Co/Al2O3/Co磁性隧道结，坡莫合金的成分为Py＝Ni79Fe21.例如：利用狭缝宽度为100?μm的金属掩模，直接制备出室温隧穿磁电阻比值为17.2%的磁性隧道结Co/Al2O3/Co，其结电阻为76Ω,结电阻和结面积的积矢为76×10_{4Ωμm2，自由层的偏转场为1114?A/m，并且在外加磁场0.1114A·m-1之间时室温磁电阻比值 关键词： 磁性隧道结 隧穿磁电阻 磁随机存储器 金属掩模}     

磁随机存储器中垂直电流驱动的磁性隧道结自由层的磁化翻转

在基于磁性隧道结(Magnetic Tunneling Junction，MTJ)的磁随机存储器(Magnetoresistantive Random Access Memory，MRAM)中利用通过MTJ的垂直电流，实现信息写入的新方法，同时给出了基于此新方法的一种新的MRAM结构和驱动原理图，并分析了它的读和写操作的可行性具体过程. 关键词： 垂直电流 磁性隧道结 磁随机存储器     

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     

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     

具有纳米氧化层的磁性隧道结的热稳定性研究

研究发现在磁隧道结的反铁磁层和被钉扎铁磁层之间插入一层纳米氧化层，可以使磁隧道结的退火温度增加了40℃，即明显地提高了磁隧道结的温度稳定性.通过卢瑟福背散射实验直接观察到产生这一效应的原因是该纳米氧化层有效地抑制了Mn元素在退火过程中的扩散，从而使TMR值在较高的退火温度下得以保持. 关键词： 磁性隧道结 隧穿磁电阻 热稳定性 纳米氧化层     

退火对IrMn基磁隧道结多层膜热稳定性的影响

采用磁控溅射方法制备了结构为IrMn/CoFe/AlO_x/CoFe的磁性隧道结多层膜,样品置于真空磁场中进行退火处理. 将在不同温度退火的磁隧道结结构多层膜置于负饱和场中等待,研究退火温度对样品热稳定性的影响. 结果表明:退火提高了多层膜反铁磁层的单轴各向异性能,增加了样品的交换偏置;随着负饱和场等待时间的延长,被钉扎层的磁滞回线向正场偏移,交换偏置单调减小,但退火减弱了这种趋势. 关键词： 磁隧道结 交换偏置 磁化反转     

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     

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.     

Spin-dependent tunneling in magnetic tunnel junctions with a layered antiferromagnetic Cr(001) spacer: role of band structure and interface scattering

The tunnel magnetoresistance effect (TMR), which is intrinsically determined by the interface monolayer of an electrode, was realized by using magnetic tunnel junctions (MTJs) with a single-crystal Cr(001) layer inserted between a tunnel barrier and an electrode. The MTJs showed an oscillation of the TMR ratio as a function of the thickness of the Cr(001) layer with a period of 2 monatomic layers, which corresponds to the layered antiferromagnetic structure of Cr(001). These oscillations originate from electron scattering at the interface, due to the mismatching of the symmetry of the wave functions and band structure in Cr(001).     

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.     

Magnetic tunnel junctions comprising amorphous NiFeSiB and CoFeSiB free layers

Magnetic and magneto-transport behaviors of magnetic tunnel junctions (MTJs) incorporating newly developed amorphous NiFeSiB and CoFeSiB as free layers were investigated. Both experimental and theoretical approaches were carried out to understand the details of switching characteristics. The MTJs with traditional free layer materials such as NiFe and CoFe were also fabricated and compared. The studied amorphous ferromagnets appeared beneficial for reducing switching fields without loosing tunnelling magnetoresistive outputs. Further layer thickness, composition, and MTJ structure optimization are expected.     

Non-equilibrium Green's function approach to the quasiparticle transport and magnetoresistance in hybrid magnetic tunnel junctions

A generalized approach to study quasiparticle transport across hybrid magnetic tunnel junctions (MTJs) is formulated using the non-equilibrium Green's function technique. This formalism allows for arbitrary thicknesses of the electrodes and the central scattering region comprising of materials with multiple electronic bands, and incorporates the many body interactions present in the electrode regions. While the method can be used to study the transport characteristics of various types of MTJs, we have used it to study the tunneling characteristics and magnetoresistance (MR) of MTJs in which s-f interaction is present at the electrode layers. It is also used to study the transport characteristics of MTJs with hybrid electrodes and double barrier. The magnetic correlation present in the electrodes is found to strongly influence the TMR. Eventhough the magnetic correlation in general suppress the TMR, the TMR is found to be enhanced strongly for certain band occupations of the electrodes. We observe a fall of TMR with increase in the number of layers in the insulating region. Band occupation of the metallic layer present at the middle of the insulating layers in the double barrier MTJ is found to be important in deciding its tunnel characteristics. Origin of the different types of behavior of TMR is analyzed in terms of the spin-dependent tunnel currents.     

Kondo effect in magnetic tunnel junctions

Tunneling magnetoresistance was found to be suppressed with decreasing temperature for magnetic tunnel junctions (MTJs) oxidized under high plasma power. A strong temperature dependence of the junction resistance was observed, along with zero-bias anomalies of dynamic resistance at low temperatures. Resistance shows a logarithmic dependence on temperature, and resistance versus temperature exhibits a scaling behavior. Our experimental data can be explained in a consistent way by the Kondo effect in the MTJs with the Kondo temperature TK=20-30 K.     

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.     

Two-dimensional XSe_2(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 X Se_2(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 MnSe_2. 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 X Se_2(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.     

Controlling the demagnetizing field of an in-plane free layer by exchange coupling with a perpendicular multilayer

The demagnetizing field of a Co₅₀Fe₅₀ free layer in an in-planemicron-sized magnetic tunnel junction (MTJ) can be partially compensated by exchangecoupling with a [Co₉₀Fe₁₀/Pt] _N multilayer with perpendicular magnetic anisotropy via a Ru interlayer. The perpendicularanisotropy for N = 5 is optimized for nominal CoFe and Pt thicknessof 0.4 nm and 1.0 nm, respectively. An increase of tunnel magnetoresistance (TMR) from 2%to 75% is observed in MTJs as the free layer thickness, t variesfrom 1.0 nm to 3.0 nm. A phenomenological model is developed to interpret the TMRdependence in terms of the free layer magnetization rotation from in-plane to out-of-planewith decreasing t, a consequence of interlayer exchange coupling with theperpendicular multilayer. We suggest that this strategy could significantly reduce theswitching current density in such MTJs.