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 $-$0.27% without magnetic-field bias. Moreover, when a typical magnetic field ($\sim \pm 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.     

Influence of interface within the composite barrier on the tunneling electroresistance of ferroelectric tunnel junctions with symmetric electrodes

The interface with a pinned dipole within the composite barrier in a ferroelectric tunnel junction（FTJ） with symmetric electrodes is investigated.Different from the detrimental effect of the interface between the electrode and barrier in previous studies,the existence of an interface between the dielectric SrTiO₃ slab and ferroelectric BaTiO₃ slab in FTJs will enhance the tunneling electroresistance（TER） effect.Specifically,the interface with a lower dielectric constant and larger polarization pointing to the ferroelectric slab favors the increase of TER ratio.Therefore,interface control of high performance FTJ can be achieved.     

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     

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     

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.     

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.     

Multiferroic and magnetoelectric properties of MnFe2O4/(Pb0.8Sr0.2)TiO3 composite films

Abstract

MnFe₂O₄/(Pb_0.8Sr_0.2)TiO₃ (MFO/PST20) heterostructured composite films with three different structures have been grown on Pt/TiO₂/SiO₂/Si substrates by metal–organic decomposition processing via spin coating technique. The structural analysis revealed that the crystal axes of the MnFe₂O₄ are aligned with those of the PST20 ferroelectric matrix with obvious interfaces and no diffusions exist in all the three composite films. These composite films exhibit simultaneously multiferroic and magnetoelectric responses at room temperature. The growth structure of MFO and PST20 layers has an effect on multiferroic and magnetoelectric coupling behaviours of the composite films. The bi- and four-layered MFO/PST20 composite films exhibit superior ferroelectric properties compared to the tri-layered film. The increasing MFO and PST20 layers in the composite films enhance ferromagnetic properties and are closely related to the strain release in MnFe₂O₄ phase. The MFO/PST20 bi-layered composite film shows a high magnetoelectric voltage co-efficient α_E ~ 194 mVcm^?1Oe^?1 at a dc magnetic field H_dc ~ 2.5 kOe. A significant decrease in α_E value has been observed for tri- and four- layered composite films. A close correlation between phase selective residual stress and magnetoelectric properties has been emerged. The results are reasonably encouraging for employing MnFe₂O₄ for growing multiferroic–magnetoelectric composite films.     

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     

单相ABO3型多铁材料的磁电耦合及磁电性质研究

实验发现多铁性钙钛矿物质YMnO₃和BiMnO₃在接近磁有序相变温度时,其介电常数和正切损失会出现异常,这些现象说明在物质的磁性和介电性质之间存在耦合.通过对系统磁性和铁电性之间可能磁电耦合方式的分析,考虑在系统哈密顿量中加入与自旋关联和极化相关的耦合项,对铁电子系统应用软模理论,对磁性运用基于海森伯模型的量子平均场近似,研究了外磁场诱导的极化、介电的变化和外电场诱导的磁化的变化等,并将以上结果与实验进行了比较和分析,较为合理地解释了一些多铁钙钛矿物质中的磁电现 关键词： 多铁 磁电耦合 铁电 铁磁     

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.     

Magnetization reversal mechanism of magnetic tunnel junctions

Using the ion-beam-sputtering technique,we have fabricated Fe/Al2O3/Fe magnetic tunnelling junctions (MTJs).We have observed double-peaked shapes of curves,which have a level summit and a symmetrical feature,showing the magnetoresistance of the junction as a function of applied field.We have measured the tunnel conductance of MTJs which have insulating layers of different thicknesses.We have studied the dependence of the magnetoresistance of MTJs on tunnel conductance.The microstructures of hard-and soft-magnetic layers and interfaces of ferromagnets and insulators were probed.Analysing the influence of MJT microstructures,including those having clusters or/and granules in magnetic and non-magnetic films,a magnetization reversal mechanism(MRM) is proposed,which suggests that the MRM of tunnelling junctions may be explained by using a group-by-group reversal model of magnetic moments of the mesoscopical particles.We discuss the influence of MTJ microstructures,including those with clusters or/and granules in the ferromagnetic and non-magnetic films,on the MRM.     

Electrical conductance properties for magnetic tunnel junctions with MgO barriers

We measured inelastic electron tunneling (IET) spectra and conductance for MgO tunneling magnetoresistance (TMR) films to obtain information on the ferromagnetic/barrier layer interface. The IET spectra showed the difference between amorphous and crystalline structures in the barrier. In the magnetic tunnel junction (MTJ) with a crystalline barrier the IET spectra indicated an Mg-O phonon peak at a low bias voltage by measurement with a parallel magnetization configuration. On the other hand, no peak was observed in the MTJ with an amorphous barrier.     

Colossal magnetoresistance in manganites and related prototype devices＊

We review colossal magnetoresistance in single phase manganites, as related to the field sensitive spin-charge interactions and phase separation; the rectifying property and negative/positive magnetoresistance in manganite/Nb：SrTio3 p-n junctions in relation to the special interface electronic structure; magnetoelectric coupling in manganite/ferroelectric structures that takes advantage of strain, carrier density, and magnetic field sensitivity; tunneling magnetoresistance in tunnel junctions with dielectric, ferroelectric, and organic semiconductor spacers using the fully spin polarized nature of manganites; and the effect of particle size on magnetic properties in manganite nanoparticles.     

磁性隧道结Ni80Fe20/Al2O3/Co的制备和物性

用等离子体氧化形成绝缘层的方法,重复性地制备出了Ｎｉ８０Ｆｅ２０／Ａｌ２Ｏ３／Ｃｏ磁性隧道结。样品的隧道磁电阻（ＴＭＲ）比值在室温下最高可达６．０％,翻转场（ｓｗｉｔｃｈ ｆｉｅｌｄ）可低于８００Ａ／ｍ,平台宽度约２４００Ａ／ｍ。结电阻的变化范围从几百欧姆到几百千欧。     

Output voltage calculations in double barrier magnetic tunnel junctions with asymmetric voltage behavior

In this paper we study the asymmetric voltage behavior (AVB) of the tunnel magnetoresistance (TMR) for single and double barrier magnetic tunnel junctions (MTJs) in range of a quasi-classical free electron model. Numerical calculations of the TMR–V curves, output voltages and I–V characteristics for negative and positive values of applied voltages were carried out using MTJs with CoFeB/MgO interfaces as an example. Asymmetry of the experimental TMR–V curves is explained by different values of the minority and majority Fermi wave vectors for the left and right sides of the tunnel barrier, which arises due to different annealing regimes. Electron tunneling in DMTJs was simulated in two ways: (i) Coherent tunneling, where the DMTJ is modeled as one tunnel system and (ii) consecutive tunneling, where the DMTJ is modeled by two single barrier junctions connected in series.     

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     

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.     

Temperature dependence of transport properties in ZnS-based magnetic tunnel junctions

We have recently evidenced a junction magnetoresistance (JMR) signal of about 5% in magnetic tunnel junctions (MTJs) with ZnS as tunnel barrier layer. The MTJ were grown by magnetron sputtering on Si (1 1 1) substrate at room temperature and have the following structure: Fe_6 nmCu_30 nmCoFe_1.8 nmRu_0.8 nmCoFe_3 nmZnS_2 nmCoFe_1 nmFe_4 nmCu_10 nmRu_3 nm.

The hard magnetic bottom electrode consists of an artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through an Ru spacer layer. The magneto-transport for these MTJ has been studied at various temperatures to gain understanding of the transport mechanism in such junctions. A strong and linear increase of the JMR is observed as the temperature is decreased to reach 10% at a low temperature, while the conductance decreases with decreasing temperature. To understand the mechanism at the origin of these behaviors, the contribution of magnon is taken into account. It is concluded that the observed behaviors are not only related to the magnon contribution but that resonant low-level states inside the barrier can assist the tuneling transport.     

多层结构双自旋过滤隧道结中的电子输运特性

在转移矩阵方法及Mireles和Kirczenow的量子相干输运理论的基础上,研究了正常金属层/磁性半导体层/非磁绝缘层/磁性半导体层/正常金属层型双自旋过滤隧道结中Rashba自旋轨道耦合效应和自旋过滤效应对自旋相关输运的影响.讨论了隧穿磁电阻（TMR）、隧穿电导与各材料层厚度、Rashba自旋轨道耦合强度以及两磁性半导体中磁矩的相对夹角θ之间的关系.研究表明：含磁性半导体层的双自旋过滤隧道结由于磁性半导体层的自旋过滤效应和Rashba自旋轨道耦合作用可获得极大的TMR值.另外TMR和隧穿电导随着Rashba自旋轨道耦合强度的变化而振荡,振荡周期随Rashba自旋轨道耦合强度的增大逐渐减小. 关键词： 双自旋过滤隧道结 Rashba自旋轨道耦合 隧穿磁电阻 隧穿电导     

自支撑多铁性薄膜材料研究的机遇与挑战

多铁性材料兼具铁电、铁磁等两种或两种以上铁性有序,并且通过不同铁性之间的多物理场耦合实现新奇磁电效应,在信息存储、换能、传感等方面具有广阔的应用前景。当前,在基础及应用研究方面仍存在许多亟待解决的科学技术问题,譬如寻找及制备室温以上具有强磁电耦合效应的多铁性材料,以及解决与半导体的器件集成问题等,而近年来迅速发展的自支撑薄膜制备技术为此提供了新的机遇。与块体材料及束缚在刚性衬底上的外延薄膜相比,自支撑薄膜具有极为优异的晶格调控自由度,可获得前所未有的极端一维、二维应变（应变梯度）,并能够实现人工异质结的构建与器件集成等,为多铁性材料的研究提供了新的材料体系和契机。文章围绕自支撑钙钛矿氧化物（多）铁性材料及人工异质结,总结了最近的重要研究进展,并尝试初步探讨该方向未来可能面临的机遇与挑战。