电子全息对磁隧道结势垒层的研究

利用电子全息显微学方法,从理论和实验两方面,系统研究了磁隧道结势垒层的内势分布,指出了一些在实验过程中应予以注意的实验现象,提出了可能的解决方法.关键词：电子全息磁隧道结平均内势     

具有不同交换偏置方向的外延FeGa/IrMn双层膜的磁各向异性与磁化翻转

采用磁控溅射方法在MgO(001)单晶衬底上制备了交换偏置分别沿着FeGa [100]和[110]方向的FeGa/IrMn外延交换偏置双层膜,研究了交换偏置取向对磁化翻转过程与磁化翻转场的影响.铁磁共振场的角度依赖关系的测量与拟合,表明样品存在不同取向的四重对称磁晶各向异性、单向交换磁各向异性和单轴磁各向异性的叠加.矢量磁光克尔效应测量表明交换偏置沿着[100]方向的样品在不同磁场方向下表现矩形、非对称和单边两步磁滞回线;交换偏置沿着[110]方向的样品在不同磁场方向下表现单边两步和双边两步磁滞回线.考虑不同交换偏置方向的畴壁形核和位移模型,能够很好地解释磁化翻转路径随磁场方向的变化规律和拟合磁化翻转场的角度依赖关系,表明交换偏置方向的改变使得畴壁形核能发生显著变化.     

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     

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     

金属-氧化物界面在磁隧道结中的作用

磁隧道结通常是指由两层磁性金属和它们所夹的一层氧化物绝缘层 (Ⅰ )所组成的三明治结构 .通过绝缘层势垒的隧穿电子是自旋极化的 ,这种自旋极化能够反映作为电极的铁磁金属费米能级处态密度 (ＤＯＳ)的变化 .起源于自旋极化的隧道磁阻(ＴＭＲ)效应已成为当代磁学的一个研究热点 .尽管目前在ＴＭＲ的研究和应用方面已有所发展 ,但在理解隧穿自旋极化方面还存在一定的分歧 ,争论的焦点是金属 -氧化物界面对自旋极化的影响 .对于Ｃｏ、Ｎｉ等铁磁金属 ,由于多数自旋电子所处的ｄ带位于费米能级之下 ,理论预期其自旋极化应该是负的 .然而Ｆ/Ａ…     

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     

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.     

基于工艺偏差的电压调控磁各向异性磁隧道结电学模型及其在读写电路中的应用

电压调控磁各向异性磁隧道结(voltage controlled magnetic anisotropy magnetic tunnel junction, VCMA-MTJ)作为磁随机存储器(magnetic random access memory, MRAM)的核心器件,具有读写速度快、功耗低、与CMOS工艺相兼容等优点,现已得到国内外学者的广泛关注.然而随着VCMA-MTJ尺寸不断缩小、MRAM存储容量不断增大,工艺偏差对MTJ性能的影响变得越来越显著,甚至会引起VCMA-MTJ电路的读写错误.本文在充分考虑磁控溅射薄膜生长工艺中自由层厚度偏差(γ_tf)、氧化势垒层厚度偏差(γ_tox)以及离子束刻蚀工艺中由侧壁再沉积层引入的刻蚀工艺稳定因子(α)偏差影响的情况下,给出了基于工艺偏差的VCMA-MTJ电学模型,并将该模型应用到VCMA-MTJ读写电路中,研究了工艺偏差对上述电路读写错误率的影响.结果表明:当γ_tf≥13%, γ_tox≥11%时, VCMA-MTJ将无法实现磁化状态的有效切换;当...     

反铁磁耦合硬磁-软磁-硬磁三层膜体系的不可逆交换弹性反磁化过程

采用一维原子链模型研究了反铁磁耦合的硬磁/软磁/硬磁三层膜体系的反磁化过程. 研究结果表明,当考虑了软磁层的磁晶各向异性能后,软磁层厚度和界面交换耦合强度的改变都有可能导致软磁层的交换弹性反磁化过程由可逆过程转变为不可逆过程. 对软磁层很薄的体系,其反磁化过程是典型的可逆交换弹性反磁化过程. 然而,当软磁层厚度超过某一临界厚度t_c时,反磁化过程转变为不可逆的交换弹性反磁化过程. 软磁-硬磁界面交换耦合强度A_sh对反磁化行为也有很大的影响. 对于软磁层厚度小于临界厚度t_c的体系,也存在一个临界界面交换耦合强度A_sh^c. 当A_sh大于A_sh^c时,软磁层的反磁化过程是可逆的交换弹性反磁化过程;而当A_sh小于A_sh^c时,这一过程变为不可逆. 给出了体系的可逆与不可逆交换弹性反磁化过程随软磁层厚度和界面交换耦合强度变化的磁相图. 同时还研究了偏转场随软磁层厚度的变化关系.关键词：反铁磁耦合三层膜交换弹性反磁化过程反磁化机理磁相图     

磁性多层膜磁特性的表面效应

利用Monte-Carlo方法和转移矩阵法研究了具有不同表面交换耦合J_s和薄膜厚度 磁性多层 膜的表面和尺寸对磁相变的影响.模拟结果表明,系统的相变温度随薄膜层数的变化取决于J_s/J(J为体内交换耦合),当J_s/J大于某一临界值时,由于表面磁 有序先于体内磁有序 ,系统的相变温度随薄膜层数的增多而降低,反之,表面磁无序可与体内磁有序共存,系统 的相变温度随薄膜层数的增多而升高;当J_s/J较小时,随J_s增大 ,系统的居里温度缓慢 升高,趋近于体内相变温度,而当J_s/J较大时,随J_s增大,系统的 居里温度 呈线性升高.模拟结果与用转移矩阵法推导出的结果相当符合,且很好地解释了实验事实.关键词：磁星多层膜交换耦合Monte-Carlo模拟转移矩阵法     

Nd-Ce-Fe-B纳米复合薄膜的磁性及交换耦合作用

采用磁控溅射技术制备了具有永磁特征的Nd-Ce-Fe-B多层纳米复合薄膜,并对其进行了退火处理.通过改变退火温度,研究其对薄膜磁性能和晶体结构的影响.结果表明,随着退火温度的提高薄膜磁性能逐渐增大,但当温度达到695℃以上时,薄膜的磁性能急剧下降.当退火温度为675℃时,薄膜的矫顽力Hci=10.1 kOe(1Oe=79.5775 A/m),垂直于薄膜表面方向的剩余磁化强度4πM_(r⊥)=5.91 kG(1 G=10~3/(4π)A/m).薄膜的X射线衍射结果表明,磁性薄膜具有较好的c轴取向.通过对薄膜磁化反转过程的研究,发现随着外加磁场的增大,M_(rev)的极小值向M_(irr)减小的方向移动,这与畴壁弯曲模型类似,表明在薄膜中存在较强烈的局部钉扎作用,而剩余磁化强度曲线表明这种钉扎作用在薄膜矫顽力机制中并不占支配作用.此外,薄膜的Henkel曲线结果表明在薄膜中存在较强的交换耦合作用,在经过685℃退火的薄膜中磁相互作用更加显著.     

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.     

Magnetic anisotropy and magnetization reversal of ultrathin iron films with in-plane magnetization on Si（111） substrates

The magnetic anisotropy and magnetization reversal of single crystal Fe films with thickness of 45 monolayer (ML) grown on Si(111) have been investigated by ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM). Owing to the significant modification of the energy surface in remanent state by slight misorientation from (111) plane and a uniaxial magnetic anisotropy, the azimuthal angular dependence of in-plane resonance field shows a six-fold symmetry with a weak uniaxial contribution, while the remanence of hysteresis loops displays a two-fold one. The competition between the first and second magnetocrystalline anisotropies may result in the switching of in-plane easy axis of the system. Combining the FMR and VSM measurements, the magnetization reversal mechanism has also been determined.     

Types of the jump phenomenon in the angular dependence of the noncollinear exchange bias

Based on the principle of minimal energy and the coherent rotation model, two types of the jump phenomena, complete and incomplete jump phenomenon, are proved to exist in the angular dependence of the exchange bias with noncollinear unidirectional and uniaxial anisotropies. It is found that the transition between complete and incomplete jump phenomena occurs on condition that the exchange-coupling constant exceeds a critical value. Additionally, two different modes of the magnetization rotation, the whole-plane rotation, and the half-plane rotation are present in the magnetization reversal process, and they are dependent on the direction of the external field. Furthermore, the equations of the critical angle, at which orientation the exchange bias field reaches a maximum value and the coercivity disappears, are also derived in this paper. The numerical calculations in this paper are consistent with the relevant experimental observations, indicating that our method to study the angular dependence of the exchange bias as well as the magnetization reversal behaviors is valid. Our discussion about the jump phenomenon, the critical angle, and the modes of the magnetization reversal can explain the observed differences in results between different experiments.     

纳米复合永磁Pr9Fe74Co12B5Snx(x=0, 0.5)的磁化行为与磁黏滞性

用熔体快淬法制备了纳米复合永磁样品Pr₉Fe₇₄Co₁₂B₅ 与Pr₉Fe₇₄Co₁₂B₅Sn_0.5,分析了样品的起始磁化、反磁化过程,测得样品的总磁化率、可逆磁化率以及样品的磁黏滞性.结果表明,两样品在室温下均表现为单一硬磁相磁化行为,在低温下表现为双相行为,且由于添加Sn后使晶粒均匀化从而导致样品低温下的双相行为更加明显.添加Sn后引起样品中软磁相含量和软磁相晶粒尺寸的增加,使磁化反转中可逆磁化部分增多,且使反磁化形核场降低.磁黏滞性研究表明,热激活体积与软磁相晶粒的大小有关.关键词：纳米复合永磁磁化反转磁粘滞     

处理具有任意形状势垒的磁性隧道结中电子输运的一个简单方法

在Slonczewski自由电子模型的基础上,提出了一个可用于处理具有任意形状势垒的磁性隧道结中磁电子输运的简单方法,并以三种常见构形的势垒,即梯形势垒,计入了镜像势的梯形势垒和抛物线势垒为例,讨论了势垒形状对隧穿磁电阻及其随偏压变化的影响.关键词：磁性隧道结隧穿磁电阻任意形状势垒非零偏压     

Bias Dependence in Spin-Polarized Tunneling of Ferromagnet/Ferromagnetic Insulator (Semiconductor)/Ferromagnet Junctions

Based on the nearly-free-electron approximation, the bias dependencies of electron transport properties of ferromagnet/ferromagnetic insulator (semiconductor)/ferromagnet junctions have been studied. Resonances appear in electron transmission probability. These resonances cause oscillations in the zero-temperature tunnel current and the resonances occur in tunnel conductance. Tunnel magnetoresistance (TMR) is an oscillatory function of bias. The TMR can reach a value as high as 100%. The bins dependencies of electron transport properties relate to the magnetic configurations of the junctions.     

Efect of Varying Dzyaloshinskii–Moriya Interaction on the Bistable Nano-Scale Soliton Switching

The efect of Dzyaloshinskii–Moriya(D-M) interaction on the bistable nano-scale soliton switching ofers the possiblity of developing a new innovative approach for data storage technology. The dynamics of Heisenberg ferromagnetic spin system is expressed in terms of generalized inhomogeneous higher order nonlinear Schro¨dinger(NLS) equation. The bistable soliton switching in the ferromagnetic medium is established by solving the associated coupled evolution equations for amplitude and velocity of the soliton using the fourth order Runge–Kutta method numerically.