磁性夹层交换耦合中传导电子的间接交换作用分析

应用量子干涉理论和第一性原理研究了Co/Cu/Co夹层结构在[100]和[110]取向上振荡交换耦合及对磁层厚度的依赖,对夹层交换耦合振荡的计算结果表明：当相应的周期不变时,磁性层（Co）厚度的变化将导致在不同的RKKY振荡特征中势的再分配,磁层厚度发生变化时,磁性层上电子结构也发生变化,当磁性层较厚时,交换耦合仅在某一渐近值附近有一小的振荡。耦合振荡的周期、振幅和相位的测量与实验相一致,也与Barnas和Bruno 理论吻合。     

铁磁材料界面磁耦合表面格林函数方法

基于表面格林函数理论,考虑二维平移对称,通过紧束缚线性糕模轨道(TB-LMTO)表面格林函数方法和完全避开一般的块格林函数方法(GF)方法,研究了铁磁材料界面磁耦合,以Co/Cu/Co的3层膜为例,对铁磁体之间的层间交换耦合作用进行了讨论.研究表明:磁性多层结构交换耦合振荡依赖于夹层厚度,当磁性层厚度改变,单独RKKY势的贡献显著的依赖于界面耦合.这结论与一种从头计算方法,在磁性多层膜中评价交换振荡耦合的结果相一致.     

磁性多层膜层间交换耦合的自由电子模型研究

基于自由电子模型,考虑到不同膜厚和不同磁化排列的费密能的不同,自洽地给出费密能。利用这种方法,计算了材料铁磁层和非磁层电子密度对层间耦合振荡周期的影响,并比较了我们所得结果与他人按巨势法得到的结果的异同,发现振荡周期随着电子密度的变化而改变,并且当铁磁层和非磁层的电子密度不同时,将出现不等周期的振荡。这一新的理论结果与最近的实验相符。此外,还计算了极化强度和由于能带不匹配造成的接触势对耦合强度及振荡相位的影响:前者是决定耦合强度的主要因素,后者则会影响振荡相位,甚至周期。 关键词：     

不连续Co/SiO_2多层膜的结构及其输运性质的研究

用射频磁控溅射方法制备了系列Co SiO_2 不连续磁性金属绝缘体多层膜 (DMIM) .经研究发现 :对 [SiO_2 (2 4nm) Co(t) ]2 0 体系 ,在Co层厚度小于 2 5nm时 ,Co层由连续变为不连续 ;Co层不连续时 ,其导电机理为热激发的电子隧穿导电 ,lnR与T- 1 2 接近正比关系 ;隧道磁电阻 (TMR)在Co层厚度为 1 4nm时出现极大值 - 3% .DMIM的性质不仅与磁性金属层厚度密切相关 ,而且与绝缘层厚度有密切的关系 .在固定Co层厚度为 1 9nm的情况下 ,研究了TMR随SiO_2 层厚度的变化关系 ,并给出定性的解释 .对 [SiO_2 (2 4nm) Co(2 0nm) ]2 0 的样品研究了TMR随温度的变化关系 ,发现TMR随温度的变化有一极大值 ,结合Helman的理论 (Phys.Rev .Lett,37,14 2 9(1976 ) ) ,认为是颗粒之间存在磁性耦合的结果     

Nd28Fe66B6/Fe50Co50双层纳米复合膜的结构和磁性

利用磁控溅射法制备了Nd28F66B6/Fe50Co50双层纳米复合磁性薄膜,研究了其结构和磁性.经873K退火处理15min后,利用x射线衍射仪测定薄膜晶体结构,采用俄歇电子能谱仪估算薄膜厚度和超导量子干涉仪测量其磁性.磁性测量表明,1)该系列薄膜具有垂直于膜面的磁各向异性.从起始磁化曲线和小回线的形状特征可知,矫顽力机制主要是由畴壁钉扎控制.2)对于固定厚度(10nm)层的硬磁相Nd-Fe-B和不同厚度(dFeCo=1-100nm)层软磁相FeCo双层纳米复合膜,剩磁随软磁相FeCo厚度的增加快速增加,而矫顽力则减少.当dFeCo=5nm时,最大磁能积达到160×103A/m.磁滞回线的单一硬磁相特征说明,硬磁相Nd-Fe-B层和软磁相FeCo层之间的相互作用使两相很好地耦合在一起.剩磁和磁能积的提高是由于两相磁性交换耦合所致.     

CoFeB/Pt多层膜的垂直磁各向异性研究

具有垂直磁各向异性的磁性纳米结构是自旋转移力矩器件的重要研究内容, 本文采用反常霍尔效应系统地研究了磁控溅射法制备的[CoFeB/Pt]_n多层膜的垂直磁各向异性. 当CoFeB的厚度小于0.6 nm时, 可以在[CoFeB/Pt]_n多层膜中观察到清晰的垂直磁各向异性, 其垂直磁各向异性强烈依赖于CoFeB和Pt层厚度及多层膜周期数. 当多层膜周期数n ≥ 5时, 出现零剩磁现象. 另外, [CoFeB/Pt]_n多层膜的矫顽力均小于2 kA·m^-1, 有望作为垂直自由层的重要侯选材料应用于垂直磁纳米结构中.     

磁性d波超导/铁磁/磁性d波超导结中的约瑟夫森效应

通过求解磁性d波超导中的能隙和磁交换能的自洽方程, 研究磁性d波超导/铁磁/磁性d波超导结中的约瑟夫森电流. 计算结果表明: 1)临界电流随中间的铁磁层厚度呈现出两种不同周期的振荡混合, 通过增强铁磁层中的磁交换能q₀和铁磁/磁性d波超导界面处的势垒强度z₀, 短周期分量可从长周期中分离出来, 反之, 通过降低q₀和z₀, 长周期分量可从短周期中分离出来; 2)在两边磁性d波超导的磁化方向取平行时, 在取一些特定的铁磁层厚度下, 磁性d波超导中的磁交换能可增强系统的临界电流. 关键词： 磁性d波超导体 铁磁体 约瑟夫森电流     

具有垂直各向异性(Pt/Co)n/FeMn多层膜的交换偏置

采用磁控溅射方法制备了以Pt为缓冲层和保护层的具有垂直各向异性(Pt/Co)n/FeMn多层膜.研究结果表明,多层膜的垂直交换偏置场Hex和反铁磁层厚度的关系与其具有平面各向异性的交换偏置场随反铁磁层厚度变化趋势相近.随着铁磁层调制周期数的增加,垂直交换偏置场Hex相应减小,并且与铁磁层的调制周期数近似成反比关系.(Pt/Co)3/FeMn的垂直交换偏置场Hex已经达到22.3kA/m.为了进一步提高Hex,在Co/FeMn的界面插入Pt层,当Pt层厚度为0.4nm时,Hex达到最大值39.8kA/m.     

Co/Ti金属磁性多层膜的激光布里渊散射研究(英文)

在室温下利用直流磁控溅射法制备出了具有不同Co层厚度的 [Co(dConm) /Ti(dTinm) ]n 金属磁性多层膜 (其中dCoanddTi是Co和Ti的厚度 ,n薄膜调制周期数 )。在 30 0K下用X射线衍射法 (XRD)和布里渊散射方法研究了Co层厚度分别为dCo=1 ,2 ,2 5 ,3 5nm ,Ti层厚度为dTi=2 5nm的Co/Ti多层膜。XRD的结果所示在Co/Ti多层膜中Co层厚度超过 2 5nm ,其结构类似块材Co的多晶结构。对比之下 ,Co/Ti多层膜中Co层厚度低于 2 5nm ,其XRD衍射峰随Co层厚度减少变的峰形加宽 ,衍射峰的强度减少 ,峰位移动和消失。在Co/Ti多层膜中自旋波的布里渊散射结果表明 :在Co/Ti多层膜中 ,对于dCo>2 5nm磁性呈铁磁性耦合 ,在dCo≤ 2 5nm时 ,其呈反铁磁性耦合。除表面自旋波外 ,在呈反铁磁性耦合的材料中观测到了两个有体特征的自旋波 ,而在呈铁磁耦合的材料中只观测到一个有体特征的自旋波。     

磁性金属多层膜中的巨磁电阻效应

在许多磁性金属多层膜系统中都存在巨磁电阻效应，这些系统是由厚度为几个纳米的磁层与非磁层交替重叠构成，出现巨磁电阻效应的必要条件是系统的磁化状态能被外加磁场所改变。该效应的物理是传导电子在界面处或磁层内的所谓自旋相关散射，层间耦合随隔离层厚度变化而振荡的现象，在隔离层为非磁过渡金属和贵金属的系统中普遍存在，自旋阀多层结构在信息存储技术中磁电阻“读出”头方面极具应用前景。     

Tracking pore to pore exchange using relaxation exchange spectroscopy

We observe the movement of water over time between pores of differing sizes in Castlegate sandstone. To achieve this, we perform an NMR transverse relaxation exchange experiment for several mixing times. The resulting data are converted to 2D T2 distributions using a 2D inverse Laplace transform (ILT). We show for the first time that quantitative analysis of ILT distributions enables one to extract characteristic times for different pores sizes. This information is potentially useful for permeability determination as well as better understanding of exchange between specific pore subpopulations.     

Muon charge exchange and muonium spin exchange in gases

The spin dynamics of the positive muon undergoing cyclic charge exchange (μ⁺⇌Mu) while it is slowing down is compared with that in the case of repeated muonium spin exchange after thermalization. The expectation value of the spin polarization at timet aftern spin exchange collisions (att ₁,t ₂, ...,t _n ) are calculated explicitly from time dependent wave functions, and the quantity is averaged over the statistical distribution of the times of collisions and over all possible numbers of collisions betweent=0 and the observation timet. This result is complementary and equivalent to the conventional density matrix formalism, but offers an insight into the roles of spin flip and spin non-flip processes. The neutral fraction during slowing down is also discussed.     

Characteristics of double-pomeron exchange

A detailed kinematic study is made of the cross section for double-pomeron exchange. We consider reactions of the type a + b → c + d + (h₁ + h₂ + …) where a → c and b → d are quasi-elastic transitions with limited momentum transfer and (h₁ + h₂ + …) is a centrally produced hadron cluster. The dependence of the cross section on the relative azimuthal angle φ between the final hadrons c and d provides an important test for double-pomeron exchange. We calculate this φ-dependence for all cases of interest, using a model in which pomeron exchange is represented by an effective current-current interaction. For limited momentum transfer, where the pomeron spin is nearly equal to one, this model is equivalent to Reggeized pomeron exchange. We give a complete formalism for the partial-wave analysis of the double-pomeron reaction ab → cd + h₁₂ which differs from the one given by Rushbrooke and Webber. Furthermore, we discuss the two-pomeron vertex, and the construction of detailed and realistic models for double-pomeron reactions.     

Contribution of the σ meson exchange to pionic double charge exchange reaction

The contribution of the σ meson exchange to the pionic double charge exchange (DCX) reaction is investigated. A concrete calculation on the forward excitation function of the low energy DCX reaction ¹⁴C(π⁺, π⁻)¹⁴O has been performed. It shows that the contribution of the σ meson exchange can reproduce well the resonance like excitation function of low energy DCX reaction. Received: 5 August 1998     

Resolution exchange simulation

We extend replica-exchange simulation in two ways and apply our approaches to biomolecules. The first generalization permits exchange simulation between models of differing resolution--i.e., between detailed and coarse-grained models. Such "resolution exchange" can be applied to molecular systems or spin systems. The second extension is to "pseudoexchange" simulations, which require little CPU usage for most levels of the exchange ladder and also substantially reduce the need for overlap between levels. Pseudoexchanges can be used in either replica or resolution exchange simulations. We perform efficient, converged simulations of a 50-atom peptide to illustrate the new approaches.     

Baryon exchange processes

The theory and phenomenology of baryon exchange processes are critically reviewed. As regards theory the main topics are the parity doublet problem and the patterns of exchange degenerate baryon trajectories implied by duality. In the phenomenological part, all baryon exchange reactions are considered, with the emphasis being on seeing how far simple ideas of pole exchange with the constraints implied by duality and broken SU(3) can be taken in forming an understanding of the data.