x射线磁性圆二色吸收谱分析Co膜厚度对原子磁矩和自旋磁矩的影响

利用软x射线磁性圆二色吸收谱(XMCD)研究了Si衬底上沉积的不同厚度的Co膜的轨道磁矩和 自旋磁矩.样品是磁控溅射方法制备的，膜的厚度分别是2nm,10nm和30nm，并在表面覆盖0.8 —1nm厚的金膜防止样品的氧化.根据XMCD求和定则计算得到的轨道磁矩和自旋磁矩分别是0. 249—0.195μ_B(玻尔磁子)和1.230—1.734μ_B.随着膜厚的减小，C o原子的轨道磁矩增加，而自旋磁矩下降.轨道磁矩与总磁矩的比值由0.101上升至0.168，即 2nm膜中Co原子的轨道磁矩对总磁矩的贡献比30nm膜中Co原子的大了83%. 关键词： x射线磁性圆二色 磁性薄膜 轨道磁矩和自旋磁矩 厚度效应     

Co0.9Fe0.1薄膜面内元素分辨的磁各向异性

利用X射线磁性圆二色技术对Co_0.9Fe_0.1薄膜面内元素分辨的磁各向异性进行了研究，通过剩磁模式测量不同磁化方向的样品组分原子单位空穴磁矩的变化，发现除了在生长的磁诱导方向存在易磁化轴外，在与该轴垂直的方向还存在一个类似易轴的软磁化轴；面内的两个难磁化轴与易磁化轴取向大约成66°夹角，从而构成了面内双轴磁各向异性；对不同组分元素，其单位空穴磁矩随磁化方向的变化趋势基本相同，不同磁化方向Fe原子单位空穴的磁矩值约为Co的对应值的87%，反映了Fe原子和Co原子之间存在着强烈的铁磁性耦合. 关键词： 磁各向异性 X射线磁性圆二色 铁磁耦合 CoFe合金薄膜     

Co0.9Fe0.1薄膜面内元素分辨的磁各向异性

利用X射线磁性圆二色技术对Co0.9Fe0.1薄膜面内元素分辨的磁各向异性进行了研究,通过剩磁模式测量不同磁化方向的样品组分原子单位空穴磁矩的变化,发现除了在生长的磁诱导方向存在易磁化轴外,在与该轴垂直的方向还存在一个类似易轴的软磁化轴;面内的两个难磁化轴与易磁化轴取向大约成66°夹角,从而构成了面内双轴磁各向异性;对不同组分元素,其单位空穴磁矩随磁化方向的变化趋势基本相同,不同磁化方向Fe原子单位空穴的磁矩值约为Co的对应值的87%,反映了Fe原子和Co原子之间存在着强烈的铁磁性耦合.     

X光磁圆二色谱及其应用

介绍了一种新近发展起来的、以同步辐射技术为基础的磁学测量方法———Ｘ光磁圆二色谱．该方法不仅具有元素分辨和空间分辨等突出优点，而且在一些场合下还能分别确定轨道磁矩和自旋磁矩对总磁矩的贡献，为当今非常活跃的磁性薄膜和多层膜研究提供了有力的手段．     

X射线磁圆二色谱

 同步辐射真不愧为优秀的新一代光源。它的出现和不断发展,极大地促进了很多新的实验研究工作的开展。它具有频率范围宽、高强度、极好的准直性、偏振性以及包含时间结构等优良特性,如果没有同步辐射,很多实验都将无法进行。因此,它对很多学科的发展起着很大的推动作用。本文将要介绍的X射线磁圆二色谱就是利用同步辐射研究磁性材料,特别是磁性薄膜和多层膜磁性新的、有力的研究手段。通过测定材料中某一组元的X射线磁圆二色谱,可以获得该组元的自旋磁矩、轨道磁矩、电荷密度分布以及它们的磁各向异性等信息,其中有些是传统的磁测量方法无法获得的。     

铋层状氧化物单晶薄膜多铁性研究进展

室温单相多铁材料非常稀缺,磁性元素掺杂的铋层状钙钛矿结构Aurivillius相氧化物是一类重要的单相室温多铁材料,但由于缺少单晶类样品,这一类多铁材料研究主要是围绕多晶类块体或者多晶薄膜展开,它们的磁、电等性能研究大都采用宏观探测方式,因此这类多铁材料的多铁性机理研究进行得非常困难.近年来在高质量单晶薄膜的基础上,研究了多种磁性元素掺杂和不同周期结构的铋层状氧化物多铁单晶薄膜.这些单晶薄膜在室温下大都具有层状面面内方向的铁电极化,以及比较小的室温磁化强度,低温区存在第二个磁性相变.通过X射线共振非弹性散射实验发现元素掺杂会改变金属和氧原子之间的氧八面体晶体场的劈裂,能够增强铁磁性.另一方面,通过极化中子反射实验发现薄膜主体的磁化强度远小于通常探测的宏观磁化强度,说明单晶薄膜中磁的来源及其磁电耦合机理和多晶块体很可能是不同的.铋层状单晶薄膜的多铁性对未来继续改善这类材料的多铁性能有很好的指导作用.     

正交各向异性双层交换弹簧薄膜的磁矩分布

本文以Pt_(84)Co_(16)/TbFeCo双层交换弹簧体系为研究对象,利用微磁学连续模型,研究了软/硬磁层易轴方向相互垂直的新型体系中磁矩的分布特征.研究结果表明,磁矩偏离薄膜法线方向的角度在软磁层中沿膜厚方向的变化速率比硬磁层中的快.通过调节软磁层参数来增加软/硬磁的各向异性常数比、交换能常数比、饱和磁化强度比或外磁场强度,都可有效改变磁矩偏角在软/硬磁层中的变化速率.特别是当软/硬磁各向异性常数比值和交换能常数比值同时增大时,可以使得磁矩在硬磁层中的变化速率快于软磁层中的.而饱和磁化强度比值对磁矩变化速率的影响源于饱和磁化强度的变化会相应地改变各向异性常数,进而改变磁矩在软/硬磁层中磁矩方向变化速率的比值.此体系的磁滞回线显示磁性参数的改变可以显著改变体系的剩磁及饱和磁场.软磁层中的退磁场能及体系的正交各向异性可导致负的成核场.     

PdMn/Co多层膜的磁性和磁光特性研究

用磁控溅射法制备了不同Mn含量的PdMn/Co磁性多层膜，通过x射线衍射对该多层膜系列进行结构分析；测定了不同Mn含量系列样品的磁滞回线、垂直各向异性及磁力显微镜图，分析了饱和磁化强度、磁畴和垂直各向异性变化的原因；通过测定该多层膜体系的克尔谱，简要分析了一定波长下克尔角随Mn含量增加而变化的物理机制. 关键词： 多层膜 磁性 磁光     

重金属缓冲层和覆盖层对TbFeCo超薄膜磁性及热稳定性的影响

非晶态稀土-过渡金属合金亚铁磁薄膜具有很强的垂直磁各向异性、超快的磁矩翻转速度以及磁矩和角动量补偿的特性,是当前自旋电子学以及超快信息存储领域的重要研究对象.本文采用磁控溅射制备了系列X/Tb_x(Fe_0.75Co_0.25)_1–x/X三明治结构薄膜(0.13≤x≤0.32, X=SiO₂, Pt和W),系统地研究了重金属Pt, W作为TbFeCo超薄膜的缓冲层和覆盖层(统称包覆层)对其室温下磁性和热稳定性的影响.实验结果显示,被SiO₂包覆的TbFeCo薄膜具有垂直磁各向异性,磁矩补偿成分在0.21 相似文献   

利用X射线磁性圆二色吸收谱计算3d过渡族磁性原子的轨道和自旋磁矩

在4个方面研究了实验数据的预处理和应用加和定则中的问题.1)外磁场对样品电流法测量的吸收谱强度的影响.发现外磁场H<200×10^-4T时，信号强度正比于H^-β；当H>200×10^-4T时，尽管外磁场继续增加，但信号强度基本保持不变.2)不同方向的电磁铁剩磁会导致吸收谱的分离.这种分离与入射光的偏振态和样品的磁性无关，可以通过乘以一个常数很好地消除这种分离.3)通过XPSPEAK 4.1对实验数据拟合，写出了吸收谱的解析函数.利用解析函数的积分值，建立一种相对“客观"的标准，判断在一定的实验条件下，不同的数值积分方法的准确性.4)以误差函数作为吸收谱的背景函数，建立了一套完整的X射线磁性圆二色的数据处理方法.最后用Bode积分法计算出20nm厚Co膜的轨道和自旋磁矩分别为0.141μ_B和1.314μ_B. 关键词： X射线磁性圆二色 加和定则 台阶函数 吸收谱拟合     

Synchrotron radiation studies of mass-selected Fe nanoclusters deposited in situ

A portable UHV-compatible gas aggregation cluster source, capable of depositing clean mass-selected nanoclusters in situ, has been used at synchrotron radiation facilities to study the magnetic behaviour of exposed and Co-coated Fe clusters in the size range 250 to 540 atoms. X-ray magnetic circular dichroism (XMCD) studies of isolated and exposed 250-atom clusters show a 10% enhancement in the spin magnetic moment and a 75% enhancement in the orbital magnetic moment relative to bulk Fe. The spin moment monotonically approaches the bulk value with increasing cluster size but the orbital moment does not measurably decay till the cluster size is above ∼ 400 atoms. The total magnetic moments for the supported particles though higher than the bulk value are less than those measured in free clusters. Coating the deposited particles with Co in situ increases the spin moment by a further 10% producing a total moment per atom close to the free cluster value. At low coverages the deposited clusters are super-paramagnetic at temperatures above 10 K but a magnetic remanence at higher temperature emerges as the cluster density increases and for cluster films with a thickness greater than 50 ?(i.e. 2-3 layers of clusters) the remanence becomes greater than that of an Fe film of the same thickness produced by a conventional deposition source. Thick cluster-assembled film show a strong in-plane anisotropy. Received 14 December 2000     

Effect of surface chemisorption on the spin reorientation transition in magnetic ultrathin Fe film on Ag(0 0 1)

We have investigated the effect of surface chemisorption on the spin reorientation transitions in magnetic ultrathin Fe films on Ag(0 0 1) by means of the polar and longitudinal magneto-optical Kerr effect (MOKE) and X-ray magnetic circular dichroism (XMCD) measurements. It is found by the MOKE that adsorption of O₂ and NO induces the shift of the critical thickness for the transitions to a thinner side, together with the suppression of the remanent magnetization and the coercive field of the Fe film. This implies destabilization of the perpendicular magnetic anisotropy. On the other hand, H₂ adsorption is found not to change the magnetic anisotropy, though the enhancement of the coercive field is observed. The XMCD reveals that although both the spin and orbital magnetic moments along the surface normal are noticeably reduced upon O₂ and NO adsorption, the reduction of the orbital magnetic moments are more significant. This indicates that the destabilization of the perpendicular magnetic anisotropy upon chemisorption of O₂ and NO originates from the change of the spin-orbit interaction at the surface.     

磁性薄膜自旋重取向行为的Monte Carlo模拟

利用Monte Carlo方法模拟了二维简单立方结构磁性薄膜的自旋重取向行为，重点研究了各向异性和偶极相互作用对系统自旋取向的影响．通过计算，获得了系统的相图以及系统组态、磁分量、比热等随偶极相互作用和温度的变化规律．模拟结果表明，在一定的参数范围内，随着温度的升高，系统的自旋取向将由垂直向平行方向转变。     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

Theoretical study of the magnetic moments and anisotropy energy of CoRh nanoparticles

The role of size, structure and chemical order on the magnetic moments and magnetic anisotropy energy (MAE) of CoRh nanoparticles are studied in the framework of a self-consistent real-space tight-binding method. Our results show that a Rh core in a geometry having a large surface/volume ratio and with Co–Rh mixing at the interface is the most likely chemical arrangement. A local analysis reveals that the orbital and spin moments at the Co–Rh interface are largely responsible for the increase of the magnetic moments and magnetic anisotropy. Moreover, the local moments induced at the Rh atoms, which amount to about 20% of the moment per Co atom [ μ_Rh = (0.2–0.3) μ_B] and the orbital moments of Co atoms play a crucial role on the interpretation of experiment. The results are discussed in the context of the interplay between chemical order and magnetic properties.     

Strongly enhanced orbital moment by reduced lattice symmetry and varying composition of Fe1-xCox alloy films

We studied tetragonally distorted Fe(1-x)Co(x) alloy films on Rh(001), which show a strong perpendicular anisotropy in a wide thickness and composition range. Analyzing x-ray magnetic circular dichroism spectra at the L_(3,2) edges we found a dependence of the Co magnetic orbital moment on the chemical composition of the Fe(1-x)Co(x) alloy films, with a maximum at x=0.6. For this composition, we observed an out-of-plane easy axis of magnetization at room temperature for film thickness up to 15 monolayers. Since both the magnetic orbital moment and the anisotropy energy show similar composition dependence, it confirms that both quantities are directly related. Our experiments show that the adjustment of the Fermi level by a proper choice of the alloy composition is decisive for the large magnetic orbital moment and for a large magnetic anisotropy in a tetragonally distorted lattice.     

Tuning the coercive field of Ni and CuNi thin films with the embedding of Co nanoparticles: An element-specific study

Cobalt nanoparticles have been embedded either within magnetic thin films of CuNi or Ni, or at their interface with an Al capping layer. Soft X-ray magnetic circular dichroism (XMCD) was used to measure element-specific hysteresis loops, allowing discrimination between the nanoparticles and the host film. The hysteresis shows coupling between the nanoparticles and the films and a reduction in coercivity is observed when positioning the nanoparticles within the film, as opposed to at its interface with the cap. Sum rule analysis of the XMCD spectra makes it possible to determine the orbital to spin moment ratios for Co and Ni in the samples. The difference in the calculated ratios is consistent with the differences in the coercivities, the films appearing more homogenous due to hybridization of the 3d bands of Co and Ni when the nanoparticles are embedded deeper within the films, creating less pinning sites and hence a lower coercivity.     

First-principles studies of the magnetic hyperfine field in Fe multilayers

A detailed theoretical study of the magnetic moments and magnetic hyperfine fields in several Fe multilayers (Fe fcc(001)/5X fcc(001), X=Cu and Ag, and Fe bcc(001)/5X fcc(001), X=Ag and Au) as well as in bulk Fe is presented. The calculations have been performed using the spin-polarized, relativistic linear muffin-tin orbital (SPR-LMTO) method of band structure calculation. Therefore, not only the contribution to the hyperfine fields due to the conventional Fermi contact interaction but also due to the spin dipolar and orbital contributions induced by the crystal field and by spin-orbit coupling are accounted for. To decompose the hyperfine field of non-s-electrons into these contributions it has been assumed that they are proportional to the corresponding so-called magnetic dipole moment and the orbital magnetic moment, respectively. In contrast to previous results for pure metals and alloys not only the orbital but also the spin dipolar hyperfine field was found to be non-negligible. The anisotropy of the hyperfine field determined by calculations for in-plane and perpendicular orientation of the magnetisation was found to be very pronounced and closely connected with the corresponding anisotropy of the magnetic dipole moment and the orbital moment.