Structure and magnetic properties of Fe4/Cun (n=2,4) superlattices from first-principles study

The structures and magnetic properties of Fe4/Cun (n=2, 4) superlattices have been investigated by the first-principles pseudopotential plane-wave method based on spin density approximation. Compared with the ideal fcc-Cu bulk structure, for the optimized Fe4/Cu2 model, obvious contraction of interlayer distances occurs on the interior Fe layers, whereas the interlayer distances of Fe layers in Fe4/Cu4 are expanded. The anti-parallel alignment magnetic moment and negative polarization of the interior Fe layer have been found in the Fe4/Cu2 model. This can be explained in terms of the magnetic-volume effect, and the moment of anti-parallel alignment attributes to the contracted interlayer distances between the interior Fe layers. The MR ratio has also been evaluated by means of the two-current model. The MR ratio of the Fe4/Cu2 model (4.89%) is much small than that of the Fe4/Cu4 one (23.65%).     

Effects of interfacial roughness on the magnetoresistance of Co/Cu multilayers

Using a semi-classical approach, Hood, Falicov and Penn have studied the effects of interfacial roughness on the magnetoresistance (MR) of iron based trilayers (Fe/Cr/Fe and Fe/Cu/Fe). We extend their theory to magnetic metallic multilayers composed of N bilayers ferromagnetic-normal metal. The in plane MR of Co/Cu multilayers is calculated for correlated quasiperiodic interfaces. The averaged effects due to impurities, interdiffusion, band structure, etc. are included in a simple way using two phenomenological parameters S^↑ and S^↓ for two directions of spin. MR variation with S^↑, S^↓ and relaxation time is reported. We analyse also recent experimental data giving the influence of number of bilayers on the MR of Co/Cu multilayers for different temperatures.     

Mössbauer investigation of the Fe/Cu interfaces in BCC Fe/Cu/Fe(0 0 1) structures

A RHEED study shows quasi layer-by-layer growth in BCC Fe/Cu/Fe(0 0 1) structures. The BCC stacking of Cu layers is maintained up to a critical thickness of 11 layers. The different iron sites at the Fe/Cu interfaces can be identified by Mössbauer spectroscopy from the distinct values measured for the magnetic hyperfine fields H_hf and isomer shifts at the ⁵⁷Fe nuclei. This makes it possible to determine the concentration of ⁵⁷Fe atoms in the different iron sites. The roughness of the Cu on Fe interface estimated from the Mössbauer study is more pronounced than that estimated from a RHEED study of the structure. The growth of Fe on Cu produces CuFe alloy layers at the Cu/Fe interface.     

Interactive effect of impurities on giant magnetoresistance of Co–Fe/Cu multilayers

Giant magnetoresistance of Co–Fe–B/Cu multilayers fabricated in the sputtering atmosphere, where the amount of oxygen impurity is varied, is discussed in connection with their interfacial roughness. The magnetoresistance (MR) ratio of Co–Fe/Cu multilayers is enhanced by up to 33% when the oxygen content is varied between 10 and 100 ppm of processing Ar gas. The enhancement of the MR ratio was due to the flattening effect of impurity oxygen on the multilayer interfaces: the root mean square roughness of the multilayer was decreased from 7.5 to 5 Å. With increasing boron content in Co–Fe layers, however, the enhancing effect of MR ratio by oxygen diminished and nearly vanished for 12 at%-B–(Co–Fe) case. The strong affinity of boron for oxygen is suggested as a probable mechanism.     

Distribution of the magnetic scattering amplitudes in the Fe/Cu multilayer investigated by resonant magnetic diffraction with circularly polarized hard X-rays

Resonant X-ray magnetic diffraction profiles were measured for an epitaxial Fe/Cu multilayer using circularly polarized X-rays near the Fe and Cu K-edges. Diffraction intensities were compared with those obtained from the theoretical and empirical models. It is found that the interface Fe moment is reduced to 70% of the inner-layer moment. Concerning the Cu layer, the observed energy dependence of the magnetic diffraction intensities is consistent with that derived from the first-principle band calculation, indicating that magnetic proximity effect in the Cu layer is confined within a few atomic layers near the interface.     

多弧离子镀TiN与不同金属基材间的接触界面与表面特性

用多弧离子镀技术在不同金属基材上进行TiN镀膜实验 ,制备了TiN/Fe、TiN/Cu和TiN/Cr/Cu复合膜 .借助扫描电子显微镜 (SEM)、X射线衍射仪 (XRD)和光电子能谱 (XPS) ,研究了TiN与Fe、Cu和Cr/Cu三种不同衬底接触界面的形貌、结构及其表面特性 .SEM观察发现 ,在一定离子镀膜条件下 ,TiN涂层可与Fe、Cu和Cr/Cu金属基材形成均匀平整的接触界面 ,在铜基上TiN界面清晰 ,在Fe与Cr/Cu界面有明显的层状晶界微结晶分布 .XRD分析显示 ,Fe、Cu和Cr/Cu表面生成的薄膜都包含TiN、Ti2 N等多晶相 ,在Cr/Cu界面还包含Ti-Cr的金属间化合物 .XPS结果表明 ,表面除了TiN膜外 ,还生成TiO2 和TiOxNy 等氧化膜 .Ar+ 刻蚀 5min后 ,TiO2 消失 ,TiOxNy 减少 ,TiN则呈增加趋势 .TiN与Cr/Cu界面形成明显的Ti-Cr和Cr-Ni互扩散层 ,这有助于增强薄膜附着力 ,形成较牢固的TiN涂层 .     

Asymmetric transport due to spin injection into a Kondo alloy

Spin injection is found to have a significant effect on the transport properties of the Kondo alloy Cu(Fe). When a spin-polarized electron current flows from Co into Cu(Fe) wires through the Co/Cu(Fe) interface, the resistivity of the Cu(Fe) wire is suppressed near the interface, as distinct from the ordinary logarithmic increase in the resistivity at low temperatures. For the opposite current direction, no significant changes are observed. The asymmetry of the resistivity with respect to the current direction decays with a characteristic length of 1.5+/-0.4 microm at 2.5 K as the distance from the interface is increased. Possible mechanisms for the asymmetry are discussed.     

Spin-polarized edge and magnetoresistance in graphene flake

We examine spin-polarized edge and magnetoresistance (MR) in Fe/graphene flake (GF)/Fe junctions. For simulating various edge designs, we use the tight-binding approximation, mean-field scheme for the Hubbard model and the Landauer-Büttiker formalism. The Fe electrodes strongly affect electronic states and magnetic properties of the GF and induce magnetism in the edge atoms even in case of armchair interfaces. Also, the edge magnetic moments of the zigzag interface rotate and couple antiferromagnetically with the Fe electrodes. The conductivity of the junctions strongly depend on the relative magnetic orientation of the Fe electrodes, so, the junctions show high MR ratios. Moreover, edge geometry and localized edge state in the GF alter the MR ratios and produce large (small) variation in the MR at low (high) bias voltages.     

Magnetic and transport properties of sputtered Fe/Si multilayers

The structural, magnetic and transport properties of sputtered Fe/Si multilayers were studied. The analyses of the data of the X-ray diffraction, resistance and magnetic measurements show that heavy atomic interdiffusion between Fe and Si occurs, resulting in multilayers of different complicated structures according to different sublayer thicknesses. The nominal Fe layers in the multilayers generally consist of Fe layers doped with Si, ferromagnetic Fe-Si silicide layers and nonmagnetic Fe-Si silicide interface layers, while the nominal Si spacers turn out to be Fe-Si compound layers with additional amorphous Si sublayers only under the condition either for the series or for the series multilayers. A strong antiferromagnetic (AFM) coupling and negative magnetoresistance (MR) effect, about 1%, were observed only in multilayers with iron silicide spacers and disappeared when -Si layers appear in the spacers. The dependences of MR on and on bilayer numbers N resemble the dependence of AFM coupling. The increase of MR ratio with increasing N is mainly attributed to the improvement of AFM coupling for multilayers with N. The dependence of MR ratio is similar to that in metal/metal system with predominant bulk spin dependent scattering and is fitted by a phenomenological formula for GMR. At 77 K both the MR effect and saturation field increase. All these facts suggest that the mechanisms of the AFM coupling and MR effect in sputtered Fe/Si multilayers are similar to those in metal/metal system. Received: 11 February 1998 / Revised: 9 March 1998 / Accepted: 9 March 1998     

Coupled perpendicular magnetization in Fe/Cu/Fe trilayers

Ultrathin epitaxial Fe films on Cu(1 0 0) with perpendicular magnetization have been used as templates for the preparation of FCC Fe/Cu/Fe trilayers. The magnetic anisotropy and the coupling of these films have been studied by in-situ magneto optical Kerr effect measurements and Kerr microscopy. The magnetic coupling of both Fe layers is found to be dominated by magnetostatic interaction. Adsorbate-induced spin reorientation in the top layer also causes spin reorientation in the bottom layer. The governing role of the Fe-vacuum interface for the magnetism of the whole trilayer is demonstrated.     

Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: first-principles calculations

A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with the voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin-density approximation, we calculate the spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of the applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions.     

Long-range chemical interaction in solid-state synthesis: The Kirkendall effect and solid-state reactions in Cu/β-CuZn and Cu/Fe/β-CuZn film systems

The results of the experimental investigations of the solid-state reaction of Cu with β brass, which is associated with the Kirkendall, in Cu/β-CuZn and Cu/Fe/β-CuZn film systems are reported. It has been shown that the initiation temperature of the solid-state synthesis of α brass at the Cu/β-CuZn interface is about 200°C. The chemically inert Fe barrier layers 420 and 850 nm in thickness between the Cu and β-CuZn films do not suppress the solid-state synthesis of the α brass, but increase the initiation temperature to about 250°C. This behavior indicates that the chemical interaction between the Cu and Zn atoms through the chemically inert Fe layer is long-range. The X-ray photoelectron investigations reveal the migration of Zn atoms from the β-CuZn layer through the Fe barrier to the Cu layer. The results are interpreted under the assumption that the migration of Zn atoms in the Kirkendall is initiated by the strong chemical interaction between the Cu and Zn atoms in the Cu and β-CuZn layers, which is due to the synthesis of α brass, rather than by the diffusion random walk, as is commonly accepted at present.     

Annealing effect on magnetoresistance in NiO-Co-Cu based spin valves

We have fabricated two sets of NiO-Co-Cu based spin valves by the magnetosputtering technique with different deposition parameters. Magnetoresistance (MR) measurements show that the MR value for the NiO layer under the bottom of Co/Cu/Co spin valve (BSV) is larger than that for the NiO layer at the top of Co/Cu/Co (TSV). The MR value of BSV decreases with increasing annealing temperature in air or in vacuum, which disappears at the blocking temperature of NiO, i.e., about 250 °C. There is maximum MR value for TSV annealing at a temperature range from room temperature to 350 °C. The different thermal behavior for BSV and TSV is explained by the competition between the interface roughness of NiO/Co, which was determined by the grazing incident X-ray reflectivity and X-ray diffuse scattering, and the coupling effect between Co layer and NiO layer. PACS 68.60.Dv; 61.10.Eg; 68.35.Ct; 73.43.Qt; 75.25.+z     

Direct observation of exchange bias related uncompensated spins at the CoO/Cu interface

Magnetization-induced optical second harmonic generation (MSHG) from the exchange-biased CoO/Cu-(X)/Fe multilayer shows the presence of pinned uncompensated spins at the CoO/Cu interface. For increasing Cu spacer thickness, the exchange bias measured via the hysteresis loop shift diminishes and disappears at X = 3.5 nm, while the MSHG signal still shows a strong magnetic contribution from the CoO interface. This indicates that the magnetic interaction between Fe and CoO layers is sufficiently strong to induce order in the antiferromagnetic layer even at a spacer thickness for which there is no observable hysteresis loop shift.     

The deposition of Fe or Co clusters on Cu substrate by molecular dynamic simulation

A molecular dynamic method is used to simulate the film growth process of Fe or Co clusters depositing on Cu substrate with low energy. The tight-binding (TB-SMA) many-body potential is used to simulate the interaction between atoms. The effects of different incident energies and/or substrate temperatures on the surface roughness, layer coverage function, radial distribution function (RDF), and residual stress are investigated. From the simulation results, as the substrate temperature and/or incident energy is increased, the surface roughness of the grown film could be reduced, and the interface intermixing is increased. Also, as compared to Co–Cu system, Fe–Cu system has better surface roughness, less interface intermixing, and similar radial distribution function as well as average stresses.     

Frozen low-spin interface in ultrathin Fe films on Cu(111)

In ultrathin film systems, it is a major challenge to understand how a thickness-driven phase transition proceeds along the cross-sectional direction of the films. We use ultrathin Fe films on Cu(111) as a prototype system to demonstrate how to obtain such information using an in situ scanning tunneling microscope and the surface magneto-optical Kerr effect. The magnetization depth profile of a thickness-driven low-spin to high-spin magnetic phase transition is deduced from the experimental data, which leads us to conclude that a low-spin Fe layer at the Fe/Cu interface stays live upon the phase transition. The magnetically live low-spin phase is believed to be induced by a frozen fcc Fe layer that survives a thickness-driven fcc-->bcc structural transition.     

The γ-Fe formation in epitaxial Cu(0 0 1)/Fe(0 0 1) thin films by the solid-state synthesis: Structural and magnetic features

The γ-Fe formation in epitaxial Cu(0 0 1)/Fe(0 0 1) bilayers with the annealing temperature increasing has been studied. Using Mössbauer spectroscopy, X-ray diffraction, and magnetic analysis, structural and chemical characterization of the interface between cooper and iron layers has been performed. After annealing at 850 °C and subsequent cooling to room temperature, paramagnetic γ-Fe(0 0 1) precipitates with an average size of 30 nm coherent to a Cu(0 0 1) matrix form. Conditions of intermixing and the γ-Fe formation at the Cu/Fe interface are explained in terms of the solid-state synthesis in thin films. Specific features of the martensitic transition γ↔α are discussed.     

Fe基体中包含Cu团簇的Fe-Cu二元体系在升温过程中结构变化的原子尺度计算

采用基于嵌入原子方法的分子动力学方法模拟了具有体心立方晶格结构的Fe基体中包含小尺寸Cu纳米粒子的Fe-Cu二元体系在升温过程中的原子堆积结构变化.进行了Cu原子均方位移、Cu原子对分布函数和原子的径向密度分布函数的计算,并对纯Cu原子区、Fe-Cu界面区和纯Fe基体区的分区域原子堆积结构进行了分析.结果表明,Fe基体内Cu团簇的尺寸及其在Fe基体内所能占据区域的大小,对不同温度下的Cu团簇内原子堆积结构及Fe基体的原子堆积结构具有影响.升温过程中不同尺寸受基体约束Cu团簇对Fe基体结构改变的影响表现出很大差异.对于Fe_(bulk)-Cu_(135)体系,基体的应变临近Fe-Cu界面区,同时在团簇中间的基体区域出现大量空位缺陷和应变集中区;对于Fe_(bulk)-Cu_(141)体系,随温度升高,基体中出现的应变区域表现为小尺寸、数量多向大尺寸、小数量的变化.     

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     

Role of interface scattering on the giant magnetoresistance of magnetic multilayers

A discussion is presented of the effect of interface scattering on the giant magnetoresistance of magnetic multilayers. It has recently been found experimentally that increased interface scattering causes the magnetoresistance to increase for Fe/Cr multilayers, but to decrease for Co/Cu multilayers. An explanation is proposed for this striking difference in behaviour for these two systems.