Giant Magnetoresistance Effect of [bcc-Fe(M)/Cu](M=Co,Ni) Multilayers

1. IlltroductionGiant magnetoresistance (GMR) effect of metallic multilayers has been widely investigated after thefinding by Baibich et al.11], as a new phenomenon tobreak through the memory density in ultra high density magnetic recording, high sensitivity in magnetichead, and so on. Metallic multilsyers of 3d transition elements could be classified into three groups of[bee/bcc], [fee/fccl and [bee/fcc] from the standpointof combination of crystal structure of constituting elements of metal…     

Fe/V多层膜中亚稳结构Fe的形成及其磁学性能

采用电子束蒸镀的方法,通过改变多层膜的周期结构,成功地制备出具有不同晶格常数的bcc亚稳结构铁相的Fe/V多层膜,并研究了亚稳结构铁相形成对其磁性影响的规律.实验结果表明,多层膜中Fe与V层均由纳米晶粒组成.Fe层厚度小于2nm时,受多层膜界面自由能作用,Fe与V相互准外延生长,多层膜由点阵常数一致的体心立方相组成,其点阵常数随样品V/Fe层厚度比的增大而增加.多层膜平均原子磁矩随铁或钒层厚度的改变发生明显变化:当钒层厚度固定为6nm时,铁原子磁矩随铁层厚度的增加逐渐下降,在2nm处出现极小值后又随铁层增厚而回升;对于铁层厚度固定为1.6nm的样品,磁矩在钒层厚度为3nm时出现极大值.     

磁性多层膜的X射线光电子能谱研究

用射频 /直流磁控溅射法制备了NiOx/Ni81Fe19和Co/AlOx/Co磁性薄膜。利用X射线光电子能谱研究了NiOx 对Ni81Fe19耦合交换场Hex与NiOx 化学状态的关系以及Co/AlOx/Co磁性薄膜中AlOx 对Co膜的覆盖状况。结果表明 :Hex的大小只与 2价镍有关 ,单质镍和 3价镍对Hex没什么作用 ;在Co/AlOx/Co磁性薄膜中 ,Al层将Co膜完全覆盖所需要的最小厚度为 2 .0nm ,用角分辨XPS测出的Al氧化厚度为 1 15nm     

Oscillatory interlayer magnetic coupling and induced magnetism in Fe/Nb multilayers

We present an ab initio calculation of interlayer magnetic coupling for Fe/Nb multilayers using the self-consistent full-potential linearized augmented-plane-wave (FLAPW) method. For this calculation, we have constructed supercells consisting of bcc Fe and Nb multilayers in Fe/Nb/Fe sandwich geometry stacked along (001) direction. In the supercells two Fe layers are separated by Nb layers ranging from 1 to 11 layers. We have calculated the total energy of the system as a function of Nb spacer layer thickness. For each spacer layer thickness, we have done three calculations corresponding to para, ferro and antiferromagnetic ordering of Fe atoms. The interlayer magnetic coupling is obtained from the energy difference of the systems in which Fe layers are antiferromagnetically and ferromagnetically ordered. We find that the interlayer magnetic coupling oscillates with increasing Nb spacer thickness in agreement with the experimental results. The induced magnetic moment is also found to be oscillating with increasing Nb spacer layer thickness.     

Microstructure,Magnetic and Magnetoresistance Properties of Electrodeposited [Fe/Pt] Granular Multilayers

In this paper, we report experimental results concerning the magnetic properties and the magnetoresistance effect of [Fe/Pt] _n and [Pt/Fe] _n electrodeposited multilayers. Two series of multilayers starting with Pt and Fe layers, respectively, were grown onto glass substrate covered with electroless deposited amorphous Ni. We investigated the effect of the seed layer (Pt or Fe) and Pt layer thicknesses on the magnetic and magnetoresistance properties of electrodeposited multilayers. The structure and morphology of the samples were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The hysteresis loops of the [Fe/Pt] _n and [Pt/Fe] _n multilayers showed that the magnetic properties strongly depend on the Pt thickness and on the seed layer (the first layer deposited onto the glass substrate). [Fe/Pt] _n and [Pt/Fe] _n electrodeposited multilayers display magnetoresistance (∼15%) effect which can be explained mainly by the spin-dependent scattering of conduction electrons between Fe layers through a Pt layer and by the existence of anti-ferromagnetic coupling between subsequent Fe layers. The existence of a GMR effect in Fe/Pt multilayers is very promising for technological applications (e.g., magnetoresistance sensors).     

Characterizations of NiCu/Cu Multilayers: Dependence of Nonmagnetic Layer Thickness

A series of NiCu/Cu multilayers were grown on (110) textured polycrystalline Cu substrates from a single electrolyte under potentiostatic deposition conditions. Microstructure, magnetoresistance and magnetic properties of the multilayers were investigated as a function of the nonmagnetic layer thicknesses. The structural studies by X-ray diffraction revealed that the multilayers have face-centered-cubic structure with preferred (110) crystal orientation as their substrates. The composition of the deposits determined by energy dispersive X-ray spectroscopy showed that the Cu content of the films increased as the Cu layer thickness increased. The scanning electron microscope studies showed that samples have homogeneous and smooth surfaces. Multilayers exhibited either anisotropic magnetoresistance (AMR) or giant magnetoresistance (GMR) depending on the non-magnetic Cu layer thickness. The multilayers with Cu layer thickness thicker than 0.7 nm exhibited GMR, but the AMR effect was observed to be dominant for the Cu layer thickness less than 0.7 nm. The GMR curves are broad in shape and the nonsaturated curves indicated the predominance of a superparamagnetic contribution. The GMR magnitudes of NiCu/Cu multilayers are found to be about 1–1.5 %. The vibrating sample magnetometer measurements revealed that the saturation magnetization decrease with increasing nonmagnetic layer thickness. The changes in the magnetic and magnetotransport properties might arise from the change in the Ni and Cu content of the samples caused by the variation of Cu layer thicknesses.     

Interface defects and formation of non-collinear magnetic ordering in Fe/Cr multilayers

Non-collinear magnetic structure of Fe/Cr multilayers was investigated within the framework of Periodic Anderson model (PAM) in mean field approximation for Coulomb repulsion on sites. Self-consistent calculations were performed for the superlattices with different step width at the interface. It is shown that due to frustration in the interface region the ground state corresponds to non-collinear orientation of magnetic moments near steps. This non-collinear ordering penetrates on a large distance from the interface both in Fe and Cr layers and leads to the non-collinear magnetic coupling between Fe layers through the Cr spacer. Angle between magnetic moments of Fe slabs depends strongly not only on the thickness of the Cr spacer but also on the interface structure at atomic scale. It is found that only very specific types of the interface defects with plural frustrations can give out-of-plane orientation of magnetic moments.     

Vickers hardness and deformation of Ni/Cu nano-multilayers electrodeposited on copper substrates

Ni/Cu nano-multilayers were fabricated by an electrodeposition technique. Ratio of the Ni:Cu layer thickness was kept at 1:1. By laminating nickel and copper layers at a very narrow spacing, we obtained highly-densified parallel interfaces which can give rise to high strength. Dependence of Vickers hardness and tensile deformation on individual layer thickness h was investigated on the Ni/Cu multilayers. The Vickers hardness increased with decreasing layer thickness for the multilayers of h≥ 10 nm. This change in the hardness was consistent with the Hall-Petch relation. At the 10 nm layer thickness, the hardness attained more than three times higher than that of the copper substrate. On the other hand, the hardness decreased rapidly with the layer thickness at h < 10 nm. The tensile deformation tests were also carried out at the substrates coated with the multilayer of h = 5, 20 and 100 nm. The SEM observations revealed that the slip lines of the deformed substrates were terminated by the multilayer at the multilayers of h = 20 and 100 nm. On the other hand, a lot of slip lines penetrated into the multilayer of h = 5 nm. These slip observations were compatible with the layer thickness dependence of the hardness.     

Wear-induced microstructure in Ni/Cu nano-multilayers

Sliding wear tests were carried out in order to investigate wear resistance and resultant microstructure of Ni/Cu multilayers. The Ni/Cu multilayers having the component layer thickness h ranging from 5 to 100 nm were fabricated on copper substrates using the elecrodeposition technique. It was found that the wear depths in the multilayers were less than one-fifth of that of a conventional nickel coating at a high load condition. The wear resistance of the multilayer was almost independent of the component layer thickness, except the multilayer of h = 100 nm whose resistance was lower than those of the others. The observation of cross section revealed that the grains were generated locally near the worn surface in the Ni/Cu multilayers. Surface cracks were grown in such grained areas. The multilayer having a large grained area showed relatively low wear resistance. From the TEM observation, there were many equiaxed grains without the laminated structure. It is conceivable that the equiaxed grains without the laminated structure were formed due to dynamic recrystallization occurring after the laminated structure was annihilated by severe deformation. Assuming that the annihilation period is required for the wear of the Ni/Cu multilayer, the high wear resistance can be obtained regardless of the strengths of the multilayers.     

First-Principles Investigation of Electronic Structure Features of TbOFeAs Compound

We report a theoretical investigation on the electronic and magnetic properties of rare-earth pnictide parent compound, such as TbOFeAs. Employing first-principles method supplemented by the local spin density approximation (LSDA), we discuss the electronic structure with the incorporation of the role of Coulomb on-site repulsion (U) of Tb 4f states as well as the spin-orbit (SO) coupling on the magnetic and nonmagnetic phases. For ferromagnetic (FM) and antiferromagnetic (AFM) phases, we have determined the spin and orbital magnetic moments of Tb ions and confer the significance of the spin-orbit interaction of Tb 4f states in this parent compound. In the FM state, the reduction of Fe moment is about a factor of 3.5 with respect to AFM configuration. The most energetically favorable state is AFM configuration. Our theoretical findings surmise that the magnetic moments on Fe sites carry an AFM order. Based on LSDA + U + SO approximation, we infer that the Tb magnetic moments also carry an AFM order, albeit the spin Tb sites in TbO layer possess the same orientation as the Fe spins in FeAs layer. With the incorporation of on-site Coulomb repulsion and spin-orbit interaction in AFM state, the Fe 3d states are large near the Fermi level and this phase is illustrating a metallic behavior. Moreover, the Fermi surface topology and nesting features are presented.     

Atomistic modeling of plastic deformation in B2-FeAl/Al nanolayered composites

In this work, the deformation response of the B2-FeAl/Al intermetallic composites, as a model material system for nanolayered composites comprised of intermetallic interfaces, has been explored. We use atomistic simulations to study the deformation mechanisms and the interface misfit dislocation structure of B2-FeAl/Al nanolayered composites. It is shown that two sets of dislocations are contained in the interface misfit dislocation network and are correlated with the initial dislocation nucleation from the interfaces. The effects of layer thickness on the uniaxial deformation response of the B2-FeAl/Al multilayers are investigated. We observed that under compressive loading the smaller proportion of the FeAl layers leads to the lower overall flow stress. Under tensile loading, the void formation mechanism is investigated, suggesting the interface structure and the dislocation activities in the FeAl layers playing a significant role to trigger the strain localization which leads to void nucleation commencing at the interface. It is also found that the deformation behavior in the “weak” Fe/Cu interface behaves substantially different than that of the “strong” FeAl/Al interface. The atomistic modeling study of the nanolayered composites here underpinned the mechanical response of “strong” intermetallic interface material systems. There is no void nucleation during the entire plastic deformations in the Fe/Cu simulations, which is attributed to much higher dislocation density, more slip systems activated, and relative uniformly distributed dislocation traces in the Fe phase of the Fe/Cu multilayers.

     

Influence of a Pb buffer layer on structural and magnetotransport properties of Co/Cu multilayers

In this paper changes of structure and magnetotransport properties of Co/Cu multilayers were observed as a function of the Pb buffer layer thickness. Structural analysis indicated that the Pb buffer leads to the decay of superlattice periodicity. Surface topography of the top layer of the Co/Cu multilayers observed by SFM allowed the determination of surface roughness which is relatively large and weakly depends on buffer thickness. This effect is accompanied by the continuous rise of island size that reaches a diameter around 200 nm for Co/Cu multilayers deposited on 40 nm Pb buffer. AES experiments show significant segregation of Pb to the surface. A small magnetoresistance effect ΔR/R measured for Co/Cu multilayers deposited on an Pb buffer is almost independent of the thickness of the buffer layer. This behavior of ΔR/R could be understood by assuming that discontinuous ferromagnetic layers, bridged through the Cu spacer, are formed.     

Magnetic effect in multilayer Fe/Cr films

Ferromagnetic resonance measurements on different Fe/Cr multilayer samples with the same thickness of the spacing Cr layers suggest that these multilayers have a different magnetic behavior depending on the thickness of the active Fe layers. In this work we show that the change of magnetic behavior from bulk mode to surface mode is observed only when the thickness of the Fe layers is less than 4 nm. This is attributed to the Fe layer thickness and to the relative thickness of the magnetic and non-magnetic layers. ©1999Kluwer Academic Publishers     

Room-temperature ferromagnetism in Cu doped GaN nanowires

We report magnetism in Cu doped single-crystalline GaN nanowires. The typical diameter and the length of the Ga1-xCuxN nanowires (x = 0.01, 0.024) are 10-100 nm and tens of micrometers, respectively. The saturation magnetic moments are measured to be higher than 0.86 microB/Cu at 300 K, and the Curie temperatures are far above room temperature. Anomalous X-ray scattering and X-ray diffraction measurement make it clear that Cu atoms substitute the Ga sites, and they largely take part in the wurtzite network of host GaN. X-ray absorption and X-ray magnetic circular dichroism spectra at Cu L(2,3) edges show that doped Cu has local magnetic moment and the electronic configuration of it is mainly 3d9 but mixed with a small portion of trivalent component. It seems that the ionocovalent bonding nature of Cu 3d orbital with surrounding semiconductor medium makes Cu atom a mixed electron configuration and local magnetic moments. These outcomes suggest that the Ga1-xCuxN system is a room-temperature ferromagnetic semiconductor.     

Monte Carlo Investigation of the Magnetization Distribution in Fe/Cu Multilayer

By applying Monte Carlo simulations in the canonical ensemble, investigation of the magnetic properties of Fe/Cu multilayers is carried out in comparison with Mössbauer experimental results. From differing relaxation rates for spins with few near-neighbor spins and particularly those located at the interface, the Mössbauer result can be simulated. Our model consists of an alternate stacking of iron and copper layers (Fe _nFe/Cu _nCu) with disordered interface. The simulation results confirm that the concentration of interface alloys (Fe_1?x Cu _x ), measurement temperature and magnetic layer thickness modify systemically the magnetization distribution in the Fe/Cu multilayers. The result is compared with Mössbauer experimental analysis. Meaningful results are obtained by analysing the dynamical microscopic behaviour of individual spins.     

Magnetoresistance behaviour in CoFe/Cu multilayers: thin Cu layer effect

The magnetoresistance properties of the CoFe/Cu multilayers have been investigated as a function of thin non-magnetic Cu layer thickness (from 2.5 to 0.3 nm). CoFe/Cu multilayers were electrodeposited on Ti substrates from a single electrolyte containing their metal ions under potentiostatic control. The structural analysis of the films was made using X-ray diffraction. The peaks appeared at 2θ ≈ 44°, 51°, 74° and 90° are the main Bragg peaks of the multilayers, arising from the (111), (200), (220) and (311) planes of the face-centered cubic structure, respectively. The magnetic characterization was performed by using vibration sample magnetometer in magnetic fields up to ±1600 kA/m. At 0.6, 1.2 and 2.0 nm Cu layer thicknesses, the high saturation magnetization values were observed due to antiferromagnetic coupling of adjacent magnetic layers. Magnetoresistance measurements were carried out using the Van der Pauw method in magnetic fields up to ±1000 kA/m at room temperature. All multilayers exhibited giant magnetoresistance (GMR), and the similar trend in GMR values and GMR field sensitivity was observed depending on the Cu layer thickness.     

Depth profile analysis of electrodeposited nanoscale multilayers by SNMS

As early as 10 years after the discovery of the giant magnetoresistance (GMR) the magnetic/non-magnetic multilayers found their first application in the read-out units of magnetic recording media, and the hard disk drives with GMR-based sensors since gained a dominating market share. In spite of the large number of works published on nanoscale multilayers, data on the depth profile of electrodeposited multilayer samples are very scarce. This work deals with the depth profile analysis of electrodeposited CoNiCu/Cu and Co/Cu multilayers films. Commercial Cu sheet and a Cr/Cu layer evaporated onto Si (1 1 1) surface were used as substrates with high and low roughness, respectively. The Secondary Neutral Mass Spectrometry (SNMS) depth profile analysis clearly revealed the layered structure of the samples. The resolution of the individual layers varied with the initial roughness of the substrate. The SNMS spectra showed that the oxygen incorporation into the layers is insignificant. When both Ni and Co are present in the magnetic layer, the composition of the samples is influenced by both the anomalous codeposition properties of the iron-group elements and the mass transport of the corresponding ions in the electrolyte. This observation draws the attention to the possible inhomogeneity of the magnetic layers in electrodeposited samples.     

Substrate-induced magnetic ordering and switching of iron porphyrin molecules

To realize molecular spintronic devices, it is important to externally control the magnetization of a molecular magnet. One class of materials particularly promising as building blocks for molecular electronic devices is the paramagnetic porphyrin molecule in contact with a metallic substrate. Here, we study the structural orientation and the magnetic coupling of in-situ-sublimated Fe porphyrin molecules on ferromagnetic Ni and Co films on Cu(100). Our studies involve X-ray absorption spectroscopy and X-ray magnetic circular dichroism experiments. In a combined experimental and computational study we demonstrate that owing to an indirect, superexchange interaction between Fe atoms in the molecules and atoms in the substrate (Co or Ni) the paramagnetic molecules can be made to order ferromagnetically. The Fe magnetic moment can be rotated along directions in plane as well as out of plane by a magnetization reversal of the substrate, thereby opening up an avenue for spin-dependent molecular electronics.     

Microstructure,Magnetic and Magnetoresistance Properties of Electrodeposited [Co/Zn]<Subscript>50</Subscript> Multilayers

In this work, we report experimental results concerning the giant magnetoresistance effect and the magnetic properties of [Co/Zn] _n and [Zn/Co] _n electrodeposited multilayers. Two series of multilayers starting with Co and Zn layers, respectively, were grown onto Cu (100) substrate under identical conditions. We investigated the effect of the seed layer (Co or Zn) and Zn layer’s thicknesses on the magnetic and magnetoresistant properties of electrodeposited multilayers. We found out that the magnetic anisotropy and the shape of the hysteresis cycle are strongly influenced by the multilayer features. The magnetic properties of the [Co/Zn] _n and [Zn/Co] _n strongly depend on the Zn thickness and on the seed layer (the first layer deposited onto the Cu substrate). The coercivity fields varied between (145 Oe–208 Oe) as a function of the Zn layer’s thicknesses. [Co/Zn] _n and [Zn/Co] _n electrodeposited multilayers display magnetoresistance (14%) effect which can be explained mainly by the exchange interaction among neighboring layers and by the spatially inhomogeneous magnetic structure of the nanostructured multilayer; in addition, the thickness of the Zn interlayer has an important role on the transportation and diffusion processes.