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.     

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.     

Magnetoresistance enhancement of amorphous CoNbZr-buffered magnetoresistive multilayers

Amorphous CoNbZr alloys are thermally stable and thus have been intensively studied as soft layers of a pseudo-spin-valve (PSV). By depositing a wedge-shaped Co inset layer (IL) between the CoNbZr and Cu layer, we were able to simultaneously fabricate CoNbZr(t_CNZ)/Co(0-3 nm)/Cu/Co PSVs with various CoNbZr and Co IL thicknesses. We have investigated the dependence of magnetic properties, giant magnetoresistance (GMR) effect, and microstructure on the thickness of the amorphous CoNbZr buffer layer. The GMR enhancement behaviour of the PSVs with different CoNbZr thickness was also studied along the inset Co wedge. By optimizing the thickness of CoNbZr and Co IL, a maximum GMR ratio of 7% was obtained in the stack of CoNbZr(4 nm)/Co(1.2 nm)/Cu(2.2 nm)/Co(4 nm).     

Properties of Electrodeposited CoFeNi/Cu Superlattices: The Effect of CoFeNi and Cu Layers Thicknesses

CoFeNi/Cu superlattices were grown on Ti substrate by electrodeposition as a function of the ferromagnetic and non-magnetic layer thicknesses. In order to examine the effect of the Cu layer thickness on the film properties, the Cu layer thickness was changed from 0.5 to 6 nm, while the CoFeNi layer thickness was kept constant at 4 nm. Also, for the CoFeNi layer effect, the CoFeNi layer thickness was changed from 2 to 15 nm, while the Cu layer thickness was fixed at 4 nm. The structural analysis studied by X-ray diffraction indicated that the superlattices have face-centered-cubic structure. Magnetic characteristics were investigated by vibrating sample magnetometer. From the hysteresis curves, the coercivity and the saturation magnetization were determined. It was found that the easy-axis of the films is parallel to the film plane. Magnetoresistance measurements were made by the Van der Pauw method at the room temperature with magnetic fields up to ±12 kOe. All superlattices exhibited giant magnetoresistance (GMR). As the ferromagnetic layer thickness increased up to 4 nm, the GMR value increases up to 22 % and then decreases. The superlattices saturated at the lower magnetic layers with increasing ferromagnetic layer thickness. The maximum GMR value was obtained to be 22 % for a superlattice with 375[CoFeNi(4 nm)/Cu(4 nm)].     

Microstructure and giant magnetoresistance of FeCo–Cu nanogranular films

A series of (Fe₅₀Co₅₀) _x Cu_1−x granular films were prepared using a magnetron controlled sputtering system. The microstructure and giant magnetoresistance (GMR) of FeCo–Cu films deposited at room temperature and then annealed at various temperatures were investigated through transmission electron microscope and conventional four probes method under room temperature, respectively. The results revealed that all FeCo–Cu films consisted of fine FeCo granules uniformly dispersed in the Cu matrix and formed fcc structure. By a non-linear least-squares method, the size distribution of FeCo granules in all as-deposited films satisfied a log-normal function. Upon varying the magnetic volume fraction (x), the GMR of as-deposited FeCo–Cu films reached a maximum of about 0.7% at the volume fraction of 31% FeCo, corresponding to the fact that the GMR has a non-monotonic relationship with the granule size. With increasing the annealing temperature, the GMR of films with lower volume fraction reached a peak at higher temperature, while for films with higher volume fraction the GMR reached a peak at lower temperature. In addition, the relationships between the full width at half maximum (FWHM) or the sensitivity of the GMR and the volume fraction were discussed in detail.     

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…     

NiFe／Co／Cu／Co结构自旋阀GMR效应及Co夹层的影响研究

用射频磁控溅射发射法成功制备了ＮｉＦｅ／Ｃｕ／Ｃｏ自旋阀多层膜材料,改变Ｃｕ层的厚度,研究材料的ＧＭＲ效应与Ｃｕ层厚度的关系,结果表明Ｃｕ为２．５ｎｍ时样品的ＭＲ值最大,其磁电阻效应ＭＲ可达１．６％,在ＮｉＦｅ和Ｃｕ之间插入一Ｃｏ薄夹层,通过对不同温度厚度Ｃｏ夹层的样品的ＭＲ曲线及磁滞回线的研究,讨论了Ｃｏ夹层对样品磁电阻的影响并分析了原因,结果表明插入适当的Ｃｏ层将提高材料的磁电阻效应,可达２．     

Asymmetry of Magnetization Reversal of Pinned Layer in NiFe/Cu/NiFe/IrMn Spin-Valve Structure

Two types of asymmetry in giant magnetoresistance (GMR) are observed which are not related to a training effect, but indicate different mechanisms of magnetization reversal of the pinned layer in spin-valve (SV) structures for ascending and descending field scans. GMR, exchange bias and coercivity in Si/Ta/NiFe/Cu/NiFe/IrMn/Ta SV-structures were investigated as functions of the thickness of the nonmagnetic spacer. The spacer thickness effects are discussed in correlation with layers microstructure and interfaces morphology variations.     

Growth and characterisation of electrodeposited Co/Cu superlattices

Ferromagnetic/non-ferromagnetic Co/Cu superlattices were grown on polycrystalline Titanium (Ti) from a single electrolyte by electrodeposition. Microstructure and magnetoresistance (MR) of the superlattices were investigated as a function of the electrolyte pH as well as the layer thicknesses. Structural characterisation by X-ray diffraction (XRD) showed that the superlattices have face-centred cubic (fcc) structure with a strong (111) texture at the studied pH levels, but the texture degree is affected by the electrolyte pH. The scanning electron microscope (SEM) studies revealed that the superlattices grown at low pH (2.0) have smoother surfaces compared to those grown at high pH (3.0). The superlattices exhibited either anisotropic magnetoresistance (AMR) or giant magnetoresistance (GMR) depending on the Cu layer thickness. The shape of MR curves changes depending on the combination of Co and Cu layer thicknesses. The superlattices with Co layers less than 3 nm and Cu layers less than 2 nm have broad and non-saturating curves, indicating the predominance of a superparamagnetic contribution, possibly due to the discontinuous nature of the ferromagnetic (Co) layer. For superlattices with the same bilayer and total thicknesses, the GMR magnitude decreased as the electrolyte pH increased. Besides possible structural differences such as the texture degree and the surface roughness, this may arises from the variation in the Cu content of the ferromagnetic layers caused by the electrolyte pH.     

Magnetic and Structural Properties in Co/Cu/Co Sandwiches with Ni and Cr Buffer Layers

1. IntroductionThe gial magnetoresistance (GMR) effect occursin multilayers of ferromagnet ic / nonmagnet ic met almultilayers and sandwiches[1'2]. Many material systems, such as Fe/Crl'], Co/Cut'l, have exhibited theGMR properties. The saturation field in these multilayers is usually very large due to the strong exchange coupling field between the adjacent magneticlayers. Non-coupled type multilayers consisting of twomagnetic components with different coercive forces,and relatively thick …     

用FTMS方法制备Co／Cu巨磁阻超晶格多层薄膜

本文用双靶相对磁控溅射法（ＦＴＭＳ）在云母单晶基板上原位生长了巨磁阻Ｃｏ／Ｃｕ超晶格薄膜，研究了放电气压及背景真空对薄膜结构和电磁性质的影响。     

Structural evolution during growth of electrodeposited Co-Cu/Cu multilayers with giant magnetoresistance

The maximum room-temperature giant magnetoresistance (GMR) of electrodeposited Co-Cu/Cu multilayers produced during this work was approximately 9% at 8 kOe, and it was found to decrease with increasing bilayer repeat number. A transmission electron microscopy study has revealed the fine details of the microstructure formed during growth. At the beginning of the deposition very small, nano-sized crystallites formed with both hexagonal close-packed (hcp) and face-centred cubic (fcc) crystal structures containing a high level of internal stress. The Cu-content of these small crystallites was found to depend strongly on their crystal structure (fcc or hcp). After this initial polycrystalline region, the size of crystallites increases, forming an fcc superlattice with increasing average Cu concentration at the first hundreds of repeat periods. This increase is not monotonous across the whole sample thickness. As another effect, the bending of layer planes becomes more remarkable as the growth progresses. The above inhomogeneities formed during the deposition of hundreds of bilayers could be responsible for the decrease in GMR with increasing total thickness of the multilayered samples.     

Thermal and Electrical Conductivity of Copper-Graphene Heterosystem: An Effect of Strain and Thickness

Copper-graphene (Cu/Gr) composite carries high thermal (κ) and electrical (σ) conductivities compared with pristine copper film/surface. For further improvement, strain is applied (compressive and tensile) and thickness is changed (of both copper and graphene). It is observed that electronic thermal conductivity (κ_e) and σ enhance from 320.72 to 869.765 W mK⁻¹ and 5.28 × 10⁷ to 23.01 × 10⁷ S m⁻¹, respectively, by applying 0.20% compressive strain. With the increase in copper thickness (three to seven layers) in Cu(111)/single-layer-graphene (SLG) heterosystem, κ_e increases from 320.72 to 571.81 W mK⁻¹ while electrical resistivity (ρ ∝ (1/σ)) decreases from 0.189 × 10⁻⁷ to 0.117 × 10⁻⁷ Ωm. Furthermore, with the increase in graphene thickness (one to four layers) in seven-layer Cu(111)/multilayer-graphene (MLG) heterosystem, κ_e enhances upto 126% while ρ decreases upto 70% compared with the three-layer Cu(111)/SLG. A large available state near Fermi level (of Cu/Gr heterosystem) offers the conduction of more electrons from valence to conduction bands. The increasing thickness broadens this state and enhances conduction electrons. The electron localization function decreases with increasing thickness, suggesting electrons are delocalized at copper-graphene junction, resulting in an increase of free electrons that enhance κ_e and σ. Herein, it is useful in advancing the thermal management of electronic chips and in applying hybrid copper-graphene interconnects.     

Cu层和Mo层对NiFe多层膜巨磁电阻（GMR）的影响

采用磁控溅射方法制备了ＮｉＦｅ／Ｃｕ和ＮｉＦｅ／Ｍｏ两个系列的多层膜，进行了结构，磁性和磁电阻测量，并对部分ＮｉＦｅ／Ｃｕ多层膜样品作了电镜分析，对于ＮｉＦｅ／Ｃｕ多层膜，在室温下的测量到巨磁电阻随Ｃｕ层厚度振荡的第一，二三峰。在ＮｉＦｅ／Ｍｏ多层膜样品中未发现巨磁电阻效应，讨论了非磁性 多层膜的磁性，界面结构和巨磁电阻效应。     

Giant magnetoresistance, microstructure, and application characteristics of amorphous CoNbZr-based pseudo-spin valves

We have fabricated pseudo-spin-valve (PSV) multilayers with amorphous CoNbZr alloy as a soft magnetic layer and a buffer layer by magnetron sputtering. We investigated the multilayers' giant magnetoresistance (GMR), microstructure,thermal annealing effects, and application characteristics. Our results show that the film microstructure, consequently the magnetostatic coupling effect and the magnetization reversal process, strongly depends on the CoNbZr thickness. We observed antiparallel magnetization alignments in the samples with a 2-4nm CoNbZr layer and a measured maximum GMR ratio of 6.5%. The PSV with 4 nm CoNbZr has a superior thermal stability to 400 /spl deg/C as a result of the dense and homogeneous Cu spacer. After patterning with a 6 /spl mu/m/spl times/1 /spl mu/m elliptic stripe, the structure forms a single domain. The dynamic GMR behavior under a 10 kHz sinusoidal magnetic field indicates the patterned stripe has a linear and stable GMR response. We therefore believe that PSVs with amorphous CoNbZr have good potential for spintronic devices.     

Negative Magnetoresistance in Dual Spin Valve Structures With a Synthetic Antiferromagnetic Free Layer

Giant magnetoresistance (GMR) in spin valves is due to spin-dependent scattering occurring at ferromagnet/normal metal (F/N) interfaces and/or in the ferromagnetic layers. In a spin valve with a typical F/N/F structure where the spin scattering asymmetry factor $(alpha)$ of both F/N interfaces is the same (more or less than 1), the GMR is expected to be positive. If $alpha$ is greater than one at one F/N interface and less than one at the other F/N interface, however, the GMR is expected to be negative. Here, we show that the F1/Cu/SAF/Cu/F2/IrMn dual spin valve structure exhibits negative GMR, where F1 and F2 are CoFe and ${rm SAF} = {rm CoFe}/{rm Ru} t/{rm CoFe}$, due to both opposite electron spin scattering asymmetry factor at the CoFe/Ru/CoFe interfaces as well as the electrical separation of the overall structure into two GMR spin valves connected in parallel. A GMR of 6% is observed in the structure without the Ru spacer layer, insertion of a 0.6 nm thick Ru in the SAF results in a negative GMR ratio of ${-}3hbox{%}$ , which becomes positive again at the Ru thickness of 0.8 nm, the oscillation from positive to negative MR is consistent with interlayer exchange coupling period across the Ru spacer.      

Deviations from Matthiessen's rule and longitudinal magnetoresistance in cold-worked and neutron-irradiated copper

Deviations from Matthiessen's rule (MR) in cold-worked Cu samples have been measured as a function of temperature between 4°K and room temperature. They can amount up to 100% of the residual resistivity. A second group of measurements concerns deviation from the additivity of the residual resistivities that arise in the deformed specimens when point defects are added at 4.6° K during a neutron irradiation. In a third group of experiments we investigated deviations from MR as a function of temperature in undeformed Cu samples that have been neutron irradiated at 4.6°K and annealed at different temperatures. There are two essential reasons for these deviations: (1) Defects can change the vibrational behavior of the lattice and so alter the temperature-dependent part of the electrical resistivity. The additional resistivity is found experimentally and theoretically to depend asT ⁵ on temperature at low temperatures. (2) Anisotropies in the Fermi surface of Cu and in the scattering potentials of the defects give rise to deviations from MR. These deviations can be described by a two-band model for the conduction electrons. Within the framework of this model we were able to describe the deviations that were measured in the deformed Cu samples and in Cu alloys (earlier experiments) in a consistent way, using the same parameters (ratio _N / _B of the relaxation times for the neck and belly of the Fermi surface) for the two-band model. Furthermore we could show that the enhanced resistivity increase during neutron irradiation observed in deformed Cu samples is not the consequence of an enhanced defect production rate but that it is a result of deviations from MR due to the different scattering behavior of the defects induced by irradiation and cold work. Measurements of the longitudinal magnetoresistance of the deformed and irradiated copper samples give additional information about the scattering anisotropy of the defects and therefore allow for separating the contributions of the two types of defects the combination of which causes deviations from MR.     

Stretchable magnetoelectronics

We fabricated [Co/Cu] multilayers revealing a giant magnetoresistance (GMR) effect on free-standing elastic poly(dimethylsiloxane) (PDMS) membranes. The GMR performance of [Co/Cu] multilayers on rigid silicon and on free-standing PDMS is similar and does not change with tensile deformations up to 4.5%. Mechanical deformations imposed on the sensor are totally reversible, due to the elasticity of the PDMS membranes. This remarkable performance upon stretching relies on a wrinkling of GMR layers on top of the PDMS membrane.     

Nanostructure and giant magnetoresistive properties of granular systems

This article aims to make a connection between the microstructures of various nanostructured alloys and giant magnetoresistive (GMR) properties. The GMR behavior of nanoclusters embedded in a nonmagnetic matrix differs considerably from an alloy with the content of a magnetic phase above the percolation threshold; that is to say, an increase of GMR effect upon going from 300 to 10 K for the former and a decrease of the GMR effect for the latter. The following materials systems were examined with high-resolution transmission electron microscopy and magnetoelectrical resistance measurements: magnetic Co and CoFe nanoclusters in a Au matrix, NiFe clusters in a Cu matrix, and NiFe/Cu spinodal decomposition waves with interconnection of the magnetic phase. After annealing (> or = 300 degrees C), Co particles in Au become semi- or incoherent, whereas under other conditions and in all other systems, the interfaces remain coherent. This state of coherency at the interface between magnetic particles and a nonmagnetic matrix turned out to have a detectable influence on the GMR behavior.