Cu-Co Alloys Research Articles

電析磁性ナノ構造の機能性

Electrodeposition is a promising candidate for the preparation of magnetic nanostructures. Multilayered fcc Co/Pt and CoNi/Pt nanostructures grown on a Pt (111) or Cu (111) single-crystal substrate by electrodeposition under potential control respectively exhibit remanent perpendicular magnetization and large coercivity, which depend on the deposition overpotential and hence the multilayer growth mechanism. Giant magnetoresistance and oscillatory interlayer coupling have been observed ire a fcc (111) textured Co/Cu multilayered nanostructure. Moreover, a large saturation magnetoresistance of more than 20% has been achieved at room temperature for a heterogeneous Co-Cu alloy, which consists of ultrafine fcc Co-rich clusters in a nonmagnetic Cu matrix.

Microstructural Characterization of Granular CuCo Rapidly Solidified Ribbons

The microstructure and magnetic properties of granular CuCo alloys obtained by planar flow casting in controlled atmosphere have been characterised by transmission electron microscopy (TEM), small angle neutron scattering (SANS), and magnetisation measurements. The magnetoresistance measurements have been performed using the classical dc four probe method. The influence of quenching rate on the structural and magnetic properties has been investigated. The TEM and SANS data indicate a marked change in the morphology for samples obtained with wheel velocities greater than 19 m/s, and in the same samples a significant decrease of the magnetic moment is observed. The as cast samples, as well as the ones subjected to isothermal annealing at 713 K or to Joule heating up 7 A, show little variation of their microstructure. Treatments with 10 A Joule currents induce the formation of well defined Co precipitates. The effect of cold lamination on the giant magnetoresistance properties has also been investigated.

Magnetic Properties of Nano-Particulate Cu-Co Alloy on the Route of Bulk Mechanical Alloying

Bulk mechanical alloying was used to fabricate the supersaturated solid solution of Cu 80 Co 20 . The nano-Co particulates Cu-matrix alloy was made by low-temperature annealing. Both the annealing temperature and time were parametrically varied to describe the relationship between process condition and magnetic properties. The maximum magnetoresistance ratio under 1.0 MA/m at room temperature of 4.9 % was measured for this nano-particulate material with the mean Co-particulate size of 6 nm. From the measured magnetic properties, the effect of particulate size of Co in Cu-matrix on the magnetic properties was discussed.

Kinetics of Decomposition in Copper–Cobalt: A Time-Resolved ASAXS Study

The decomposition kinetics of dilute Cu-Co alloys containing 0.5 at.% Co were investigated by time-resolved anomalous small-angle X-ray scattering (ASAXS). No evidence for the existence of compositional fluctuations preceding the formation of Co precipitates was found. The separated scattering curves of the Co precipitates show 'self-similarity'. The results are interpreted in terms of a classical process of decomposition and values of 0.18 J m -2 for the interfacial energy and 10.9 A for the critical radius are calculated.

Mesoscopic 'volcano' structures formed in the epitaxial growth of Co on Cu

Co has been grown by molecular beam epitaxy on Cu (111), with the Cu made pinhole-free by use of Au surfactant. We find that 'volcano'-like structures of metastable Cu-Co alloy self-assemble during Co growth at 500oC. This happens because the 'crater' affords a pathway for free energy reduction as the Cu migrates to the surface by a surface pump mechanism discussed earlier by the present authors.

Correlation effects among nanometre-sized clusters in Cu-Co melt-spun alloys with giant magnetoresistance

Abstract Magnetic correlation effects in nearly superparamagnetic granular Cu100_xCox systems exhibiting giant magnetoresistance (GMR) are studied by analysing the flat-top parabolae obtained when the GMR is plotted as a function of reduced magnetization. The nature of GMR in granular metallic systems is discussed in detail, taking into account the typical time constants involved in the short-time dynamics of magnetic moments in a local magnetic field. An outline of a theory providing the first quantitative explanation of the effect of magnetic correlation on GMR is given. The role of Co content in the sizes and distances of Co particles and in their degree of magnetic correlation are discussed, examining, for studied composition each, the samples characterized by the best GMR values.

Properties of Magneto-Optical Co-Pt and Magnetoresistive Co-Cu Alloy Films

ABSTRACTCo-Cu films of 1000 – 1500 Å thick were produced by means of flash-evaporation technique at 150 K, 400 K, 520 K and 720 K. The Co022Cu078 films produced at 400 K or 520 K exhibit the maximum value of MR of 16.5% at 77 K and 1.5 T. Co-Pt films of 200 Å thick with Pt buffer layers of 200 Å thick were prepared at room temperature (RT) by ultrahigh-vacuum cosputtering. The saturation polar Kerr rotation angles (θk) at RT of CoPt3 film are all larger than those of the Co/Pt multilayered (ML) film of the corresponding composition. The surface morphology, mean grain and magnetic domain sizes, and size distributions of the grains and magnetic domains of the Co alloy films have been investigated by magnetic force microscopy (MFM) and atomic force microscopy (AFM). The results were correlated with those obtained by using MO and optical spectroscopy at both RT and low temperatures (LT) in a spectral ramge of 0.5 – 4.4 eV, and magnetic investigations at both RT and LT including field-dependence measurement of MR. The overall results reflect the changes in the electronic structures and magnetic states of the constituent elements, induced by the structural transformation or correlated with the structures of the films.

Structural study by NMR in Co/Cu multilayers at second antiferromagnetic maximum

A 59Co NMR study has been carried out in a series of Co/Cu multilayers with t Cu = 20 Å and 2 Å < t Co < 30 Å. The Co layer is found to have a strained fcc structure with hcp stacking faults above 15 Å Co. The lateral size of columnar grains is estimated to be 45 Å. A transformation from continuous to granular Co layer is evidenced below 10 Å Co. Below 3 Å all deposited Co exists in form of a diffused CoCu alloy and Co clusters disappear. These structural changes, together with the decrease of a number of domain walls inferred from the rapid drop of NMR enhancement factor are to be related with the previously observed disappearance of magnetoresistive hysteresis.

Cu-Co単結晶からの強磁性体Co粒子の析出・成長に及ぼす外部磁場効果

The effects of magnetic field on the growth of ferromagnetic Co precipitate particles in Cu-2.7 mass%Co alloy single crystals have been examined by the magnetic anisotropy measurement. The (110) disk specimens of the solution-treated Cu-Co alloy were aged at 973 K for 1∼24 h under the magnetic field of 1.25 MA/m parallel to the [001] direction. The magnetic anisotropy was examined by measuring magnetic torque around the (110) plane. Considering the crystal and shape magnetic anisotropies, the torque curves were analyzed theoretically. It is found that the Co particles which precipitate under the magnetic field slightly elongate to the [001] direction parallel to the magnetic field. Furthermore, the torque curves were also sensitive to the coherent→incoherent shape change of the Co particles. The results can be explained by considering the minimization of the magnetostatic energy of the Co particles.

Microstructure of Cu-Co alloys solidified at various supercoolings

The effects of supercooling on the microstructure of Cu-Co alloys containing 10 to 65 wt pct Co were investigated. Supercooling of the alloys below a characteristic temperature,t SEP, resulted in a metastable phase separation into two liquids: one Co rich (L1) and the other Cu rich (L2). The microstructure of the phase-separated alloys consisted of spherulites of one phase embedded in a matrix of the other. The spherulites in alloys containing less than 40 wt pct Co were solidified from the L1 melt and from L2 in alloys containing more than 55 wt pct Co. Supercooling of copper alloys containing around 50 wt pct Co resulted in a duplex structure of fine and coarse dendrites. Microstructural evidence was presented for the formation of ae-Cu metastable phase in alloys containing less than 30 wt pct Co.

Epitaxial Growth of Metastable Co-Cu Alloys by a Surface Pump Mechanism.

We report the operation for Co-Cu alloys of a surface pump mechanism whereby bulk metastable alloys are formed during epitaxial growth. This happens when a material with a larger surface energy (Co) is grown on a film with a lesser surface energy (Cu). The specific pathway transports Cu through pinholes by surface diffusion at temperatures which are too low for the metastable alloy to decompose by bulk diffusion. {copyright} {ital 1996 The American Physical Society.}

Static and Dynamic Magnetic Properties of Melt-Spun Granular Cu&lt;sub&gt;100-x&lt;/sub&gt;Co&lt;sub&gt;x&lt;/sub&gt; Alloys

The magnetic properties of granular Cu-Co alloys produced by melt-spinning have been investigated by DC and AC susceptibility measurements. The results indicate the occurrence of blocking processes of interacting superparamagnetic particles. The effects of annealing by Joule heating at different current densities are discussed in relation to the giant magnetoresistance properties of the system.

Dynamic scaling of phase separation in CuCo alloys

The Langer-Bar-Miller (LBM) theory and the non-linear Cahn theory on phase separation dynamics in alloys have been reviewed briefly and applied to calculate dynamics of phase separation of homogenized CuCo alloys. The three-parameter technique of the LBM theory for evaluating compositional fluctuations is applicable only in a narrow range of alloy composition. In this narrow range, the as-calculated equal time structure function S(k,t), where k is the wave vector and t is time, does not follow the universal scaling hypothesis in the late stage of phase separation: km3S(k/km) = f(k/km), where f is a scaling function and km is the peak position of S(k,t). The kinetic growth exponent, defined by km as a power function of time, does not agree with prediction of the scaling analysis. The non-linear Cahn approach has been applied to evaluate the profile and amplitude spectra of the compositional fluctuations during phase separation in both meta-stable and non-stable regions. Overall range of alloy composition, the structure function S(k,t) in the late stage of phase separation shows scaling behaviours quite well consistent with the universal scaling hypothesis. The kinetic growth exponent equals to 0.22 in the non-stable region, well consistent with prediction of the scaling analysis. In the meta-stable region, this exponent increases up to 0.28 as the alloy composition decreases down to 0.01, exhibiting good agreement with theory of coarsening kinetics.

X-Ray investigation of one-dimensional disorder in supersaturated Co-Cu Alloys

Single crystalline grains of coarse polycrystals of Co alloyed with 8 to 10 wt\% Cu were studied by X-ray diffractometry. A martensite phase with one-dimensionally disordered h.c.p. and long-period polytype structures is observed. Up to four martensite variants (domains) are formed in one crystal, where the h.c.p. and the polytypic phases are stacked along any one of the four 111c directions. Stacking disorder in these martensite structures was analysed by Monte Carlo simulation. The polytype structures are formed both after quenching and slow cooling from 1100 °C. The long-period polytype in quenched samples has a disordered 7T1 structure which can be described in terms of a non-random distribution of h.c.p. lamellae within the f.c.c. matrix. The high-temperature f.c.c. phase has a perfect structure before and after transformation. It is suggested that the formation of polytypes is related to small variations of chemical composition in different grains of the material.

Thermoremanence and zero-field-cooled susceptibility measurements of electrodeposited granular CuCo alloys

Magnetic measurements have been made on the electrodeposited, thin-film alloy Cu 1− x Co x with x = 0.06-0.35 in the temperature range 5–300 K. Values of remanence derived from hysteresis loops are compared with the thermoremanence. Zero-field-cooled and field-cooled measurements of the susceptibility have also been made. It is shown that the size of the Co-rich clusters increases with Co concentration, and that particle interactions play an important role in this system.

Structure function and dynamic scaling of CuCo alloys during phase separation

Based on the nonlinear Cahn theory and the Langer equation of motion for the structure function, a simple computational technique of structure function S(k, t), where k is the wave vector in Fourier space and t the time, has been proposed. This technique has been applied to calculate the dynamics of phase separation of CuCo alloys over the whole range of composition. A series of dynamic characters and scaling properties involved in the evolution of the structure function have been analysed and a comparison with the current theories has been made, demonstrating that the nonlinear Cahn theory makes sense as a general dynamic theory of phase separation in alloys.

Theoretical Analysis of Phase Decomposition in Real Alloy Systems based on the Non-Linear Diffusion Equation

A Khachaluryan type diffusion equation including solute atom occupation probability in atom site which should be so called discrete type non-linear diffusion equation, is proposed. The composition dependence of atomic interchange energy is taken into account so as to be applicable for the real alloy system, and the non-linear composition fluctuation is also introduced. The two dimensional computer simulations are performed for the phase decomposition of the Al-Zn, Cu-Co and Fe-Mo binary alloy systems by utilizing the thermodynamic data on the equilibrium phase diagrams. The microstructures theoretically obtained are coincident with experimental results of the real alloys. A long and loose solute concentration is developed with progress of annealing at the single phase region of phase diagram, that is caused by a non-linear effect of the composition fluctuation. Such the solute concentration effectively assists the nucleation of precipitate particularly in the lower solute content alloys.

Microstructure development during rapid solidification of highly supersaturated Cu-Co alloys

We report a detailed microstructure analysis of rapidly quenched Cu 1− x Co x (0.1 ⩽ × ⩽ 0.5) alloys. The chemical homogeneity of the alloys was investigated on nanometer scale using a combination of AP/FIM- and TEM-studies. Cu-Co alloys with a Co concentration up to 10 at.% Co can be prepared homogeneously by rapid quenching. For larger Co contents, the microstructure is determined by a competition between polymorphic solidification, nucleation of the Co-rich solid solution from the melt, and spinodal decomposition of the unstable supersaturated solid solution. The microstructures are discussed in terms of the kinetics of the Cu-Co system, provided quantitatively by recent thermodynamic calculations.

On the structure and homogeneity of solid solutions: The limits of conventional X-ray diffraction

Abstract The characterization of solid solutions by conventional X-ray diffraction (XRD) is critically assessed using the example of Cu-Co alloys. In particular, the limits of this technique for investigating compositional inhomogeneities are discussed. Coherent Cu-Co multilayer films are taken as a model for a decomposed solid solution, with the layer thicknesses being taken as representative for the sizes of heterogeneous domains. It is shown that the XRD patterns of these multilayer films continuously change with decreasing layer thickness from that of two bulk phases to a single pattern identical to that of a homogeneous solid solution, the transition occurring at a layer thickness of about 10 nm. Below this layer thickness, the lattice spacing derived from the single diffraction pattern exhibits an average value determined by the overall composition. By definition, this average lattice spacing varies continuously between the pure elements with a Vegard law dependence when the overall composition is changed. However, since the multilayer is heterogeneous, the average lattice spacing is nowhere present in the specimen. The results indicate that coherent inhomogeneities in Cu-Co solid solutions must be larger than several nanometres before they can be detected using conventional XRD. With these conclusions XRD measurements of Cu-Co alloys after preparation as well as during subsequent annealing are considered. The results demonstrate the important role of structural coherence on diffraction measurements. It is concluded that it is not possible to determine the supersaturation of a solid solution using conventional XRD. Complementary measurements have to be performed in order to prove the homogeneity of a supersaturated alloy.

Microstructure and magnetoresistance in Cu-Co alloy thin films

The microstructure and magnetoresistance of as-prepared and annealed Cu-Co alloy films were investigated to explore the origin of giant magnetoresistance and to examine the development of new magnetic materials. The Cu matrix in as-prepared Cu-Co film consists of small grains that are less than 20 nm in diameter. The Co atoms are dissolved in Cu. For alloy films with a Co concentration below 35%, the magnetoresistance ratio increases with higher Co concentrations. Magnetoresistance then decreases for films containing more than 35% Co. After annealing at 400 °C for 1 h, Cu grains increase from 20 to 50 nm in diameter. The Co grains precipitate along the grain boundary, which has a maximum size of approximately 20 nm in diameter. Magnetoresistance decreases according to Co grain growth.