Co-rich Regions Research Articles

Effects of liquid separation on the microstructure formation and hardness behavior of undercooled Cu–Co alloy

Applying a melt-fluxing method, a substantial degree of undercooling up to 220 K has been obtained for Cu50Co50 immiscible alloy and the non-equilibrium solidification microstructure is investigated considering the successive processes of undercooling, liquid separation, non-equilibrium and near-equilibrium solidification. For samples undercooled by 62, 79 and 105 K, grain refinement occurs and becomes more serious without the occurrence of liquid separation and peritectic transformation. In contrast, metastable liquid demixing occurs upon the undercooling reaching 125 K, and it proceeds completely with the further increase of undercooling to 188 and 220 K, which can be ascribed to the sufficient time through the immiscible region. After liquid separation, versatile solidification microstructure for Co-rich and Cu-rich regions can be observed due to the different non-equilibrium effect. Moreover, second liquid separation becomes more serious for higher undercooling because of the enhanced immiscible effect. Finally, the solute content of primary Co-rich phase was analyzed with the consideration of solute trapping and liquid immiscible effects, which gives an excellent explanation for the variation of hardness behavior.

Uncompensated antiferromagnetic moments in Mn-Ir/FM (FM = Ni-Co, Co-Fe, Fe-Ni) bilayers: Compositional dependence and its origin

Ferromagnetic (FM) material dependence of the uncompensated (UC) antiferromagnetic (AF) moments in AF/FM exchange biased bilayers has been studied using the x-ray magnetic circular dichroism technique in the AF/FM (AF = γ-Mn-Ir, FM = Ni-Co, Co-Fe, Fe-Ni) bilayers. The direction and magnitude of the UC-Mn moment change significantly when the composition of the FM layer changes. The crystal structure of the FM layer affects the magnitude of the UC-Mn moments. The UC-Mn moments and the FM moments of Fe-rich alloys prefer the anti-parallel alignment. Conversely, the UC-Mn moments align parallel to the FM moments in Co-rich or Ni-rich regions. A first-principles calculation pertaining to the L12-Mn3Ir/FM (FM = Ni4-nCon, Co4-nFen, Fe4-nNin; n = 0, 1, 2, 3) bilayer system was carried out to characterize the UC-Mn moments near the interface. It was found that the UC-Mn moments originate from the reorientation of the magnetic moments of Mn and other ferromagnetic atoms near the AF/FM interface. The calculated result for the compositional dependence of the UC-Mn moment is in good agreement with the obtained experimental data. As a result, the dependence of the UC-Mn moment on the composition of the FM layer can be explained qualitatively based on the model that the band filling fraction modifies the direction and the magnitude of exchange coupling between AF and FM atoms, depending on the crystal structure and the composition of the FM layer.

Enhancement of perpendicular coercivity in L11 CoPt thin films by replacement of Co with Cu

Magnetic properties and microstructures of L11 (Co50−xCux)Pt50 films sputter-deposited at 350 °C on MgO(111) substrates are reported. The addition of Cu significantly improves the alignment of c-axis and chemical ordering. Perpendicular coercivity (Hc⊥) also increases markedly from 0.1 to 1.9 kOe while in-plane coercivity declines from 0.5 to 0.07 kOe for the 20 nm thick films at x=26. Similar phenomena are observed with larger effects for the 50 nm thick films. The coercive mechanism is attributed to domain-wall pinning produced by the compositional segregation of nanoscaled nonmagnetic Cu-rich and magnetic Co-rich regions within a coherent L11 crystal domain. Therefore, an intermediate value of Hc⊥ can be obtained from this hardening mechanism when further microstructure modifications are enforced, which largely increases the potential for the use in spintronic devices or patterned media.

Polyacrylonitrile and cobalt–tin compounds based composite and its electrochemical properties in lithium ion batteries

A novel composite electrode material has been prepared by easy and inexpensive route. Nanocrystalline grains of the tin-based phases CoSn 2, CoSn and SnO 2 are embedded in an amorphous layer based on the polyacrylonitrile (PAN) polymer. Maximum reversible capacities around 600 mAh/g are observed. The PAN-based layer and the oxygen atoms contribute to stabilize the intermetallic phases upon electrochemical cycling, thus improving capacity retention. The mechanisms of the reactions are explored by using X-ray diffraction (XRD) and Mössbauer spectroscopy. In the first discharge process, the cassiterite phase is destroyed and amorphous Li y CoSn x phases are formed, while some grains of CoSn and CoSn 2 remained unaffected. Lithium–tin intermetallic phases are not detected. In the charge process, the segregation of tin-based phases into Co-rich and Co-poor regions takes place.

Fabrication of a nanocrystalline Ni–Co/CoO functionally graded layer with excellentelectrochemical corrosion and tribological performance

Nanocrystalline (NC) Ni–Co/CoO functionally graded materials with excellent lubricating,high anti-corrosion and anti-wear performance were fabricated by electrodepositionand subsequent cyclic thermal oxidation and quenching. Transmission electronmicroscopy and energy dispersive x-ray spectroscopy investigations show thatbulk Ni–Co gradient deposits with an average grain size in the range of 13–40 nmdemonstrated a graded structure transition from face-centred cubic to hexagonal closepacked and graded composition changes from Ni-rich to Co-rich regions with theincrease in deposit thickness. X-ray diffraction and x-ray photoelectron spectroscopyanalysis indicated the surface layer of NC Ni–Co graded materials to be mainlycomposed of dense and ultrafine CoO with a (111) preferred orientation. The NCNi–Co/CoO functionally graded materials exhibited significantly enhanced corrosionresistance in both NaOH and NaCl solutions and remarkably improved wearresistance and dry self-lubricating performance when compared with the NC Ni andNi–Co graded deposits under dry sliding wear conditions. The higher corrosion andtribological performance of NC Ni–Co/CoO graded materials can be attributed tothe graded microstructure within the deposits, the anti-corrosion barrier of adense oxide layer and the solid lubrication effect of CoO-rich tribo-surface films.

Oxygen vacancies and ferromagnetism in CoxTi1−xO2−x−y

Cobalt-doped titanium dioxide, or CTO, has emerged in the past 2 years as a semiconducting, transparent, room-temperature ferromagnet. Very recently it has been shown that the magnetism in CTO often originates in surface nanoparticles or Co-rich regions that have a much-enhanced substitutional Co content up to 40% of Ti sites, so that magnetic CTO is not a true dilute magnetic semiconductor (DMS), but rather a fairly high-density spin system. In this work we describe a computational study of Co-rich CTO using the generalized gradient approximation to the density functional theory within the supercell model. Our total energy calculations show a strong tendency for Co-atom clustering or segregation on Ti sites. There is also a strong tendency for the oxygen vacancies to form complexes with the Co atoms. In addition, we find that the oxygen stoichiometry plays an essential role in determining the system’s magnetic order. The largest ordered moments require at least enough oxygen vacancies to put all of the Co atoms in the +2 charge state, as they indeed appear to be experimentally, so that the conventional DMS mechanism could only apply via n-type carriers. We find a small but not negligible spin density associated with Ti atoms near the vacancy sites, suggesting an F-center-mediated interaction between the much larger Co moments. We also present experimental data showing that the ferromagnetic remanence and coercive field increase with the n-type conductivity.

Corrosion Behavior of Sm–Co-Based Permanent Magnets in Oxidizing Environments

Investigations of the oxidation behavior of Sm(Co, Fe, Cu, Zr)/sub z/-type high temperature permanent magnets are of particular importance for the assessment of their stability under application conditions in the temperature range up to 500/spl deg/C. Sintered magnets of the Sm/sub 2/Co/sub 17/-type were studied regarding the dependence of surface on the exposure temperature and time in air. For the investigation of the oxide layer growth processes at elevated temperatures, time-dependent mass changes were determined using thermo-gravimetrical analysis. Oxidized magnet surfaces were characterized by means of X-ray diffraction, optical microscopy and scanning electron microscopy (incl. energy dispersive analysis). It was found from time-dependent mass changes that a parabolic growth law controls the oxidation on the Sm/sub 2/Co/sub 17/-type magnet surfaces. An oxide layer thickness of 135 /spl mu/m was measured after 312 h exposure time at a temperature of 500/spl deg/C. The outermost oxidic surface layer is mainly formed by oxidized Fe and Co species. The Sm concentration at the surface is depleted due to vaporization. The oxide layer is separated in Co-rich and Sm-rich regions as well as Cu-rich segregations near microcracks.

Electron microscopy on Co–CoO obliquely evaporated magnetic tapes

Microstructures of Co–CoO obliquely evaporated tapes were studied by transmission electron microscopy with an energy filter. The structures were the following. The columnar structure consisted of Co-rich regions enclosed by O-rich regions. CoO crystallites were densely packed between Co-rich columns where they had been considered “interstices”. The columnar structure of single layered film was different from that of double layered film.

Three-Dimensional Structuring of Electrodeposited Cu-Co Multilayer Alloys

Scanning and transmission electron microscopy (TEM) were used to characterize the microstructure of electrodeposited multilayer Cu-Co alloys. Both columnar alloys (where the Co is localized to the columns) and three-dimensional arrays (where the Co is present in regions less than 1 μm diam) were studied. The latter type was produced by successively electrodepositing a columnar alloy layer between Cu spacer layers, resulting in a semiordered, three-dimensional network of Co-rich regions in a principally Cu matrix. The composition gradient between the Co-rich regions and the Cu matrix was verified by electron energy loss spectroscopy and energy-dispersive spectroscopy using TEM. It was found that the size of the Co-rich regions was affected more by the overall alloy layer composition than by the deposition parameters. Under appropriate conditions, it was possible to reduce the individual alloy layer thickness to 25 nm while maintaining chemical segregation between the Co-rich areas. Annealing at 250°C and higher caused the Co-rich regions to merge together. © 2001 The Electrochemical Society. All rights reserved.

Fine structure and possible growth mechanisms of some electrodeposited CuCo granular films

Granular thin-films of the alloy Cu100−xCox within the composition range 6<x<35 have been grown by electrodeposition. The films have been investigated both magnetically and structurally and a comparison made between both types of measurements. Low field susceptibility measurements have been made which, in principle, allow us to determine the distribution of blocking temperatures and thus make an estimate of particle sizes. Before structural examination, the films were thinned either electrochemically or by ion-etching; both standard electron diffraction and TEM techniques have been employed. We believe that we are able to see Co-rich regions in the vicinity of sub-grain boundaries and that it is these regions that account for the magnetic behaviour of the samples. Whereas electron diffraction patterns from thin sections correspond to the FCC phase only, in thicker sections, additional reflections corresponding to interplanar spacings that cannot be indexed in terms of an FCC phase are observed. The possible origins of these extra reflections are discussed in terms of either double-diffraction at microtwins or, alternatively, in terms of extended, Co-rich HCP phases formed at matrix/twin interfaces. A possible mechanism for the growth of the HCP phase, which is a consequence of the electrodeposition technique, is discussed.

Study of boron effects on the reaction of Co and Si1−xGex at various temperatures

The effects of boron on Co and Si1−xGex interfacial reaction were studied. Undoped and in situ boron-doped strained Si0.91Ge0.09 and Si0.86Ge0.14 layers prepared at 550 °C by an ultrahigh vacuum chemical vapor deposition system were subjected to Co silicidation at various rapid thermal annealing (RTA) temperatures ranging from 500 to 1000 °C. The resulting films were characterized by a sheet resistance measurement, Auger electron spectroscopy, x-ray diffractometry (XRD), high resolution x-ray diffractometry, secondary ion mass spectroscopy, scanning electron microscopy, and transmission electron microscopy. Seen from XRD spectroscopy, a Co(Si1−yGey) cubic structure was formed with RTAs ranging from 500 to 700 °C. For boron-doped samples, the CoGe fraction in Co(Si1−yGey) was less than that in undoped samples, indicating that boron atoms retarded the incorporation of Ge into the Co(Si1−yGey) ternary phase. It also led to a large Ge pileup at the interface between the Co-rich and silicidation regions. On the other hand, from the high resolution x-ray spectra, the presence of boron led to less relaxation of the strained Si1−xGex lattice. It is the first time that small boron atoms inhibiting the relaxation of the Si1−xGex layer during silicidation was observed. Furthermore, from the sheet resistance measurement, the formation of CoSi2 was found to be slightly retarded in boron-doped samples, due probably to the decrease of Co or Si diffusivities as a result of boron accumulation at the Co/SiGe reaction interface. At temperatures above 800 °C, CoSi2 formed and Ge segregated to the silicide boundaries and the upper reaction region was discovered. These phenomena caused by B dopants are explained in detail.

The effect of particle size and surface-to-volume ratio distribution on giant magnetoresistance (GMR) in melt-spun Cu–Co alloys

Giant magnetoresistance (GMR), magnetization and structure of granular Cu 90Co 10 alloy were investigated. The annealed sample is inhomogeneous with respect to the Co-particle sizes and interparticles distances and, therefore, not all particles behave superparamagnetically. The observed deviation from the expected theoretically quadratic dependence of GMR on magnetization can be attributed to differentiated surface-to-volume ratio of superparamagnetic particles as well as to the presence of a small amount of ferromagnetic phase which is created by relatively large Co-rich regions.

Characterizations of NdFe&lt;sub&gt;0.5&lt;/sub&gt;Co&lt;sub&gt;0.5&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Trimetallic Oxide Prepared by Thermal Decomposition of Heteronuclear Complex, Nd[Fe&lt;sub&gt;0.5&lt;/sub&gt;Co&lt;sub&gt;0.5&lt;/sub&gt;(CN)&lt;sub&gt;6&lt;/sub&gt;

NdFe0.5Co0.5O3, a trimetallic perovskite-type oxide, was prepared by thermal decomposition of a synthesized heteronuclear complex of Nd[Fe0.5Co0.5(CN)6]⋅4H2O. Using XRD, the perovskite-type phase was detected in the sample heated at 500°C and the complex was completely decomposed to NdFe0.5Co0.5O3 at 700°C. Distribution of the elements Nd, Fe, and Co on the surface of the complex that decomposed at 800°C were homogeneous according to Auger electron spectroscopy (AES) analysis, For a solid-state reaction method using a mixture of single oxides (Nd2O3, Fe2O3 and CoO), a high heating temperature (>1000°C) is needed to form a single phase of the perovskite-type oxide. Fe-rich and Co-rich regions on the surface were clearly detected even in a mixture calcined at 1100°C. Surface concentration and structure of samples obtained by both methods were investigated by X-ray photoelectron spectroscopy (XPS). For both samples, XPS revealed that the (Fe+Co)/Nd ratio was less than unity.

Nanostructure and Co-rich regions of Plasma-free sputtered Co-Cr films

150nm thick Co-20at%Cr films with c-axis oriented hcp structure prepared by a new facing targets sputtering coater were evaluated on nanostructure of initial-, middle-, and surface-layers. 5-10 nm sizes Co-rich regions separeted by 3-10 sized Cr-rich once were confirmed to be distributed within 10-30 nm sized crystalites and across crystallite boundaries on all specimens of any layers.

Evaluation of spinodal decomposition process on sputtered Co-Cr films by heat flux method

Spinodal decomposition characteristics of Co-Cr films with highly oriented c-axis crystallites of hcp phase were evaluated from heat flux measurement of adatoms as well as plasma bombardment on deposition plane. Distribution and nano-size of Co-rich and Cr-rich regions from spinodal decomposition may be controlled under plasma free conditions with highly energized adatoms and also quantified from heat flux measurement.

Nanocrystalline Solid Solutions of Cu/Co and Other Novel Nanomaterials

ABSTRACTA brief review will be given of the preparation, synthesis and properties of the Cu/Co system. The special case of the synthesis of nanocrystalline Cu.80Co.20 by precipitation and reduction of hydroxides is discussed in more detail. It was found that the lattice constant of nanocrystalline Cu.80Co.20. determined from x-ray diffraction measurements, approximately fits Vegard's Law and the average nearest neighbor distance from both the Cu and Co atoms, determined from EXAFS measurements, ir shifted from their bulk values. Samples given the minimum heat treatment needed to reduce the hydroxides contained Co-rich regions. Heat treatments cause the Co to segregate preferentially onto the surface of the Cu crystals. The presence of the Co delays the oxidation of the Cu surfaces.

Microstructure and giant magnetoresistance in granular and multilayer magnetic thin films

The microstructure and magnetic properties of phase-separated (FeAg, CoAg) granular and multilayer Permalloy (Py)/Au magnetic thin films exhibiting giant magnetoresistance (GMR) havebeen investigated. Surprisingly, two Fe-Ag films of similar composition grown under identicalconditions and having substantially different microstructures yet display similar GMR. The microstructure of these films is characterized by Fe-rich or Co-rich regions respectively, 350–700 nm in extent and surrounded by a Ag-rich matrix. Within the Ag-rich regions, the Fe concentration varies from 20 to 25 atomic % and the Co concentration is ~16 at%. Within these regions essentially pure fcc Co particles and bcc Fe particles are in parallel and rotated epitaxial alignment respectively with the fcc Ag matrix. The Co and Fe particles are ~15 to 25Å in diameter. It is these small particles which account for the GMR exhibited by these alloys. This suggests that a size distribution of magnetic particles, sharply peaked at the optimum size with limited bulk segregation might give rise to larger GMR values. On the other hand, high resolution electron microscopy (HREM) analysis of as-grown permalloy (Py)/Au multilayers in (111) orientation revealed that the Au and Py layers have sharp interfaces with defects such as twin boundaries and misfit dislocations. The density of twin boundaries decreases from the Pt-seed layer to the top surface of the multilayer. An average of 1 twin boundary/195 nm 2 has been observed. On mild annealing, the twin density decreases and the multilayer interfaces become rough. The magnetoresistance of the as-grown Py/Au multilayer decreased from 11% to 8% on annealing at 250 °C for 40 minutes. From the measured roughness and total thickness of Au layers it was found that Au preferentially intermixes with Py, which reduces the magnitude of GMR.

Giant magnetoresistance and microstructural characteristics of epitaxial Fe-Ag and Co-Ag granular thin films

The microstructure and magnetic properties of phase-separated Fe-Ag and Co-Ag granular alloy films, grown epitaxially on (001) NaCl substrates and exhibiting giant magnetoresistance (GMR), have been investigated. Surprisingly, two Fe-Ag films of similar composition grown under identical conditions are found to have substantially different microstructures yet display similar GMR. The microstructure of the films is characterized by Fe-rich or Co-rich regions, respectively, 350–700 nm in extent, surrounded by a Ag-rich matrix. Within the Ag-rich regions, the Fe concentration varies from 20 to 25 at % and the Co concentration is ≂16 at %. Within these regions essentially pure fcc Co particles and bcc Fe particles are in parallel and rotated epitaxial alignment respectively with the fcc silver matrix. The Co and Fe particles are ≂15–25 Å in diameter. It is these small particles which most likely account for the giant magnetoresistance exhibited by these alloys. This suggests that a size distribution of magnetic particles, sharply peaked at the optimum size with limited bulk segregation, might give rise to larger GMR values.

An Analysis of the Segregated Microstructures of Electroless-Deposited CoNiP Perpendicular Magnetic Recording Media Using Selective Chemical Etching

The segregated microstructures of electroless-deposited CoNiP perpendicular magnetic recording media with various H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c(⊥)</sub> values-500, 1000 and 1500 Oe-were investigated using a selective chemical etching technique, in which Co-rich ferromagnetic regions were preferentially dissolved with an etchant of sodium hydroxide-m-nitrobenzenesulfonic acid-ethylenediamine aqueous solution. TEM observations of the etched 1500 Oe film revealed an "asteroid" pattern with sixfold symmetry, which was inferred to be a substructure with Ni-rich composition. It is thought that the Co-rich regions are segregated between the "arms" of the asteroid substructure. The shape of the substructure varied with the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c(⊥)</sub> of the films; no substructure was observed in the 500 Oe film, while a substructure with a needlelike shape as well as a quasi-asteroid shape were observed in different regions of the 1000 Oe film. The differences in the segregation conditions are strongly dependent on the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c(⊥)</sub> of the films. Further investigation of the 1500 Oe film in the thickness direction clearly indicated that the segregation occurs even in the initial deposition region (30 nm), and that the size of the substructure becomes larger with increasing film thickness.

選択化学エッチング法による無電解ＣｏＮｉＰ垂直磁気記録媒体の微細偏析構造の解析

The segregated microstructures of electrolessdeposited CoNiP perpendicular magnetic recording media with various Hc (⊥) of 500, 1000 and 1500 Oe were investigated by using the selective chemical etching technique; Co-rich ferromagnetic regions were preferentially dissolved with an etchant of sodium hydroxide-m-nitrobenzenesulfonic acidethylenediamine aqueous solution. TEM observation of the etched 1500 Oe film revealed an “asteroid“ pattern with hexa-symmetry, which was expected to be a substructure with a Ni-rich composition. It is considred that the Co-rich regions are segregated between the “arms” of the asteroid substructure. The shape of the substructure varied with the Hc (⊥) of the films:; no substructure was observed in the 500 Oe film, while a substructure with a needlelike shape and also a quasi-asteroid one were observed in different regions of the 1000 Oe film. The differences in the segregation conditions are strongly dependent on the Hc (⊥) of the films. Further investigation of the 1500 Oe film in the thicknees direction clearly indicated that the segregation occurs even in the initial deposition region (30 nm) and that the size of the substructure becomes larger with an increase in the film thickness.