bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Study on High Coercivity and L10Ordered Phase in CoPt and FePt Thin Films
Authors: Ristau R. A. , Barmak K. , Lewis L. H. , Coffey K. R. , Howard J. K. 
Published: April 2011

ABSTRACTThis study relates the microstructure of equiatomic binary alloys of CoPt and FePt with their room-temperature hysteretic magnetic properties, particularly their high coercivity. A transformation from an atomically disordered, face-centered-cubic structure to the Li0ordered structure occurred during post-deposition annealing and was characterized using digital analysis of dark-field transmission electron microscopy (TEM) images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from a superconducting quantum interference device (SQUID) magnetometer. Although the relationship most commonly described in the literature is that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study was found to correspond to the fully ordered state. Furthermore, in samples that were less than fully ordered, a direct relationship between ordered volume fraction and coercivity was observed. The proposed mechanism for the high coercivity in these films is an increasing density of magnetic domain wall pinning sites concurrent with an increasing fraction of ordered phase.
Magnetic Exchange-Coupling in CoPt/Co Bilayer thin Films
Authors: Kim J. , Barmak K. , Lewis L. H. , Crewt D. C. , Welcht D. O. 
Published: April 2011

ABSTRACTThin film CoPt/Co bilayers have been prepared as a model system to investigate the relationship between microstructure and exchange coupling in two-phase hard/soft composite magnets. CoPt films, with a thickness of 25 nm, were sputter-deposited from a nearly equiatomic alloy target onto oxidized Si wafers. The films were subsequently annealed at 700°C and fully transformed from the FCC phase to the magnetically hard, ordered L1 0 phase. The coercivity of the films increased rapidly with annealing time until it reached a plateau at approximately 9.5 kOe. Fully-ordered CoPt films were then used as substrates for deposition of Co layers, with thicknesses in the range of 2.8-225 nm, in order to produce the hard/soft composite bilayers. As predicted by theory, the magnetic coherency between the soft Co phase and the hard, ordered CoPt phase decreased as the thickness of the soft phase increased. This decrease in coupling was clearly seen in the magnetic hysteresis loops of the bilayers. At small thicknesses of Co (a few nanometers), the shape of the loop was one of a uniform material showing no indication of the presence of two phases with extremely different coercivities. At larger Co thicknesses, constricted loops, i.e., ones showing the presence of a mixture of two ferromagnetic phases of different hardnesses, were obtained. The magnetic exchange present in the bilayer samples was qualitatively analyzed using magnetic recoil curves and the dependence of exchange coupling on the soft phase dimension in the bilayer hard/soft composite magnet films is discussed.
Controlled Assembly of Monodisperse ε-Cobalt-Based Nanocrystals
Authors: Sun S. , Murray C. B. , Doyle H. 
Published: April 2011

ABSTRACTGeneral synthetic routes to monodisperse c-cobalt (β-Mn type phase) nanocrystals (ε-Co) and controlled assembly of these nanocrystals are presented in this paper. The ε-Co particles are obtained by superhydride reduction of cobalt chloride (anhydrous or hexahydrate) in a high temperature solution phase (200°C) in the presence of a combination of long chain diol, oleic acid and trialkylphosphine. Monodisperse nanocrystals are isolated by size selective precipitation. As synthesized cobalt particles are each a single crystal with a complex cubic structure related to the beta phase of elemental manganese (β-Mn). Self-assembly of these uniform cobalt particles on solid substrates is induced by evaporation of the carrier solvent producing 2-D and 3-D magnetic superlattices. Annealing of assembled ε-Co nanocrystal arrays converts them to the hcp cobalt crystal arrays. The inter-particle distance can be adjusted by selected thermal treatments or by chemical ligand exchange. This control over particle dimensions, crystallinity and assembly offers a model system for the study of ultra-high density recording media.
A Novel Apparatus for the Synthesis of Graphite Encapsulated Metallic Nanocrystals
Authors: Klug K.L. , Johnson D.L. , Dravid V.P. 
Published: April 2011

ABSTRACTGraphite encapsulated metal nanocrystals (GEM nanocrystals) are a promising new class of materials with both scientific and industrial significance. Several encapsulating graphite layers protect the nanoscale metallic core without compromising the inherent magnetic properties of the metal, thereby enhancing studies of small particle magnetism. The size and single domain nature of these particles immediately suggest a wide variety of potential applications ranging from increased density magnetic data storage to immunoassays. Encapsulated nanoparticles have traditionally been synthesized by evaporating a composite metal/graphite anode via an electric arc. While effective, that method also results in a great deal of unwanted amorphous carbon debris in the product. Based on past synthesis runs and observations, an improved apparatus for GEM production has been designed with the goals of increasing GEM yield and reducing soot production. The four segment system consists of I ) a chamber capable of arc or resistive evaporation of raw material(s), 2) a tube furnace for the chemical vapor deposition of carbon on raw material(s) via hydrocarbon dissociation, 3) an increased-efficiency powder collection unit, and 4) a final powder reservoir. Details of the new apparatus are presented here.
Gmr Effect and Properties of CoAg Granular Films Formed by Implantation with a Metal Vapor Vacuum Arc Ion Source
Authors: Wong S.P. , Chiah M.F. , Cheung W.Y. , Ke N. , Xu J.B. , Zhang X.X. 
Published: April 2011

ABSTRACTCobalt-silver granular thin films were formed by Co implantation into Ag using a metal vapor vacuum arc (MEVVA) ion source. The magnetic field dependence and the temperature variation of the giant magnetoresistance (GMR) effect and their relation with the processing conditions were studied and discussed in conjunction with results of Rutherford backscattering spectrometry, atomic force microscopy, magnetic force microscopy (MFM), and SQUID measurements. Anomalous temperature dependence of the coercive field Hc in the perpendicular-to-film direction determined from GMR measurements was observed for some samples. For one sample, Hc shows a maximum value at around 240K and decreases with decreasing temperature from 240K to 20K. The temperature variation of the magnetization M of this sample exhibits a minimum. The maximum in the Hc-T curve corresponds well with the minimum in the M-T curve. The M-T curve suggests that there is more than one magnetic phase present in this sample. The domain structures of the implanted granular films as revealed by MFM images exhibit very different features compared with those of sputter deposited CoAg granular films.
Crystalline Texture Of Cocrpt Films On Crmn//ial And Cr/Nial Underlayer Structures
Authors: Zou J. , Lu B. , Bayer A.E. , Laughlin D.E. , Lambeth D.N. 
Published: April 2011

ABSTRACTIn this study, we examine the effects of CrMn/NiAI and Cr/NiAl underlayer structures on the crystalline texture and microstructure of CoCrPt magnetic films using x-ray and electron diffraction. The former underlayer structure was found to induce better CoCrPt (1010) texture. The stress in the CoCrPt layer was measured. The effective in-plane anisotropy energy densities of the CoCrPt films on the above two underlayers were also measured by using out-of-plane torque and Miyajima methods and the result was consistent with the texture analysis.
C-Axis Perpendicularly Oriented Barium Ferrite Thin Film Media On Silicon Substrate
Authors: Zhuang Zailong , Kryder Mark H. , White Robert M. , Laughlin David E. 
Published: April 2011

ABSTRACTBarium ferrite thin films with excellent perpendicular c-axis orientation were successfully fabricated on Si substrate without any buffer layer. To compensate for possible barium deficiency due to the inter-diffusion between films and substrate, a barium-rich target was used. For a 900 Å-thick film, the perpendicular remanent squareness is about 0.9, while the in-plane remanent squareness is about 0.3. The saturation magnetization (Ms) is about 220 emu/cc, while the coercivity is around 3500 Oe. X-ray diffraction (XRD) results show the (001) perpendicular c-axis texture in the films. It was also found that the rapid thermal annealing conditions greatly affect the magnetic properties of barium ferrite films. With a certain flow rate of oxygen gas in the rapid thermal annealer (RTA), barium ferrite films generally crystallize with good perpendicular c-axis texture. Without oxygen gas, the hexagonal barium ferrite phase fails to develop; instead spinel Fe 30 4 phase forms. The reason for the collapsing of hexagonal barium ferrite texture is thought to be an oxygen deficiency in the barium ferrite films due to the reduction of oxygen in the films during the high temperature annealing.
Microstructural and Magnetic Improvements in Tube-Cast Pr-Fe-B-Cu Alloys Via Heat Treatments and Rapid Upset Forging
Authors: Hatch G P , Williams A J , Harris I R 
Published: April 2011

ABSTRACTAlloys of Pr-Fe-B-Cu were cast into Cu and stainless steel tubes. It could be seen that a preferred crystallographic orientation was obtained after cooling. Fine platelets of Pr2Fe1 4B matrix phase were formed, surrounded by various grain boundary phases, and there was a significant reduction in the amount of free Fe present, in comparison to conventional slab cast alloys. In order to improve the magnetic properties in the as-cast state, two alternative routes were used. The first involved a series of two step heat treatments to develop an improved microstructure. After an optimum heat treatment of 1000 'C for 24 hours + 500°C for 3 hours, significantly improved magnetic properties were obtained for a 17.5 % Pr alloy; Br = 752 mT, Hci = 613 kAm−1 and BHmax = 96 kJm−3. The second route involved a rapid upset forging [RUF] process, with a strain rate of 11.5 s−1, to hot deform the alloys. Following post-forging heat treatments, properties of Br = 966 mT, Hci = 780 kAm−3 and BHmax = 160 kJm−3 were obtained for a 15% Pr alloy. That nature of the improvements in properties as a result of heat treatments and RUF were investigated and are discussed in the present work.
Dislocation Forest Interactions: Simulation and Prediction
Authors: Wickham L. K. , Schwarz K. W. , Stölken J. S. 
Published: April 2011

AbstractUsing linear elastic dislocation dynamics simulations, we show that junction formation between dislocations from various interacting slip systems can be predicted by a simple self-energy calculation. We find that this prediction is robust: dislocation curvature and external stress produce little change in the simulation results for junction formation. One key to this success appears to be a separation of timescales, where movement of the far away dislocation arms (under, for example, external stress) is typically slow compared to the process of making a junction. The self-energy calculation we describe gives a rule for dislocation encounters which should allow a considerable saving in computational effort, allowing one to impose correct interaction outcomes without calculating the interactions in detail. We also find that dislocations often come together under attraction without forming a junction. The resulting “cross-linked” state provides an additional type of connection between dislocations. We include preliminary results on the persistence of junctions and cross-linked states under stress.
Simulation of Dislocation Dynamics in Ni3Al: A Study of Velocity Autocorrelations
Authors: Erdonmez C. K. , Chrzan D. C. 
Published: April 2011

AbstractThe yield strength anomaly in some L12 compounds has been linked to the thermally assisted cross slip of screw superdislocations. This work continues earlier efforts to understand the yield strength anomaly in L12 alloys using computer simulations of dislocation motion. Dislocations are modelled within isotropic elasticity theory, and simple rules are used to model the cross-slip process in the two dimensional geometry of the simulation. The velocity of a single dislocation in Ni3Al is studied as a function of the applied stress. The observed velocities vary nonlinearly with the applied stress. Further, dislocations are observed to become immobile for small applied loads. At high stresses, the dislocations are observed to advance relatively unhindered by the thermally activated cross slip process. Fluctuations in the velocity of the dislocations are studied, and their autocorrelation function shows an increased correlation time near a threshold stress. This threshold stress is identified with the critical stress proposed in earlier works.
Transition Temperatures in Plastic Yielding and Fracture of Semiconductors
Authors: Pirouz P. , Kubin L. P. , Demenet J. L. , Hong M. H. , Samant A. V. 
Published: April 2011

AbstractRecent experiments on deformation of semiconductors show an abrupt change in the variation of the critical resolved shear stress, τy, with temperature, T. This implies a change in the deformation mechanism at a critical temperature Tc. In the cases examined so far in our laboratory and elsewhere, this critical temperature appears to coincide approximately with the brittle-ductile transition temperature, TBDT. In this paper, the deformation experiments performed on the wide bandgap semiconductor, 4H-SiC, over a range of temperatures and strain rates are described together with the characterization of induced dislocations below and above Tc by transmission electron microscopy. Based on these results, and those of Suzuki and coworkers on other compound semiconductors, some understanding of the different mechanisms operating at low and high temperatures in tetrahedrally coordinated materials has been gained, and a new model for their brittle-ductile transition has been proposed.
Dislocation Mobility in Two-Dimensional Lennard-Jones Material
Authors: Bailey Nicholas P. , Sethna James P. , Myers Christopher R. 
Published: April 2011

AbstractIn seeking to understand at a microscopic level the response of dislocations to stress we have undertaken to study as completely as possible the simplest case: a single dislocation in a two dimensional crystal. The intention is that results from this study will be used as input parameters in larger length scale simulations involving many defects. We present atomistic simulations of defect motion in a two-dimensional material consisting of atoms interacting through a modified Lennard-Jones potential. We focus on the regime where the shear stress is smaller than its critical value, where there is a finite energy barrier for the dislocation to hop one lattice spacing. In this regime motion of the dislocation will occur as single hops through thermal activation over the barrier. Accurate knowledge of the barrier height is crucial for obtaining the rates of such processes. We have calculated the energy barrier as a function of two components of the stress tensor in a small system, and have obtained good fits to a functional form with only a few adjustable parameters.
Modeling of the Dislocation Formation at Pores and Inclusions under Thermo-Mechanical Shear Loads
Authors: Rentsch R. , Vitek V. 
Published: April 2011

AbstractMaterials used in many technical applications contain a broad variety of defects. In both metals and ceramics, pores and inclusions influence significantly the elastic and plastic response. They can limit or broaden the range of applicability of the materials for high performance components and play, therefore, a very significant role in engineering applications.This study focuses on the onset of dislocation formation and its intensity at pores and hard inclusions during the deformation process. For this purpose a molecular dynamics model of an fcc metal containing pores and/or inclusions was developed. This model material was then sheared at different strains and related development of the dislocation substructures investigated.
Symmetry Investigation of Textured Polycrystal Properties (Invited)
Authors: Maudlin P. J. , Bingert J. F. , Gray G. T. , Garrett R. K. 
Published: April 2011

AbstractTantalum plate and rod materials that demonstrate mild-to-strong anisotropic plastic flow during large deformation are analyzed in terms of tensorial property symmetry. Texture interrogations of these materials reveal duplex orientation components that have implications with regard to the symmetry realized during plastic deformation; specifically these materials show less symmetry than one would expect from a cursory examination of the texture. Mesoscale polycrystal simulations are performed to probe a general shape for the yield surface function based on a discrete orientation distribution representation of the material texture and previously established single-crystal deformation modes. The yield surface shape is mathematically represented in terms of second and higher-order tensors. A plastic compliance analysis is presented and applied to graphically map the deformation symmetry contained in these tensors for both ideal and real materials. Compliance results are shown to be consistent with finite element simulations of r-value specimens loaded in uniaxial tension.
Identification of 2D Boundaries from 3D Atom Probe Data, and Spatial Correlation of Atomic Distributions with Interfaces
Authors: Hellman O.C. , Vandenbroucke J.A. , Rüsing J. , Isheim D. , Seidman D.N. 
Published: April 2011

AbstractThe Three Dimensional Atom Probe produces a real space map of the elemental identities and positions of atoms field-evaporated from a sharply pointed specimen. The analyzed volume is on the order of 20 nm × 20 nm × 100 nm. This is large enough to enclose microstructural features such as grain- or heterophase boundaries. Correlation of the measured atomic positions with such features results in an atom-by-atom description of the chemical environment of these crystallographic defects. We describe here a method for identifying these interfaces and profiling the composition in the vicinity of the interfaces without any assumptions about the interface geometry. This approach is applied to quantitative determination of interfacial segregation of Ag at a MgO/Cu(Ag) heterophase interface. We discuss the implications of our technique with respect to classical treatments of segregation at interfaces.
Modelling of Grain Refinement in Aluminium Alloys
Authors: Greer A. L. , Tronche A. , Vandyoussefi M. 
Published: April 2011

AbstractCommercial grain refiners for aluminium solidification are so potent that the barrier forgrain initiation is that for free growth rather than for nucleation itself. In this case quantitative prediction of grain size is possible. For small melt volumes a successful isothermal-melt model is presented. This is extended to directional solidification in a temperature gradient using cellular-automaton modelling of grain growth with finite-element heat-flow calculations.
Characterization and Modeling of Precipitation Kinetics in Aluminium 7000 Alloys
Authors: Werenskiold J.C. , Deschamps A. 
Published: April 2011

AbstractThe precipitation kinetics of 7108.70 aluminum alloy has been investigated in a wide range of temperatures by in situ Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM), and computer modeled by use of an internal-state variable model which predicts the evolution of microstructural parameters. The modeling and experiments were done for isothermal heat treatment at 120, 140, 150, 160 and 170°C. The industrial T6 and T7 treatments have also been investigated.
The Strain Gradient Effects in Micro-Indentation Hardness Experiments
Authors: Xue Z. , Huang Y. , Gao H. , Nix W.D. 
Published: April 2011

AbstractMicro-indentation experiments have shown very strong size dependence of the indentation hardness, a phenomenon that cannot be explained by classical plasticity theories. A theory of mechanism-based strain gradient (MSG) plasticity has been developed based on the Taylor model in dislocation theories, and is intended for materials and structures whose dimension controlling plastic deformation falls roughly between 0.1 to 10 microns. The MSG plasticity theory is used in the present study to investigate the size effects observed in micro-indentation experiments. It is shown that the theory can indeed reproduce micro-indentation experimental data, thus providing an important self-consistent check of the MSG plasticity theory.
The Role of Boron in the Mechanical Milling of Titanium-6 %Aluminium-4% Vanadium Powders
Authors: Brown A.P. , Brydson R. , Hammond C. , Godfrey T.M.T. , Wisbey A. 
Published: April 2011

ABSTRACTThe reduction in grain size of a metal can lead to significant improvement in mechanical properties. Mechanical alloying (MA) with a second phase is a possible route to producing fine-grained, particulate reinforced material. This study describes the microstructural development of Ti-6%Al-4%V milled with increasing concentrations of boron. Mechanical milling of Ti-6%Al-4%V powder produces a nanocrystalline material. MA of Ti-6%Al-4%V with boron results in the alloying of the two to form either a boride or an amorphous phase when the local concentration of boron is ∼ 50 at.%. During milling, the boron tends to remain near to its original particle form and in these boron-rich regions TiB is formed. Beyond these regions small amounts of boron (a few at.%) mix with the titanium matrix and reduce further the grain size of the titanium. An increase in the global concentration of boron increases the volume fraction of boride produced.
Formation of bulk magnetic nanostructured Fe40Ni40P14B6 alloys by metastable liquid state phase separation
Authors: Li Q. , Kui H.W. 
Published: April 2011

ABSTRACTMETGLAS alloy #2826 (Fe40Ni40P14B6) is a eutectic alloy. Its melt can undergo metastable liquid state spinodal decomposition deep in the undercooling regime ΔT defined as ΔT = T1 - T where T1 is the liquidus of the alloy and T is the temperature at which the decomposition reaction takes place. Micrographs of undercooled Fe40Ni40P14B6 of ΔT = 209, 260 and 307 K were displayed. All of them exhibit refined microstructures, esp. the one with the largest ΔT.
Residual Stress Determination and Subsurface Microstructure in Ground and Polished Alumina/Silicon Carbide Nanocomposites and Monolithic Alumina Ceramics
Authors: Wu H.Z. , Roberts S.G. , Winn A.J. , Derby B. 
Published: April 2011

ABSTRACTThe surface residual stress state induced by grinding and polishing an alumina/silicon carbide nanocomposite and monolithic alumina has been investigated using Hertzian indentation and fluorescence spectroscopy. Specimens were ground and then polished with diamond slurry with grit sizes ranging between 8 μm and 1 μm. The results show that the surface residual stress state in the nanocomposites is more sensitive to surface treatment than that in the monolithic alumina. Surfaces of both ceramics were examined in cross-section by TEM and direct observations were made of the plastic deformation induced by different surface treatments. There is a change in the predominant deformation micromechanism from twinning in the alumina to dislocation generation in the nanocomposites.