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Field driven ferromagnetic phase nucleation and propagation from the domain boundaries in antiferromagnetically coupled perpendicular anisotropy films

Description: We investigate the reversal process in antiferromagnetically coupled [Co/Pt]{sub X-1}/{l_brace}Co/Ru/[Co/Pt]{sub X-1}{r_brace}{sub 16} multilayer films by combining magnetometry and Magnetic soft X-ray Transmission Microscopy (MXTM). After out-of-plane demagnetization, a stable one dimensional ferromagnetic (FM) stripe domain phase (tiger-tail phase) for a thick stack sample (X=7 is obtained), while metastable sharp antiferromagnetic (AF) domain walls are observed in the remanent state for a thinner stack sample (X=6). When applying an external magnetic field the sharp domain walls of the thinner stack sample transform at a certain threshold field into the FM stripe domain wall phase. We present magnetic energy calculations that reveal the underlying energetics driving the overall reversal mechanisms.
Date: December 9, 2008
Creator: Hauet, Thomas; Gunther, Christian M.; Hovorka, Ondrej; Berger, Andreas; Im, Mi-Young; Fischer, Peter et al.
Partner: UNT Libraries Government Documents Department

Studies of the viscoelastic properties of water confined between surfaces of specified chemical nature.

Description: This report summarizes the work completed under the Laboratory Directed Research and Development (LDRD) project 10-0973 of the same title. Understanding the molecular origin of the no-slip boundary condition remains vitally important for understanding molecular transport in biological, environmental and energy-related processes, with broad technological implications. Moreover, the viscoelastic properties of fluids in nanoconfinement or near surfaces are not well-understood. We have critically reviewed progress in this area, evaluated key experimental and theoretical methods, and made unique and important discoveries addressing these and related scientific questions. Thematically, the discoveries include insight into the orientation of water molecules on metal surfaces, the premelting of ice, the nucleation of water and alcohol vapors between surface asperities and the lubricity of these molecules when confined inside nanopores, the influence of water nucleation on adhesion to salts and silicates, and the growth and superplasticity of NaCl nanowires.
Date: September 1, 2010
Creator: Houston, Jack E.; Grest, Gary Stephen; Moore, Nathan W. & Feibelman, Peter J.
Partner: UNT Libraries Government Documents Department

Final technical report. [Heterogeneous nucleation and growth in metal alloys]

Description: We have refined a heat treatment to obtain coherent, heterogeneous nucleation of precipitates on dislocations in a high purity binary alloy. This allowed, for the first time, a quantitative comparison to be made for coherent heterogeneous nucleation. This part of the research resulted from our concern about the role of dislocations in sub-boundaries in aluminum alloys and directed us to first examine isolated dislocations in the binary Al-Li systems. We were able to design the experiment so the heterogeneous nucleation of AL{sub 3}Li occurred. Previously, only homogeneous nucleation of Al{sub 3}Li had been examined.
Date: September 14, 1997
Creator: Shiflet, G.J.
Partner: UNT Libraries Government Documents Department

Large melting point hysteresis of Ge nanocrystals embedded inSiO2

Description: The melting behavior of Ge nanocrystals embedded within SiO{sub 2} is evaluated using in situ transmission electron microscopy. The observed melting point hysteresis is large ({+-} 17%) and nearly symmetric about the bulk melting point. This hysteresis is modeled successfully using classical nucleation theory without the need to invoke epitaxy.
Date: May 4, 2006
Creator: Xu, Q.; Sharp, I.D.; Yuan, C.W.; Yi, D.O.; Liao, C.Y.; Glaeser,A.M. et al.
Partner: UNT Libraries Government Documents Department

Preparation of pHEMA-CP composites with high interfacial adhesionvia template-driven mineralization

Description: We report a template-driven nucleation and mineral growth process for the high-affinity integration of calcium phosphate (CP) with a poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogel scaffold. A mineralization technique was developed that exposes carboxylate groups on the surface of crosslinked pHEMA, promoting high-affinity nucleation and growth of calcium phosphate on the surface along with extensive calcification of the hydrogel interior. External factors such as the heating rate, the agitation of the mineral stock solution and the duration of the process that affect the outcome of the mineralization were investigated. This template-driven mineralization technique provides an efficient approach toward bonelike composites with high mineral-hydrogel interfacial adhesion strength.
Date: December 5, 2002
Creator: Song, Jie; Saiz, Eduardo & Bertozzi, Carolyn R.
Partner: UNT Libraries Government Documents Department

Molecular Dynamics Simulations of Spinodal-Assisted Polymer Crystallization

Description: Large scale molecular dynamics simulations of bulk melts of polar (poly(vinylidene fluoride) (pVDF)) polymers are utilized to study chain conformation and ordering prior to crystallization under cooling. While the late stages of polymer crystallization have been studied in great detail, recent theoretical and experimental evidence indicates that there are important phenomena occurring in the early stages of polymer crystallization that are not understood to the same degree. When the polymer melt is quenched from a temperature above the melting temperature to the crystallization temperature, crystallization does not occur instantaneously. This initial interval without crystalline order is characterized as an induction period. It has been thought of as a nucleation period in the classical theories of polymer crystallization, but recent experiments, computer simulations, and theoretical work suggest that the initial period in polymer crystallization is assisted by a spinodal decomposition type mechanism. In this study we have achieved physically realistic length scales to study early stages of polymer ordering, and show that spinodal-assisted ordering prior to crystallization is operative in polar polymers suggesting general applicability of this process.
Date: July 8, 2005
Creator: Gee, R H; Lacevic, N M & Fried, L
Partner: UNT Libraries Government Documents Department

Stifling of Crevice Corrosion in Alloy 22

Description: Artificially creviced Alloy 22 (N06022) specimens may be susceptible to crevice corrosion in presence of hot chloride containing solutions. The presence of oxyanions in the electrolyte, especially nitrate, may inhibit the nucleation and growth of crevice corrosion. Constant potential tests were performed using tightly creviced specimens of Alloy 22. It was found that crevice corrosion may initiate when a constant potential above the crevice repassivation potential is applied. It was found that as the crevice corrosion nucleated, the current initially increased but later decreased. The net measured current can be converted into penetration following a power law fit of the experimental data. The average power law coefficient ''n'' was found to be 0.439, suggesting that even under constant applied potential crevice corrosion penetration is diffusion controlled.
Date: June 8, 2005
Creator: Mon, K G; Gordon, G M & Rebak, R B
Partner: UNT Libraries Government Documents Department

ISOTHERMAL (DELTA)/(ALPHA-PRIME) TRANSFORMATION AND TTT DIAGRAM IN A PLUTONIUM GALLIUM ALLOY

Description: Differential scanning calorimetry (DSC) is used as an alternative approach to determining the tine-temperature-transformation (TTT) diagram for the martensitic delta to alpha-prime transformation in a Pu-2.0 at% Ga alloy. Previous work suggests that the TTT diagram for a similar alloy exhibits an unusual double-C curve for isothermal holds of less than 100 minutes. Here, we extend this diagram to 18 hours, and confirm the double-C curve behavior. When the sample is cooled prior to the isothermal holds, the delta to alpha-prime transformation is observed as several overlapping exothermic peaks. These peaks are very reproducible, and they are believed to be the result of different kinds of delta to alpha-prime martensitic transformation. This may be due to the presence of different nucleation sites and/or different morphologies.
Date: November 11, 2005
Creator: Oudot, B P; Blobaum, K M; Wall, M A & Schwartz, A J
Partner: UNT Libraries Government Documents Department

Metal-inducd assembly of a semiconductor-island lattice: Getruncated pyramids on Au-patterned Si

Description: We report the two-dimensional alignment of semiconductor islands using rudimentary metal patterning to control nucleation and growth. In the Ge on Si system, a square array of sub-micron Au dots on the Si (001) surface induces the assembly of deposited Ge adatoms into an extensive island lattice. Remarkably, these highly ordered Ge islands form between the patterned Au dots and are characterized by a unique truncated pyramidal shape. A model based on patterned diffusion barriers explains the observed ordering and establishes general criteria for the broader applicability of such a directed assembly process to quantum dot ordering.
Date: August 28, 2005
Creator: Robinson, J.T.; Liddle, J.A.; Minor, A.; Radmilovic, V.; Yi,D.O.; Greaney, P.A. et al.
Partner: UNT Libraries Government Documents Department

Transverse field-induced nucleation pad switching modes during domain wall injection

Description: We have used magnetic transmission X-ray microscopy (M-TXM) to image in-field magnetization configurations of patterned Ni{sub 80}Fe{sub 20} domain wall 'injection pads' and attached planar nanowires. Comparison with micromagnetic simulations suggests that the evolution of magnetic domains in rectangular injection pads depends on the relative orientation of closure domains in the remanent state. The magnetization reversal pathway is also altered by the inclusion of transverse magnetic fields. These different modes explain previous results of domain wall injection into nanowires. Even more striking was the observation of domain walls injecting halfway across the width of wider (>400 nm wide) wires but over wire lengths of several micrometers. These extended Neel walls can interact with adjacent nanowires and cause a switching in the side of the wire undergoing reversal as the domain wall continues to expand.
Date: March 12, 2010
Creator: Bryan, M. T.; Fry, P. W.; Schrefl, T.; Gibbs, M. R. J.; Allwood, D. A.; Im, M.-Y. et al.
Partner: UNT Libraries Government Documents Department

Crossover from dimer nucleation to adatom exchange during submonolayer epitaxy

Description: The nucleation and growth of islands in the early stages of epitaxial growth is studied with kinetic Monte Carlo Simulations and self-consistent mean field rate equations. Specifically, adatom exchange and irreversible dimer formation are allowed to compete equally as the origin of two-dimensional islands. The island size distribution and number density are found to satisfy a dynamic crossover scaling form. The critical island size evolves from one to zero with increasing temperature, decreasing flux, and increasing coverage.
Date: November 1, 1995
Creator: Bales, G.S.
Partner: UNT Libraries Government Documents Department

Modeling of the recrystallization textures of Al-alloys after hot deformation

Description: The recrystallization textures of Al-alloys can be explained by a growth selection of grains with an approximate 40{degree}<111> orientation relationship out of a limited spectrum of preferentially formed nucleus orientations. Accordingly, recrystallization textures can be modeled by the multiplication of a function f(g){sup nucl} describing the probability of nucleation of the various orientations with a function f(g){sup grow} representing their growth probability. Whereas the growth probability can be accounted for by a 40{degree}<111> transformation of the rolling texture, the nucleation probability of the respective grains is given by the distribution of potential nucleus orientations, which is known from local texture analysis for the most important nucleation sites in rolled Al-alloys, cube-bands, grain boundaries and second-phase particles. The contribution of each of these nucleation sites are determined according to an approach to calculate the number of nuclei forming at each nucleation site, which is based on microstructural investigations on the evolution of the various nucleation sites during deformation. The paper describes the model for recrystallization texture simulation in Al-alloys and gives examples of recrystallization textures of AA3004 deformed in plane strain compression at a variety of different deformation temperatures and strain rates.
Date: December 1, 1998
Creator: Engler, O. & Vatne, H.E.
Partner: UNT Libraries Government Documents Department

Nucleation and Growth of GaN on GaAs (001) Substrates

Description: The nucleation of GaN thin films on GaAs is investigated for growth at 620 "C. An rf plasma cell is used to generate chemically active nitrogen from N<SUB>2</SUB>. An arsenic flux is used in the first eight monolayer of nitride growth to enhance nucleation of the cubic phase. Subsequent growth does not require an As flux to preserve the cubic phase. The nucleation of smooth interfaces and GaN films with low stacking fault densities is dependent upon relative concentrations of active nitrogen species in the plasma and on the nitrogen to gallium flux ratio.
Date: May 3, 1999
Creator: Drummond, Timothy J.; Hafich, Michael J.; Heller, Edwin J.; Lee, Stephen R.; Liliental-Weber, Zuzanna; Ruvimov, Sergei et al.
Partner: UNT Libraries Government Documents Department

Time-Resolved Studies of Laser Damage Processes in DKDP Crystals

Description: The authors have used a 3-color imaging technique to obtain time-resolved series of images during nanosecond laser damage in bulk DKDP crystals. In contrast to single-pump, single-probe time-resolved imaging techniques, they are able to correlate behavior during single damage events. This enables us to observe a range of morphological dynamics that is otherwise difficult to study, including: the propagation of elastic sound waves and the liquid/solid melt front from the damage nucleation site and the dynamics of crack formation and propagation.
Date: February 12, 2000
Creator: Jiang, H; McNary, R; Tom, H; Yan, M; Radousky, H & Demos, S
Partner: UNT Libraries Government Documents Department

Phase transitions: An overview with a view

Description: The dynamics of phase transitions plays a crucial role in the so- called interface between high energy particle physics and cosmology. Many of the interesting results generated during the last fifteen years or so rely on simplified assumptions concerning the complex mechanisms typical of nonequilibrium field theories. After reviewing well-known results concerning the dynamics of first and second order phase transitions, I argue that much is yet to be understood, in particular in situations where homogeneous nucleation theory does not apply. I present a method to deal with departures from homogeneous nucleation, and compare its efficacy with numerical simulations. Finally, I discuss the interesting problem of matching numerical simulations of stochastic field theories with continuum models.
Date: October 1, 1997
Creator: Gleiser, M.
Partner: UNT Libraries Government Documents Department

Experimental and Theoretical Studies of Nucleation in Supercooled Liquid Silicon. Final Program Report

Description: The original objectives of the present program consisted of two specific nucleation-related research activities; (1) to provide a set of experimental data that will enable the quantitative examination of classical nucleation theory, and (2) to describe the phenomenon of nucleation by developing general expressions of nucleation that include both the thermal and athermal components and that correctly consider and incorporate the transient effects that arise from the nonstationary cluster distribution profile.
Date: April 29, 2004
Creator: Im, J.
Partner: UNT Libraries Government Documents Department

Heterogeneous nucleation of ice on anthropogenic organic particles collected in Mexico City

Description: This study reports on heterogeneous ice nucleation activity of predominantly organic (or coated with organic material) anthropogenic particles sampled within and around the polluted environment of Mexico City. The onset of heterogeneous ice nucleation was observed as a function of particle temperature (Tp), relative humidity (RH), nucleation mode, and particle chemical composition which is influenced by photochemical atmospheric aging. Particle analyses included computer controlled scanning electron microscopy with energy dispersive analysis of X-rays (CCSEM/EDX) and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). In contrast to most laboratory studies employing proxies of organic aerosol, we show that anthropogenic organic particles collected in Mexico City can potentially induce ice nucleation at experimental conditions relevant to cirrus formation. The results suggest a new precedent for the potential impact of organic particles on ice cloud formation and climate.
Date: June 20, 2010
Creator: Knopf, D.A.; Wang, B.; Laskin, A.; Moffet, R.C. & Gilles, M.K.
Partner: UNT Libraries Government Documents Department

Synthesis of silicon and germanium nanowires.

Description: The vapor-liquid-solid growth process for synthesis of group-IV semiconducting nanowires using silane, germane, disilane and digermane precursor gases has been investigated. The nanowire growth process combines in situ gold seed formation by vapor deposition on atomically clean silicon (111) surfaces, in situ growth from the gaseous precursor(s), and real-time monitoring of nanowire growth as a function of temperature and pressure by a novel optical reflectometry technique. A significant dependence on precursor pressure and growth temperature for the synthesis of silicon and germanium nanowires is observed, depending on the stability of the specific precursor used. Also, the presence of a nucleation time for the onset of nanowire growth has been found using our new in situ optical reflectometry technique.
Date: November 1, 2007
Creator: Clement, Teresa J. (Arizona State University) & Hsu, Julia W. P.
Partner: UNT Libraries Government Documents Department

Program Abstracts: Formation and Growth of Atmospheric Aerosols

Description: DOE provided $11,000 to sponsor the Workshop on New Particle Formation in the Atmosphere, which was held at The Riverwood Inn and Conference Center near Minneapolis, MN from September 7 to 9, 2006. Recent work has shown that new particle formation is an important atmospheric process that must be better understood due to its impact on cloud cover and the Earth's radiation balance. The conference was an informal gathering of atmospheric and basic scientists with expertise pertinent to this topic. The workshop included discussions of: • atmospheric modeling; • computational chemistry pertinent to clustering; • ions and ion induced nucleation; • basic laboratory and theoretical studies of nucleation; • studies on neutral molecular clusters; • interactions of organic compounds and sulfuric acid; • composition of freshly nucleated particles. Fifty six scientists attended the conference. They included 27 senior scientists, 9 younger independent scientists (assistant professor or young associate professor level), 7 postdocs, 13 graduate students, 10 women, 35 North Americans (34 from the U.S.), 1 Asian, and 20 Europeans. This was an excellent informal workshop on an important topic. An effort was made to include individuals from communities that do not regularly interact. A number of participants have provided informal feedback indicating that the workshop led to research ideas and possible future collaborations.
Date: September 7, 2006
Creator: McMurry, Peter H. & Kulmala, Markku
Partner: UNT Libraries Government Documents Department

Direct real-space observation of nearly stochastic behavior in magnetization reversal process on a nanoscale

Description: We report a non-deterministic nature in the magnetization reversal of nanograins of CoCrPt alloy film. Magnetization reversal process of CoCrPt alloy film is investigated using high resolution soft X-ray microscopy which provides real space images with a spatial resolution of 15 nm. Domain nucleation sites mostly appear stochastically distributed within repeated hysteretic cycles, where the correlation increases as the strength of the applied magnetic field increases in the descending and ascending branches of the major hysteresis loop. In addition, domain configuration is mostly asymmetric with inversion of an applied magnetic field in the hysteretic cycle. Nanomagnetic simulation considering thermal fluctuations of the magnetic moments of the grains explains the nearly stochastic nature of the domain nucleation behavior observed in CoCrPt alloy film. With the bit size in high-density magnetic recording media approaching nanometer length scale, one of the fundamental and crucial issues is whether the domain nucleation during magnetization reversal process exhibits a deterministic behavior. Repeatability of local domain nucleation and deterministic switching behavior are basic and essential factors for achieving high performance in high-density magnetic recording [1-3]. Most experimental studies on this issue reported so far have been mainly performed by indirect probes through macroscopic hysteresis loop and Barkhausen pattern measurements, which provide the ensemble-average magnetization. Thus, they are inadequate to gain insight into the domain-nucleation behavior on a nanometer length scale during the magnetization reversal process [4-6]. Very recently, coherent X-ray speckle metrology, where the speckle pattern observed in reciprocal space acts as a fingerprint of the domain configurations, was adopted to investigate stochastic behavior in the magnetization reversal of a Co/Pt multilayer film [7,8]. However, no direct observation on the stochastic behavior of domain nucleation during magnetization reversal in real space at the nanometer scale has yet been reported. The main reason is due to limitations of ...
Date: June 1, 2007
Creator: Im, M.-Y.; Kim, D.-H.; Lee, K.-D.; Fischer, P. & Shin, S.-C.
Partner: UNT Libraries Government Documents Department