Physical Sciences and Mathematics Commons^™

Direct Visualization Of Dislocation Dynamics In Grain Boundary Scars, Mark Bowick, Peter Lipowsky, Jan H. Meinke, David R. Nelson, Andreas R. Bausch

Physics - All Scholarship

Mesoscale objects with unusual structural features may serve as the analogues of atoms in the design of larger-scale materials with novel optical, electronic or mechanical behaviour. In this paper we investigate the structural features and the equilibrium dynamics of micron-scale spherical crystals formed by polystyrene particles adsorbed on the surface of a spherical water droplet. The ground state of sufficiently large crystals possesses finite-length grain boundaries (scars). We determine the elastic response of the crystal by measuring single-particle diffusion and quantify the fluctuations of individual dislocations about their equilibrium positions within a scar determining the dislocation spring constants. We observe …

Baselines In Requirements Tracing, Senthil Karthikeyan Sundaram, Jane Huffman Hayes, Alexander Dekhtyar

Computer Science and Software Engineering

We summarize the results of our requirements tracing work to date, focusing on our empirical results with open source datasets. Specifically, we describe the problem of after-the-fact requirements tracing for Verification and Validation (V&V) analysts, we provide a brief overview of Information Retrieval methods we have applied as well as measures used to evaluate them, we describe our tracing tool, and we present the results of a number of empirical studies. Two of the open source datasets that we have used are available to the research community at http://promise.site.uottawa.ca/SERepository/.

Magnetic Properties Of Dilute Fept:C Nanocluster Films, Yinfan Xu, M.L. Yan, J. Zhou, David J. Sellmyer

David Sellmyer Publications

Nanocluster-assembled dilute (FePt) _xC_100-x films with 30≥x≥5 (x denotes volume fraction) were produced using a gas-aggregation technique. FePt clusters with an average size of about 4.0 nm (with standard deviation σ /d=0.09) were embedded in high volume fraction of carbon matrix, which is used to isolate the FePt clusters. Postdeposition annealing was used to realize the high-anisotropy L1₀ phase. Single-crystal features of well-isolated clusters were observed by transmission electron microscope (TEM) in annealed dilute films. The coercivity of the films annealed at 700 °C for 5 min was a few hundred Oersteds at room temperature, while …

Relaxation In Magnetic Nanostructures, Ralph Skomski, J. Zhou, David J. Sellmyer

David Sellmyer Publications

The damping of magnetization processes in nanostructures is investigated by Fokker–Planck-type model calculations and quantum-mechanical considerations based on Fermi's golden rule. In the absence of energy barriers, the problem reduces to a magnetic diffusion equation with a particle-size-dependent diffusivity which is indirectly proportional to the relaxation-time parameter τ_o. For small particles, the relaxation time is proportional to the particle volume, but when the particle size reaches a few nanometers, it approaches a constant bulk value.

Rapidly Annealed Exchange-Coupled Sm-Co/Co Multilayers, J. Zhou, Ralph Skomski, Yi Liu, Yucheng Sui, W. Liu, David J. Sellmyer

David Sellmyer Publications

In exchange-coupled two-phase permanent magnets, the length scale of soft phase is limited to about twice of the domain-wall width of the hard phase. To optimize the energy product, it is important to realize this length experimentally. In this work, we investigate the Sm–Co/Co hard/soft multilayers with varying thickness of soft phase layers. On rapidly annealing, the multilayered hard-soft structure forms. Transmission electron microscopy micrography confirms that the multilayer structure is retained after the annealing. Single-phase-like hysteresis loops are obtained for samples with Co layers up to 13 nm thick. This behavior indicates that the soft phase is well exchange …

Template-Mediated Assembly Of Fept L10 Clusters Under External Magnetic Field, Yucheng Sui, W. Liu, Lanping Yue, Xingzhong Li, J. Zhou, Ralph Skomski, David J. Sellmyer

David Sellmyer Publications

FePt L1₀-structured clusters were synthesized by hydrogen reduction of Fe(NO₃)₃•9H₂O and H₂PtCl₆•6H₂O mixtures within the pores of alumina. They were released by dissolving the alumina matrix and capped with organic surfactants. After a series of chemical treatments, clusters with an average diameter of about 11 nm were precipitated. The clusters were drop casted onto ordered nanoporous arrays. Clusters reaching the bottom of the pores formed an ordered magnetic array. The corresponding magnetic film exhibits distinct anisotropic behavior caused by the external magnetic field.

Magnetic Studies Of Iron Oxide Nanoparticles Coated With Oleic Acid And Pluronic® Block Copolymer, Marco Morales Torres, Tapan Kumar Jain, V. Labhasetwar, Diandra Leslie-Pelecky

Diandra Leslie-Pelecky Publications

We have prepared and studied iron-oxide nanoparticles coated with oleic acid (OA) and Pluronic® polymer. The mean diameter of the iron-oxide nanoparticles was 9.3(±)0.8 nm. Saturation magnetization values measured at 10 K varied from 66.1(±0.7) emu/g to 98.7(±0.5) emu/g. At 300 K the loops showed negligible coercive field. The peaks in zero-field-cooled susceptibility decreased from 280 to 168 K with increasing OA concentration up to 10.6 wt %, and remained nearly constant for higher concentrations. This suggests that incomplete coverage of the OA allows small, interacting agglomerates to form.

Magnetization And Neutron Diffraction Studies On Dy5si2ge2, R. Nirmala, V. Sankaranarayanan, K. Sethupathi, A. V. Morozkin, Q. Cai, Z. Chu, J. B. Yang, William B. Yelon, S. K. Malik

Physics Faculty Research & Creative Works

The compound Dy₅Si₂Ge₂ crystallizes in an orthorhombic structure (Sm₅Ge₄ type, space group Pnma). Magnetization measurements performed in the temperature range of 2-300 K in applied fields up to 7 T reveal that this compound orders antiferromagnetically at 56 K (T_N) but with a positive paramagnetic Curie temperature θP. Magnetization-field isotherms, obtained at 5 K and 20 K, display a field-induced antiferromagnetic to ferromagnetic transition. the magnetization approaches saturation in a field of 6 T with a moment value of ~8μ_BDy³⁺. Neutron diffraction measurements, carried out at …

Cluster-Assembled Exchange-Spring Nanocomposite Permanent Magnets, X. Rui, Zhiguang Sun, Yinfan Xu, David J. Sellmyer, Jeffrey E. Shield

David Sellmyer Publications

Isotropic nanocomposite magnetic structures were produced via cluster assembly routes with energy products reaching 18 MG Oe. Gas aggregation produces Fe clusters with an average size less than 10 nm with a very narrow size distribution, and these were embedded in a hard magnetic matrix by cosputtering of FePt. Structures produced were crystallographically isotropic with Fe cluster phase content ranging from 0 to 30 vol %. The coercivity decreased from 13 kOe as Fe cluster content increased. Single-phase-type hysteresis loops indicating excellent exchange coupling at low Fe cluster content give way to a stepped demagnetization loop at an Fe cluster …

Anisotropic Exchange, Ralph Skomski, Arti Kashyap, Jian Zhou, David J. Sellmyer

David Sellmyer Publications

The origin and physical nature anisotropic exchange interactions is investigated. Emphasis is on nonrelativistic exchange anisotropies, as encountered, for example, in intermetallics with layered crystal structures. The summation of site-resolved exchange interactions is analyzed, and it is shown that Ruderman–Kittel-type long-range exchange yield converging exchange-stiffness expressions down to atomic length scales. In general, the resulting exchange stiffness is anisotropic, even if the interaction is mediated by an isotropic free electron gas. The determination of the mean-field Curie temperature from pair-exchange interactions requires the diagonalization of an interaction matrix, as opposed to simple site averaging.

The Effects Of The Thickness Of Magnetically Hard- And Soft-Phase Layers On Magnetic Properties And Exchange Coupling In Multilayer Magnets, W. Liu, Yucheng Sui, J. Zhou, X.K. Sun, C.L. Chen, Z.D. Zhang, David J. Sellmyer

David Sellmyer Publications

The structure and magnetic properties of multilayer magnets containing a hard phase [HP≡ (Nd_0.95Dy_0.05)(Fe,Co,Nb,B) _5.5] and a soft phase (SP≡Fe and Fe₆₅Co₃₅) have been investigated. For the nanocomposite multilayer of Si(substrate)/Ti(30 nm)/[HP(x nm)/SP(y nm)] n/Ti(30 nm), the remanence enhancement has been observed at different thickness and volume fractions of hard- and soft-phase layers. The remanence increases and the coercivity decreases with the addition of soft phase, in comparison with the results of the single-layer film. For the multilayer of Si(substrate)/Ti(30 nm)/[HP(15 nm)/Fe(5 nm)] n/Ti(30 nm) annealed at …

Enhanced Coercivity In Thermally Processed (Nd,Dy)(Fe,Co,Nb,B) 5.5/ Α-Fe Nanoscale Multilayer Magnets, W. Liu, Xingzhong Li, J Ping Liu, X.K. Sun, C.L. Chen, Ralph Skomski, Z.D. Zhang, David J. Sellmyer

David Sellmyer Publications

Structural and magnetic properties of laminated (Nd,Dy)(Fe,Co,Nb,B)_5.5/Fe nanocomposites are investigated. Normally, the addition of the soft phase to the hard phase enhances the remanence but deteriorates the permanent-magnet performance of the material by reducing the coercivity. In the present system, the coercivity increases to 1608 kA/m (20.2 kOe) in thermally processed Nd–Dy–Fe–Co–Nb–B(15 nm)/Fe(4 nm) multilayered nanocomposites, which is higher than that of the single-layer hard-magnetic film. The abnormally high coercivity is achieved by annealing at relatively high temperature, which breaks the laminated structure of the as-deposited multilayer. A likely physical explanation of the enhanced coercivity is the introduction …

Sample Preparation And Annealing Effects On The Ferromagnetism In Mn-Doped Zno, Jun Zhang, Ralph Skomski, David J. Sellmyer

David Sellmyer Publications

Magnetic properties of Mn-doped ZnO strongly depend on sample preparation and annealing. While the samples sintered in air at 500 °C show ferromagnetism at room temperature, those sintered at 900 °C are paramagnetic. The samples sintered in vacuum show a ferromagnetic transition at 45 K, which is attributed to ferrimagnetic Mn₃O₄. With an increase of annealing temperature, the ferromagnetism observed at room temperature in the low-temperature-sintered samples is gradually suppressed and finally completely removed. These results can be understood by considering a metastable ferromagnetic phase and its stability under varying temperatures and atmospheres.

Highly (001)-Oriented Ni-Doped L10 Fept Films And Their Magnetic Properties, M.L. Yan, Yinfan Xu, Xingzhong Li, David J. Sellmyer

David Sellmyer Publications

We report on Ni-doped nonepitaxial L1₀ FePt thin films with strong (001) texture. The influences of Ni doping on L1₀ ordering, orientation, and the magnetic properties of the FePt films have been investigated. In-plane and out-of-plane x-ray diffractions (XRD) were used to analyze the texture of FeNiPt films. For [Fe(0.38 nm)/Ni(0.04 nm)/Pt(0.4)] ₁₃ sample, the out-of-plane XRD data showed only (00l) peaks and in-plane data showed (hk0) peaks after annealing, indicating high (001) texture of the FeNiPt films. In comparison with FePt, the (00l) peak positions shifted to higher angle, …

Magnetic Properties And L10 Phase Formation Of Fept Films Prepared By High Current-Density Ion-Beam Irradiation And Rapid Thermal Annealing Methods, T. Yokota, M.L. Yan, Yinfan Xu, L. Gao, R. Zhang, Leighann Nicholl, Lu Yuan, Ralph Skomski, David J. Sellmyer, Sy_Hwang Liou, Chih-Huang Lai, Cheng-Han Yang, Sheng-Huang Huang

Sy-Hwang Liou Publications

We investigated magnetic properties and L1₀ phase formation of FePt films by rapid thermal annealing (RTA) and high current-density ion-beam irradiation. The sample prepared by RTA at 550 °C has (001) texture and strong magnetic perpendicular anisotropy with H_c equal to 6 kOe. The sample irradiated at 5.04 µA/cm² has H_c equal to 10 kOe but has isotropic magnetic properties due to the (111) texture. The magnetic correlation length of the ion-irradiated sample was about twice as large as that of the RTA sample. This may be due to the inhomogeneity of the L …

Spin Glass Or Random Anisotropy?: The Origin Of Magnetically Glassy Behavior In Nanostructured Gdal2, P. M. Shand, C. C. Stark, D. Williams, Marco Morales Torres, T. M. Pekarek, Diandra Leslie-Pelecky

Diandra Leslie-Pelecky Publications

Initially crystalline GdAl₂ was mechanically milled for long times to produce a highly chemically disordered phase with approximately 8-nm grains. Analysis of dc magnetization measurements using an Arrott plot and the approach to saturation suggest the presence of significant random anisotropy. ac susceptibility measurements showed that the shift in the peak temperature with frequency usually seen in magnetically glassy and superparamagnetic systems was virtually undetectable in the 10–1000-Hz frequency range. Based on these results, we believe that this material represents an interacting system with random anisotropy, where the anisotropy is the result of surface and interface asymmetries. ©2005 American …

Magnetic Materials For Finite-Temperature Quantum Computing, Ralph Skomski, J. Zhou, A.Y. Istomin, Anthony F. Starace, David J. Sellmyer

Anthony F. Starace Publications

The potential use of interacting magnetic nanodots for quantum computing (qubit) operations is investigated by model calculations. The quantum entanglement of the low-lying ferromagnetic states, as quantified by the concurrence, exhibits a resonant peak whose position and width depend on parameters such as dot anisotropy, interdot exchange, and external field gradient. The maximum operation temperature is proportional to the magnetocrystalline anisotropy of the dot material. A specific condition is that the dots are sufficiently small so that the interatomic exchange ensures a coherent magnetization state and quantum coherence at finite temperatures. From a material point of view, there is a …

Deep-Uv Light Emitting Diodes: An Experimental Investigation Of Characterization And Optimization Techniques, Eric M. Fraser

Pomona Senior Theses

Light emitting diodes (LEDs) and laser diodes (LDs) have many advantages over conventional light sources. Current commercial LEDs span the spectrum from IR to near- UV. There are a variety of applications for devices that extend into the deep-UV, including biological agent detection and optical storage. The nitride material system is a set of semiconducting compounds that have wavelengths that span a broad range, from yellow to deep-UV. AlGaN has a direct bandgap that extends into the deep-UV range; we will try to grow device-quality material, deposited epitaxially using metalorganic chemical vapor deposition on sapphire substrates.

Cyclopropylcarbinyl → Homoallyl-Type Ring Opening Of Ketyl Radical Anions. Structure/Reactivity Relationships And The Contribution Of Solvent/Counterion Reorganization To The Intrinsic Barrier, J. M. Tanko, Jason G. Gillmore, Robert Friedline, M'Hamed Chahma

Faculty Publications

Following a protocol developed by Mathivanan, Johnston, and Wayner (J. Phys. Chem. 1995, 99, 8190−8195), the radical anions of several cyclopropyl- and oxiranyl-containing carbonyl compounds were generated in an effort to measure the rate constants for their ring opening (ko) by laser flash photolysis. The results of these experiments are compared to those obtained from earlier electrochemical studies, and the combined data set is used to rationalize the kinetics of radical anion ring opening in a general context by using Savéant's theory pertaining to stepwise dissociative electron transfer (Acc. Chem. Res. 1993, 26, 455−461). Compared to cyclopropylcarbinyl → homoallyl rearrangements …

Two-Neutron Transfer In The 6he + 209bi Reaction Near The Coulomb Barrier, Paul A. Deyoung, Patrick J. Mears, J. J. Kolata, E. F. Aguilera, F. D. Becchetti, Y. Chen, M. Cloughesy, H. Griffin, C. Guess, J. D. Hinnefeld, H. Jiang, Scott R. Jones, U. Khadka, D. Lizcano, E. Martinez-Quiroz, M. Ojaniega, Graham F. Peaslee, A. Pena, J. Reith, S. Vandendriessche, J. A. Zimmerman

Faculty Publications

The cross section for α-particle emission in the He6+Bi209 reaction at energies near the Coulomb barrier is remarkably large. Possible reactions that may produce the observed α particles include two-neutron transfer, one-neutron transfer, and direct projectile breakup. Each of these mechanisms results in a distinctive angular correlation between the α particle and the outgoing neutron(s). A neutron-α-particle coincidence experiment was performed to study two-neutron transfer to unbound states of Bi211. It is shown that approximately 55% of the observed α-particle yield at and beyond the grazing angle is because of this process. This is more than 2.5 times the fraction …

A Multiwavelength View Of The Tev Blazar Markarian 421: Correlated Variability, Flaring, And Spectral Evolution, P. T. Reynolds, Veritas

Physical Sciences Publications

We report results from an intensive multiwavelength monitoring campaign on the TeV blazar Mrk 421 over the period of 2003-2004. The source was observed simultaneously at TeV energies with the Whipple 10 m telescope and at X-ray energies with the Rossi X-Ray Timing Explorer (RXTE) during each clear night within the Whipple observing windows. Supporting observations were also frequently carried out at optical and radio wavelengths to provide simultaneous or contemporaneous coverages. The large amount of simultaneous data has allowed us to examine the variability of Mrk 421 in detail, including cross-band correlation and broadband spectral variability, over a wide …

Epitaxial Growth And Magnetic Properties Of Cr-Doped Aln Thin Films, Jun Zhang, Sy_Hwang Liou, David J. Sellmyer

Department of Physics and Astronomy: Faculty Publications

Cr-doped AlN thin films were epitaxially grown on Al₂O₃(001) substrates at low temperature by reactive magnetron sputtering,and their magnetic properties were investigated. Extensive x-ray diffraction studies indicated that the films have a wurtzite-type hexagonal structure and are (001) oriented, with an epitaxial relationship of the [100] direction of the films along the [110] direction of Al₂O₃. The c axis lattice parameter of the films showed a linear dependence on the Cr concentration for Cr concentrations below 0.15. Room temperature ferromagnetism was observed, and the magnetic properties showed strong dependence on the Cr …

Total Suspended Solids In Las Vegas Wash, Mika Koponen

UNLV Theses, Dissertations, Professional Papers, and Capstones

Erosion along the Las Vegas Wash (Wash) has increased steadily along with the rapidly increasing population. The focus of this study is to find out if there is a relationship between lower total suspended solids rate (TSS) and the construction of erosion control structures along the Wash. The study was conducted between 7/15/2003 and 11/29/2004.

The method was water sampling on 7 sites and the samples were analyzed at Southern Nevada Water Systems (SNWS) laboratory at Saddle Island in Boulder City. Additional water quality data was also collected using Quanta, a device measuring different parameters in water. In this study …

Analyzing The Effects Of Various Sign Types Used By The Bureau Of Land Management For Desert Cleanup And Restoration Projects, Christopher Linehan

UNLV Theses, Dissertations, Professional Papers, and Capstones

What is the effectiveness of attempts to curb illegal dumping with prompts on signs posted in the area? Do positive friendly language prompts such as “Restoration in Progress Please Do Not Disturb” generate more positive attitudes and intentions towards illegal dumping problems, than less friendly prompts such as “No Dumping Allowed”?

Results, although inconclusive, did however show a slight trend towards the negative language signs.

Integrated Recombinant Protein Expression And Purification Platform Based On Ralstonia Eutropha, Gavin C. Barnard, Jesse D. Mccool, David W. Wood, Tillman U. Gerngross

Dartmouth Scholarship

Protein purification of recombinant proteins constitutes a significant cost of biomanufacturing and various efforts have been directed at developing more efficient purification methods. We describe a protein purification scheme wherein Ralstonia eutropha is used to produce its own “affinity matrix,” thereby eliminating the need for external chromatographic purification steps. This approach is based on the specific interaction of phasin proteins with granules of the intracellular polymer poly

A Qualitative Study Investigating The Effectiveness Of The Nts Cab To The U.S. Department Of Energy, Cheryl Freire

UNLV Theses, Dissertations, Professional Papers, and Capstones

Environmental Management Site Specific Advisory Boards (SSAB) were Implemented by the U.S. Department of Energy (DOE) as means for stakeholder involvement in decision-making and recommendations related to environmental management activities at DOE sites throughout the United States. As part of this process, the Nevada Test Site Community Advisory Board (NTS CAB) was formed in 1995. This project examines the effectiveness of the NTS CAB in aiding the DOE in the decisions made concerning environmental management activities at facilities located in Nevada using secondary data sources and background information. Results show that the NTS CAB is an effective entity and proves …

Using A Systems Dynamics Approach To Determine What Can Be Done To Minimize Ozone Production, Jusdi Mcdonald

UNLV Theses, Dissertations, Professional Papers, and Capstones

Ground level ozone is an air pollutant that affects people of all ages and origins in many urban areas including the Las Vegas Valley. The purpose of this study is to examine the best strategy to minimize tropospheric ozone generation through using a system dynamics approach via LUTAQ is used to formulate a solution to the reduction of ozone in the Clark County non-attainment area. By increasing housing density, decreasing average distance per trip, and decrease average number of trips per person per day is the most effective strategies to improving ozone levels in Las Vegas Valley.

Study Examining A Doe Proposal To Dispose Of Mixed Low Level Waste At The Nevada Test Site Using An Alternative Landfill Design, Deborah Hart

UNLV Theses, Dissertations, Professional Papers, and Capstones

The Department of Energy has set forth a proposal to use an Alternative Landfill Design (ALD) for the Mixed Low Level Waste disposal facility, in Area 5 of the Nevada Test Site in place of a traditional engineered liner for the facility. The purpose of this thesis is to examine the impasse between the DOE and the State of Nevada regarding the proposal of placing a traditional liner versus the alternative landfill design, using a case study method to provide recommendations to policy makers and the NTS CAB. This project used secondary data to evaluate, and determine the effectiveness of …

General Existence Principles For Nonlocal Boundary Value Problems With Ø-Laplacian And Their Applications, Ravi P. Agarwal, Donal O'Regan, Svatoslav Staněk

Mathematics and System Engineering Faculty Publications

The paper presents general existence principles which can be used for a large class of nonlocal boundary value problems of the form (ø(x′)) ′ = f1(t,x,x′) + f2(t,x,x′)F 1X + f3(t,x,x′)f2x,α(x) = 0, β(x) = 0, where fj satisfy local Carathéodory conditions on some [0,T] × Dj ⊂ ℝ, fj are either regular or have singularities in their phase variables (j = 1,2,3), f i, : C1[0.T] → C0[0,T] (i = 1,2), and α,β : C1[0.T] → ℝ are continuous. The proofs are based on the Leray-Schauder degree theory and use regularization and sequential techniques. Applications of general existence principles …

Preparation, Structural Characterization, And Dynamic Properties Investigation Of Permalloy Antidot Arrays, Andriy Vovk, Leszek M. Malkinski, Scott L. Whittenburg, Charles O'Connor, Jin-Seung Jung, Suk-Hong Min

Chemistry and Biochemistry Faculty Publications

Regular nanosized structures are considered to be promising materials for magnetic information storage media with high density of information. Recently attention was paid to static and dynamic magnetic properties arising from dimensional confinement in such nanostructures. Here we present an investigation of permalloy antidot arrays of different thicknesses. Thin permalloyfilms of thickness ranging from were deposited on nanoporous membranes with a pore size of . It was found that additional ferromagnetic resonance peaks appear for film thicknesses below , while films with larger thicknesses show resonanceproperties similar to continuous films. A comparison between the filmsdeposited onto Si wafers and porous …