Physical Sciences and Mathematics Commons^™

On The Role Of Zn In Sm2Fe17NX Permanent Magnets, P.A.P. Wendhausen, D. Eckert, A. Handstein, K.-H. Müller, G. Leitner, Ralph Skomski

Ralph Skomski Publications

Coercivities up to 1.9 T were achieved by bonding micron-size Sm₂Fe₁₇N_x particles with zinc. X-ray analysis of Zn-bonded samples indicates the presence of the ZnFe Γ phase and no unreacted Zn, after heat treatment at about 390 °C for 100 min for samples with Zn content varying from 5 to 35 wt. %. The increase of coercivity occurs only for a more intense heat treatment, e.g., at 390 °C! for 300 min or 425 °C for 100 min. Differential scanning calorimetry (DSC) shows two exothermic peaks at 388 and 426 °C, but no endothermic peak …

Magnetization Reversal And Defects In Co/Pt Multilayers, J.X. Shen, Roger D. Kirby, K. Wierman, Z.-S. Shan, David J. Sellmyer, T. Suzuki

Roger Kirby Publications

(Co 3 Å/Pt 10 Å)× N (N=8, 16, and 30 bilayers) multilayer thin films were prepared by sputtering onto an 850-Å-thick SiN layer that had been deposited on a silicon (111) substrate. We used the polar Kerr effect to measure the time dependence of magnetization reversal over the temperature range 90–300 K. Direct domain observations were also carried out. The results show that the domain expansion process depends strongly on the number of bilayers. Uniform domain expansion was found only in the thinner samples. From the time dependence of the magnetization reversal measurements over the temperature range 90–300 …

Experimental Determination Of The Unoccupied Bands Of W(110), Dongqi Li, Peter A. Dowben, J.E. Ortega, F.J. Himpsel

Peter Dowben Publications

Inverse photoemission from W(110) has been used to determine the dispersion of the unoccupied energy bands along the tungsten ΓΣN line. The critical points are determined to be Γ₁₂ at 2.6 eV, N₁ at 2.2 eV, and the lower limit of N₄ at 3.2 eV. The upper bound of the lifetime broadening is estimated as 0.3-0.4 eV at 2-3 eV above E_F. The image state is observed at 4.5 eV above E_F, or 0.7 eV in binding energy, relative to the vacuum level.

Behavior Of Disordered Co-Pd, Co-Ag, And Co-Mo Alloys In Multilayer Interfaces, Z.S. Shan, Ping He, C. Moore, John A. Woollam, David J. Sellmyer

David Sellmyer Publications

Local environment effects are important for understanding interfacial magnetism in artificially structured multilayers. For this reason, stable and/or metastable disordered alloys Co_xM_1-x (M=Pd, Ag, Mo) were fabricated in a dc cosputtering system. The magnetization as a function of composition and temperature were measured and analyzed in terms of a mean-field model. The local environment effect on the Co moment from its Ag (or Mo) neighbors and the Pd moment from its Co neighbors are studied. It is found that the polarized Pd moment increases linearly with an increasing number of nearest Co neighbors and the Co moment …

Electronic Structure Of Sm2Fe17NX Compounds, J.P. Woods, A.S. Fernando, Sitaram Jaswal, B.M. Patterson, D. Welipitiya, David J. Sellmyer

David Sellmyer Publications

Sm₂Fe₁₇N_x is of considerable current interest as a permanent-magnet material because of its enhanced Curie temperature and uniaxial anisotropy. The electronic structures of Sm₂Fe₁₇N_x for x=0 and x~2.6 have been studied with photoemission and spin-polarized calculations. The materials are prepared by arc melting and nitrogen is introduced by ion implantation. The nitrogen concentration is quantified with Auger electron spectroscopy. The Sm 4f electrons with binding energies between 6 and 10 eV are investigated with resonant photoemission using photon energies near 140 eV. The major features of ultra-violet …

Characterization, Transport, And Magnetic Properties Of Co/Pd Multilayers Produced By Pulsed Laser Deposition, M. Enrech, Ralph Skomski, J.M.D. Coey, J.G. Lunney

Ralph Skomski Publications

Metallic Co/Pd multilayers with equal layer thickness and periods ranging from 5 to 150 A were deposited using pulsed laser deposition. They have an fcc (111) texture and the diffraction patterns give clear evidence of a modulated period structure. From the period dependence of the in-plane electrical resistivity a diffuse interface between cobalt and palladium of thickness b= 6 ± 2 Å has been deduced. Magnetic, magnetoresistance, and Kerr-effect measurements indicate in-plane magnetization. In low fields (B₀ < 10 mT) the magnetoresistanceis highly anisotropic, varying by ~2% in the transverse configuration and + 1% in the longitudinal configuration due to orbital moment of cobalt in the interface region. The shape of the Kerr-effect major hysteresis loop can be explained by the in-plane component of the anisotropy of the cobalt layers.

Magnetic Reversal In Sm2Fe17NY Permanent Magnets, Ralph Skomski, K.-H. Müller, P.A.P. Wendhausen, J.M.D. Coey

Ralph Skomski Publications

Coercivity and nucleation fields for three-dimensional inhomogenities are calculated and discussed. For soft spherical inclusions in a hard magnetic matrix a (δ /R) ² dependence of the nucleation field on inclusion radius R and domain wall width δ has been obtained. To calculate propagation fields for spherical inhomogenities, a nonlinear model has been used. The corresponding coercive field can be much higher than the nucleation field, particularly if the magnitude of the anisotropy changes very steeply. Soft inclusions with sharp interfaces are ineffective at producing complete magnetic reversal. For partly nitrided particles coercivity and nucleation field are predicted …

Magnetic Properties Of Epitaxial Single Crystal Ultrathin Fe3Si Films On Gaas (001), Sy_Hwang Liou, S.S. Malhotra, J.X. Shen, M. Hong, J. Kwo, H.S. Chen, J.P. Mannaerts

Sy-Hwang Liou Publications

Magnetic properties of Fe₃Si films with thickness from 2 to 210 monolayers (ML) epitaxially grown on GaAs (001) were studied using a superconducting quantum interference device and alternating gradient force magnetometers. Growth of these single-crystal intermetallic compound films were carried out in a multichamber molecular beam epitaxy (MBE) system. The samples were covered in situ with Au 50 Å thick to prevent oxidation when the samples were removed from the MBE chamber. All the films are ferromagnetic even for samples as thin as 2 ML. The easy magnetization direction of the films is parallel to the film surface. …

Interface Magnetism And Superparamagnetism Of Co/Cu Multilayers, Z.S. Shan, S. Nafis, John A. Woollam, Sy_Hwang Liou, David J. Sellmyer

Sy-Hwang Liou Publications

The magnetic properties of Co/Cu multilayers were investigated from 5 to 380 K and analyzed in terms of the interface magnetism and mean-field model. The interface, which is about 1.2 Å thick, is nonmagnetic at room temperature and becomes magnetic at 5 K with the average magnetization of 40% of the pure Co magnetization. The samples of X Å Co/10 Å Cu behaved superparamagnetically as X ranged from 4 to 6.5 Å and did not show superparamagnetic behavior for thinner or thicker Co layer thickness.

Photoluminescence And Absorption Studies Of Defects In Cdte And Znxcd1-Xte Crystals, Cheryl Barnett Davis, David D. Allred, A. Reyes-Mena, Jesus González-Hernández, Ovidio González, Bret C. Hess, Worth P. Allred

Faculty Publications

We have studied at cryogenic temperatures photoluminescence features which lie more than 0.15 eV below the band edge in ZnxCd1-xTe (0≤x≤0.09) crystals. The same features, namely a defect band which lies at about 0.13-0.20 eV below the band-gap energy and a peak at 1.1 eV, that are observed in pure CdTe samples are observed in these alloy materials. In annealed samples we observe that the 1.1 eV feature, which has been attributed to tellurium vacancies, increases with fast cooling. Increased concentrations of tellurium vacancies can be understood in terms of the phase diagram of CdTe which indicates that higher concentrations …

Far-Ultraviolet Stellar Photometry: A Field In Monoceros, Edward G. Schmidt, George R. Carruthers

Edward Schmidt Publications

Far-ultraviolet photometry of stars in a field in Monoceros in the wavelength rang from 1230 Å to 1600 Å has been carried out using data from an electrographic Schmidt camera carried on a sounding rocket. Ultraviolet magnitudes were extracted for 602 objects in the field. Fifty-eight percent were tentatively identified with visible stars using the SIMBAD data base while another 25% are blends of objects too close together to separate with our resolution. Eleven of the uv objects coincide with parts of the star clusters NGC 2169, NGC 2244, and KGC 2264 in which individual stars cannot be resolved. As …

Photoquenching And Thermal Recovery Of A Thermally Stimulated Current Peak In Semi-Insulating Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

A prominent thermally stimulated current peak T₅ appearing in semi‐insulating GaAs is shown to photoquench under infrared illumination, and then thermally recover at a rate r=2.0×10⁸ exp(−0.26 eV/kT) s⁻¹, exactly the same as that observed for EL2, within experimental error. Two possible explanations exist: (1) T₅ and EL2 are microscopically very similar, probably each with an As_Ga core; or (2) T₅ is an electron trap that only appears to quench and recover with EL2 because EL2 controls the electron lifetime. Several other traps show similar quenching and recovery behavior.

Canted Magnetic Moments At The Gd(0001) Surface, Dongqi Li, Jiandi Zhang, Peter A. Dowben, K. Garrison, P.D. Johnson, H. Tang, T.G. Walker, H. Hopster, J.C. Scott, D. Weller, D.P. Pappas

Peter Dowben Publications

With spin polarized electron spectroscopies, we have investigated ordered GD(0001) films deposited on W(110). The photoemission features of the gadolinium 5d surface state, the 4f levels, and the background exhibit considerable spin polarization along the same direction in the plane of the film, indicative of ferromagnetic coupling between the surface and the bulk. The 4f spin polarized photoemission data provides strong evidence that the surface 4f polarization differs from the bulk 4f polarization for Gd(0001). Our temperature dependent measurements with spin polarized secondary electron spectroscopy conclusively establishes that the surface of clean Gd(0001) possesses a perpendicular polarization component which persists …

Investigation Of Spin-Orbit Effects In The Excitation Of Noble Gases By Spin-Polarized Electrons, J. E. Furst, W. M. K. P. Wijayaratna, D. H. Madison, Timothy J. Gay

Timothy J. Gay Publications

We have measured the integrated Stokes parameters of the light emitted following the impact excitation by polarized electrons of the np⁵ (n+1)p [5/2] ₃ states in Ne (n=2), Ar (n=3), Kr (n=4), Xe (n=5), and the np⁵ (n+1) p[5/2] ₂ state in Kr. The near-threshold linear-polarization fractions η1 were consistent with zero for all of the J=3 states measured, providing no evidence of Mott scattering. For J=2 excitation, η₁ shows the clear influence of the intermediately coupled nature of the state. …

A Neutron Diffraction And Mössbauer Effect Study Of The Magnetic Structure Of Y₂(Fe₁₋ₓmnₓ)₁₄B, Gaya Kanishka Marasinghe, Oran Allan Pringle, Gary J. Long, William Joseph James, William B. Yelon, Fernande Grandjean

Physics Faculty Research & Creative Works

The magnetic structures of a series of Y₂(Fe _1-xMn_x)₁₄B samples, with x equal to 0.03, 0.10, 0.25, and 0.37, have been studied by powder neutron diffraction and Mössbauer spectroscopy. Y₂(Fe_0.97Mn _0.03)₁₄B and Y₂(Fe_0.9Mn _0.1)₁₄B are ferromagnetic at both 295 and 85 K, Y ₂(Fe_0.63Mn_0.37)₁₄B is paramagnetic at both 295 and 85 K, whereas Y₂(Fe_0.75Mn _0.25)₁₄B is paramagnetic at 295 K and is partially ordered at …

Two Dimensional Power Spectral Estimation Using Constrained Iterative Spectral Deconvolution, Jerome Leslie Coggins

University of New Orleans Theses and Dissertations

Two-dimensional power spectral estimation is an important tool for seismic data analysis and other applications. Some datasets, however, have a limited number of points in one or both dimensions. In seismic applications, there are typically fewer points in the spatial domain as compared to the temporal domain. Conventional spectral estimation techniques suffer from poor resolution on short datasets due to inherent smoothing or bias.

Ramaswamy and loup developed a one-dimensional method for the estimation of power spectra for short datasets. Constrained Iterative Spectral Deconvolution (CISD) greatly improves the power spectral resolution using a straight-forward algorithm. In a comparison to other …

Modification Of The Eikonal Relation For Chemical Waves To Include Fluid Flow, J. W. Wilder, D. A. Vasquez, Boyd F. Edwards

All Physics Faculty Publications

Propagating wave fronts resulting from autocatalytic chemical reactions have been the focus of much recent research. For the most part, the hydrodynamics resulting from such reactions has been neglected. In this work, a relation is derived for the normal speed of a propagating wave front as a function of the local curvature when fluid motion is allowed. This ‘‘eikonal’’ equation is a generalization of one which was derived in the absence of fluid flow. It is also shown that small variations in the fluid density due to the chemical reaction do not change the form of the relation.

A Lattice Gas Approach To The Structure And Dynamics Of Electrorheological Fluids, Jie Chen '93

Honors Projects

Electrorheological fluids consist of a colloidal suspension of dielectric particles in a continuous fluid of smaller dielectric constant. Molecular dynamics simulations of these fluids in an applied electric field have recently been shown to produce percolated, columnar structures. No systematic attempt has been made so far to simultaneously include the effects of temperature and the viscous drag due to the continuous fluid. We propose a dipolar lattice gas model for electrorheological fluids and study the resulting structures and dynamics. We attempt to incorporate the effect of the viscosity of the continuous medium by a dynamic ansatz that determines the range …

Resonant Inhibition Of Multiphoton Ionization, J. Greg Story, D. I. Duncan, Thomas F. Gallagher

Physics Faculty Research & Creative Works

When exposed to intense light of ~580 nm, the ground state of K shifts up in energy, passing through two-photon resonances with Rydberg states and finally crossing the two-photon ionization limit. Using laser pulses of 0.5 to 13 ps duration, we have shown experimentally that ionization occurs for short pulses, but for long pulses the population is diverted into the Rydberg states where some population survives the peak intensity of the pulse, in excellent agreement with a dynamic Floquet model.

120kev Ar8+-Li Collisions Studied By Near Uv And Visible Photon Spectroscopy, E. Jacquet, P. Boduch, M. Chantepie, M. Druetta, D. Hennecart, X. Husson, D. Lecler, Ronald E. Olson, J. Pascale, N. Stolterfoht, M. Wilson

Physics Faculty Research & Creative Works

A spectroscopic analysis of light emitted in the 200-600 nm wavelength range by Ar⁷⁺, Ar⁶⁺ and Ar⁵⁺ ions after charge exchange in 120keV Ar⁸⁺-Li collisions is performed. Transitions with Δn = 1 and Δn = 2 for n = 8, 9, 10 and 11 states of Ar8 following single electron capture are identified and the production cross sections for n = 8 and n = 9 are deduced from emission cross sections and compared with those calculated by the three-body classical trajectory Monte-Carlo method. Lines due to double capture process were observed and identified …

Adjustable Levels Of Strong Turbulence In A Positive/Negative Ion Plasma, D. P. Sheehan, R. Mcwilliams, N. Rynn

Physics and Biophysics: Faculty Scholarship

Positive/negative ion plasmas, composed of Ba+, SF6−, and residual electrons, were observed to display characteristics of strong turbulence.Experiments on the UCI Q machine linked the presence of negative ions (and the depletion of electrons) with large density fluctuations (δn/n≂1), large‐amplitude, low‐frequency electrostatic noise (f≤20 kHz), and rapid transport of ions across magnetic field lines (D⊥≂104 cm2/sec). Ion velocity distributions were heated parallel to and cooled perpendicular to the confining magnetic field. The partial pressure of gaseous SF6 was shown to serve as a regulator of plasma turbulence. Turbulence levels could be smoothly varied from quiescent states (δn/n≂0.01) to strongly turbulent …

Unusual Thermal Stability Of A Site-Ordered Mc60 Rocksalt Structure (M=K, Rb, Or Cs), Q. Zhu, O. Zhou, J. E. Fischer, Andrew R. Mcghie, W. R. Romanow, Robert M. Strongin, M. A. Cichy, Amos B. Smith Iii

Chemistry Faculty Publications and Presentations

X-ray diffraction and differential scanning calorimetry of M_xC₆₀, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to K_xC₆₀ with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas …

A Novel Technique For Studying The Shear Elastic Properties Of Weak Solids, Jason A. Payne '93

Honors Projects

We have developed a simple, inexpensive, and precise technique to measure the shear elastic modulus of weak solids using electromagnetic and optical tools. This technique can be easily adapted to measure the viscosity of a liquid also. A Helmholtz pair was used to produce a torque on a permanent magnet mounted on the smaller of two concentric cylinders, coupled by the material to be studied. The torque was controlled precisely and measured accurately in terms of the current flowing through the coils of the Helmholtz pair. An optical lever was employed to measure the angular displacement of the inner cylinder …

Evidence Of A Nondiffusive Transport In A Monodisperse Screened Coulomb System By A Molecular-Dynamics Simulation, Michael Wild, Ras B. Pandey

Faculty Publications

A molecular-dynamics simulation is carried out for particles (tracers) interacting with a screened Coulomb potential in three dimensions. A phase transition is observed from a liquid to an fcc solid on reducing the temperature at a fixed density, consistent with previous studies. In the liquid phase, the variation of the rms displacement R_tr of the charged particles with time seems to depend on the density and the temperature. In the short-time regime, the effective exponent k for the subdiffusive power-law behavior, i.e., R_tr∼t^k, decreases on increasing the density and lowering the temperature. A crossover from …

Cp-Odd Nucleon Potential, Vladimir Gudkov, Xiao-Gang He, Bruce H.J. Mckellar

Faculty Publications

The P-odd and CP-odd nucleon potential for different models of CP violation in the one-meson exchange approximation is studied. It is shown that the main contribution is due to the π-meson exhcange which leads to a simple one-parameter CP-odd nucleon potential.

Properties Of Rigid Foams For Application As Materials For Light Weight Structures In Space, Huichen Chi

All Graduate Theses and Dissertations, Spring 1920 to Summer 2023

The mechanical and mesoscopic structural properties of rigid cellular foam made of polystyrene have been investigated. Basic properties (e.g., density, total and available porosity, permeability, surface area, isotropicity, and cell size and cell wall thickness distributions) were measured. In most cases, alternative methods were used to determine which methods were most appropriate for the type of samples we studied. Standard compression and deflection mechanical tests were performed. The stress-strain curves and related mechanical properties were found to agree with standard cellular structural models of open-cell foams.

We investigated the effects of small (~ < 5 atm) hydrostatic stress applied to foam samples for long periods of time (~one day). We observed large changes (up to a factor of three) in the stress-strain diagrams, Young's modulus, elastic collapse stress, ultimate strength, resilience, Poisson's ratio, permeability, penetration depth, and available porosity. Effects were most pronounced above 2 atm applied pressure differential, but were observed even for 1 atm loads. Short-term exposure to loads up to ten times as large did not cause comparable changes. These changes were interpreted as resulting from observed changes in the mesoscopic structure occurring near the surface using standard cellular structural models.

This work was originally motivated by applications …

Three-Dimensional Quantum Gravity Coupled To Gauge Fields, Simon Catterall, Ray L. Renken, John B. Kogut

Physics - All Scholarship

We show how to simulate U(1) gauge fields coupled to three-dimensional quantum gravity and then examine the phase diagram of this system. Quenched mean field theory suggests that a transition separates confined and deconfined phases (for the gauge matter) in both the negative curvature phase and the positive curvature phase of the quantum gravity, but numerical simulations find no evidence for such transitions.

The Xy Model On A Dynamical Random Lattice, Simon Catterall, John B. Kogut, Ray L. Renken

Physics - All Scholarship

We study the XY model on a lattice with fluctuating connectivity. The expectation is that at an appropriate critical point such a system corresponds to a compactified boson coupled to 2d quantum gravity. Our simulations focus, in particular, on the important topological features of the system. The results lend strong support to the two phase structure predicted on the basis of analytical calculations. A careful finite size scaling analysis yields estimates for the critical exponents in the low temperature phase.

Metastability In Two Dimensions And The Effective Potential, Mark Alford, Marcelo Gleiser

Dartmouth Scholarship

We study analytically and numerically the decay of a metastable phase in (2+1)-dimensional classical scalar field theory coupled to a heat bath, which is equivalent to two-dimensional Euclidean quantum field theory at zero temperature. By a numerical simulation we obtain the nucleation barrier as a function of the parameters of the potential, and compare it to the theoretical prediction from the bounce (critical bubble) calculation. We find the nucleation barrier to be accurately predicted by theory using the bounce configuration obtained from the tree-level (“classical”) effective action. Within the range of parameters probed, we found that using the bounce derived …

Multiplet Fine Structure In The Photoemission Of The Gadolinium And Terbium 5p Levels, B.T. Thole, X.D. Wang, B.N. Harmon, Dongqi Li, Peter A. Dowben

Peter Dowben Publications

Fine structure is observed in the photoemission of the gadolinium and terbium 5p levels. The 5p levels are split into multiplets due to spin-orbit splitting and to Coulomb and exchange interactions with the f shell. The calculated theoretical spectra are in good agreement with the experimental results.