Theses, Dissertations, and Student Research: Department of Physics and Astronomy

Ab-initio and model studies of spin fluctuation effects in transport and thermodynamics of magnetic metals

James K. Glasbrenner — Mon, 22 Apr 2013 07:20:22 PDT

Magnetic materials are vital to many devices and the manipulation of spins is central to the operation of novel devices such as spin transistors. It is important to understand the effect of spin fluctuations on such systems. In this dissertation, first-principles calculations and models further the understanding of spin fluctuation effects in the transport and thermodynamics of magnetic metals.

A simple classical spin-fluctuation Hamiltonian with a single itinerancy parameter is studied using the mean-field approximation, Monte Carlo simulations, and a generalized Onsager cavity field method. The results of these different methods are in agreement. It is found that the thermodynamics are sensitive to the choice of phase space measure and that short-range order is weak for all degrees of itinerancy.

Spin injection from a half-metallic electrode in the presence of thermal spin disorder is analyzed using a combination of random matrix theory, spin-diffusion theory, and explicit simulations for the tight-binding s-d model. It is shown that spin-flip scattering from the interface destroys spin coherence. Spin injection is possible and is constrained by the mean-free path and spin diffusion length in the semiconductor.

The spin-disorder resistivity (SDR) is calculated for the Gd-Tm series in the paramagnetic state using two complimentary first-principles approaches. The SDR in the series follows an almost universal dependence on the exchange splitting and is underestimated when compared with experiment. Frozen atomic displacements (phonons) are then introduced along with spin disorder and the total resistivity is calculated as a function of the mean-square displacement for Fe and Gd. The resistivity increases non-linearly for small displacements and transitions to a linear dependence at larger displacements that, when fitted, enhances the SDR. The enhancement observed in Gd is substantial. The enhancements are electronic in origin, and the rapid increase observed in Gd is traced to a strong, disorder-induced interaction between the electron and hole Fermi surfaces, while the linear trend at large displacements is a saturation effect brought on by strong disorder.

Adviser: Kirill D. Belashchenko

Electron-Phonon Coupling and Structural Phase Transitions on Au/Mo(112)

Keisuke Fukutani — Wed, 14 Nov 2012 13:20:29 PST

The electronic structures, many-body interactions and Fermi surface topologies of Au/Mo(112) were investigated in detail and were found to play important roles in the newly discovered order-disorder structural phase transition of the system. First, the high-resolution angle-resolved photoemission spectroscopy was utilized to characterize the electronic band structure of Mo(112) in far greater details than before. This elucidated the existence of several surface-derived states and their dispersion relations in high precisions near the Fermi level, as well as the symmetries of the bulk and surface electronic states, which are in good quantitative agreement with the ab-initio calculations. Such thorough understanding of the electronic states on Mo(112) made it possible to investigate the more complex electronic structure and many-body interactions in the Au overlayers formed on the Mo(112) surface and their interface. Upon the Au adsorption on Mo(112) substrate, the Au overlayer states are seen to hybridize with those of Mo substrate, which resulted in the formation of the several surface resonance bands, exhibiting high electronic localization near the surface and interface of the combined system. Furthermore, the electron-phonon coupling, involving these surface resonance states, is found to cause strong effective mass enhancement of the electrons near the Fermi level, which can contribute significantly to the surface lattice instability. In particular, for the (4x1) Au overlayer on Mo(112), the noticeable temperature-dependent changes in the Fermi surface contours were observed near the room temperature and were seen to act in favor of the stronger nesting condition and phonon-induced lattice distortions. The combination of the identified strong electron-phonon coupling and the critical Fermi surface topology near the room temperature likely relates to the overlayer lattice instability on the Au/Mo(112) system. In accord with the above general expectation, the order-disorder structural phase transitions were identified on Au/Mo(112) above the room temperature, which is characterized by the abrupt changes in the effective surface Debye temperature, indicative of significant softening of phonons on Au/Mo(112) across the transition. The sequence of these studies likely evidences that the strong electron-phonon coupling and the temperature-dependent Fermi surface topology are indispensable in driving the order-disorder transitions on Au/Mo(112).

Advisor: Peter A. Dowben

Ultrafast intense-field photoionization and photofragmentation of systematic series of substituted organic molecules

Timothy D. Scarborough — Mon, 23 Apr 2012 13:50:30 PDT

The abundance and relevance of organic molecules similar to benzene makes their study important. Studying the interactions of such molecules with intense light fields has implications for the generation of short-wavelength radiation, attosecond science, high-harmonic generation, and many other fields. However, the computing power necessary to complete fully ab initio calculations describing molecules of this size does not exist; this leaves theoretical studies to rely on assumptions and approximations just to calculate the energies of the ground state. Including any sort of dynamics in these calculations is prohibitively complicated, and this makes experimental observations important. Since many organic molecules are similar, it is possible to construct systematic series out of groups of molecules which differ from each other by a single structural parameter. Any differences in the intense-field behaviors between two molecules in such a series must then be a result of the structural parameter that is different. This dissertation is dedicated to the study of systematic series of organic molecules similar to benzene (C₆H₆). Our experiments are sensitive to the dynamics of photoionization and photofragmentation processes which result from interacting molecules in the gas phase with ultrafast (50 fs, 800 nm) laser pulses.

Adviser: Professor Cornelis Uiterwaal

An Exploration of Neutron Detection in Semiconducting Boron Carbide

Nina Hong — Wed, 18 Apr 2012 12:52:50 PDT

The ³He supply problem in the U.S. has necessitated the search for alternatives for neutron detection. The neutron detection efficiency is a function of density, atomic composition, neutron absorption cross section, and thickness of the neutron capture material. The isotope ¹⁰B is one of only a handful of isotopes with a high neutron absorption cross section—3840 barns for thermal neutrons. So a boron carbide semiconductor represents a viable alternative to ³He. This dissertation provides an evaluation of the performance of semiconducting boron carbide neutron detectors grown by plasma enhance chemical vapor deposition (PECVD) in order to determine the advantages and drawbacks of these devices for neutron detection. Improved handling of the PECVD system has resulted in an extremely stable plasma, enabling deposition of thick films of semiconducting boron carbide. A variety of material and semiconducting characterization tools have been used to investigate the structure and electronic properties of boron carbide thin films, including X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, infrared/Raman spectroscopy, current-voltage measurements and capacitance-voltage measurements. Elemental concentrations in the boron carbide films have been obtained from Rutherford backscattering and elastic recoil detection analysis.

Solid state neutron detection devices have been fabricated in the form of heterostructured p-n diodes, p-type boron carbide/n-type Si. Operating conditions, including applied bias voltage, and time constants, have been optimized for maximum detection efficiency and correlated to the semiconducting properties investigated in separate electronic measurements. Accurate measurements of the neutron detection efficiency and the response of the detector to a wide range of neutron wavelengths have been performed at a well calibrated, tightly collimated, “white” cold neutron beam source using time-of-flight neutron detection technique. Because neutron detection measurements indicate that charge capture in boron carbide is affected by the nanocrystalline/amorphous nature of the semiconductor, the effects of incomplete charge collection efficiencies on the neutron detection efficiencies and pulse height spectra in heterostructured p-n diode neutron detectors have been modeled using a Monte Carlo GEANT4 simulation.

The dissertation ends with suggestions for devices with improved neutron detection efficiencies.

Advisor: Shireen Adenwalla

MAGNETOELECTRIC INTERACTIONS BETWEEN AN ORGANIC FERROELECTRIC AND A TRANSITION METAL FERROMAGNET

Abhijit Mardana — Wed, 11 Jan 2012 06:57:59 PST

The interaction between ferromagnetic and ferroelectric films, the magnetoelectric effect, is a fascinating fundamental research area as well as having potential applications in magnetic data storage devices. We have investigated magnetoelectric coupling effects in thin film heterostructures, consists of metallic ferromagnet, cobalt, and the polymer ferroelectric [P(VDF-TrFE) 70:30]. The work described here encompasses changes in ferroelectric polarization with magnetic field as well as changes in the magnetic anisotropy with ferroelectric polarization.

In samples of Co overlayers on P(VDF-TrFE), in which the Co is not constrained by the substrate, the polarization shows a large change on application of a perpendicular magnetic field. This magnetoelectric effect is reversible, repeatable and possesses odd symmetry with respect to positive and negative magnetic field. Magnetic saturation destroys the effect, implying the presence of multiple magnetic domains is essential for the effect. The flexoelectric effect, the change in polarization due to strain gradients in the ferroelectric film, is a possible candidate for the cause of this effect.

In samples consisting of Co layers overlaid with P(VDF-TrFE), large changes in the magnetic coercivity with changes in ferroelectric polarization are observed. The out-of-plane coercivity is significantly larger for up polarization (i.e. polarization pointing away from the Co layer), whereas the opposite is true for the in-plane coercivity. The magnetic anisotropy, calculated using the areas of magnetization hysteresis loops, is shown to change by as much as 50% as the ferroelectric polarization is switched from up to down. For the thinnest films, the easy axis switches from out-of-plane to in-plane as the ferroelectric polarization is switched. The change in coercivity is proportional to the ferroelectric polarization, as confirmed by taking magnetization loops at intermediate polarization values. Rotation of the magnetization through a large angle, using only electric fields is demonstrated. These large changes in the anisotropy arise from the large electric field at the surface of the Co layer.

Two dimensional electron gas at oxide interfaces

Karolina Janicka — Mon, 05 Dec 2011 08:39:11 PST

Extraordinary phenomena can occur at the interface between two oxide materials. A spectacular example is a formation of a two-dimensional electron gas (2DEG) at the SrTiO₃/LaAlO₃ interface. In this dissertation the properties of the 2DEG are investigated from first principles.

The spatial extent of the 2DEG formed at the SrTiO₃/LaAlO₃ n-type interface is studied. It is shown that the confinement of the 2DEG is controlled by metal induced gap states formed in the band gap of SrTiO₃. The confinement width is then determined by the attenuation length of the metal induced gap states into SrTiO₃ which is governed by the lowest decay rate evanescent states of bulk SrTiO₃ which in turn can be found from the complex band structure of bulk SrTiO₃.

Magnetic properties of the 2DEG formed at the n-type interface of the SrTiO₃/LaAlO₃ superlattices are investigated. It is found that for a thin SrTiO₃ film the interface is ferromagnetic but for a thicker SrTiO₃ film the magnetic moment decreases and eventually disappears. This is a result of delocalization of the 2DEG that spreads over thicker SrTiO₃ film which leads to violation of the Stoner criterion. Further, it is shown that inclusion of the Hubbard U interaction enhances the Stoner parameter and stabilizes the magnetism.

The effect of the 2DEG and the polar interfaces for the thin film ferroelectricity is investigated using both first principles and model calculations. Using a TiO₂-terminated BaTiO₃ film with LaO monolayers at the two interfaces it is shown that the intrinsic electric field produced by the polar interface forces ionic displacements in BaTiO₃ to produce the electric polarization directed into the interior of the BaTiO₃ layer. This creates a ferroelectric dead layer near the interfaces that is non-switchable and thus detrimental to ferroelectricity. It is found that the effect is stronger for a larger effective ionic charge at the interface and longer screening length due to a stronger intrinsic electric field that penetrates deeper into the ferroelectric.

Adviser: Evgeny Y. Tsymbal

Thermodynamics of Magnetic Multilayers

Tathagata Mukherjee — Thu, 01 Dec 2011 14:34:27 PST

Our interest in thermodynamics of magnetic thin film heterostructure began by exploring the possibility to use magnetic nanostructures in the search for optimized magnetocaloric materials for potential room temperature refrigeration. In the present thesis magnetic thin film heterostructures are experimentally realized by Molecular Beam Epitaxy (MBE) and Pulsed Laser Deposition (PLD). Co/Cr and Fe/Cr superlattices were fabricated using mean-field theoretical concepts as guiding principles. The potential of artificial antiferromagnets for near room-temperature refrigeration is explored. Magnetocaloric properties are deduced from measurements of the temperature and field dependence of the magnetization of our samples. The effects of intra-plane and inter-plane exchange interactions on the magnetic phase diagram in Ising-type model systems are revisited in mean-field considerations with special emphasis on tailoring magnetocaloric properties. The experimental results are discussed in light of our theoretical findings, and extrapolations for future improved nanostructures are provided. Further, magnetization relaxation is investigated in a structurally ordered magnetic Co/Cr superlattice. Magnetization transients are measured after exposing the heterostructure to a magnetic set-field for various waiting times. Scaling analysis reveals an asymptotic power-law behavior in accordance with a full aging scenario. The temperature dependence of the relaxation exponent shows pronounced anomalies at the equilibrium phase transitions of the antiferromagnetic superstructure and the ferromagnetic to paramagnetic transition of the Co layers. The latter leaves only weak fingerprints in the equilibrium magnetic behavior but gives rise to a prominent change in non-equilibrium properties. Our findings suggest scaling analysis of non-equilibrium data as a probe for weak equilibrium phase transitions. In addition some misleading interpretations concerning the rigorousness of phenomenological thermodynamics are clarified. Specifically, it is shown that the Maxwell relation incorporates contributions from the spin degrees of freedom and potential lattice degrees of freedom into the isothermal entropy change. A minimalist model involving pairs of exchange coupled, mobile Ising spins is investigated. It is explicitly shown that lattice degrees of freedom can be activated via applied magnetic fields and the integrated Maxwell relation contains this lattice contribution. A simple and intuitive analytic expression for the isothermal entropy change in the presence of field-activated lattice degrees of freedom is provided.We quantify the impact of quantum corrections in the low-temperature limit. To this end, we compare calculations which include elastic interaction with the rigid exchange model in the high-temperature limit. We find that quantum effects provide quantitative corrections in the low-temperature limit. In addition we show that the elastic contributions to the isothermal entropy change can be additive but, remarkably, it can also give rise to reduced isothermal entropy change in certain temperature regions.

Adviser: Christian Binek

First-principles Studies on Physical and Chemical Properties of Nanostructures

Menghao Wu — Thu, 01 Dec 2011 06:46:38 PST

The physical and chemical properties of decorated graphene and graphene ribbons, single-layer III-V systems, three-dimensional carbon and BN foam, and transition-metal-molecular sandwich nanowires have been investigated by first-principle calculations and their potential applications have been predicted. First, it is shown that zigzag graphene nanoribbons (ZGNRs) can be converted into half metal when their edges are decorated by some chemical functional groups, and the half-metalicity is induced by chemical potential difference between two edges when one edge is decorated by electron-donating group like –OH and the other edge is decorated by electron-accepting group like –F, -NH₂, -N(CH₃)₂, -SO₂, -NO₂ and –CN, or by spin-polarized impurity state induced by isolated SO₂ group. In addition, no matter how trivial the potential difference between two edges is, the decorated ZGNR can be half metal as long as the width of ZGNR is sufficiently large. As ZGNRs are decorated by copper atoms, they are shown to be a unique host system for the realization of an extended planar tetracoordinate carbon (ptC) strips due to its highly delocalized in-plane π-electrons and intrinsic rigid structure. When they are decorated by scandium atoms, they are shown to be good candidates for hydrogen storage and the adsorption energy can be controlled by electric field. Second, graphene is also revealed to become half metal through selective chemical decorations, and by selective hydrogenation, it can become magnetic quantum dot arrays and its magnetic coupling or band gap can be tuned, which can be applied for magnetic data storage and light-emitting devices. Third, single-layer BN and some other hexagonal systems like AlN, GaN, BP, SiC, ZnO are studied. It is demonstrated that upon charge-injection some of them like BN and AlN can become magnetic or even half-metallic, which is clarified by using Stoner Criterion. As they are cut into zigzag nanoribbons with one or two edges unpassivated, some of them may become half metals, and the edge-reconstructions of the unpassivated edges are studied. Fourth, two kinds of 3D carbon and BN foam, one with hexagonal holes and the other with triangular holes and only pure sp2 carbon bonds, are designed, and they are shown to be stable and porous low-density carbon and BN allotrope with large internal surface area and strong bulk modulus. Finally, some transition-metal-molecular sandwich nanowires can become magnetic or even half-metallic from nonmagnetic through charge-injection, which is also clarified by using Stoner Criterion. For Ti-benzene nanowires its magnetic coupling can be tuned either by charge-injection or tension.

Adviser: Xiao Cheng Zeng

Self Assembly and Interface Chemistry of non-Metallated Tetraphenyl Porphyrin

Geoffrey Rojas — Tue, 18 Oct 2011 13:23:50 PDT

The study of the electronic properties and geometrical arrangement of 5, 10, 15, 20-tetraphenyl-21H, 23H-porphine on metal is presented. The systems were analyzed using both scanning tunneling microscopy and photoelectron spectroscopy and compared across surfaces to determine how the interface chemistry between the metal and molecule affect the self-assembly and band structure of the adsorbed species. The molecules are found to self-assemble and grow on the Ag(111) surface in a manner described by similar models to weakly bound metal/metal surface systems. The CH-pi bonds between molecules are found to largely determine the relative inter-molecular arrangement, while the more isotropic van der Waals interactions drive the self-assembly. The 2H-TPP however remains isolated and equally dispersed despite any increases in coverage, observed motion, or annealing on the Cu(111) surface, indicating an electrostatic repulsion between adsorbates. Through calculation, spectroscopic observations of state shifts and mapping of the local work function, the limiting factor in the inter-molecular repulsion is found to be due to a combination of charge transfer between molecule and surface and perturbation of the surface electrons due to frontier orbital overlap. By comparing this molecule across surfaces and temperatures, the complex interplay between band structure matching, charge transfer, surface barriers, and self-assembly is described. Controlling the charge transferred to the adsorbed species by the underlying metal, these properties are tailored without changing the atomic constituents or general band structure of the adsorbed species.

Adviser: Professor Axel Enders

In Situ Measurement of Three-Dimensional Ion Densities in Focused Femtosecond Pulses

James Strohaber et al. — Wed, 14 Sep 2011 14:38:34 PDT

We image spatial distribution of xenon ions in the focus of a laser beam of ultrashort, intense pulses in all three dimensions, with a resolution of three by twelve microns in the two transverse directions. This allows for studying ionization processes without spatially averaging ion yields. Our in situ ion imaging is also useful to analyze focal intensity proﬁles and to investigate the transverse modal purity of tightly focused beams of complex light. As an example, the intensity proﬁle of a Hermite-Gaussian beam mode HG recorded with ions is found to be in good agreement with optical images.

FUNCTIONAL TWO-DIMENSIONAL ELECTRONIC GASES AT INTERFACES OF OXIDE HETEROSTRUCTURES

Yong Wang — Thu, 28 Jul 2011 14:22:14 PDT

A quasi-two dimensional electron gas (2DEG) in oxide heterostructures such as LaAlO₃/SrTiO₃ has unique properties that are promising for applications in all-oxide electronic devices. In this dissertation, we focus on understanding and predicting novel properties of the 2DEG by performing first-principles electronic calculations within the frame work of density-functional theory (DFT).

The effects of polarization in all-oxide heterostructures incorporating different ferroelectric constituents, such as KNbO₃/ATiO₃ (A = Sr, Ba, Pb), are investigated. It is found that screening charge at the interface that counteracts the depolarizing electric field in the ferroelectric material significantly changes the free electron density of 2DEG at the interface. Nonvolatile metal-insulating transition can be achieved at the interface by switching the ferroelectric spontaneous polarization.

Growing on different substrates, LaAlO₃/SrTiO₃ heterostructures experience different epitaxial strains. It is found that compressive epitaxial strain introduces a polarization in SrTiO₃ pointing away from the interface. This polarization strongly affects the 2DEG carrier density through a polarization charge formed at the interface. The critical thickness to form a 2DEG at the interface of the heterostructure increases with the compressive strain, while the saturated carrier density decreases.

Adding a spin degree of freedom to 2DEG may be interesting for the application of 2DEGs in a spintronic device. We explore a LaAlO₃/EuO interface as a potential candidate to create a spin-polarized 2DEG. The exchange splitting of unoccupied Eu-5d conduction band in bulk EuO makes it possible to realize spin-polarized 2DEG. We hope that this prediction will stimulate experimental investigations to achieve the spin-polarized 2DEG.

Adviser: Evgeny Y. Tsymbal

THE INTERPLAY BETWEEN SYMMETRY AND STATIC DIPOLES WITH ADSORPTION ON MOLECULAR SUBSTRATES

Zhengzheng Zhang — Mon, 25 Apr 2011 17:08:19 PDT

This thesis presents evidence of preferential adsorption and the associated dipole-dipole interactions that can occur at molecule to molecule interfaces. The results are discussed in the context of the possibility of interactions caused by strong intrinsic dipoles when adsorbed on electrostatically biased substrates. Key is the discovery of lock and key adsorption chemistry by comparing the reversible absorption of the three isomers of di-iodobenzene (1,2-di-iodobenzene, 1,3-di-iodobenzene, and 1,4-di-iodobenzene) on molecular films of a quinonoid zwitterion. There is unequivocal evidence that the molecular adsorption and absorption of 1, 3-diiodobenzene is strongly favored at 150 K over the other isomers of di-iodobenzene. Our experiments also demonstrate that reversible isomer-selective adsorption chemistry of small molecules is indeed possible, with a preferential adsorption mechanism illustrating that symmetry does matter.

Evidence of selective adsorption on specific ferroelectric domains of the molecular ferroelectric, copolymers of polyvinylidene fluoride with trifluoroethylene (PVDF-TrFE) is presented. The adsorption of di-iodobenzene depends not only on the dipole orientation of the PVDF-TrFE ferroelectric domains, but also the di-iodobenzene isomer.

Foundational to this work is the investigation of the interaction and orientation of a strongly dipolar zwitterionic p-benzoquinonemonoimine-type molecule, with a large intrinsic dipole of 10 Debye, on both conducting (gold) and on polar insulating substrates (lithium niobate). I have studied surface electronic spectroscopic properties and the preferential absorption pattern of these unusual zwitterionic molecules C₆H₂(···NHR)₂(···O)₂, where R = H, n-C₄H₉, C₃H₆-S-CH₃, C₃H₆-O-CH₃, CH₂-C₆H₅on gold and demonstrated the selective deposition of molecules onto specific ferroelectric domains for a spatially periodically poled ferroelectric surface (lithium niobate).

Advisor: Peter A. Dowben

The Photofragmentation Processes of the closo-Carborane and the Local Structure of Transition Metal Doped Semiconducting Boron Carbide Thin Films

Jing Liu — Fri, 22 Apr 2011 13:14:40 PDT

I investigated the photofragmentation processes of various closo-carboranes in an effort to understand the radical-induced polymerization of the closo-carboranes (i.e., semiconducting film growth), based on their partial dehydrogenation during plasma-enhanced chemical vapor deposition. The chemistry of vacuum ultraviolet VUV assisted dehydrogenation processes of both the closo-carboranes and related closo-phosphacarboranes were compared by photoionization mass spectrometry studies. The dominant ion pairs were identified and compared with the energetics constructed by theoretical modeling for the possible dissociation pathways.

Transition metal (Mn, Fe, Co) doped boron carbides thin films produced by plasma-enhanced chemical vapor deposition of orthocarborane (closo-1,2-C₂B₁₀H₁₂) and metallocenes were investigated by performing K-edge extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements. The Mn, Fe and Co transition metal atoms dope boron carbide pairwise on adjacent icosahedra. Each transition metal atom occupies one of the icosahedral boron or carbon apical site atomic site within the icosahedral cage on adjacent edge bonded icosahedral cages. There is good agreement with experiment and theoretical models. The local spin configurations of all the 3d transition metal doped boron carbides, Ti through to Cu, are compared using theoretical cluster or icosahedral chain calculations.

The chromium doped boron carbide thin films, fabricated by boron carbide-chromium co-deposition, were then studied by current-voltage (I-V) characteristics and temperature dependence of moment measurements. The results provide some reason to believe that magneto-resistive effects are indeed present at room temperature.

Advisor: Peter A. Dowben

Upgrade To The Pierre Auger Cosmic Ray Observatory's Lidar System

Emily B. Petermann — Fri, 23 Apr 2010 07:17:50 PDT

The Pierre Auger Cosmic Ray Observatory currently operates four elastic lidar systems in order to characterize the atmospheric aerosol content above the observatory. The atmospheric information gained by the lidar system is then used in the calibration of the observatory’s four fluorescence detectors. Currently the four lidars in operation are unable to accurately determine the aerosol content below a distance of 1 km. A project is currently underway to upgrade the current lidar system by adding an additional detector to each of the existing lidar systems. The considered designs for this upgrade and the initial results from the upgrade prototype are the subject of this thesis.

Search for the Standard Model Higgs Boson in p-pbar Interactions with the decay mode H -> W+ W- -> mu+ nu mu- nu at the D0 Experiment

Dale M. Johnston — Fri, 23 Apr 2010 06:43:50 PDT

A search for the standard model Higgs boson in p-pbar collisions resulting in two muons and large missing transverse energy is presented. The analysis uses 4.2 fb-1 of integrated luminosity at a center-of-mass energy of sqrt(s) = 1.96 TeV collected between April 2002 and December 2008 with the D0 detector at the Fermilab Tevatron collider. No significant excess above the background estimation is observed and limits are derived on Higgs boson production.

Finite temperature effects in magnetic materials: Model and ab initio studies

Aleksander L. Wysocki — Fri, 04 Dec 2009 12:56:33 PST

The understanding of finite temperature behavior of magnetic materials is of vital importance for spintronic applications. In this dissertation different theoretical techniques for studying magnetic thermodynamics of various materials are discussed. Cr₂O₃ is an antiferromagnetic insulator that was proposed to be a key component of new spintronic devices. The magnetic properties of Cr₂O₃ were studied using the LDA+U method. Magnetism was found to be very well described by the Heisenberg model. Subsequently, magnetic thermodynamics was explored using quantum pair cluster approximation. Overall, very good agreement with experiment was found for the ground state and thermodynamics properties.

The magnetism at the (0001) surface of Cr₂O₃ was investigated using first principles. The description of magnetic properties required a detailed knowledge of the surface structure that was found to be very nontrivial. In particular, an order-disorder structural phase transition was shown to exist at the surface. In addition, the existence of the reentrant phase transition due to a magneto-structural coupling was hypothesized. The magnetic properties of the Cr₂O₃ (0001) surface were found to be very unique; an uncompensated magnetic moment exists at the surface and persists even with surface roughness. The finite temperature behavior of this surface magnetism was studied using the Heisenberg model and the mean-field approximation. The surface magnetization was found to exist up to almost room temperature. This effect makes Cr₂O₃ a very promising material for exchange bias applications.

In itinerant magnets both transverse and longitudinal spin fluctuations are very important for thermodynamics. A classical model containing both types of fluctuations was introduced with a single parameter controlling the degree of itinerancy, i.e., relative importance of longitudinal and transverse spin fluctuations. The thermodynamics was studied using the Monte Carlo method, mean-field approximation, and Onsager method. In general, magnetic short-range order was found to be weak even for strongly itinerant systems and Monte Carlo was in a good agreement with mean-field approximation. The Onsager cavity field method was extended to models with longitudinal spin fluctuations and was shown to be in excellent agreement with Monte Carlo. The ambiguity of the choice of the phase space measure for longitudinal spin fluctuations for classical models was emphasized.

In magnetic metals the resistivity has an additional contribution due to scattering on the thermally induced spin fluctuations. This spin-disorder resistivity was studied from first principles for Fe and Ni. Various models of thermal spin disorder were considered, including the mean-field approximation and the nearest-neighbor Heisenberg model. In general, spin-disorder resistivity was found to depend very weakly on magnetic short-range order. For local moments frozen to their zero-temperature values, a good agreement with experiment was obtained for Fe, but for Ni the resistivity at elevated temperatures was significantly overestimated. This overestimation of spin-disorder resistivity for Ni was attributed to the reduction of the local moment due to longitudinal spin fluctuations.

EXCHANGE BIAS TRAINING EFFECT IN MAGNETICALLY COUPLED BILAYERS

Srinivas Polisetty — Fri, 16 Oct 2009 09:14:00 PDT

Interfaces in magnetically coupled bilayer heterostructures play a vital role in novel spintronics devices. Particularly, control of the interface spin structure enables the development of progressively down-scalable magnetic read-heads which are of major importance for non volatile magnetic recording media. Exchange bias and its accompanying training effect are fundamental magnetic coupling phenomena taking place at the interfaces of antiferromagnetic/ferromagnetic and hard/soft ferromagnetic bilayers. Here, in my thesis I present the experimental results of exchange bias training in the prototypical antiferromagnetic/ferromagnetic exchange bias system CoO/Co and the corresponding coupling and aging phenomena in the all ferromagnetic hard/soft bilayer CoPtCrB/CoCr. The latter system provides experimental access to its pinning layer magnetization thereby allowing to measure fundamental properties of exchange bias and its corresponding training phenomenon. A phenomenological theory is best fitted to all experimental training data of antiferromagnetic/ferromagnetic and hard/soft ferromagnetic bilayers evidencing the universality of the theory. My studies are further extended to the temperature dependence of the exchange bias training effect. Again, excellent agreement between experiment and theory confirms the remarkable universality of the underlying phenomenological approach. Furthermore, the dependence of the exchange bias training on the ferromagnetic film thickness is investigated in a CoO/Co-wedge sample. Scaling behavior with collapse of the temperature and thickness dependent parameters onto a single master curve is presented. Magnetotransport measurements are used for complementary studies of exchange bias in CoO/Co-heterostructures. Here, exchange bias produces a shift of the magnetoresistance curve along the magnetic field axis and an additional asymmetry along the resistance axis. The dynamic non-equilibrium properties of the exchange bias training effect are investigated via the sweep rate dependence of the exchange bias field. A dynamical enhancement of the exchange bias training effect has been observed in both CoO/Co and CoPtCrB/CoCr bilayers with increasing sweep rate of the applied magnetic field. A generalized theory has been developed for the dynamical enhancement studies confirming once more the consistency and universality of the phenomenological approach.

Fluorescence Polarization of Atomic, Dissociated Atomic, and Molecular Transitions Induced by Spin-Polarized Electron Impact

Jack W. Maseberg — Tue, 04 Aug 2009 15:50:45 PDT

Excitation of atoms by spin-polarized electron impact yields fluorescence that can generally exhibit both linear and circular polarization. For experiments where the scattered electrons are not detected, symmetry requires that the electron beam be spin polarized in order for non-zero circular polarization to be observed. Extensive theoretical and experimental investigations have been performed regarding fluorescence polarizations (Stokes parameters) resulting from spin-polarized electron impact excitation of atoms. Measurement of fluorescence polarization provides insight into the angular momentum coupling that exists in the atomic state of interest. It also enables the measurement of electron spin polarization and experimental benchmarking of theoretical atomic structure calculations.

In an extension of previous atomic investigations, fluorescence polarization from polarized electron impact dissociation and excitation of simple diatomic molecules is considered. Stokes parameters are presented for dissociated atomic transitions in H, D, and N. Rotationally resolved molecular Fulcher band transitions in H2 and D2, as well as partially resolved transitions in N2, are also presented. Non-zero circular polarizations are observed for both the dissociation and molecular excitation processes. For the rotationally resolved molecular transitions, lower circular polarizations are observed for higher values of rotational states.

Exchange Coupling at Cobalt/ Nickel Oxide Interfaces

Andrew G. Baruth — Wed, 22 Apr 2009 12:21:17 PDT

Spin arrangement at interfaces in layered magnetic materials is of vital importance to the emerging field of spintronics. Knowledge of how and why the interfacial spins behave in a certain way will aid in the development of future magnetic-based memories.

Much exploration has taken place in the interlayer exchange coupling (IEC) of ferromagnetic heterostructures with in-plane anisotropy. Only recently has it become apparent that to achieve the goals of increased areal density in magnetic memory a push for exploring magnetic materials with perpendicular magnetic anisotropy (PMA) must occur. An interesting and promising candidate for such a magnetic system is [Co/Pt]/NiO/[Co/Pt], where two [Co/Pt] multilayers with PMA are separated by a thin, insulating, antiferromagnetic NiO layer and display oscillatory coupling with NiO thickness. This magnetic heterostructure displays an entirely new IEC where the Ni spins within the NiO layer cant in concert with the adjacent [Co/Pt] layers, causing the periodicity of the oscillatory coupling to coincide with the NiO antiferromagnetic ordering parameter. The strength and sign of this coupling, either positive (favoring parallel alignment) or negative (favoring anti-parallel alignment), can be tuned with slight changes in the NiO layer thickness. The origin of the oscillatory IEC was investigated using advanced microscopy and spectroscopy techniques.

For antiferromagnetically coupled [Co/Pt] layers, the competition between magnetostatic coupling and IEC gives rise to a region of overlapping domains (resulting in a ferromagnetically coupled stripe). Discovered with high resolution magnetic force microscopy and quantitatively modeled with micromagnetic simulation, the width of this overlap region scales inversely with the IEC.

Heterostructures of Co/NiO/[Co/Pt], where the Co ([Co/Pt]) has in-plane (out-of-plane) anisotropy, allow for isothermal tuning of the hysteresis loop shift along the applied field axis at room temperature, as well as display a greatly enhanced blocking temperature (increase of more than 175K). The presence of the [Co/Pt] multilayer with PMA is responsible for the enhancement. In addition, these structures display temperature dependent exchange bias training effects, which have been successfully modeled using a phenomenological thermodynamic approach.

Nanoscale Investigation of Polarization Interaction and Polarization Switching in Ferroelectric P(VDF-TrFE) Copolymer Samples

Jihee Kim — Tue, 15 Jan 2008 12:09:13 PST

Ferroelectric properties of thin films and self-assembly of copolymers of polyvinylidene fluoride with trifluoroethylene (P(VDF-TrFE) have been studied. All samples were fabricated with Langmuir-Blodegtt (LB) film deposition technique. Two main observations are presented in this dissertation. One is a polarization interaction effect in multi-layered thin films made of two different copolymers, and the other is local polarization switching of the self-assembly, called nanomesas.

The multilayer films were built with two different content ratio of P(VDF-TrFE) copolymers. They were P(VDF-TrFE 80:20) and P(VDF-TrFE 50:50) with phase transition temperatures of 133 ± 4 ºC and 70 ± 4 ºC respectively. The polarization interaction effect resulted in transition temperature changes of the materials, and the determined interaction length was approximately 11 nm, perpendicular to the film plane.

Nanomesas of P(VDF-TrFE) copolymer were found during annealing study of thinner films with less than 3 deposited layer thin films. Nanomesas are disk shaped isolated islands approximately 9 nm in height and 100 nm in diameter in average. Ferroelectric switching properties of nanomesas have been shown macroscopically in the previous studies. In this work, the switching properties of individual nanomesas were probed at nanoscale. Nanomesas, switched with ± 7 V_dc were recorded using piezoresponse force microscopy (PFM). Switching hysteresis loops from a local area of 12~15 nm² within an individual nanomesa were also obtained using switching spectroscopy PFM (SS-PFM). The coercive field determined from the well behaved switching loops is ~250 ~ 450 MV/cm.

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