Atomic, Molecular and Optical Physics Commons^™

Mesoscale Flux-Closure Domain Formation In Single-Crystal Batio3, R. G.P. Mcquaid, L. J. Mcgilly, Pankaj Sharma, Alexei Gruverman, J. M. Gregg

Alexei Gruverman Publications

Over 60 years ago, Charles Kittel predicted that quadrant domains should spontaneously form in small ferromagnetic platelets. He expected that the direction of magnetization within each quadrant should lie parallel to the platelet surface, minimizing demagnetizing fields, and that magnetic moments should be configured into an overall closed loop, or flux-closure arrangement. Although now a ubiquitous observation in ferromagnets, obvious flux-closure patterns have been somewhat elusive in ferroelectric materials. This is despite the analogous behaviour between these two ferroic subgroups and the recent prediction of dipole closure states by atomistic simulations research. Here we show Piezoresponse Force Microscopy images of …

Structure And Magnetism Of Mnau Nanoclusters, X. Wei, Damien Le Roy, Ralph Skomski, Xingzhong Li, Zhiguang Sun, Jeffrey E. Shield, M. J. Kramer, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Equiatomic MnAu clusters with average sizes of 4 and 10 nm are produced by inert-gas condensation. As-produced clusters are used to form both dense cluster films and films with clusters embedded in a W matrix with a cluster volume fraction of 25%. Both structure and magnetism are size-dependent. Structural analysis of the 10 nm clusters indicate a distorted tetragonal body-centered cubic structure with lattice parameters a=0.315 and c=0.329 nm. The 4 nm clusters have a partially ordered tetragonal L10 structure with lattice parameters a=0.410 nm and c=0.395 nm. Magnetic properties of the clusters show evidence at …

Metastability Of Free Cobalt And Iron Clusters: A Possible Precursor To Bulk Ferromagnetism, Xiaoshan Xu, Shuangye Yin, Ramiro Moro, Anthony Liang, John Bowlan, Walt A. De Heer

Xiaoshan Xu Papers

Homonuclear cobalt and iron clusters Co_N and Fe_N measured in a cryogenic molecular beam exist in two states with distinct magnetic moments (μ), polarizabilities, and ionization potentials, indicating distinct valences. The μ is approximately quantized: μ_N ~ 2Nμ_B in the ground states and μ_N* ~ Nμ_B in the excited states for Co; μ_N ~ 3N μ_B and μ_N * ~ Nμ_B for Fe. At a large size, the average μ of the two states converges to the bulk value with diminishing ionization potential differences. …

Magnetism Of Cluster-Deposited Y–Co Nanoparticles, Balamuruga Balamurugan, Ralph Skomski, Xingzhong Li, V. R. Shah, George C. Hadjipanayis, Jeffrey E. Shield, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Nanoparticles of YCo₂, YCo_3, and YCo₅ are produced by cluster-deposition and investigated both structurally and magnetically. The nanoparticles have sizes of less than 10 nm and are superparamagnetic at 300 K, irrespective of stoichiometry. As-produced nanoparticles exhibit disordered structures with magnetic properties differing from those of the bulk particles. The temperature-dependent magnetization curves of the nanoparticles reveal blocking temperatures from 110 to 250 K, depending on stoichiometry. The magnetic anisotropy constant K₁ of disordered YCo₅ nanoparticles of 7.8 nm in size is 3.5×10⁶ergs/cm³, higher than those of the disordered YCo …

Vibrational Frequencies Of Magnetic Random Access Memory Materials., Lee Li Ling

Student Works (2010-2019)

The advantages of MRAM are so overwhelming which acts as motivation for scientist in further investigation for better read and write operation. The types of materials used are closely related to the cell performance in forming the ferromagnetic layers. Thus,the vibrational frequencies of the MRAM materials are of interest. Clusters for the two types of combinations, FexNiy and FexCoyBz are built. Subsequently, vibrational frequencies of each cluster are calculated theoretically from the first principles. The secular determinant is large when the number of electrons in a cluster is large. For the simplest combination of Fe-Ni cluster, (atomic number of an …

Partial Phonon Density Of States Of 57-Iron And 161-Dysprosium In Dyfe3 By Nuclear Resonant Inelastic X-Ray Scattering Under High Pressure, Elizabeth Anne Tanis

UNLV Theses, Dissertations, Professional Papers, and Capstones

The dual partial phonon density of states (DOS) from two different Mossbauer isotopes (161Dy and 57Fe) in the same material (DyFe3) was successfully measured using the nuclear resonant inelastic x-ray scattering (NRIXS) technique at high pressure. Nuclear inelastic scattering measurements yield an in-depth understanding of the element-specific dynamic properties. The Debye temperatures , the Lamb-Mossbauer factor, and the vibrational contributions to the Helmholtz free energy, specific heat , entropy and internal energy are calculated directly from the phonon density of states.

Magnetodielectric Coupling Of Infrared Phonons In Single-Crystal Cu2oseo3, K. H. Miller, X. S. Xu, H. Berger, E. S. Knowles, D. J. Arenas, M. W. Meisel, D. B. Tanner

Xiaoshan Xu Papers

Reflection and transmission as a function of temperature (5–300 K) have been measured on a single crystal of the magnetoelectric ferrimagnetic compound Cu₂OSeO₃ utilizing light spanning the far infrared to the visible portions of the electromagnetic spectrum. The complex dielectric function and optical properties were obtained via Kramers-Kronig analysis and by fits to a Drude-Lortentz model. The fits of the infrared phonons show a magnetodielectric effect near the transition temperature (T_c~60 K). Assignments to strong far-infrared phonon modes have been made, especially those exhibiting anomalous behavior around the transition temperature.

Tunable Band Gap In Bi(Fe1−Xmnx)O3 Films, X. S. Xu, J. F. Ihlefeld, J. H. Lee, O. K. Ezekoye, E. Vlahos, R. Ramesh, V. Gopalan, X. Q. Pan, D. G. Schlom, J. L. Musfeldt

Xiaoshan Xu Papers

In order to investigate band gap tunability in polar oxides, we measured the optical properties of a series of Bi(Fe_1−xMn_x)O₃ thin films. The absorption response of the mixed metal solid solutions is approximately a linear combination of the characteristics of the two end members, a result that demonstrates straightforward band gap tunability in this system.

Adsorption-Controlled Growth Of Bimno3 Films By Molecular-Beam Epitaxy, J. H. Lee, X. Ke, R. Misra, J. F. Ihlefeld, X. S. Xu, Z. G. Mei, T. Heeg, M. Roeckerath, J. Schubert, Z. K. Liu, J. L. Musfeldt, P. Schiffer, D. G. Schlom

Xiaoshan Xu Papers

We have developed the means to grow BiMnO₃ thin films with unparalleled structural perfection by reactive molecular-beam epitaxy and determined its band gap. Film growth occurs in an adsorption-controlled growth regime. Within this growth window bounded by oxygen pressure and substrate temperature at a fixed bismuth overpressure, single-phase films of the metastable perovskite BiMnO₃ may be grown by epitaxial stabilization. X-ray diffraction reveals phase-pure and epitaxial films with w rocking curve full width at half maximum values as narrow as 11 arc sec (0.003°). Optical absorption measurements reveal that BiMnO₃ has a direct band gap of 1.1±0.1 …

Lattice Dynamical Probe Of Charge Order And Antipolar Bilayer Stacking In Lufe2o4, X. S. Xu, J. De Groot, Q.-C. Sun, B. C. Sales, D. Mandrus, M. Angst, A. P. Litvinchuk, J. L. Musfeldt

Xiaoshan Xu Papers

We investigated the infrared response of LuFe₂O₄ through the series of charge, magnetic, and structural transitions. All vibrational modes couple strongly to the charge order, whereas the LuO zone-folding modes are also sensitive to magnetic order and structural distortion. The dramatic splitting of the LuO₂ layer mode is attributed to charge-rich/poor proximity effects and its temperature dependence reveals the antipolar nature of the W layer pattern.

Optical Properties Of Quasi-Tetragonal Bifeo3 Thin Films, P. Chen, N. J. Podraza, X. S. Xu, A. Melville, E. Vlahos, V. Gopalan, R. Ramesh, D. G. Schlom, J. L. Musfeldt

Xiaoshan Xu Papers

Optical transmission spectroscopy and spectroscopic ellipsometry were used to extract the optical properties of an epitaxially grown quasi-tetragonal BiFeO₃ thin film in the near infrared to near ultraviolet range. The absorption spectrum is overall blue shifted compared with that of rhombohedral BiFeO₃, with an absorption onset near 2.25 eV, a direct 3.1 eV band gap, and charge transfer excitations that are ~0.4 eV higher than those of the rhombohedral counterpart. We interpret these results in terms of structural strain and local symmetry breaking.

Robust Isothermal Electric Control Of Exchange Bias At Room Temperature, Xi He, Ning Wu, Anthony N. Caruso, Elio Vescovo, Kirill D. Belashchenko, Peter A. Dowben, Christian Binek

Kirill Belashchenko Publications

Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with highspeed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr₂O₃ has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr₂O₃ single crystal, we achieve reversible, room-temperature isothermal switching …

Reflectivity Of A Cholesteric Liquid Crystal, Justin Lawson

Physics

In this paper we investigate the light properties of a chiral liquid crystal or a crystal for which the director angle relative to some fixed axis changes as a function of the crystal depth. Sometimes a dopant can introduce a chirality or "twist" in a nematic liquid crystal. For such cases of non-linear depth dependence (where chirality is determined by a diffusion equation) we may use this research to work backwards from a crystal's light properties to intensity and duration of dopant exposure.

Pressure Induced Dynamical Instabilities In Body Center Cubic Crystals, Oscar Guerrero

Open Access Theses & Dissertations

Large-scale atomistic simulations of shock-wave propagation in single crystals exhibit large anisotropies in the elastic-plastic and solid-liquid transitions. Characteristic of this type of simulations are the large strains at which the crystal yields plastically, regardless of crystal orientation. At these large strains, uniaxial deformations, such as those produced in planar shock loading generate dynamical instabilities. We have investigated the directional anisotropy of the elastic limit in bcc crystals, in particular Tantalum (Ta), employing molecular dynamics (MD) simulations. We show that the elastic - plastic transition in BCC defect-free crystals is caused by the appearance of soft-phonon modes and not via …

Dynamical Theory Calculations Of Spin-Echo Resolved Grazing-Incidence Scattering From A Diffraction Grating, Rana Ashkar, P. Stonaha, A. L. Washington, V. R. Shah, M. R. Fitzsimmons, B. Maranville, C. F. Majkrzak, W. T. Lee, W. L. Schaich, Roger Pynn

Nebraska Center for Materials and Nanoscience: Faculty Publications

Neutrons scattered or reflected from a diffraction grating are subject to a periodic potential analogous to the potential experienced by electrons within a crystal. Hence, the wavefunction of the neutrons can be expanded in terms of Bloch waves and a dynamical theory can be applied to interpret the scattering phenomenon. In this paper, a dynamical theory is used to calculate the results of neutron spin-echo resolved grazing-incidence scattering (SERGIS) from a silicon diffraction grating with a rectangular profile. The calculations are compared with SERGIS measurements made on the same grating at two neutron sources: a pulsed source and a continuous …

Optical Properties And Magnetochromism In Multiferroic Bifeo3, X. S. Xu, T. V. Brinzari, S. Lee, Y. H. Chu, L. W. Martin, A. Kumar, S. Mcgill, R. C. Rai, R. Ramesh, V. Gopalan, S. W. Cheong, J. L. Musfeldt

Xiaoshan Xu Papers

In order to investigate spin-charge coupling in multiferroic oxides, we measured the optical properties of BiFeO₃. Although the direct 300 K charge gap is observed at 2.67 eV, absorption onset actually occurs at much lower energy with Fe³⁺ excitations at 1.41 and 1.90 eV. Temperature and magnetic-field-induced spectral changes reveal complex interactions between on-site crystal-field and magnetic excitations in the form of magnon sidebands. We employ the sensitivity of these magnon sidebands to map out the magnetic-fieldtemperature phase diagram which demonstrates optical evidence for spin spiral quenching above 20 T and suggests a spin domain reorientation near …

Absence Of Spin Liquid Behavior In Nd3ga5sio14 Using Magneto-Optical Spectroscopy, X. S. Xu, T. V. Brinzari, S. Mcgill, H. D. Zhou, C. R. Wiebe, J. L. Musfeldt

Xiaoshan Xu Papers

We measured the low-lying crystal field levels of Nd³⁺ in Nd₃Ga₅SiO₁₄ via magneto-optical spectroscopy and employed the extracted energies, magnetic moments, and symmetries to analyze the magnetic properties and test the spin liquid candidacy of this material. The exchange interaction is surprisingly small, a discovery that places severe constraints on models used to describe the ground state of this system. Further, it demonstrates the value of local-probe photophysical techniques for rare-earthcontaining materials where bulk property measurements can be skewed by low-lying electronic structure.

Magnon Sidebands And Spin-Charge Coupling In Bismuth Ferrite Probed By Nonlinear Optical Spectroscopy, M. O. Ramirez, A. Kumar, S. A. Denev, N. J. Podraza, X. S. Xu, R. C. Rai, Y. H. Chu, J. Seidel, L. W. Martin, S. -Y. Yang, E. Saiz, J. F. Ihlefeld, S. Lee, J. Klug, S. W. Cheong, M. J. Bedzyk, O. Auciello, D. G. Schlom, R. Ramesh, J. Orenstein, J. L. Musfeldt, V. Gopalan

Xiaoshan Xu Papers

The interplay between spin waves (magnons) and electronic structure in materials leads to the creation of additional bands associated with electronic energy levels which are called magnon sidebands. The large difference in the energy scales between magnons (meV) and electronic levels (eV) makes this direct interaction weak and hence makes magnon sidebands difficult to probe. Linear light absorption and scattering techniques at low temperatures are traditionally used to probe these sidebands. Here we show that optical secondharmonic generation, as the lowest-order nonlinear process, can successfully probe the magnon sidebands at room temperature and up to 723 K in bismuth ferrite, …

Charge Order, Dynamics, And Magnetostructural Transition In Multiferroic Lufe2o4, X. S. Xu, M. Angst, T. V. Brinzari, R. P. Hermann, J. L. Musfeldt, A. D. Christianson, D. Mandrus, B. C. Sales, S. Mcgill, J. -W. Kim, Z. Islam

Xiaoshan Xu Papers

We investigated the series of temperature and field-driven transitions in LuFe₂O₄ by optical and Mossbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the ‘‘order by fluctuation’’ mechanism for the development of charge order superstructure. Bragg splitting and large magneto-optical contrast suggest a low-temperature monoclinic distortion that can be driven by both temperature and magnetic field.

Photoconductivity In Bifeo3 Thin Films, S. R. Basu, L. W. Martin, Y. H. Chu, M. Gajek, R. Ramesh, R. C. Rai, X. S. Xu, J. L. Musfeldt

Xiaoshan Xu Papers

The optical properties of epitaxial BiFeO₃ thin films have been characterized in the visible range. Variable temperature spectra show an absorption onset near 2.17 eV, a direct gap (2.667±0.005 eV at 300 K), and charge transfer excitations at higher energy. Additionally, we report photoconductivity in BiFeO₃ films under illumination from a 100 mW/cm² white light source. A direct correlation is observed between the magnitude of the photoconductivity and postgrowth cooling pressure. Dark conductivities increased by an order of magnitude when comparing films cooled in 760 and 0.1 Torr. Large increases in photoconductivity are observed in light.

Optical Band Gap Of Bifeo3 Grown By Molecular-Beam Epitaxy, J. F. Ihlefeld, N. J. Podraza, Z. K. Liu, R. C. Rai, X. Xu, T. Heeg, Y. B. Chen, J. Li, R. W. Collins, J. L. Musfeldt, X. Q. Pan, J. Schubert, R. Ramesh, D. G. Schlom

Xiaoshan Xu Papers

BiFeO₃ thin films have been deposited on (001) SrTiO₃ substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase BiFeO₃ films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with w rocking curve full width at half maximum values as narrow as 29 arc sec (0.008°). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at 2.74 eV for stoichiometric as well as 5% bismuth-deficient single-phase BiFeO₃ films.

Magnetic Enhancement In Cobalt-Manganese Alloy Clusters, Shuangye Yin, Ramiro Moro, Xiaoshan Xu, Walter A. De Heer

Xiaoshan Xu Papers

Magnetic moments of Co_NMn_M and Co_NV_M clusters (N ≤ 60; M ≤ N/3) are measured in molecular beams using the Stern-Gerlach deflection method. Surprisingly, the per atom average moments of Co_NMn_M clusters are found to increase with Mn concentration, in contrast to bulk CoMn. The enhancement with Mn doping is found to be independent of cluster size and composition in the size range studied. Meanwhile, Co_NV_M clusters show reduction of average moments with increasing V doping, consistent with what is expected in bulk CoV. The results are …

Magnetic Moments And Adiabatic Magnetization Of Free Cobalt Clusters, Xiaoshan Xu, Shuangye Yin, Ramiro Moro, Walt A. De Heer

Xiaoshan Xu Papers

Magnetizations and magnetic moments of free cobalt clusters CoN (12< N <200) in a cryogenic (25 K ≤ T ≤ 100 K) molecular beam were determined from Stern-Gerlach deflections. All clusters preferentially deflect in the direction of the increasing field and the average magnetization resembles the Langevin function for all cluster sizes even at low temperatures. We demonstrate in the avoided crossing model that the average magnetization may result from adiabatic processes of rotating and vibrating clusters in the magnetic field and that spin relaxation is not involved. This resolves a longstanding problem in the interpretation of cluster beam deflection experiments with implications for nanomagnetic systems in …

Measurement Of Magnetic Moments Of Free BiNMnM Clusters, Shuangye Yin, Xiaoshan Xu, Ramiro Moro, Walt A. De Heer

Xiaoshan Xu Papers

Magnetic properties of free Bi_NMn_M clusters (N=2–20, M=0–7) are determined from Stern-Gerlach deflections at low temperature (46.5 K). Pure bismuth clusters with odd number of atoms exhibit paramagnetic deflections. The addition of manganese atoms produces a ferromagnetic response which is strongly size dependent. Certain combinations have very large magnetic moments such as Bi₅Mn₃, Bi₉ Mn₄, Bi₁₀Mn₅, and Bi₁₂Mn₆.

Specification Of Jecp/Holz, An Interactive Computer Program For Simulation Of Holz Pattern, Xingzhong Li

Nebraska Center for Materials and Nanoscience: Faculty Publications

1. Purpose of the program 2. Graphic user interface and program design 3. Formulas for calculating the positions of the HOLZ lines 4. System requirement 5. Installation and user instruction 6. How to contact the author 7. References

1. Purpose of the program JECP/HOLZ (Li, 2005a) is one computer program in the Java Electron Crystallography Package (JECP), which is designed and written by Dr. XingZhong Li. The package is developed for quantitative electron diffraction and image processing purpose, JECP/HOLZ is an interactive program for simulation of the higher-order Laue zone (HOLZ) lines using kinematical approximation and a first-order dynamic correction. …

Condensation On (002) Graphite Of Liquid Bismuth Far Below Its Bulk Melting Point, M. K. Zayed, H. E. Elsayed-Ali

Electrical & Computer Engineering Faculty Publications

Condensation of thermally evaporated Bi on (002) graphite, at temperatures of 300-523K, was studied using in situ reflection high-energy electron diffraction (RHEED) and room temperature ex situ atomic force microscopy (AFM). For deposition at temperatures below 415±5K, transmission RHEED patterns of Bi appeared at an average thickness of ∼0.5 monolayer (ML). AFM images showed that the film consisted of crystallites in the shape of triangular step pyramids with step heights corresponding to single and double Bi layers in the [111] direction. This morphology indicates crystallization from the vapor. For deposition at higher temperatures, diffuse RHEED patterns appeared independent of the …

Anisotropy Of Exchange Stiffness And Its Effect On The Properties Of Magnets, K. D. Belashchenko

Kirill Belashchenko Publications

Using the spin-spiral formulation of the tight-binding linear muffin-tin orbital method, the principal components of the exchange stiffness tensor are calculated for typical hard magnets including tetragonal CoPt-type and hexagonal YCo₅ alloys. The exchange stiffness is strongly anisotropic in all studied alloys. This anisotropy makes the domain wall surface tension anisotropic. Competition between this anisotropic surface tension and magnetostatic energy controls the formation and dynamics of nanoscale domain structures in hard magnets. Anisotropic domain wall bending is described in detail from the general point of view and with application to cellular Sm–Co magnets. It is shown that the repulsive …

Spin Uncoupling In Free Nb Clusters: Support For Nascent Superconductivity, Ramiro Moro, Shuangye Yin, Xiaoshan Xu, Walt A. De Heer

Xiaoshan Xu Papers

Molecular beam Stern-Gerlach deflection measurements on Nb clusters (Nb_N, N <100) show that at very low temperatures the odd-N clusters deflect due to a single unpaired spin that is uncoupled from the cluster. At higher temperatures the spin is coupled and no deflections are observed. Spin uncoupling occurs concurrently with the transition to the recently found ferroelectric state, which has superconductor characteristics [Science 300, 1265 (2003)]. Spin uncoupling (also seen in V, Ta, and Al clusters) is analogous to the reduction of spin-relaxation rates observed in bulk superconductors below T_c.

Tem Study Of Crystalline Structures Of Cr–N Thin Films, Xingzhong Li, J. Zhang, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Cr–N films were grown on Si (001) substrates by reactive magnetron sputtering under an N2/Ar atmosphere at room temperature. The composition of the films, expressed as Cr_1–xN_x, can be varied by changing the N₂/Ar pressure ratio during the synthesis process. Crystalline states of Cr–N films have been studied using electron diffraction. It is well known that two intermediate phases, Cr₂N (hexagonal) and CrN (cubic), exist in the Cr–N system, and small variations around the ideal stoichiometry are tolerated. The present study shows that cubic CrN with vacancies rather than hexagonal Cr_{2 …}

Ferroelectricity In Free Niobium Clusters, Ramiro Moro, Xiaoshan Xu, Shuangye Yin, Walt A. De Heer

Xiaoshan Xu Papers

Electric deflections of gas-phase, cryogenically cooled, neutral niobium clusters [Nb_N; number of atoms (N) = 2 to 150, temperature (T) = 20 to 300 kelvin], measured in molecular beams, show that cold clusters may attain an anomalous component with very large electric dipole moments. In contrast, room-temperature measurements show normal metallic polarizabilities. Characteristic energies k_BT_G(N) [Boltzmann constant k_B times a transition temperature T_G(N)] are identified, below which the ferroelectric-like state develops. Generally, T_G decreases [110 > T_G(N) > 10K] as …