Nanoscience and Nanotechnology Commons^™

Texture Formation In Fept Thin Films Via Thermal Stress Management, P. Rasmussen, X. Rui, Jeffrey E. Shield

Nebraska Center for Materials and Nanoscience: Faculty Publications

The transformation variant of the fcc to fct transformation in FePt thin films was tailored by controlling the stresses in the thin films, thereby allowing selection of in- or out-of-plane c-axis orientation. FePt thin films were deposited at ambient temperature on several substrates with differing coefficients of thermal expansion relative to the FePt, which generated thermal stresses during the ordering heat treatment. X-ray diffraction analysis revealed preferential out-of-plane c-axis orientation for FePt films deposited on substrates with a similar coefficients of thermal expansion, and random orientation for FePt films deposited on substrates with a very low coefficient of …

Wave Attenuations In Solids With Perfectly Aligned Cracks, Liyong Yang, Joseph A. Turner

Nebraska Center for Materials and Nanoscience: Faculty Publications

The theory of wave propagation and scattering in cracked media is applied to study the wave attenuations in an isotropic solid medium containing perfectly aligned penny-shaped microcracks. The unit normals of all cracks are assumed parallel to a given direction. The wave scattering model is formulated using an anisotropic Green’s dyadic approach. Explicit expressions are derived for attenuations of the three wave modes in terms of the microcrack density. Numerical results are presented and discussed. In particular, comparisons of the attenuation results presented in this letter with previous results for the Rayleigh limit are given.

Imaging Using Lateral Bending Modes Of Atomic Force Microscope Cantilevers, A. Caron, U. Rabe, M. Reinstadtler, Joseph A. Turner, W. Arnold

Nebraska Center for Materials and Nanoscience: Faculty Publications

Using scanning probe techniques, surface properties such as shear stiffness and friction can be measured with a resolution in the nanometer range. The torsional deflection or buckling of atomic force microscope cantilevers has previously been used in order to measure the lateral forces acting on the tip. This letter shows that the flexural vibration modes of cantilevers oscillating in their width direction parallel to the sample surface can also be used for imaging. These lateral cantilever modes exhibit vertical deflection amplitudes if the cantilever is asymmetric in thickness direction, e.g., by a trapezoidal cross section.

Attenuation Of Ultrasonic Waves In Rolled Metals, Liyong Yang, Joseph A. Turner

Nebraska Center for Materials and Nanoscience: Faculty Publications

Scattering of ultrasonic waves in polycrystals with texture is studied in this article. The attenuations of the three wave modes are determined as a function of dimensionless frequency and propagation direction, respectively, for given orientation distribution coefficients (ODCs). The calculation is done in the case of a statistically orthorhombic sample made up of cubic crystallites. The wave propagation and scattering model is formulated by the Dyson equation using an anisotropic Green’s function approach.Within the limits of the first-order smoothing approximation, the Dyson equation is solved in the spatial Fourier transform domain. The results presented are shown to be directional dependent, …

Nanotube Magnetism, Yucheng Sui, Ralph Skomski, Kory D. Sorge, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

FePt and Fe3O4 nanotubes are produced by hydrogen reduction in nanochannels of porous alumina templates and investigated by electron microscopy, x-ray diffraction analysis, and magnetic measurements. Loading the templates with a Fe chloride and Pt chloride mixture followed by hydrogen reduction at 560 °C leads to the formation of ferromagnetic FePt nanotubes in the alumina pores. Using a Fe nitrate solution, thermally decomposed at 250 °C and reduced in hydrogen for 2.5 h at the same temperature, yields Fe3O4 tubes. The length of the nanotubes is about 50 mm and their diameters range from about 150 to 220 nm, depending …

Humidity Effects On The Determination Of Elastic Properties By Atomic Force Acoustic Microscopy, D.C. Hurley, Joseph A. Turner

Nebraska Center for Materials and Nanoscience: Faculty Publications

We have investigated how ambient humidity can affect quantitative measurements of elastic properties on the nanoscale. Using an emerging technique called atomic force acoustic microscopy (AFAM), two samples were examined: a thin film of fluorosilicate glass and a section of borosilicate glass. When experimental results were analyzed using a simple model of the atomic force microscope cantilever dynamics, values of the tip–sample contact stiffness k* increased approximately linearly with relative humidity. The effect is believed to be due to the presence of a humidity-dependent layer of water on the sample. To account for this, the data analysis model was …

Ultrasound Diffusion For Crack Depth Determination In Concrete, S.K. Ramamoorthy, Y. Kane, Joseph A. Turner

Nebraska Center for Materials and Nanoscience: Faculty Publications

The determination of the depth of surface-breaking cracks in concrete specimens using an ultrasound diffusion technique is discussed. Experiments were carried out on precracked concrete specimens of varying crack depths (0%–40% of the specimen thickness). Contact transducers were placed at the specimen surface with source and receiver separated by the crack. Tone-burst excitations over a frequency range of 400–600 kHz were used. At these frequencies, ultrasound is scattered considerably by the heterogeneities in the concrete. In the limit of many scattering events, the evolution of energy may be modeled as a diffusion process. The arrival of the peak diffuse energy …

Jecp/Sp: A Computer Program For Generating Stereographic Projections, Applicable To Specimen Orientation Adjustment In Tem Experiments, Xingzhong Li

Nebraska Center for Materials and Nanoscience: Faculty Publications

In the present work, a computer program was written for generating stereographic projections and was also extended as an application for specimen orientation adjustment in TEM experiments. JECP/SP is a fully functional tool for generating stereographic projections in direct and reciprocal space. JECP/SP has been extended as an application for specimen orientation adjustment using a TEM holder. JECP/SP can be used as a teaching aid for students in crystallography as well as a practical tool for scientists performing TEM experiments.

Atomic Force Acoustic Microscopy Methods To Determine Thin-Film Elastic Properties, D.C. Hurley, K. Shen, N.M. Jennett, Joseph A. Turner

Nebraska Center for Materials and Nanoscience: Faculty Publications

We discuss atomic force acoustic microscopy (AFAM) methods to determine quantitative values for the elastic properties of thin films. The AFAM approach measures the frequencies of an AFM cantilever’s first two flexural resonances while in contact with a material. The indentation modulus M of an unknown or test material can be obtained by comparing the resonant spectrum of the test material to that of a reference material. We examined a niobium film (d=280±30 nm) with AFAM using two separate reference materials and two different cantilever geometries. Data were analyzed by two methods: an analytical model based on conventional …

Elastic Wave Propagation And Scattering In Solids With Uniaxially Aligned Cracks, Liyong Yang, Joseph A. Turner

Nebraska Center for Materials and Nanoscience: Faculty Publications

In this article, elastic wave propagation and scattering in a solid medium permeated by uniaxially aligned penny-shaped microcracks are studied. The crack alignment refers to the case in which the unit normals of all cracks are randomly oriented within a plane of isotropy. The analysis is restricted to the limit of the noninteraction approximation among individual cracks. Explicit expressions for attenuations and wave speeds of the shear horizontal, quasilongitudinal, and quasishear vertical waves are obtained using stochastic wave theory in a generalized dyadic approach. The ensemble average elastic wave response is governed by the Dyson equation, which is solved in …

Jecp/Ed — A Computer Program For Simulation Of Selected-Area And Precession Electron Diffraction Patterns, Xingzhong Li

Nebraska Center for Materials and Nanoscience: Faculty Publications

Crystal structure determination by electron crystallography has been very successful in many fields, ranging from organic to inorganic materials. Collection of electron diffraction data is still a crucial step in electron crystallography. The curvature of the Ewald sphere limits the data available in each electron diffraction pattern. The dynamical multi-scattering effect of electron diffraction increases the complexity of the structure determination procedure.

A precession method (Vincent & Midgley, 1994) has been used to solve the crystal structure of AlmFe by Gjonnes and coworker (Berg et al., 1998; Gjonnes et al., 1998). The advantages of precession electron diffraction are (i) the …

Scattering Of Elastic Waves In Damaged Media, Liyong Yang, Joseph A. Turner

Nebraska Center for Materials and Nanoscience: Faculty Publications

The scattering of elastic waves in a medium with damage from microcracking is discussed. The influence of damage from penny-shaped microcracks within a homogeneous medium is considered. The microcracks are assumed to be randomly oriented and uniformly distributed. Explicit expressions are derived for the attenuation of longitudinal and shear elastic waves in terms of the damage parameter and the effective elastic moduli of the medium. A generalized tensor-based approach is used such that the results are coordinate free. The derivation is based upon diagrammatic methods. The problem is formulated in terms of the Dyson equation, which is solved for the …

Multiscale Modeling Of Hysteretic Phenomena In Magnets „(Invited), Vladimir P. Antropov, Kirill D. Belashchenko

Nebraska Center for Materials and Nanoscience: Faculty Publications

Methodology of multiscale modeling of hysteretic phenomena in magnets is discussed. A practical combination of first-principles, micromagnetic, and microstructural calculations is constructed which allows one to study the hysteretic phenomena in hard magnets. Advantages and shortcomings of this approach are discussed. Multiscale nature of coercivity in CoPt type magnets is elucidated. Two sources of coercivity in polytwinned CoPt type magnets developing at different length scales, domain wall pinning at antiphase boundaries and splitting at twin boundaries, are illustrated for a realistic microstructure

Multiscale Nature Of Hysteretic Phenomena: Application To Copt-Type Magnets, Kirill D. Belashchenko, Vladimir P. Antropov

Nebraska Center for Materials and Nanoscience: Faculty Publications

We suggest a workable approach for the description of multiscale magnetization reversal phenomena in nanoscale magnets and apply it to CoPt-type alloys. We show that their hysteretic properties are governed by two effects originating at different length scales: a peculiar splitting of domain walls at twin boundaries and their strong pinning at antiphase boundaries. We emphasize that such multiscale nature of hysteretic phenomena is a generic feature of nanoscale magnetic materials.

Structure Of Macrodomain Walls In Polytwinned Magnets, Kirill D. Belashchenko, Vladimir P. Antropov

Nebraska Center for Materials and Nanoscience: Faculty Publications

We propose a microscopic approach to the studies of magnetic configurations in hard magnets which may be conveniently used for nanoscale systems; the microstructure of the magnet is easily and naturally included in the calculations. This approach is applied to find the structure of macrodomain walls in polytwinned magnets of the CoPt family. Magnetostatic fields are small compared to the anisotropy field in these magnets; direct simulation shows that in this case the macrodomain wall is not continuous, but rather comprised of segments held together by relatively small magnetostatic forces. This segmentation is expected to have a strong effect on …

Magnetization Reversal Of Individual Nanowires With Controlled Defects, Andrei Sokolov, Renat F. Sabirianov, W. Wernsdorfer, Bernard Doudin

Nebraska Center for Materials and Nanoscience: Faculty Publications

Low temperature magnetization reversal measurements were performed on individual permalloy nanowires of diameters between 30 and 60 nm. During the electrochemical growth of the wire, a defect was induced by a short pulse in the deposition potential modifying locally the microstructure and composition. Magnetic measurements performed with micro-superconducting quantum interference devices were performed. The angular dependence of switching field revealed significant deviations from classical predictions. For specific angles, magnetization curves indicate a reversal occurring in two steps.

Characterization Of Crbn Films Deposited By Ion Beam Assisted Deposition, S.M. Aouadi, F. Namavar, E. Tobin, N. Finnegan, R.T. Haasch, R. Nilchiani, Joseph A. Turner, S. L. Rohde

Nebraska Center for Materials and Nanoscience: Faculty Publications

This article reports on the growth and analysis of CrBN nanocrystalline materials using an ion beam assisted deposition process. In addition, this article addresses the utilization of spectroscopic ellipsometry for in situ analysis of ternary nitrides. Coatings, with a total thickness of 1.5 ±0.2 μm, were deposited at low temperatures (<200 °C) on silicon substrates using ion beam assisted deposition. These coatings were characterized postdeposition using x-ray diffraction (XRD), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), visible-light spectroscopic ellipsometry (VIS-SE), infrared spectroscopic ellipsometry (IR-SE), and nanoindentation. The primary phases in the films were investigated using XRD. The surface morphology and nanocrystalline nature of the coatings (grain size of 5–7 nm) were deduced using AFM. The elemental composition and phase composition of the samples were determined from XPS and AES measurements and were subsequently deduced from the analysis of the VIS-SE data, and these correlated well. XPS, AES, and IR-SE revealed the crystal structure of the BN phase in the ternary compounds. The correlation of the results from these various techniques indicates that in situ SE may be a potential technique to control the growth of ternary nitride coatings in the future. The mechanical properties of the coatings were evaluated using nanohardness testing. The hardness and elastic modulus were measured to be 19–22 GPa and 250–270 GPa, respectively.

Boron Carbide/N-Silicon Carbide Heterojunction Diodes, Shireen Adenwalla, P. Welsch, A. Harken, Jennifer I. Brand, A. Sezer, Brian W. Robertson

Nebraska Center for Materials and Nanoscience: Faculty Publications

The fabrication, initial structural characterization, and diode measurements are reported for a boron carbide/silicon carbide heterojunction diode. Current–voltage curves are obtained for operation at temperatures from 24 to 351 °C. Plasma-enhanced chemical-vapor deposition (PECVD) -deposited undoped boron carbide material is highly crystalline and consists of a variety of polytypes of boron carbide (BC) with crystal sizes as large as 110 nm. Crystal phases are similar to those for PECVD BC on Si but only partially match known boron and boron-rich BC phases.

Anisotropy Of Conducting P States And 11B Nuclear Spin-Lattice Relaxation In Mg1-XAlXB2, Kirill D. Belashchenko, Vladimir B. Antropov, S.N. Rashkeev

Nebraska Center for Materials and Nanoscience: Faculty Publications

We calculated the nuclear spin-lattice relaxation rate in the Mg_1-xAl_xB₂ system and found that the orbital relaxation mechanism dominates over the dipolar and Fermi-contact mechanisms in MgB₂, whereas in AlB₂ due to a smaller density of states and strong anisotropy of boron p orbitals the relaxation is completely determined by Fermi-contact interaction. The results for MgB₂ are compared with existing experimental data. To validate the theory, nuclear resonance experiments for the studied diboride alloy system are needed.

Coexistence Of Covalent And Metallic Bonding In The Boron Intercalation Superconductor Mgb2, Kirill D. Belashchenko, Mark Van Schilfgaarde, Vladimir P. Antropov

Nebraska Center for Materials and Nanoscience: Faculty Publications

Chemical bonding and electronic structure of MgB₂, a boron-based newly discovered superconductor, is studied using self-consistent band-structure techniques. Analysis of the transformation of the band structure for the hypothetical series of graphite–primitive graphite–primitive graphitelike boron–intercalated boron, shows that the band structure of MgB₂ is graphitelike, with π bands falling deeper than in ordinary graphite. These bands possess a typically delocalized and metallic, as opposed to covalent, character. The in-plane σ bands retain their two-dimensional (2D) covalent character, but exhibit a metallic hole-type conductivity. The coexistence of 2D covalent in-plane and three-dimensional (3D) metallic-type interlayer conducting bands is …

Superconductivity Of Metallic Boron In Mgb2, J. Kortus, I.I. Mazin, Kirill D. Belashchenko, Vladimir P. Antropov, L.L. Boyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Boron in MgB₂ forms stacks of honeycomb layers with magnesium as a space filler. Band structure calculations indicate that Mg is substantially ionized, and the bands at the Fermi level derive mainly from B orbitals. Strong bonding with an ionic component and considerable metallic density of states yield a sizable electron-phonon coupling. Together with high phonon frequencies, which we estimate via zone-center frozen phonon calculations to be between 300 and 700 cm^-1, this produces a high critical temperature, consistent with recent experiments. Thus MgB₂ can be viewed as an analog of the long sought, but still …

Scattering Of Elastic Waves In Heterogeneous Media With Local Isotropy, Joseph A. Turner, Phanidhar Anugonda

Nebraska Center for Materials and Nanoscience: Faculty Publications

The scattering of elastic waves in heterogeneous media is discussed. Explicit expressions are derived for the attenuation of longitudinal and transverse elastic waves in terms of the statistics of the density and Lame´ parameter fluctuations. The derivation is based upon diagrammatic methods with the problem posed in terms of the Dyson equation. The Dyson equation is solved for the mean field response. The results are given here in a straightforward manner, in which the attenuations reduce to simple integrals on the unit circle. The medium is assumed statistically homogeneous and statistically isotropic. This model, with assumed local isotropic properties, is …

Order–Disorder Effects In Nitrided Sm–Fe Permanent Magnets, B.E. Meacham, Jeffrey E. Shield, D.J. Branagan

Nebraska Center for Materials and Nanoscience: Faculty Publications

Rapidly solidified Sm₁₁Fe₈₉ was observed to form partially ordered Sm₂Fe₁₇. The ordering increased systematically with annealing temperature, reaching an order parameter of 0.8 after annealing at 950 °C for 15 min. The coercivity of the corresponding interstitial compound Sm₂Fe₁₇N_x varied with annealing temperature and thus order parameter. The coercivity reached a maximum for an order parameter of 0.45–5, then dramatically decreased as the order increased. This behavior is characteristic of demagnetization controlled by domain wall pinning by antiphase boundaries.

Phase Formation In Hypostoichiometric Sm2Fe17 Alloys Modified With Ti And C, Jeffrey E. Shield, B.E. Meacham

Nebraska Center for Materials and Nanoscience: Faculty Publications

The phase formation, crystallization products, microstructure, and magnetic behavior of Fe-rich Sm–Fe alloys modified with Ti and C have been determined. Alloys of nominal composition (Sm _0.11-xFe _0.89+x) ₉₄Ti ₃C ₃ with x=0, 0.02, and 0.04 were produced by melt spinning. Crystalline phases observed included SmFe ₇ and α -Fe and an increasing amorphous fraction was observed with increasing x. Upon crystallization the amorphous fraction converted to Sm₂Fe₁₇ and a-Fe, with a grain size of 30 nm for the sample with x=0.02. The magnetic measurements of nitrided alloys …

Elastic Wave Propagation And Scattering In Heterogeneous, Anisotropic Media: Textured Polycrystalline Materials, Joseph A. Turner

Nebraska Center for Materials and Nanoscience: Faculty Publications

The propagation of elastic waves through heterogeneous, anisotropic media is considered. Appropriate ensemble averaging of the elastic wave equation leads to the Dyson equation which governs the mean response of the field. The Dyson equation is given here in terms of anisotropic elastic Green’s dyadics for the medium with and without heterogeneities. The solution of the Dyson equation for the mean response is given for heterogeneities that are weak. The formalism is further specified for the case of equiaxed cubic polycrystalline metals with a single aligned axis. The Green’s dyadics in this case are those for a transversely isotropic medium. …

Asymptotic Decay Of Radius Of A Weakly Conductive Viscous Jet, A.F. Spivak, Yuris A. Dzenis

Nebraska Center for Materials and Nanoscience: Faculty Publications

Motion of a weakly conductive viscous jet accelerated by an external electric field is considered. Nonlinear rheological constitutive equation applicable for polymer fluids (Oswald–deWaele law) is applied. A differential equation for the variation of jet radius with axial coordinate is derived. Asymptotic variation of the jet radius at large distances from the jet origin is analyzed. It is found that the well-known power-law asymptote for Newtonian fluids with the exponent 1/4 holds for more general class of fluids, i.e., pseudoplastic (shear thinning) and dilatant (shear thickening) fluids with the flow index between 0 and 2. Dilatant fluids with the flow …

The Effect Of Grain Size On Magnetic Properties In Sm2Fe17NX, Jeffrey E. Shield, D.J. Branagan, C.P. Li, R.W. Mccallum

Nebraska Center for Materials and Nanoscience: Faculty Publications

The microstructures of melt spun Sm11Fe89 and (Sm₁₁Fe₈₉)₉₄Ti₃C₃ have been investigated under different solidification conditions. The magnetic properties of the nitrided materials have been related to the observed microstructures. Melt spinning of the Sm₁₁Fe₈₉ alloys resulted in grain sizes at or above the single domain limit. The addition of Ti and C resulted in an order of magnitude refinement in the microstructural scale. The magnetic properties of the samples with grain sizes below the single domain limit decreased with decreasing grain size due to increased intergranular exchange coupling.

A Generalized Solidification Model And Microstructural Verification For The Nd–Fe–B–Ti–C System Processed By Rapid Solidification, Matthew J. Kramer, C.P. Li, K.W. Dennis, R.W. Mccallum, C.H. Sellers, D.J. Branagan, Jeffrey E. Shield

Nebraska Center for Materials and Nanoscience: Faculty Publications

For the Nd₂Fe₁₄B (2-14-1) compound, the optimal grain size should be smaller than the size of the single domain size of 150 nm. Transition metal carbides (TMC) also reduce the quench rate necessary to achieve the optimal or overquenched condition. This allows inert gas atomization (IGA) to produce viable magnetic materials. In this article we will demonstrate that optimal microstructure for the 2-14-1 can be produced by IGA with the addition of TiC. Moreover, a solidification model will be presented to show (1) how recalescence is a critical feature to the evolution of the microstructure in …

Explosive Compaction Of Magnequench Nd–Fe–B Magnetic Powders, S. Guruswamy, M.K. Mccarter, Jeffrey E. Shield, V. Panthanachan

Nebraska Center for Materials and Nanoscience: Faculty Publications

Magnequench NdFeB powders having high and low rare earth contents were explosively compacted to obtain cylindrical magnets. The magnetic properties were found to be isotropic and were superior to conventionally consolidated isotropic magnets. The (BH) _max was 14.7 MGOe and the remanence was 8.7 kG for the explosively compacted magnet with lower rare earth content. X-ray diffraction patterns confirm the explosively compacted magnet to be crystalline and the predominant phase to be the 2-14-1 phase. Transmission electron microscopy examination showed a microstructure to consist of 20–25 nm size equiaxed grains consistent with the magnetic measurements.

Incorporation Of Zn In Gaas During Organometallic Vapor Phase Epitaxy Growth Compared To Equilibrium, W. Reichert, C.Y. Chen, W.M. Li, Jeffrey E. Shield, R.M. Cohen, D.S. Simons, P.H. Chi

Nebraska Center for Materials and Nanoscience: Faculty Publications

The zinc concentration measured after organometallic vapor phase epitaxy (GMVPE) growth on (100)-oriented GaAs at 700 °C has been compared to the zinc concentration measured after in-diffusion under near-equilibrium conditions. During diffusion, the concentration of Zn 20 nm below the surface was found to vary with P_Zn^1/2, as expected for bulk solid-vapor equilibrium. During growth, the concentration of Zn varied linearly with P_Zn up to a maximum value which was found to correspond to the solubility limit set by second phase formation, e.g., growth of Zn₃As₂. Although large differences were observed between …