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Displaying Results 101 - 125 of 3911 on page 5 of 157
Marked
Mark
Magnetoresistance of magnetite point contacts and nanoconstrictions
(2005)
COEY, JOHN MICHAEL DAVID
Magnetoresistance of magnetite point contacts and nanoconstrictions
(2005)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
Point contacts of magnetite exhibit large, reversible increases of conductance by a factor of up to 5 in the presence of a small magnetic field (<10?mT), provided the conductance is less than the quantum of conductance G0 (12?900??)?1. Smaller effects are found in (La0.7Sr0.3)MnO3 and Co2Cr0.6Fe0.4Al. Comparable effects observed in magnetite nanoconstrictions milled using a focused-ion beam in thin films deposited on oxide substrates are only reversible on a time scale of hours. High-resistance nanogaps show evidence of field emission beyond a voltage threshold, which is itself field dependent. The results are discussed in terms of spin-polarized transport, magnetostriction, magnetic dipole strain, and Fowler?Nordheim tunneling.
http://hdl.handle.net/2262/31834
Marked
Mark
All optical, high contrast absorptive modulation in an asymmetric Fabry-Perot etalon
(1991)
HEGARTY, JOHN
All optical, high contrast absorptive modulation in an asymmetric Fabry-Perot etalon
(1991)
HEGARTY, JOHN
Abstract:
We report a 27:l switching contrast ratio with 2.5 mW of power in an asymmetric Fabry- Perot Ctalon. The modulation is achieved by optical saturation of the excitonic absorption profile of a 95 A GaAs/AlGaAs multiple quantum well structure grown on a high-reflectivity dielectric stack mirror.
http://hdl.handle.net/2262/38790
Marked
Mark
MgO-based double barrier magnetic tunnel junctions with thin free layers
(2009)
FENG, GEN; VAN, DIJKEN SEBASTIAN; COEY, JOHN MICHAEL DAVID
MgO-based double barrier magnetic tunnel junctions with thin free layers
(2009)
FENG, GEN; VAN, DIJKEN SEBASTIAN; COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
The free layer thickness (tfree) in double barrier magnetic tunnel junctions (DMTJs) based on crystalline MgO barriers and CoFeB ferromagnetic layers has been varied from 0.5 to 3.0 nm in order to investigate its effect on the magnetic and electrical properties. One obvious feature of DMTJs with tfree<=1 nm is the absence of sharp free layer switching in the TMR curves, which can be explained by the superparamagnetic nature of discontinuous CoFeB layer, which breaks into nanodots when it is very thin. Normal free layer switch is observed when tfree=2.0 and 3.0 nm. Another difference is a rapid increase in junction resistance and tunnel magnetoresistance at low temperature for DMTJs with thin tfree, which is attributed to the Coulomb blockade effect. We also observed a small conductance peak in the dI/dV curve at low bias only in the parallel configuration and at temperatures below 100 K. This is related to the Kondo scattering process on the nanodots, which con...
http://hdl.handle.net/2262/31629
Marked
Mark
Influence of the antiphase domain distribution on the magnetic structure of magnetite thin films
(2006)
SHVETS, IGOR; MURPHY, SHANE
Influence of the antiphase domain distribution on the magnetic structure of magnetite thin films
(2006)
SHVETS, IGOR; MURPHY, SHANE
Abstract:
A long-range ordered magnetic domain structure was found in magnetitelike (Fe3??O4, ? ? 0.03) thin films prepared by molecular beam epitaxy on MgO (100) substrates. The stripelike magnetic domain structure arising after suitable postprocessing differs significantly from earlier observations. The field-dependent domain structure was investigated by magnetic force microscopy in external magnetic fields. The magnetic domain structure results from a moderate perpendicular anisotropy. The domain structure is pinned, and the pinning centers arise from the magnetite antiphase domain structure.
http://hdl.handle.net/2262/40426
Marked
Mark
Influence of miscut on the anisotropic magnetoresistance of magnetite
(2009)
SHVETS, IGOR
Influence of miscut on the anisotropic magnetoresistance of magnetite
(2009)
SHVETS, IGOR
Abstract:
We present the comparative measurements of the angular dependence of the anisotropic magnetoresistance (AMR) of magnetite (Fe3O4) films on MgO(001) nonvicinal and vicinal substrates (2.86? miscut along the 100 direction). The AMR behavior for current parallel and perpendicular to the direction of step edges is quite similar to that of nonvicinal Fe3O4 films for temperatures above the Verwey transition (TV). However, we observed a dramatic change in the symmetry of the AMR's angular response below TV for the vicinal sample, which depends on the direction of current with respect to the step edges. The results highlight the influence of step edge induced spin disorder on the transport properties below Verwey transition.
http://hdl.handle.net/2262/31770
Marked
Mark
Raman scattering and Anti-Stokes emission from a single spherical microcavity with a CdTe quantum dot monolayer
(2003)
RAKOVICH, YURY
Raman scattering and Anti-Stokes emission from a single spherical microcavity with a CdTe quantum dot monolayer
(2003)
RAKOVICH, YURY
Abstract:
peer-reviewed
We have studied the Raman and luminescence spectra of a microcavity?quantum dot system consisting of a melamine formaldehyde latex microsphere coated by CdTe colloidal quantum dots. The cavity-induced enhancement of the Raman scattering allows the observation of Raman spectra from only a monolayer of CdTe quantum dots. Periodic structure with very narrow peaks in the luminescence spectra of a single microsphere was detected arising from the coupling between the emission from quantum dots and spherical cavity modes. Strong anti-Stokes emission from CdTe quantum dots coupled to the cavity modes was observed using low intensity below band-gap excitation and attributed to the strong field enhancement at the microcavity resonances.
http://hdl.handle.net/2262/23600
Marked
Mark
Fast ab initio methods for the calculation of adiabatic spin wave spectra in complex systems
(2001)
EDERER, CLAUDE
Fast ab initio methods for the calculation of adiabatic spin wave spectra in complex systems
(2001)
EDERER, CLAUDE
Abstract:
peer-reviewed
The interpretation of the physics of magnets with reduced dimensionality often requires information on the spin wave excitations at arbitrary wavelengths which is generally hard to obtain experimentally. Two powerful methods for the ab initio calculation of adiabatic spin-wave spectra are introduced, a frozen-magnon-torque method for systems with large exchange fields and a transverse-susceptibility method which may be used also for materials with smaller exchange fields. The efficiency of both methods results from the fact that the number of calculations required to obtain the spin-wave spectrum scales linearly with the number of basis atoms in the unit cell. Results are given for Fe, Co, Ni, permalloy Ni3Fe, and CoFe, materials which are often used for thin-film technologies.
http://hdl.handle.net/2262/31420
Marked
Mark
Nonexponential dielectric relaxation dynamics in supercooled liquid and glassy states of isoamyl bromide and 2-methylpentane mixtures
(2001)
VIJ, JAGDISH KUMAR
Nonexponential dielectric relaxation dynamics in supercooled liquid and glassy states of isoamyl bromide and 2-methylpentane mixtures
(2001)
VIJ, JAGDISH KUMAR
Abstract:
The dielectric spectra of mixtures of the polar solute isoamyl bromide in 2-methylpentane have been investigated in the frequency range 1 mHz to 1 MHz and in the temperature range approaching the glass transition temperature. The results obtained from the spectra are compared with those obtained recently [J. Chem. Phys. 111, 10979 (1999)] on pure isoamyl bromide. It is found that on increasing dilution with the nonpolar solvent, the width of the curves of the dielectric spectra increase significantly, and this is reflected in the increase in the nonexponential nature of the relaxation dynamics. This is found to be a consequence of the decrease in the cooperativity of the relaxation dynamics and or an increase in the heterogeneity of the solution. The data are found to fit the Havriliak?Negami equation extremely well. The data at low and high frequencies also fits the "universal law," since the latter is a low and high frequencies limiting case of the Havriliak?Negami equat...
http://hdl.handle.net/2262/40539
Marked
Mark
Effects of induced steric hindrance on the dielectric behavior and H bonding in the supercooled liquid and vitreous alcohol
(2001)
VIJ, JAGDISH KUMAR
Effects of induced steric hindrance on the dielectric behavior and H bonding in the supercooled liquid and vitreous alcohol
(2001)
VIJ, JAGDISH KUMAR
Abstract:
The reasons of Debye-type dielectric relaxation in monohydroxy alcohols were investigated by studying the effects of steric hindrance on dielectric behavior and hydrogen bonding in alcohols. Broadband dielectric spectroscopy of supercooled liquid and glassy states of alcohol was performed to study the effect of optical isomers on intermolecular hydrogen bonding. The results showed that the relaxation rates fit the power law equations with critical exponent of 13.4 compared to 2-4 required in mode-coupling theory.
http://hdl.handle.net/2262/40540
Marked
Mark
Effect of epitaxial strain on the cation distribution in spinel ferrites CoFe(2)O(4) and NiFe(2)O(4): A density functional theory study
(2011)
FRITSCH, DANIEL; EDERER, CLAUDE
Effect of epitaxial strain on the cation distribution in spinel ferrites CoFe(2)O(4) and NiFe(2)O(4): A density functional theory study
(2011)
FRITSCH, DANIEL; EDERER, CLAUDE
Abstract:
The effect of epitaxial strain on the cation distribution in spinel ferrites CoFe(2)O(4) and NiFe(2)O(4) is investigated by GGA+U total energy calculations. We obtain a very strong (moderate) tendency for cation inversion in NiFe(2)O(4) (CoFe(2)O(4)), in agreement with experimental bulk studies. This preference for the inverse spinel structure is reduced by tensile epitaxial strain, which can lead to strong sensitivity of the cation distribution on specific growth conditions in thin films. Furthermore, we obtain significant energy differences between different cation arrangements with the same degree of inversion, providing further evidence for recently proposed short range B site order in NiFe(2)O(4).
http://hdl.handle.net/2262/59503
Marked
Mark
Wall slip of bubbles in foams
(2006)
WEAIRE, DENIS LAWRENCE
Wall slip of bubbles in foams
(2006)
WEAIRE, DENIS LAWRENCE
Abstract:
We present a computational analysis of the flow of liquid foam along a smooth wall, as encountered in the transport of foams in vessels and pipes. We concentrate on the slip of the bubbles at the wall and present some novel finite element calculations of this motion for the case of fully mobile gas/liquid interfaces. Our two-dimensional simulations provide for the first time the bubble shapes and entire flow field, giving detailed insight into the distribution of stresses and dissipation in the system. In particular, we investigate the relationship between the drag force and the slip velocity of the bubble, which for small slip velocities obeys power laws, as predicted by previous semianalytical treatments.
http://hdl.handle.net/2262/41096
Marked
Mark
Study of shear force as a distance regulation mechanism for scanning near-field optical microscopy
(1996)
SHVETS, IGOR
Study of shear force as a distance regulation mechanism for scanning near-field optical microscopy
(1996)
SHVETS, IGOR
Abstract:
Results of the investigation into the suitability of the shear?force distance regulation mechanism for scanning near?field optical microscopy (SNOM) are presented. It is shown that there is a range of relative sizes of surface features to tip size that gives rise to malfunctions of the shear?force distance regulation mechanism. If the size of the tip is comparable to the size of any depressions on the sample, the corresponding shear?force image may show contrast reversal. It is also shown that the resolution obtained when imaging a surface with the shear?force scanning microscope may differ for the two perpendicular lateral directions. The resolution along the oscillation axis is usually lower than the one perpendicular to it. This has implications for SNOM images. Therefore, the interpretation of shear force, and hence SNOM images, may become complex when imaging any sample with significant surface roughness.
http://hdl.handle.net/2262/40478
Marked
Mark
Reflection-mode scanning near-field optical microscopy: Influence of sample type, tip shape, and polarization of light
(1998)
SHVETS, IGOR
Reflection-mode scanning near-field optical microscopy: Influence of sample type, tip shape, and polarization of light
(1998)
SHVETS, IGOR
Abstract:
A reflection-mode aperture-type scanning near-field optical microscope (R-SNOM) based on the external collection of the reflected light is presented. The light detection is based on an elliptical mirror setup, with the tip and sample at one focus, and a photomultiplier tube at the other. Results are presented on the general imaging properties of this microscope. The results presented concentrate on an analysis of the gap-width dependence of the optical signal, on resolution and on the contrast mechanisms which may be used in R-SNOM imaging, including reflectivity, polarization/magneto-optics, and topographic effects.
http://hdl.handle.net/2262/40477
Marked
Mark
Morphological and mechanical properties of carbon-nanotube-reinforced semicrystalline and amorphous polymer composites
(2002)
COLEMAN, JONATHAN NESBIT; BLAU, WERNER
Morphological and mechanical properties of carbon-nanotube-reinforced semicrystalline and amorphous polymer composites
(2002)
COLEMAN, JONATHAN NESBIT; BLAU, WERNER
Abstract:
In this work, multiwalled carbon nanotubes were investigated as potential mechanical reinforcement agents in two hosts, polyvinyl alcohol (PVA) and poly(9-vinyl carbazole) (PVK). It was found that, by adding various concentrations of nanotubes, both Young?s modulus and hardness increased by factors of 1.8 and 1.6 at 1 wt?% in PVA and 2.8 and 2.0 at 8 wt?% in PVK, in reasonable agreement with the Halpin?Tsai theory. Furthermore, the presence of the nanotubes was found to nucleate crystallization of the PVA. This crystal growth is thought to enhance matrix-nanotube stress transfer. In addition, microscopy studies suggest extremely strong interfacial bonding in the PVA-based composite. This is manifested by the fracture of the polymer rather that the polymer-nanotube interface.
http://hdl.handle.net/2262/40262
Marked
Mark
Dispersion and purification of Mo6S3I6 nanowires in organic solvents
(2007)
COLEMAN, JONATHAN NESBIT; BLAU, WERNER
Dispersion and purification of Mo6S3I6 nanowires in organic solvents
(2007)
COLEMAN, JONATHAN NESBIT; BLAU, WERNER
Abstract:
Sedimentation measurements have been performed on dispersions of Mo6S3I6 nanowires in a range of common solvents. By far the best solvents were N,N-dimethylformamide DMF and acetone. Stable dispersions of purified Mo6S3I6 nanowires in DMF, with concentrations as high as 0.06 g l?1, could be produced. Detailed analysis of the sedimentation curves showed that the material consisted of three phases, two insoluble phases and one which could be stably dispersed. We associate the insoluble phases with pseudospherical impurities and insoluble nanowires. The sedimenting nanowires tend to be in the form of large diameter bundles, in contrast to the dispersed phase which consists of nanowires arranged in smaller bundles. The average diameters of the nanowire bundles stably suspended in DMF are 12?14 nm. The measured sedimentation time constants of the insoluble nanowires agree very well with theory allowing us to calculate the solid-fluid interaction parameter, , to be very close to 105 Pa...
http://hdl.handle.net/2262/40257
Marked
Mark
Magneto-chiral anisotropy in charge transport through single-walled carbon nanotubes
(2002)
KRSTIC, VOJISLAV
Magneto-chiral anisotropy in charge transport through single-walled carbon nanotubes
(2002)
KRSTIC, VOJISLAV
Abstract:
Carbon nanotubes are chiral molecular objects and therefore exist in two forms that are each other?s mirror image. Many aspects of these fascinating new materials have recently been explored but their chirality has hardly been addressed. We have investigated the charge transport through individual single-walled carbon nanotubes in the presence of a magnetic field parallel to the tube axis. For most of the investigated tubes, a dependence of the resistance that is odd in both the magnetic field and in the current through the tube is observed. This effect is ascribed to the chirality of the nanotube and is called electrical magneto-chiral anisotropy.
http://hdl.handle.net/2262/38874
Marked
Mark
Growth and electrical transport of germanium nanowires
(2001)
KRSTIC, VOJISLAV
Growth and electrical transport of germanium nanowires
(2001)
KRSTIC, VOJISLAV
Abstract:
Single crystalline germanium nanowires have been synthesized from gold nanoparticles based on a vapor-liquid-solid growth mechanism. Germanium powder was evaporated at 950?C, and deposited onto gold nanoparticles at 500?C using argon as a carrier gas. The diameter of the germanium nanowires ranged from 20 to 180 nm when gold thin films were utilized as the substrate, while the nanowires grown from 10 nm Au particles showed a narrower diameter distribution centered at 28 nm. The growth direction of germanium nanowires is along the [111] direction, determined by high resolution transmission electron microscopy. Transport measurements on individual Ge nanowires indicate that the wires are heavily doped during growth and that transport data can be explained by the thermal fluctuation tunneling conduction model.
http://hdl.handle.net/2262/38873
Marked
Mark
Observation of magnetic domains using a reflection-mode scanning near-field optical microscope
(1997)
SHVETS, IGOR
Observation of magnetic domains using a reflection-mode scanning near-field optical microscope
(1997)
SHVETS, IGOR
Abstract:
It is demonstrated that it is possible to image magnetic domains with a resolution of better than 60 nm with the Kerr effect in a reflection-mode scanning near-field optical microscope. Images taken of tracks of thermomagnetically prewritten bits in a Co/Pt multilayer structure magnetized out-of plane showed optical features in a track pattern whose appearance was determined by the position of an analyzer in front of the photomultiplier tube. These features were not apparent in the topography, showing this to be a purely magneto-optic effect.
http://hdl.handle.net/2262/40475
Marked
Mark
Noise in MgO barrier magnetic tunnel junctions with CoFeB electrodes; influence of annealing temperature
(2007)
FENG, GEN; COEY, JOHN MICHAEL DAVID
Noise in MgO barrier magnetic tunnel junctions with CoFeB electrodes; influence of annealing temperature
(2007)
FENG, GEN; COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
Low frequency noise has been measured in magnetic tunnel junctions with MgO barriers and magnetoresistance values up to 235%. The authors investigated the noise for different degrees of crystallization and CoFeB/MgO interface quality depending on the annealing temperature. The authors report an extremely low 1/ f noise, compared to magnetic junctions with Al2O3 barriers. The origin of the low frequency noise is discussed and it is attributed to localized charge traps with the MgO barriers.
http://hdl.handle.net/2262/31906
Marked
Mark
Magnetic, magnetotransport, and optical properties of Al-doped Zn0.95Co0.05O thin films
(2007)
VENKATESAN, MUNUSWAMY; COEY, JOHN MICHAEL DAVID
Magnetic, magnetotransport, and optical properties of Al-doped Zn0.95Co0.05O thin films
(2007)
VENKATESAN, MUNUSWAMY; COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
Thin films of 5% Co-doped ZnO with a range of Al codoping exhibit a band-edge shift, which varies with carrier concentration as n2/3. Carrier effective mass is 0.26me and mobility is ~ 10?cm2?V?1?s?1. The doped films, which contain coherent Co clusters of 4?8?nm in size, exhibit a ferromagnetic moment of 0.3?1.0?B per cobalt. The magnetism is progressively destroyed by Al doping due to a reduction in Co-cluster formation. Magnetoresistance appears below 30?K, but these materials cannot be regarded as dilute magnetic semiconductors.
http://hdl.handle.net/2262/31893
Marked
Mark
Influence of annealing on the bias voltage dependence of tunnelling magnetoresistance in MgO double-barrier magnetic tunnel junctions with CoFeB electrodes
(2006)
FENG, GEN; VAN DIJKEN, SEBASTIAN; COEY, JOHN MICHAEL DAVID
Influence of annealing on the bias voltage dependence of tunnelling magnetoresistance in MgO double-barrier magnetic tunnel junctions with CoFeB electrodes
(2006)
FENG, GEN; VAN DIJKEN, SEBASTIAN; COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
Double-barrier magnetic tunnel junctions with two MgO barriers and three CoFeB layers exhibiting tunneling magnetoresistance (TMR) values of more than 100% were fabricated. The bias voltage dependence of the TMR ratio is highly asymmetric after annealing at low temperatures, indicating dissimilar CoFeB/MgO interfaces. The TMR effect decays very slowly for positive bias and is only reduced to half of its maximum value at V1/2 = 1.88?V when the junctions are processed at 200??C. The largest output voltage, 0.62?V, is obtained after annealing at 300??C, a temperature that combines high TMR ratios with a considerable asymmetric bias dependence.
http://hdl.handle.net/2262/31890
Marked
Mark
Point contact Andreev reflection by nanoindentation of polymethyl methacrylate
(2006)
COEY, JOHN MICHAEL DAVID
Point contact Andreev reflection by nanoindentation of polymethyl methacrylate
(2006)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
A versatile technique for performing spin polarization measurements via point contact Andreev reflection has been developed. This technique involves depositing a superconductor lead onto a thin film of the material to be studied through a nanohole formed in a layer of photoresist, using an atomic force microscope as a nanoindenter. Copper and nickel were used to demonstrate the method. The polarizations of CrO2 and Co2MnSi were also measured, the former giving a value of 95%, as expected, and the latter giving 20%, which was surprisingly low for a candidate half metal.
http://hdl.handle.net/2262/31889
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Mark
Controlled fabrication of nickel perpendicular nanocontacts using focused ion beam milling
(2006)
COEY, JOHN MICHAEL DAVID
Controlled fabrication of nickel perpendicular nanocontacts using focused ion beam milling
(2006)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
A method for the controlled fabrication of a nickel perpendicular nanocontact with diameters less than 5 nm is reported. The method involves milling pores through the 100 nm thick silicon nitride membrane using a focused ion beam, and depositing thin Ni film on the both side of the silicon nitride membrane. The shape of the resulting pore and nanocontact size is determined by the initial hole size and redeposition of the sputtered material onto the sidewalls of the holes. A sub-5 nm Ni nanocontact was prepared and the I-V and R-H characteristics measured.
http://hdl.handle.net/2262/31888
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Mark
Magnetoresistance of nickel nanocontacts fabricated by different methods
(2006)
COEY, JOHN MICHAEL DAVID
Magnetoresistance of nickel nanocontacts fabricated by different methods
(2006)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
Nickel nanocontacts have been fabricated by focused ion-beam #1;FIB#2; milling of e-beam patterned planar contacts, FIB milling of conical-shaped nanoperforations in a silicon nitride membrane, and nanoimprinting using an atomic force microscope. Their sizes ranged from 1 to 30 nm. Magnetoresistance of up to 3% is developed in a field of a few millitesla. This is interpreted in terms of ballistic magnetoresistance across a wide domain wall whose structure is determined by dipolar interactions at the contact.
http://hdl.handle.net/2262/31887
Marked
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Magnetoresistance and electrical hysteresis in stable half-metallic (La0.7Sr0.3 MnO3 and Fe3O4 nanoconstrictions
(2005)
COEY, JOHN MICHAEL DAVID
Magnetoresistance and electrical hysteresis in stable half-metallic (La0.7Sr0.3 MnO3 and Fe3O4 nanoconstrictions
(2005)
COEY, JOHN MICHAEL DAVID
Abstract:
We have studied the transport properties of mechanically stable Fe3O4 and La0.7Sr0.3MnO3 nanoconstrictions patterned by focused ion-beam milling. The magnetoresistance decreases with the square of the applied voltage and scales with the resistance of the constriction, with values up to 8000% for magnetite and 100% for La0.7Sr0.3MnO3. These results are interpreted within a model for domain-wall magnetoresistance. Some samples exhibit electrical hysteresis with discrete changes of resistance that disappear in the presence of a magnetic field, indicating domain-wall displacement driven by a spin-polarized current.
http://hdl.handle.net/2262/31850
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