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Current Search:
'Physics' in all fields;
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Displaying Results 226 - 250 of 1489 on page 10 of 60
Marked
Mark
Domain-wall dynamics in aligned bound Sm2Fe17
(1996)
COEY, JOHN MICHAEL DAVID
Domain-wall dynamics in aligned bound Sm2Fe17
(1996)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
ac susceptibility (χac=χ′-jχ″) of an aligned bound Sm2Fe17 powder sample has been measured along the easy direction as a function of temperature, frequency, warming rate, and time. Anomalous behavior has been found in that with increasing temperature, χ′ and χ″ increase steadily up to T∼150 K where a drop occurs. At higher temperatures χ′ increases again, following an S-shaped curve which is accompanied by a χ″ peak. Using a model two-level system for a thermally activated magnetic aftereffect process, τ=τ0exp(Q/kT), the high-T χ′ rise is explained by dynamic wall pinning and depinning owing to local directional ordering around domain walls with Q≊0.53 eV and τ0≊1.7×10-13 s. The low-T χ′ peak is a consequence of domain-structure reconstruction and involves a long-range thermally hysteretic pseudostatic wall pinning and depinning process with average Q≊0.42 eV and τ0≊10-10 s. The reasons for the anomalies in χac(T) often found in rare-earth–transition-metal alloys a...
http://hdl.handle.net/2262/31729
Marked
Mark
Transport properties of Bi2Sr2Can-1CunO2 n+4 thin films
(1994)
COEY, JOHN MICHAEL DAVID
Transport properties of Bi2Sr2Can-1CunO2 n+4 thin films
(1994)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
The Hall effect and resistivity have been measured in Bi2Sr2Can-1CunO2n+4 (n=2 and 3) thin films as a function of magnetic field and temperature. In the normal state, the ratio of the normal resistivity to the Hall resistivity (the cotangent of the Hall angle) is found to vary as T2. Three models are able to account qualitatively for such behavior: a bipolaron model, BCS with a Van Hove singularity, and resonating valence bond. Analyses of the resistivity and the Hall effect in the superconducting fluctuations region above Tc clearly demonstrate the two-dimensional character of the 2:2:1:2 compound. The sign reversal of the Hall constant in the vortex state is related to vortex motion. An explanation is presented based on the motion of an antivortex, or a defect of the local vortex lattice, in moving vortex bundles. The calculated activation energy of approximately 730 K agrees well with the measured value of 750 K, and the model gives the correct sign for the Nern...
http://hdl.handle.net/2262/31697
Marked
Mark
Magnetization and 57Fe hyperfine fields in Y2Fe17Z3-��(Z=H, C, or N) interstitial compounds
(1992)
COEY, JOHN MICHAEL DAVID
Magnetization and 57Fe hyperfine fields in Y2Fe17Z3-��(Z=H, C, or N) interstitial compounds
(1992)
COEY, JOHN MICHAEL DAVID
Abstract:
Measured magnetization and 57Fe hyperfine fields at T�_0 K for Y2Fe17 and Y2Fe17Z3-��, with Z=H, C, or N and ���_0.5, are analyzed to determine the influence of the interstitial atoms on the 3d magnetism. All are weak ferromagnets with a nearly-full 3d subband, although when Z=N, the magnetic moment of 38.1�_B is very close to the fully saturated value of 39.4�_B. On a local scale, the order of hyperfine fields Bhf4f(=35 T)>Bhf6g (=33 T)>Bhf12j (=30 T)>Bhf12k (=28 T) is different from that of the local magnetic moments given by several recent band calculations for Y2Fe17, �_4f (=2.5�_B)>�_12j (=2.2�_B)>�_12k (=2.0�_B)>�_6g (=1.9 �_B), reflecting a large 4s-transferred hyperfine field at 6g sites which have very short Fe-Fe distances. The volume of samples with Z=C and N are identical, and so the chemical effects of the interstitials can be distinguished; both the local magnetic moment and hyperfine fields are systematically smaller in the carbide than the nitride b...
http://hdl.handle.net/2262/31694
Marked
Mark
Observation of intermultiplet transitions in Sm(Fe11Ti) by inelastic neutron scattering
(1990)
COEY, JOHN MICHAEL DAVID
Observation of intermultiplet transitions in Sm(Fe11Ti) by inelastic neutron scattering
(1990)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
Magnetic neutron spectroscopy of SMFe11Ti reveals a dispersionless electronic excitation corresponding to transitions between levels of the exchange-split ground- and first-excited-state multiplets of the Sm3+ ion. The observed excitation energy of 175.5 meV at T=20 K may be accounted for in the framework of an exchange and crystal-field model by assuming a bare spin-orbit splitting of 129 meV for the J=5/2 and 7/2 multiplets. The neutron spectroscopy data are explained with the same parameters used to fit the experimental magnetization curves of single-crystal SmFe11Ti.
http://hdl.handle.net/2262/31689
Marked
Mark
Giant transverse hysteresis is an asperomagnet
(1990)
COEY, JOHN MICHAEL DAVID
Giant transverse hysteresis is an asperomagnet
(1990)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
Comparison of the magnetoresistance of amorphous thin films of Tb80U20 and the nonmagnetic analog Y80U20 reveals hysteresis for the terbium alloy, which extends to fields in excess of 8 T at 4.2 K. Hysteresis is found in the magnetization curve only in fields below 4 T at the same temperature. The magnetoresistance effect, attributed to scattering of electrons by the frozen transverse spin components, which have spatial fluctuations on the scale of the interatomic spacing, is interpreted in terms of the magnetization process of the amorphous alloy. Evidence of weak localization is observed in both films below 1 K.
http://hdl.handle.net/2262/31688
Marked
Mark
Electronic properties of Sr14Cu24O41
(1989)
COEY, JOHN MICHAEL DAVID
Electronic properties of Sr14Cu24O41
(1989)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
Crystals of orthorhombic Sr14Cu24O41 are semiconducting, with an activation energy of 0.18(2) eV for conduction along c, the direction lying parallel to both the (Cu2O3) planes and the (CuO2) chains. The magnetic susceptibility above 80 K follows a Curie-Weiss law with =-95 K and peff=0.65B/Cu in the ac plane and =-190 and peff=0.94B/Cu along b, the direction perpendicular to the (Cu2O3) planes and the (CuO2) chains. There is an indication of antiferromagnetic order below TN=60 K, and a Curie contribution at low temperature due to localized free spins [0.4(1) with S=(1/2 per formula]. The specific heat measured in the range 212 K is of the form AT-2+T+T3, where A=1080 mJ K/mole, =100 mJ/mole K2, and =5.93 mJ/mole K3. A 5-T magnetic field greatly enhances the low-temperature specific heat, an effect which is attributed to the localized states with S=(1/2. The linear term is attributed to a localized density of states with no net spin, possibly associated with peroxi...
http://hdl.handle.net/2262/31672
Marked
Mark
Ferromagnetic nanoparticles with strong surface anisotropy: Spin structures and magnetisation processes
(2008)
COEY, JOHN MICHAEL DAVID
Ferromagnetic nanoparticles with strong surface anisotropy: Spin structures and magnetisation processes
(2008)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
Monte Carlo simulations are used to investigate the effect of surface anisotropy on the spin configurations and hysteresis loops of ferromagnetic nanoparticles. Spherical particles of radius ρa are composed of N atoms located on a simple cubic lattice with interatomic spacing a. The particles have 2⩽ρ⩽13. A classical Heisenberg model is assumed, with surface and bulk anisotropy. When surface anisotropy is positive there are two types of ground states separated by a large energy barrier: a “throttled” configuration with reduced magnetization for intermediate values of surface anisotropy and a “hedgehog” configuration with zero magnetization in the strong surface anisotropy limit. Beyond a threshold, surface anisotropy of either sign induces ⟨111⟩ easy axes for the net magnetization. Easy-axis hysteresis loops are then square, with a continuous approach to saturation, and the effective anisotropy is deduced either from the switching field or from the initial slope of...
http://hdl.handle.net/2262/31671
Marked
Mark
Magnetic properties of iron-rich Fe-Zr glasses
(1987)
COEY, JOHN MICHAEL DAVID
Magnetic properties of iron-rich Fe-Zr glasses
(1987)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
A systematic study of the magnetic properties of melt-spun amorphous FexZr100-x alloys with 88≤x≤93 using Mössbauer and magnetization measurements has established the magnetic phase diagram. The ordering temperature Tc falls sharply with increasing x; below a second transition Txy, associated with the freezing of transverse-spin components, all the alloys order asperomagnetically (Txy<Tc). Hysteresis appears at still lower temperatures, but it is not directly related to the transverse-spin freezing. A T1.3 variation of hyperfine field observed for T<Txy is attributed to excitation of transverse magnetic modes. Contrary to the predictions of Heisenberg spin-glass theory, neither the ferromagnetic nor the asperomagnetic order appears to be long range. Extrapolation to x=100 indicates that pure amorphous iron would be a speromagnet with an iron moment of 1.7μB and a spin-freezing temperature of order 150 K. Upon hydrogenation the magnetic structures of all the a...
http://hdl.handle.net/2262/31667
Marked
Mark
Crystal Fields in Nd2Fe14B
(1984)
COEY, JOHN MICHAEL DAVID
Crystal Fields in Nd2Fe14B
(1984)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
The leading terms in the crystal field at both rare-earth sites in the new permanent magnet alloy Nd2Fe14B are B20 and B22s. The sign of B20 explains the variation of the anisotropy constant K1 reported by Sagawa et al. for the R2Fe14B series. Noncollinear magnetic structures involving the rare-earth sublattices cannot be attributed to second-order crystal-field effects.
http://hdl.handle.net/2262/31650
Marked
Mark
Magnetic structure of an amorphous rare-earth transition-metal alloy
(1976)
COEY, JOHN MICHAEL DAVID
Magnetic structure of an amorphous rare-earth transition-metal alloy
(1976)
COEY, JOHN MICHAEL DAVID
Abstract:
peer-reviewed
161Dy Mössbauer spectra show that the dysprosium moments in amorphous DyCo3.4 are strongly coupled to the local crystal field axes. The magnetic structure is one in which the cobalt "sublattice" is strongly ferromagnetic, and the dysprosium moments are almost randomly distributed in directions between antiparallel and perpendicular to the cobalt.
http://hdl.handle.net/2262/31630
Marked
Mark
Measurement of fast-electron energy spectra and preheating in laser-irradiated targets
(1979)
LUNNEY, JAMES GERARD
Measurement of fast-electron energy spectra and preheating in laser-irradiated targets
(1979)
LUNNEY, JAMES GERARD
Abstract:
peer-reviewed
Fast electrons produced by the irradiation of plane-layered targets with 20-J, 100-ps, 1015-W.cm-2, neodymium-laser pulses have been diagnosed by the Kα emission which they cause at various depths in the targets. The fast-electron energy spectrum and absolute energy deposition (target preheat) are measured.
http://hdl.handle.net/2262/31613
Marked
Mark
'Pulsed X-ray shadowgraphy of dense, cool, laser-imploded plasma
(1978)
LUNNEY, JAMES GERARD
'Pulsed X-ray shadowgraphy of dense, cool, laser-imploded plasma
(1978)
LUNNEY, JAMES GERARD
Abstract:
peer-reviewed
Laser implosion of 87-bar neon-filled glass microballoon targets produces a dense, cool plasma whose development in time and space is recorded by pulsed-x-ray shadowgraphy with soft x rays from a separate laser-produced plasma. Compressed densities of 4 g cm-3 are estimated from the data by comparison with numerical simulations of the implosion.
http://hdl.handle.net/2262/31612
Marked
Mark
Time-resolved X-ray spectroscopy of laser produced plasmas
(1980)
LUNNEY, JAMES GERARD
Time-resolved X-ray spectroscopy of laser produced plasmas
(1980)
LUNNEY, JAMES GERARD
Abstract:
peer-reviewed
X-ray emission spectra, produced by irradiation of neon-filled aluminum-coated glass microballoon and plane aluminum targets, with 100-ps Nd-glass laser pulses, have been time resolved with an x-ray streak camera. The time evolution of the Ne×L and Si xiii 1S01-3P11 linewidths has been recorded. The implosion delay time of microballoon targets and the emission histories of the Al xiii 1S01-2P11 line and its satellites from plane targets have been measured.
http://hdl.handle.net/2262/31611
Marked
Mark
Silicon satellite spectra from laser imploded microballoons
(1981)
LUNNEY, JAMES GERARD
Silicon satellite spectra from laser imploded microballoons
(1981)
LUNNEY, JAMES GERARD
Abstract:
peer-reviewed
The dielectronic satellite spectral lines, on the long-wavelength side of the hydrogenic Si XIV Lyman-α line, have been observed in the space-resolved spectra from laser-imploded glass microballoons. In the electron density range 1×1022 to 2×1023 cm-3, the intensities of the Si XIII 2s 2p P3-1s 2s S3 and 2p2 P3-1s2p P3 satellite lines increase with density in agreement with numerical modeling. These satellite lines are a promising density diagnostic for the imploded glass plasma.
http://hdl.handle.net/2262/31609
Marked
Mark
The X-ray spectroscopic diagnosis of laser-produced plasmas, with emphasis on line broadening
(1980)
LUNNEY, JAMES GERARD
The X-ray spectroscopic diagnosis of laser-produced plasmas, with emphasis on line broadening
(1980)
LUNNEY, JAMES GERARD
Abstract:
peer-reviewed
A detailed account is given of the x-ray spectroscopic diagnosis of plasmas formed by exploding pusher microballoons. The measurements of the electron temperature from the shell and from the core is described. A method for self-consistently fitting all the observed lines of one state of ionization is shown, giving a diagnosis for the electron density and the opacity. Further, it is shown that the self-consistency is only achieved when full line-broadening calculations, taking into account the shifts and asymmetries dependent on the charge of the emitter, are used for the optically thin profiles. It is also shown that non-local-thermodynamic-equilibrium effects are important in the description of the radiation transfer.
http://hdl.handle.net/2262/31610
Marked
Mark
Constant-magnetic-field effect in Néel relaxation of single-domain ferromagnetic particles
(1995)
COFFEY, WILLIAM THOMAS; KALYMYKOV, YURI PETROVICH
Constant-magnetic-field effect in Néel relaxation of single-domain ferromagnetic particles
(1995)
COFFEY, WILLIAM THOMAS; KALYMYKOV, YURI PETROVICH
Abstract:
peer-reviewed
The relaxation behavior of an assembly of noninteracting single-domain ferromagnetic particles in the presence of a constant magnetic field is studied by solving the corresponding Fokker-Planck equation. The analysis is performed by first converting that equation into a hierarchy of differential-recurrence relations by expanding the solution in Legendre polynomials. The spectrum of eigenvalues and their associated amplitudes is then determined by matrix methods where all the desired physical quantities such as the magnetization correlation time and complex magnetic susceptibility may be computed numerically. In order to ensure the accuracy of the results obtained this solution is compared with an exact solution derived in terms of matrix continued fractions. It is shown that the conventional assumption in the theory of superparamagnetism, that except in the very early stages of relaxation to equilibrium the only appreciable time constant is the one associated with ...
http://hdl.handle.net/2262/30628
Marked
Mark
Magnetism in thin films of CaB6 and SrB6
(2004)
LUNNEY, JAMES GERARD; COEY, JOHN MICHAEL DAVID; VENKATESAN, MUNUSWAMY
Magnetism in thin films of CaB6 and SrB6
(2004)
LUNNEY, JAMES GERARD; COEY, JOHN MICHAEL DAVID; VENKATESAN, MUNUSWAMY
Abstract:
peer-reviewed
Thin films of disordered hexaborides CaB6 and SrB6 deposited by pulsed-laser deposition on MgO (100) or Al2O3 (001) substrates are ferromagnetic. A typical room-temperature moment per unit area of substrate is 350 mB nm��2, with the largest values being found for CaB6 on Al2O3. Lattice defects are the likely origin of the exotic, high-temperature magnetism. The moment, which is present in films as thin as 12 nm, appears to reside in an interface layer whose polarization is approximately 0.4 Tesla.
http://hdl.handle.net/2262/31468
Marked
Mark
Langmuir probe characterization of laser ablation plasmas
(2009)
LUNNEY, JAMES GERARD
Langmuir probe characterization of laser ablation plasmas
(2009)
LUNNEY, JAMES GERARD
Abstract:
peer-reviewed
For laser ablation plumes that are significantly ionized, Langmuir probes have proved to be a useful tool for measuring the plume shape, ion energy distribution, and electron temperature. Typically in laser ablation plasmas the flow velocity is supersonic, which complicates the interpretation of the current-voltage probe characteristic. In this paper we describe some recent developments on the application of Langmuir probes for laser ablation plume diagnosis. We have investigated the behavior of the probe when it is orientated perpendicular, and parallel, to the plasma flow, and show how an analytical model developed for plasma immersion ion implantation, can quantitatively describe the variation of the ion current with probe bias for the case when the plasma flow is along the probe surface. The ion signal recorded by a probe in the parallel position is proportional to the ion density and the square root of the bias voltage. It is shown that the current varies as m...
http://hdl.handle.net/2262/31446
Marked
Mark
Ab initio treatment of noncollinear spin systems within the atomic-sphere approximation and beyond
(2000)
EDERER, CLAUDE
Ab initio treatment of noncollinear spin systems within the atomic-sphere approximation and beyond
(2000)
EDERER, CLAUDE
Abstract:
peer-reviewed
The properties of noncollinear magnets are often calculated within the framework of density-functional theory in local-spin-density approximation with the additional use of the atomic-sphere approximation for the spin directions. Thereby the intra-atomic noncollinearity is neglected for the calculation of the exchangecorrelation energy by taking into account only those components of the spin density which are parallel to spin quantization axes ~SQAs! defined for the whole atomic spheres. When the magnetic moment directions are calculated in a self-consistent manner, e.g., for systems with intrinsic noncollinearity or when calculating the response of the system to a weak external field with components perpendicular to the moment directions, the SQAs are conventionally chosen to be parallel to the magnetic moments. We present both theoretical arguments and test calculations showing that this choice of SQAs is not the best and may in certain situations lead to wrong r...
http://hdl.handle.net/2262/31421
Marked
Mark
Density-functional study of Fe3AL: LSDA versus GGA
(2002)
EDERER, CLAUDE
Density-functional study of Fe3AL: LSDA versus GGA
(2002)
EDERER, CLAUDE
Abstract:
peer-reviewed
The local-spin-density approximation and the generalized-gradient approximation ~GGA! are used to perform density-functional total-energy calculations at zero temperature for Fe3Al in the ordered D03 and L12 structures. Our calculations show that commonly used GGA functionals fail to predict the experimentally stable D03 structure as the one with the lower total energy. This qualitative discrepancy with experiment is attributed to an overestimation of the magnetic energy in GGA. The calculations were carried out using the mixed-basis pseudopotential ~MBPP! method in the frozen-core approximation and the full-potential linearized-augmented-plane-wave ~FLAPW! method, both with and without spin polarization. Although there are small differences in the magnitudes of the magnetic moments and the magnetic energies obtained with MBPP and FLAPW, both methods yield the same qualitative result.
http://hdl.handle.net/2262/31419
Marked
Mark
Theory of induced magnetic moments and x-ray magnetic circular dichroism in Co-Pt multilayers
(2002)
EDERER, CLAUDE
Theory of induced magnetic moments and x-ray magnetic circular dichroism in Co-Pt multilayers
(2002)
EDERER, CLAUDE
Abstract:
peer-reviewed
For Co-Pt multilayers, the magnetic moments of the Pt atoms and the x-ray magnetic circular dichroism (XMCD) spectra at the L2 and L3 edge of Pt are calculated-by the ab initio density-functional electron theory. The calculated magnetization profile for an ideal Co-Pt interface is in part different from the profile obtained by x-ray resonant magnetic reflectometry for the real interface. Some of the assumptions that are commonly adopted to determine the magnetic moments from the XMCD spectra via the sum rules are critically assessed for the Co-Pt system. It is shown that the orbital sum rule is strongly violated near the Co-Pt interface whereas the spin sum rule is approximately fulfilled provided the magnetic dipole term (Tz) is included in the analysis.
http://hdl.handle.net/2262/31418
Marked
Mark
From the bulk to monatomic wires: An ab initio study of magnetism in Co systems with various dimensionality
(2002)
EDERER, CLAUDE
From the bulk to monatomic wires: An ab initio study of magnetism in Co systems with various dimensionality
(2002)
EDERER, CLAUDE
Abstract:
peer-reviewed
A systematic ab initio study within the framework of the local-spin-density approximation including spin-orbit coupling and an orbital-polarization term is performed for the spin and orbital moments and for the x-ray magnetic circular dichroism (XMCD) spectra in hcp Co, in a Pt supported and a free standing Co monolayer, and in a Pt supported and a free standing monatomic Co wire. When including the orbital-polarization term, the orbital moments increase drastically when going to lower dimensionality, and there is an increasing asymmetry between the L2 and L3 XMCD signals. It is shown that spin and orbital moments can be obtained with good accuracy from the XMCD spectra via the sum rules. The {Tz} term of the spin sum rule is surprisingly small for the wires, and the reason for this is discussed.
http://hdl.handle.net/2262/31417
Marked
Mark
Ab initio electron theory for magnetism in Fe: Pressure dependence of spin-wave energies, exchange parameters, and Curie temperature
(2003)
EDERER, CLAUDE
Ab initio electron theory for magnetism in Fe: Pressure dependence of spin-wave energies, exchange parameters, and Curie temperature
(2003)
EDERER, CLAUDE
Abstract:
peer-reviewed
The spin-wave spectra for bcc Fe at various lattice constants are calculated by the ab initio density functional electron theory for various lattice constants, and the exchange parameters of a Heisenberg model are determined from the data. The spin-wave energies increase with decreasing lattice constant, and the behavior of the exchange parameters can by no means be described by the Bethe-Slater curve for an effective nearest-neighbor Heisenberg model. From the spin-wave frequencies the pressure dependence of the Curie temperature is derived in mean-field and random-phase approximation, yielding ∂TcMF/∂p≈1.6 K/kbar and ∂TcRPA/∂p ≈1.8 K/kbar, in contrast to the experimental result ∂Tc/∂p≈0. Possible reasons for this discrepancy are discussed.
http://hdl.handle.net/2262/31416
Marked
Mark
Magnetism in systems with various dimensionalities: A comparison between Fe and Co
(2003)
EDERER, CLAUDE
Magnetism in systems with various dimensionalities: A comparison between Fe and Co
(2003)
EDERER, CLAUDE
Abstract:
peer-reviewed
A systematic ab initio study is performed for the spin and orbital moments and for the validity of the sum rules for x-ray magnetic circular dichroism for Fe systems with various dimensionalities (bulk, Pt-supported monolayers and monatomic wires, free-standing monolayers and monatomic wires). Qualitatively, the results are similar to those for the respective Co systems, with the main difference that for the monatomic Fe wires, the 〈Tz〉 term in the spin sum rule is much larger than that for the Co wires. The spin and orbital moments induced in the Pt substrate are also discussed.
http://hdl.handle.net/2262/31415
Marked
Mark
Anisotropy of orbital moments and magnetic dipole term Tz in CrO2. An ab initio study
(2004)
EDERER, CLAUDE
Anisotropy of orbital moments and magnetic dipole term Tz in CrO2. An ab initio study
(2004)
EDERER, CLAUDE
Abstract:
peer-reviewed
A systematic study is performed by the ab initio density-functional theory of the anisotropy of the orbital moments (lz) and the magnetic dipole term (Tz) in bulk CrO2. Two different band-structure techniques are used (full potential linearized-augmented-plane-wave method and linear-muffin-tin-orbital method in the atomic-sphere approximation), and the electronic correlations are treated by the local-spin-density approximation (LSDA), the LSDA + orbital polarization method, and the LSDA + U method. The calculated anisotropies of (lz) and (Tz) are very large compared to Fe, Ni, and Co but still a factor of 5 and 2 smaller than the anisotropies obtained from a recently suggested analysis of the x-ray magnetic circular dichroism spectra for a thick layer of CrO2.
http://hdl.handle.net/2262/31414
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