bluuebunny/crossref_metadata_2025 huggingface dataset:
Diffuse reflectivity measurements of polyimide during argon fluoride excimer laser ablation
Authors: Ediger M. N. , Pettit G. H. , Sauerbrey R.
Published: July 2002

The diffuse reflectivity of polyimide during pulsed 193-nm excimer laser irradiation has been compared to the specular reflectivity of the material over the fluence range 0.6–190 mJ/cm2. Diffuse reflectivity is observed to decrease with increasing fluence by a factor of ∼3 over the intensity range, and remains a roughly constant fraction (0.3–0.4) of total target reflectivity. These results indicate that appreciable scattering of reflected laser radiation out of the specular direction does not occur under ablation conditions, and therefore cannot account for the specularly reflected pulse truncation observed previously.
Magnetic circular dichroism in x-ray absorption for well characterized Fe/Pt multilayers
Authors: Waddill G. D. , Tobin J. G. , Jankowski A. F.
Published: July 2002

The subtle structure-property relationship in magnetic multilayers is addressed by an array of techniques. Fe/Pt multilayers prepared by magnetron sputter deposition are characterized using x-ray diffraction, Auger depth profiling, electron microscopy, magnetometry, and x-ray magnetic circular dichroism. We concentrate on the (Fe9/Pt9)92 which is [111] textured in the growth direction, but randomly oriented in-plane, with average grain size of 30 nm. This sample is magnetized in-plane, as determined by magnetometry as well as magnetic circular dichroism (MCD). The potential of MCD to follow the magnetic axis orientation in these samples as well as provide an element specific determination of the magnetic moment will be discussed.
Growth temperature and annealing effects on deep traps of In0.52Al0.48As grown by molecular beam epitaxy
Authors: Oh Won-Ung , Oh Jae-Eung , Ryoo Seong-Ryong , Paek Su-Hyun , Chung Chun-Ki , Kang Tae-Won
Published: July 2002

The effects of substrate temperature on the deep trap property of In0.52Al0.48As layers grown by molecular beam epitaxy lattice matched on InP substrates have been investigated. Deep level transient spectroscopy measurements have been used to characterize the InAlAs layers and analyze the effects of growth kinetics on the deep traps in the epitaxial layers. Two new deep traps have been found in the samples grown at relatively low growth temperatures, which do not show in the samples grown above 450 °C. The activation energies of these traps are obtained as ΔET=0.45±0.03 and 0.62±0.02 eV for EI1 and EI2, respectively. A measurable decrease in the densities of EI1 and EI2 was detected following the heat treatment at temperatures above 500 °C.
Critical currents of ceramic superconductors induced by pulsed magnetic fields
Authors: Yasuoka H. , Mazaki H. , Tochihara S. , Kakihana M. , Yoshimura M.
Published: July 2002

We attempted to measure the critical current density Jc of ceramic superconductors with pulsed magnetic fields. Using a pair of search coils coaxially mounted just below a ring sample, we measured induced signals that were considered to be proportional to the time derivative of the sample magnetization. Four samples were used: YBa2Cu3Oy, YBa2Cu4Oy, Bi2Sr2CaCu2Oy, and Bi1.6Pb0.4Sr2Ca2Cu3Oy. These were all synthesized by the polymerized complex method. It was found that the induced signals from the paired search coils are typically of two types. One shows a monotonic decrease to zero as a function of time. The other has a minimum at a certain time and then approaches zero. The observed signals were simulated in the framework of the Bean model and the Kim–Anderson model for critical current density. Theoretical profiles by these two models reproduce the observed results well and suggest that the two types of induced signals are the same in appropriate limits. We discuss how to evaluate Jc from the observed signals.
V3Si multifilamentary superconductor produced by a modified bronze process
Authors: Takeuchi T. , Inoue K.
Published: July 2002

A V3Si multifilamentary superconductor with high overall Jc has successfully been fabricated by a modified bronze process. A double-stacked Cu-8.5 at. %Si/Ta/(Cu-8.5 at. %Si/V) composite with fine V filaments (∼1 μm) was prepared, where the primary bundle is sheathed with a Ta diffusion barrier; thus only Si in the bronze inside of the Ta barrier is available for the subsequent diffusion reaction. The combination of adjusting the total proportion of V to Si in the composite to ∼3 and reducing V filaments to ∼1 μm allows the completion of reaction in short times to decompose the initially formed V5Si3 and produce mostly V3Si without grain growth. The overall Jc(4.2 K) values so far obtained are 1.3×109 A/m2 at 5 T and 1×108 A/m2 at 12.5 T, which are 10 times larger than conventional V3Si wires. These values are comparable to those of Nb3Sn multifilamentary wires produced by the bronze process for ac use.
Transformation of a time-harmonic electromagnetic field in a suddenly created two-resonance Lorentz medium
Authors: Stanić Boz̆idar , Jelenak Zorana , Jelenak Aleksandar
Published: July 2002

The dispersion relation for the six waves generated in the suddenly created two-resonance lossy Lorentz medium has been derived and analyzed by the use of a Brillouin-like dispersion diagram. In comparison to the single-resonance Lorentz medium, another two new waves were excited. The amplitude, power transmission, and reflection coefficients for these new modes have been derived and analyzed for several values of particle density. It was shown that less than 4% of the pump wave power is transformed into the reflected modes.
Proton non-Rutherford backscattering analysis of aluminized Mylar at different incident energies
Authors: Bhuinya C. R. , Padhi H. C.
Published: July 2002

In the present work, the carbon-to-oxygen ratio of Mylar (in the form of aluminized Mylar) and the aluminum-Mylar interface character have been studied using a proton elastic backscattering method at five different incident energies ranging from 1.67 to 3.74 MeV. Using the available proton elastic scattering cross-section data of C and O, a proton backscattered spectrum of Mylar for a proton energy of 1.67 MeV was simulated. Best fitting of the simulated spectrum was achieved for a bulk carbon-to-oxygen ratio of 2.39±0.06 and a linearly varying ratio between 1.63 and 2.39 at the aluminum-Mylar interface region. The richness of oxygen at the interface region is attributed to stronger binding of aluminum with oxygen. The carbon-to-oxygen ratio of Mylar was also estimated from the C- and O-edge heights of proton backscattered spectra taken at 2.07 and 2.37 MeV, which gave a value of 2.57±0.12. Making use of the average value of the measured carbon-to-oxygen ratio from low-energy (≤2.5 MeV) data, the elastic cross-section ratios of C to O were determined for all proton energies. From the measured cross-section ratios and using the previously published oxygen cross-section data, the cross sections for carbon are estimated, which show deviation from reported data at higher energy.
Volume-diffraction analysis using bound-state perturbation theory
Authors: Friedman L. J.
Published: July 2002

One-dimensional volume diffraction may be analyzed in a manner similar to the problem of an electron subject to a periodic potential. The quantity analogous to the diffraction Nath parameter ρ is then the ratio ΔW0/V, where V is the strength of the electron potential and ΔW0 the spacing between the free-electron-energy eigenstates. The crossover between the Bragg and Raman–Nath diffraction regimes is marked by a violation of the (1/ρ)≪1 condition, and corresponds to the requirement that (V/ΔW0)≪1 in order to preclude electron eigenstate mixing. Bound-state perturbation theory as developed for use in quantum mechanics may be applied to the diffraction problem to obtain a (1/ρ) expansion for the intensities of diffracted grating orders.
Modelization of nitrogen diffusion induced by the formation of a surface carbonitride layer
Authors: Millard-Pinard N. , Moncoffre N. , Jaffrezic H. , Marest G.
Published: July 2002

Nitrogen implantations were performed into iron at a fluence of 1017 15N cm−2, an energy of 50 keV, a temperature of 150 °C, two vacuum pressures (2×10−3 and 6×10−5 Pa), and five current densities (6, 18, 24, 40, 60 μA cm−2). Experimental profiles display a preferential nitrogen migration towards the surface. In order to interpret the variation of the depth distribution shapes versus the pressure and the current density, a calculation model is elaborated. This model takes into account the presence of the contamination carbon at the sample surface. A comparison between experimental and simulated profiles is presented. An interpretation of the carbon role is proposed. The different hypothesis on the nitrogen migration mechanisms are reviewed.
Determination of viscoelastic coefficients from the optical transmission of a planar liquid crystal cell with low-frequency modulated voltage
Authors: Schmiedel H. , Stannarius R. , Grigutsch M. , Hirning R. , Stelzer J. , Trebin H.-R.
Published: July 2002

The viscoelastic response of the nematic director field to low-frequency modulations of the driving voltage was studied by means of optical transmission measurements. An external ac voltage above the critical field Uc was weakly amplitude modulated with frequencies in the range of 1 to 100 Hz. The viscosity coefficients α1, γ1, γ2, and ηb influence the time dependence of the director field. They were determined by fitting the phase and amplitude of theoretically calculated optical transmission curves to measured data.
Excimer laser processing for surface improvement of tin oxide thin films
Authors: Galindo H. , Vincent A. B. , Sánchez-R. J. C. , Laude L. D.
Published: July 2002

Results are presented on the processing of tin oxide films using an excimer laser. Commercially available films of SnO2 deposited on glass substrates were treated using a KrF excimer laser (248 nm). The results of these treatments were characterized by normal optical absorption, transmittance, and reflectance measurements in the VIS-NIR region along with x-ray diffraction and electrical conductivity measurements and a scanning electron microscopy (SEM) study. These were compared to the nontreated samples. The results inferred a relation between the amount of photoablated material with laser fluence, up to saturation. The optical spectroscopy results and the SEM study evidence modifications of the treated surface. On the other hand, the x-ray analysis indicates that the crystal structure is not altered by the laser treatment but the film morphology is. Furthermore, an increase of 15% and better in the normalized integrated optical transmittance in the visible region was obtained with this treatment, without changing the film electrical conductivity.
Control of the crystal orientation of zinc-blende MnTe epitaxial films grown on GaAs
Authors: Akinaga H. , Ando K. , Abe T. , Yoshida S.
Published: July 2002

Epitaxial films of the zinc-blende MnTe have been successfully grown directly on (001) GaAs substrates by molecular beam epitaxy. The structure and the stoichiometry of the GaAs substrate surfaces are found to be important in determining the orientation and the twin formation of the MnTe films. When the preheating treatment of the substrate was done at 580 °C and reflection high energy electron diffraction (RHEED) from the GaAs surface showed (3×1) reconstruction pattern during the treatment, the twin-free (111) oriented MnTe was obtained on it. When the preheating treatment was done at 560 °C, the weak streaked RHEED pattern with a halo was observed from the GaAs surface and the (001) oriented MnTe was obtained on it.
Magnetic excitations of sandwich structures
Authors: Hong Qiang
Published: July 2002

The spin wave excitations of sandwich structures formed by two ferromagnetic materials are studied both in the long wavelength (magnetostatic wave) and short wavelength (exchange-type magnon) regime. The dispersion relation for magnetostatic modes is obtained by making use of Maxwell’s equations. Interface modes which are localized in the interface region are examined in an external magnetic field as well as in a free space. The energy of exchange-type magnons is obtained by employing the Green’s function technique. The analysis of interface states in this regime is found to be complicated. Numerical results in two specific cases are presented, which can serve as examples to the discussions of arbitrary sandwich structures.
Feasibility of phonon-assisted electronic devices
Authors: Li S. , Khurgin J. B.
Published: July 2002

Tunneling rate due to electron–phonon interaction is calculated for a double quantum well structure. A significant negative differential conductance is expected because of the resonant behavior of longitudinal optical phonons. The results show that a negative differential conductance of up to 0.3 Ω−1 can be achieved. Based on this result, a new class of the devices that use the phonon-assisted tunneling as an operating principle is proposed.
Ambipolar diffusion and carrier lifetime measurements in all-binary (InAs)2(GaAs)5 strained quantum wells grown on GaAs
Authors: Huang X. R. , McCallum D. S. , Dawson Martin D. , Smirl Arthur L. , Boggess Thomas F. , Hasenberg T. C. , Tober R. L.
Published: July 2002

The in-well ambipolar diffusion coefficients and carrier lifetimes in ordered all-binary quantum wells, composed of (InAs)2(GaAs)5 short-period strained-layer superlattices (SPSLSs) grown on [001]-oriented GaAs substrates, are measured using picosecond optically induced transient grating and pump-probe techniques. Quantum wells containing SPSLSs may be expected to exhibit higher in-plane hole mobilities compared to InGaAs ternary alloy quantum wells because of a larger average indium content and reduced disorder scattering in the SPSLS structures. The results obtained in the SPSLSs are compared to those obtained in InGaAs alloy quantum wells of comparable well width and confinement energies. This indicates that, for a given SPSLS and its equivalent alloy, the ambipolar diffusion coefficients are comparable while the carrier lifetimes in the SPSLSs are longer. The longer carrier lifetimes in the SPSLSs suggest that these binary structures possess fewer nonradiative recombination centers than the alloys. The absence of a dramatic improvement in the transport properties, however, may indicate that imperfect interfaces in the SPSLSs may be offsetting their possible advantages, in spite of the fact that optical measurements in the SPSLSs indicate high quality and x-ray and transmission electron microscopic measurements demonstrate the binary nature of these structures.
Flux penetration of melt-processed YBa2Cu3O7−δ  : Direct observation of anisotropy
Authors: Schuster Th. , Koblischka M. R. , Kuhn H. , Glücker M. , Ludescher B. , Kronmüller H.
Published: July 2002

Flux distributions observed on melt-processed YBa2Cu3O7−δ samples are presented with the external magnetic field oriented parallel and perpendicular to the c axis using the high-resolution Faraday effect technique. In this way, flux penetration along and across the Cu–O planes is observed directly. The measured flux density profiles allow a determination of the different critical current densities and show the influence of intrinsic pinning.
Electron-interface LO phonon scattering rates in quantum wells in a quantizing magnetic field
Authors: Bhat J. S. , Mulimani B. G. , Kubakaddi S. S.
Published: July 2002

Electron scattering rates due to interaction with interface phonon modes in semiconductor quantum-well structures in a quantizing magnetic field are studied. Numerical results for scattering rates in GaAs/GaAlAs quantum wells show oscillatory behavior with magnetic field. The scattering rate due to interface phonon modes is found to decrease with the increase of well width in contrast to that seen with bulk confined phonons.
ac conductivity in Bi4Sr3Ca3CuyOx (y=0–5) and Bi4Sr3Ca3−zLizCu4Ox (z=0.1–1.0) semiconducting oxide glasses
Authors: Mollah S. , Som K. K. , Bose K. , Chaudhuri B. K.
Published: July 2002

Semiconducting oxides such as Bi4Sr3Ca3CuyOx (y=0–5) and Bi4Sr3Ca3−zLizCu4Ox (z=0.1, 0.5, and 1.0), many of which become superconductors in their glass-ceramic phases, have been studied over the wide temperature (77–450 K) and frequency (100–104 Hz) ranges. The universal power-law behavior [exponent s≤1 in σ(ω)=σ0+Aωs] is found to be valid for most of these glasses. Little deviation from this limiting behavior is also observed for some glass compositions in the low-temperature and high-frequency regions. The correlated barrier hopping model based on the pair approximation is found to be more appropriate for explaining the frequency and temperature-dependent ac conductivity data. The random walk type of model, on the other hand, is found to be valid for a limited range of temperature.
Short-cavity dye lasers with frozen ethanol solvents
Authors: Ishikawa Kazuhito , Muto Shinzo , Matsuzawa Hidenori
Published: July 2002

Short-cavity (10–500 μm) rhodamine 6G dye lasers were operated with pumping of a N2 laser (100 kW) when a solution of dye in ethanol was frozen with liquid nitrogen (77 K) into ice. Laser wavelengths shifted to shorter ones. Axial modes of the laser output were determined, as other lasers, with the optical cavity lengths. Relaxation oscillations and short pulses less than 1 ns were also obtained.
Magnetoelastic coefficients at tetragonal surfaces (abstract)
Authors: Song Oh Sung , Ballentine C. A. , O’Handley R. C.
Published: July 2002

Surfaces and thin films are generally highly strained relative to their bulk structures. The magnetoelastic (ME) coefficients coupling these strains to magnetic properties must be considered in lower symmetry than the bulk values. We outline a method of analyzing the ME problem at a surface in tetragonal symmetry (assuming the bulk is cubic). Five ME coefficients are required. Two methods have been developed to extract these surface ME coefficients from the M-H behavior of films subjected to different strains. One method fits the M-H behavior with that predicted by an equation of motion derived from a free energy which includes ME, magnetostatic, crystal field, and Zeeman terms. This method is useful in certain simple cases where canonical linear M-H behavior is observed. In more complicated cases, it can be shown that the ME coefficients are related to the area A(ΔH,Δe) between two anhysteretic M-H curves at two different strain levels and between arbitrary field limits (see Fig.). The choice of field limits allows the M-H results near the origin, H≤Hc, and near saturation H≥HK to be omitted from the analysis. The validity of the method is illustrated with measurements on thin cobalt, nickel, and permalloy films.
Details of the magnetic phase diagram of holmium from neutron diffraction in b-axis fields
Authors: Tindall D. A. , Adams C. P. , Steinitz M. O. , Holden T. M.
Published: July 2002

We report measurements of the intensity and position of the satellites arising from the helimagnetic structure of Ho in 1.4 and 3 T b-axis magnetic fields. There are a number of lock-in effects at commensurate values of the spiral wave vector τ; here we report on those in the temperature range 115–132 K. We confirm our previous observation of the lock-in at τ=5/18 between 125 K and the Néel temperature (132 K). We have discovered a two-phase region in which the 5/18 phase coexists with an incommensurate phase. In this two-phase region, the proportion of the 5/18 phase increases with temperature from onset at 119 K to full lock-in at 125 K. The existence of this two-phase region may have significant influence on interpretation of the magnetic phase diagram.
Interactions with the Dy3+ sublattice observed in high resistance ultrathin DyBa2Cu3O7−x films (abstract)
Authors: Beauchamp K. M. , Spalding G. C. , Huber W. H. , Goldman A. M.
Published: July 2002

A low-temperature anomalous magnetoresistance has previously been reported in disordered ultrathin DyBa2Cu3O7−x films. This unusual behavior was associated with interactions between the charge carriers and a magnetic lattice. Here, new results are presented showing a peak in the temperature dependence of the resistivity near the Dy3+ Néel temperature. A calculation is included which shows that the peak in R(T) is consistent with spin disorder scattering from the Dy3+ ions as they order antiferromagnetically. These observations lend support to the suggestion, based on neutron scattering studies, that the Dy3+ interactions are not strictly dipolar and that the electrons may contribute to the magnetic ordering of the Dy3+ sublattice.
Structure and magnetic properties of interstitial compounds of the series Dy2Fe17−xAlxZy (Z=N or H)
Authors: Liu J. P. , Zeng D. C. , Tang N. , Winkelman A. J. M. , de Boer F. R. , Buschow K. H. J.
Published: July 2002

The interstitial nitrides Dy2Fe17−xAlxNy with 0≤x≤6 and 0≤y≤2.7 and hydrides Dy2Fe17−xAlxHy with 0≤x≤4 and 0≤y≤4.2 have been synthesized by means of melting and by means of the gas-phase-interaction method. The effect of the interstitial atoms on the structure and the lattice parameters has been investigated. For the nitrogenated compounds, it is found that nitrogen can be introduced until a maximum volume expansion is reached. The magnetic properties of the interstitial compounds, including high-field magnetization at 4.2 K and Curie temperatures, have been measured. The Dy-Fe exchange interaction is found to decrease upon introduction of the interstitial atoms.
Evaluation of the CoCrTaPt alloy for longitudinal magnetic recording
Authors: Cheng Yuanda , Sedighi Mojtaba , Lam Irene , Gardner Richard A. , Yang ZhiJun , Scheinfein Michael R.
Published: July 2002

A quaternary alloy of CoCrTaPt with a composition of 80-10-4-6 (in at. %) was evaluated for its magnetic and recording properties. Samples of C/CoCrTaPt/Cr recording media were prepared by dc magnetron sputtering on circumferentially textured Al/NiP substrates. The effects on static magnetic properties and recording performance were studied for different substrate preheating times, Cr underlayer thicknesses, and Co-alloy layer thicknesses. It was found that both the coercivity Hc and remanent magnetization Mr increased with substrate preheating time. Hc also increased with Cr underlayer thickness, as expected. The Mrδ value depended linearly on the Co-alloy layer thickness. In addition, we found that the coercivity increased dramatically as the magnetic layer thickness decreased from ∼800 to ∼200 Å. For a sample with a 235-Å CoCrTaPt magnetic layer and ∼1000-Å Cr underlayer, the coercivity was found to be ≳2700 Oe. Parametric evaluation showed that CoCrTaPt samples have performance similar to samples of CoCrTa and, because of the Pt addition, the CoCrTaPt alloy offers significantly higher attainable coercivities than the CoCrTa alloy. Therefore, the CoCrTaPt alloy proves to be a good candidate for use in high density recording media which require coercivity of higher than 2000 Oe.
Thin film disks for high density recording with textured underlayers (abstract)
Authors: Mirzamaani Mohammad , Russak Michael A. , Jahnes Christopher V.
Published: July 2002

As higher recording densities are required on thin film disks the balance between magnetic and tribological performance becomes a delicate one.1 Optimized magnetics dictate very smooth magnetic films with small, well isolated (i.e., magnetically decoupled) grains whereas acceptable mechanical performance requires sufficient surface roughness to insure low stiction.2 This balance may not be readily obtained by mechanical texturing but can be accomplished by nonmechanical texturing.3 In this work AFM and conventional magnetic measurements have been used to characterize a variety of thin film disk surfaces and conditions found which permit the simultaneous attainment of excellent tribological and magnetic performance. In the case of nonmechanical texture a first layer of In was sputtered to provide the texture and then Cr and CoPtCr layers (single as well as laminated) deposited to provide the magnetics. The thickness and crystallographic orientation of the Cr interlayer have a marked effect on the macromagnetic properties of the magnetic layer and in turn disk recording performance. The optimized process produces disks with signal to noise (S/N) at 4000 fr/mm of ≂35 db, acceptable soft error rates for high density recording and CSS stiction values of <3 g. These parameters have been obtained on metallic and nonmetallic substrates.
Multinuclear nonmagnetic resonance studies of perfluoro poly ether lubricants (abstract)
Authors: Viswanathan K. V.
Published: July 2002

Commercial perfluoro poly ethers (PFPE) are available with different backbone structures and end groups and are being used as lubricants for particulate/thin film disks. Very little published information is available other than F-19 spectra for these polymers.1 In this abstract a systematic study was undertaken to study the structure of these PFPE polymers using multinuclear magnetic resonance (NMR). Backbone structural differences of Z. Fomblin, Y. Fomblin, Krytox, and Demnums could be clearly seen from F-19 NMR studies. Number average molecular weight of PFPE polymers was also determined from F-19 NMR and found to be between 2000 and 10 000. C-13 NMR was used to differentiate the end group from backbone structure. Associated proton test (APT) was used to distinguish the primary, secondary, and tertiary carbons. The proton connectivity of end group (e.g., Fomblin AM2001) was studied by two dimensional (2D) proton correlation spectroscopy (COSY). The connectivity of a hydrogen containing end group to the backbone was studied by heteronuclear shift correlation spectroscopy (HCORR). Even though these PFPE polymers have low vapor pressure, a significant amount of lube loss is found over a period of time under HDA (head disk assembly) conditions.

The temperature could increase from 25 to 60 °C. Additionally under certain asperity/head interaction conditions the temperature variation could be as high as 300 °C to 500 °C. It is also reported that functionalized polymeric materials (e.g., Krytox 143 AC) can be bonded to magnetic disk surface by applying heat. The effect of temperature on lubricant volatility/stability was studied for AM 2001. The molecular weight of heat treated AM 2001 (90 min at 130 °C) changed from 2100 to 4500. This could be due to preferential volatilization of low molecular weight fragments. NMR analysis showed significant conversion of piperonyl end group to -CH2OH. Thus AM 2001 is not thermally stable. Precipitation was also found to occur when AM 2001 is stored over a period of time. The precipitate is found to be a derivative of 9,10 dihydroanthracene. Unlike other PFPE polymers AM 2001 is found to be sparingly soluble in dichloromethane (contrary to the claim by the manufacturer that it is soluble only in fluorinated solvents) and hence more polar than Fomblin Z-Dol.
Femtosecond Faraday rotation in spin-engineered heterostructures (invited)
Authors: Baumberg J. J. , Awschalom D. D. , Samarth N.
Published: July 2002

We have developed a new femtosecond-resolved optical technique with which one may examine magnetic spin dynamics in near-atomic scale structures by employing a highly sensitive, ultrafast measurement of the Faraday rotation. We apply this spectroscopy to novel band-gap-engineered II–VI diluted magnetic semiconductor heterostructures of two types to demonstrate the ability to simultaneously monitor electronic and magnetic interactions in a quantum geometry. The experiments show that these dynamics evolve on widely different time scales, and reveal the onset and decay of magnetization due to carrier spin scattering with femtosecond temporal resolution (300 fs). Although photoexcited carriers recombine within several hundred picoseconds, they leave behind a magnetic ‘‘footprint’’ that persists considerably longer and relaxes through an entirely different spin-lattice mechanism. Time-resolved magnetic measurements reveal the unusual dynamical properties of low-dimensional systems incorporating magnetic spins as compared to traditional semiconductor heterostructures.
Orientation dependence of dipole gaps in the magnetostatic wave spectrum of Bi-substituted iron garnets
Authors: Chernakova Ana K. , Cash Andrew , Peruyero Jose , Stancil Daniel D.
Published: July 2002

The orientation dependence of the dipole-exchange gaps in the magnetostatic forward volume wave spectrum of Bi0.8Lu2.2Fe5O12 thin films grown from a Bi-oxide flux has been studied. The spectrum is relatively insensitive to tilting the bias field toward the direction of propagation, but significant improvements are observed when the field is tilted toward a direction perpendicular to the propagation direction. Improvements include smoothing of the dipole-exchange gaps and a reduction of the insertion loss. These improvements are nonreciprocal; either reversing the direction of propagation or tilting the bias field in the opposite direction degrades the forward volume wave spectrum.
Temperature dependence of the magnetic excitations in ordered and disordered Fe72Pt28
Authors: Rosov N. , Lynn J. W. , Kästner J. , Wassermann E. F. , Chattopadhyay T. , Bach H.
Published: July 2002

We have performed inelastic neutron scattering measurements of the long-wavelength spin wave excitations of both ordered and disordered Fe72Pt28 single crystals below their critical temperatures, TC=510 and 375 K, respectively. The spin waves followed the expected E=Dq2 dependence, and the temperature-dependent spin stiffness D decreased as (T/TC)5/2, as expected for an isotropic ferromagnet. The extrapolated zero-temperature spin stiffness was D=98(4) meV Å2 and 107(1) meV Å2 for the disordered and ordered alloy, respectively. These values are significantly higher than the zero-temperature stiffness as determined by magnetization measurements.
New perspective on the Green’s function dipole-exchange spin wave theory for thin films (abstract)
Authors: Chen Ming , Patton Carl E.
Published: July 2002

In contrast with the scalar potential theory for dipole-exchange spin waves in thin films, the Green’s function (GF) approach in combination with perturbation methods (PM) gives simple closed form dispersion equations, analytical expressions for the dipole-exchange frequency gaps, and good intuitive physics. However, in the case of in-plane magnetized films and perpendicular propagation, the GF/PM results present two major problems. (1) The multibranch dipole-exchange solutions bear little resemblance to the single dispersion curve for the well-established Damon–Eshbach magnetostatic surface modes and the nonreciprocal mode profiles predicted by the simple theory and observed experimentally. (2) The lowest order exchange branch has a curvature that is much greater than expected from either simple spin wave considerations or scalar potential theory. To resolve these problems, a simple method has been developed to solve the coupled mode equations obtained from the GF formalism without recourse to perturbation methods. The coupled mode dispersion relations obtained with this method are valid for an arbitrary field configuration and propagation direction, and are not subject to the restrictions imposed by perturbation methods. The solutions agree well with the surface mode theory for in-plane magnetized films. Good agreement is also found with the Brillouin light scattering data for iron thin films.
Composition dependence of the elastic moduli of mixed lithium–cadmium ferrites
Authors: Ravinder D.
Published: July 2002

Mixed Li–Cd ferrites having the chemical formula Li0.5−x/2CdxFe2.5−x/2O4 (where x=0.2, 0.4, 0.6, 0.8, and 1.0) are prepared by the well-known double sintering ceramic technique. From x-ray diffraction studies lattice parameters are calculated and their variation with composition is explained on the basis of the tetrahedral ionic radius. The bulk density of the specimens are determined by the hydrostatic method and the percentage porosity was calculated. The ultrasonic compressional (V1) and shear (Vs) velocities of mixed Li–Cd ferrites are measured at room temperature by the ultrasonic pulse transmission technique. The values of Young’s modulus, the rigidity modulus, Debye temperatures, and average sound velocities are calculated from the experimental values of V1 and Vs. The elastic moduli have been corrected to zero porosity. The variation of elastic moduli with composition is interpreted in terms of binding forces between the atoms.
Modeling and analysis of electric and magnetic coupled problems under nonlinear conditions
Authors: Geri A. , La Rosa M. , Veca G. M.
Published: July 2002

In this paper the authors define a numerical method to solve nonlinear combined magnetic and electric problems using reliable general purpose calculation codes. This method has been applied to analyze the behavior of a busbar system built with a saturable steel enclosure and connected with an arbitrary external circuit. The enclosure magnetic characteristics have been determined by means of experimental tests executed by the authors. To analyze this electrical system, the skin and proximity effects in the massive conductors, as well as the load conditions of the external circuit, must be taken into account. The present formulation permits us to solve the previously described coupled problems by means of an iterative procedure. Using this technique, it is possible to solve separately the field and the circuit equations by means of specific codes. In particular, an iterative procedure it is pointed out so defined: for each estimated load condition, a finite element code has been used to evaluate the relating busbar cross parameters, taking into account the nonlinear behavior of the steel enclosure, while, for each assigned set of the busbar cross parameters, a circuit simulator has been used to determine the relating load conditions of the busbar. In addition, a specialized post-processor has been developed to manage the data flow between the calculation codes. The originality of this work is linked to the use of general purpose commercial software to solve the nonlinear magnetic and electric coupled problems.
Transformation methods in computational electromagnetism
Authors: Nicolet A. , Remacle J.-F. , Meys B. , Genon A. , Legros W.
Published: July 2002

The representation of electromagnetic quantities by differential forms allows the use of nonorthogonal coordinate systems. A judicious choice of coordinate system facilitates the finite element modeling of infinite or very thin domains.
Measurements of intrinsic magnetic properties of materials from surface inspection (abstract)
Authors: Chen Z. J. , Jiles D. C.
Published: July 2002

Surface inspection of magnetic properties with a sensor is a useful and practical technique because it gives a rapid and noninvasive measurement and requires minimum material preparation. However, this technique is handicapped by the practical problems of calculating inherent magnetic properties of the material from such a measurement. A transfer function based on the first approximation was developed previously and it worked well when the dimension of the sample was comparable with the inspection head. However, the nonuniform distribution of the magnetic field is an inherent problem and gets more serious when the vertical and lateral dimensions of the test material become comparable with the pole length of inspection head. Therefore, it invalidates the application of first approximation. A more general and practical transfer function is derived in this paper, including the geometry effects of inspection head and test material. This transfer function is based on the surface magnetic charge model and fits well in the situation when the test material has a large dimension. Test results on specimens by direct measurement and measurement from surface inspection will be presented.
Dynamics of the one-dimensional spin-1 Heisenberg antiferromagnet with exchange and single-site anisotropy
Authors: Zhang Shu , Yu Yongmin , Viswanath V. S. , Stolze Joachim , Müller Gerhard
Published: July 2002

The T=0 dynamical properties of the one-dimensional s=1 XXZ model with an additional single-site term are investigated by means of the recursion method. The dynamic structure factors Sμμ(q=π,ω), μ=x,z bear the characteristic signatures of several different phase transitions. In the s=1 Heisenberg antiferromagnet, the intrinsic linewidth (at fixed q) of Sμμ(q,ω) is larger for small q than for q near π, in contrast to well-established results for the corresponding s=1/2 model.
Magnetic properties of two copper (II)-halide layered perovskites
Authors: Sivron N. , Grigereit T. E. , Drumheller John E. , Emerson K. , Willett R. D.
Published: July 2002

The magnetic properties of the powdered layered structures of 3-ammoniumpyridinium tetrabromocuprate(II) and 3-ammoniumpyridinium tetrachlorocuprate(II) have been studied from 4.2 to 150 K. The data were interpreted using both 1D and 2D series expansions for the Heisenberg model, combined with the appropriate mean-field corrections, yielding interlayer exchanges with J2h/k values of (−11±2) K for the chloride salt and (−52±7) K for the bromide salt. The intralayer exchange (J1h/k) values were found to be (14±2) K and (20±2.5) K for the chloride and bromide salts, respectively. The new results are compared with previous results for the eclipsed layered structure series NH3(CH2)nNH3CuX4, where X=Br or Cl and n=2, 3, 4, or 5. As the halide-halide separation distance decreases the transition from magnetic isolation of the layers to a strong interlayer exchange is observed. The new study allows confirmation of the power dependence on the halide-halide separation.
Depth-dependent magnetic correlation length in terbium (abstract)
Authors: Gehring P. M. , Hirota K. , Majkrzak C. F. , Shirane G.
Published: July 2002

High q-resolution neutron-scattering measurements of the critical scattering in Tb above the magnetic-spiral phase transition temperature Ts exhibit a two-component line shape, as recently documented in Ho.1 This implies the existence of a second length scale, which runs counter to our present understanding of second order phase transitions. By using a narrow beam only 300 μm wide, and then translating the crystal through the beam, we have established that the origin of the second length scale lies within the near surface volume or ‘‘skin’’ of the Tb crystal.2 This is manifested by a large enhancement of the scattering intensity at the c-axis face of the cube-shaped crystal. In this context the meaning of skin is distinct from that of surface since the second component is spread over several hundred micrometers and not just one or two. In addition, we find that not all skins are equivalent. The intensity of the second component scattering is greatest in those faces that are orthogonal to the c-axis face. Such an anisotropy is easily motivated on physical grounds given that the spins in Tb are confined to the basal plane below Ts.
Magnetic induced uniaxial anisotropy in NiFe and NiFeCr films (abstract)
Authors: Torng Terry , Liao Simon H.
Published: July 2002

We report the experimental results of magnetic uniaxial anisotropy induced during sputtering (K1) and annealing (K2) of NiFe and NiFeCr films. The NiFe and NiFeCr films of around 30 nm thickness were deposited using dc magnetron sputtering. Glass and Ta (20 nm) coated glass were used as substrates. The magnetic properties were determined using a B-H looper at 10 Hz. The crystal orientation was determined using x-ray diffraction. The results show that the predictions from a two-component anisotropy model (that is K2/[K1+K2]) for a 3 h 310 °C anneal are in good agreement with experimental data for both NiFe/glass and NiFeCr/glass, but not for NiFe/Ta/glass and NiFeCr/Ta/glass. This can be explained by the orientation effect from x-ray diffraction studies. The NiFe and NiFeCr have higher (111) preferred orientation when deposited on Ta coated glass as compared to samples deposited on glass only. The effects of annealing temperature and Ta underlayer will be discussed. The activation energy for magnetic annealing induced anisotropy will also be discussed.
Critical behavior of the anisotropic Heisenberg model by effective-field renormalization group
Authors: de Sousa J. Ricardo , Fittipaldi I. P.
Published: July 2002

A real-space effective-field renormalization-group method (ERFG) recently derived for computing critical properties of Ising spins is extended to treat the quantum spin-1/2 anisotropic Heisenberg model. The formalism is based on a generalized but approximate Callen–Suzuki spin relation and utilizes a convenient differential operator expansion technique. The method is illustrated in several lattice structures by employing its simplest approximation version in which clusters with one (N′=1) and two (N=2) spins are used. The results are compared with those obtained from the standard mean-field (MFRG) and Migdal–Kadanoff (MKRG) renormalization-group treatments and it is shown that this technique leads to rather accurate results. It is shown that, in contrast with the MFRG and MKRG predictions, the EFRG, besides correctly distinguishing the geometries of different lattice structures, also provides a vanishing critical temperature for all two-dimensional lattices in the isotropic Heisenberg limit. For the simple cubic lattice, the dependence of the transition temperature Tc with the exchange anisotropy parameter Δ [i.e., Tc(Δ)], and the resulting value for the critical thermal crossover exponent φ [i.e., Tc≂Tc(0)+AΔ1/φ ] are in quite good agreement with results available in the literature in which more sophisticated treatments are used.
Phase diagrams of diluted ferromagnetic Ising films in a transverse field
Authors: Cressoni J. C. , Tucker J. W. , Sarmento E. F.
Published: July 2002

An effective-field theory that accounts for the self-spin correlation function is used to study the effect of surface dilution on the critical behavior of a ferromagnetic Ising film in transverse surface and bulk fields. Surface exchange enhancement (or reduction) is considered. It is shown that the surface and bulk fields affect the critical ratio of surface to bulk interactions, at which the surface orders before the bulk, in opposite ways. Although it is found that for the undiluted film this leads to a critical ratio that is practically insensitive to field for common bulk and surface field strengths, this feature is relaxed to a certain degree with surface dilution. The thickness dependence of the critical temperature, and phase diagrams in the fields, interaction strength ratio, and dilution parameter space, are presented.
Optical transitions in (ZnCo)Se and (ZnFe)Se: Role of an effective p–d exchange (invited)
Authors: Mak Chee-leung , Bak J. , Sooryakumar R. , Steiner M. M. , Jonker B. T.
Published: July 2002

In this paper we discuss photoluminescence (PL) and Raman excitations in the diluted magnetic semiconductors Zn1−xCoxSe and Zn1−xFexSe. The PL spectra associated with isolated crystal field d levels are found to rapidly weaken and/or broaden as the transition metal ion concentration approaches and increases beyond a critical value. In addition a Raman continuum, that overlaps with the optical phonon, is observed to develop when the PL features are modified. In the corresponding Zn1−xCoxSe samples the optical phonons acquire a characteristic Fano line shape suggestive of its coupling to the Raman continuum. It is proposed that these changes to the optical transitions arise from the enhancement of an effective p–d hybridization between low-lying d levels and band electrons with increasing x—an effect largely due to the proximity of the Co2+ and Fe2+ crystal field ground levels to the valence band maximum.
Spin wave solitons in an antiferromagnetic film (abstract)
Authors: Boardman A. D. , Nikitov S. A. , Waby N. A.
Published: July 2002

The propagation of nonlinear dipole spin waves in a film consisting of a two sublattice, uniaxial, antiferromagnetic material has been investigated. The system when the external magnetic field is assumed to be parallel to the anisotropy axis of the antiferromagnetic film and is directed parallel, or perpendicular, to the film surface is considered. For the first case, surface and volume waves can propagate in the film, and for the second case, volume waves can propagate in the film for a rather weak external magnetic field when the magnetization of the sublattices are counter parallel to each other and are perpendicular to the film surface. The linear dispersion relations for all three types of waves are analyzed and their group velocity dispersion is calculated. The nonlinear shift of the frequency, due to the finite power of the wave, is also obtained for the three types of dipole waves for the case of a thin antiferromagnetic film, when kd≪1 (k is the wave number and d is the thickness of the film). The nonlinear Schrödinger equation governing the propagation of the nonlinear spin wave in the film is investigated. It is shown that the criterion for the existence of spin wave solitons is fulfilled, for parallel magnetization, which permits the existence of surface waves (one branch) and volume waves (both branches), but the criterion is not fulfilled for perpendicular magnetization. The power threshold for soliton creation is also calculated and estimates are given for the data appropriate to a MnF2 crystal.
Nonlinear surface spin waves (abstract)
Authors: Sukstanskii A. L. , Tarasenko S. V.
Published: July 2002

There has been extensive theoretical and experimental work devoted to investigations of surface spin waves in magnets. However, such excitations have usually been studied in the linear approximation, whereas large-scale dynamic excitations (magnetic solitons, nonlinear spin waves of different types) were discussed in infinite media only. The purpose of this work is to investigate a new type of surface excitation: nonlinear surface spin waves (surface solitons) in semi-infinite magnets. We analyzed the simplest model of a one-sublattice ferromagnet taking into account the nonhomogeneous exchange interaction and the easy-axis magnetic anisotropy. In addition, the boundary conditions on the surface describing uniaxial surface anisotropy were used. Linear surface spin waves in such a model are known to exist when the latter is of the easy-plane type only whereas nonlinear solutions of the equations of motion satisfying the boundary conditions are proved to exist whatever surface anisotropy takes place. These solutions describe nonlinear surface spin waves traveling along the surface. Moreover, for some values of precession frequency and surface anisotropy constant there are two or even three such soliton-like solutions. It should be noted that localization of these excitations near the surface is entirely due to nonlinearity, and the surface solitons under consideration have no linear analogy. We also proved the existence of so-called internal nonlinear spin waves describing soliton-like magnetic excitations localized on the interface between two semi-infinite magnets. In addition, the generalization of all results mentioned above was obtained for more complicated two- and many-sublattice magnets.
Thermal equilibrium noise with 1/f spectrum and temperature-dependent magnetic viscosity in the amorphous alloy DyNi
Authors: Barbara B. , Ratnam A. , Cavalleri A. , Cerdonio M. , Vitale S.
Published: July 2002

We report on the magnetization noise and the imaginary susceptibility for the random anisotropy amorphous alloy DyNi1.32 for frequencies f<10 Hz, temperatures 1.2<T<4.2 K, and excitation field amplitudes <150 μA/m (1.9 μOe). The noise spectra agree, without any adjustable parameter, with the fluctuation dissipation formula and the measured imaginary susceptibility of the material. The spectra show a 1/f shape coming from a frequency-independent imaginary susceptibility that it is found to be temperature-dependent extrapolating to zero at zero temperature. We discuss the connection between this approach and the usual relaxation experiments.