bluuebunny/crossref_metadata_2025 huggingface dataset:
Speckle-free phase plate (diffuser) for far-field applications
Authors: Skupsky S. , Kessler T. J.
Published: July 2002

Phase plates have been used in holography and laser fusion to produce a well-defined far-field intensity envelope that is relatively independent of the input-beam profile. However, superposed on this envelope is unwanted highly modulated speckle from the interference between different phase-plate elements. The speckle can have deleterious effects such as creating intensities in the nonlinear region of a hologram recording medium or by initiating filamentation and hydrodynamic instabilities in laser-fusion applications. A technique has been developed for choosing the phase-plate elements (in combination with polarization rotation) such that these intensity fluctuations approach zero in the limit of plane-wave near-field irradiation. For applications where the near-field phase and amplitude have only small variations, the resulting speckle is also small. Conversely, the observation of far-field intensity fluctuations can be used to estimate the quality of the near-field beam.
Pressure tuning of strain in CdTe/InSb epilayer: A photoluminescence and photomodulated reflectivity study
Authors: Boley Mark S. , Thomas Robert J. , Chandrasekhar Meera , Chandrasekhar H. R. , Ramdas A. K. , Kobayashi M. , Gunshor R. L.
Published: July 2002

The heavy-hole and light-hole excitons of a CdTe epilayer, pseudomorphically grown on an InSb epilayer by molecular beam epitaxy, are studied with a diamond anvil cell as a function of applied hydrostatic pressure via photoluminescence (PL) and photomodulated reflectivity (PR) spectroscopies. They are compared with the excitonic features in the simultaneously measured PL spectra of a sample of bulk CdTe. Under applied pressure, the lattice mismatch-induced splitting between the light-hole and heavy-hole related transitions increases in a continuous and reversible manner because of the additional pressure-induced compression due to the difference in the compressibilities of CdTe and InSb. The unusually large strain sustained by the CdTe epilayer under pressure is discussed in the light of various models. The PR signal vanishes after the InSb epilayer goes through a structural phase transition at approximately 20 kbar, while the PL signal persists until it is irreversibly quenched by the CdTe epilayer undergoing a structural phase transition at approximately 30 kbar. For pressures between 20 and 30 kbar, the behavior of the CdTe epilayer is similar to that of the bulk sample; the strain appears to have been relaxed due to the structural phase transition which has taken place in InSb. Values of the first- and second-order pressure coefficients for bulk CdTe and for the CdTe epilayer as well as values of the hydrostatic and shear deformation potentials are obtained at 14 and 80 K and compared with previously quoted values.
Raman studies of the growth mechanism of ZnSe/GaAs(001) heterostructures
Authors: Bauer S. , Berger H. , Link P. , Gebhardt W.
Published: July 2002

Monocrystalline ZnSe/GaAs(001) heterostructures have been grown by atomic layer epitaxy with typical thicknesses up to 200 nm. The 1LO Raman spectrum of ZnSe and GaAs, respectively, was recorded in situ during growth interruptions. The spectra were analyzed with respect to Raman intensity, phonon frequency, and bandwidth. Two different growth modes are distinguishable: (i) The biaxial compressive strain in the epilayers is increased in the thickness range up to 40 nm due to lattice mismatch of the heterosystem. This can be interpreted as a transition from an initial 3D island to the 2D layer by layer growth. (ii) When larger thicknesses (40–170 nm) are reached the strain is constant and the 1LO Raman intensity from the epilayer and from the substrate oscillates significantly with thickness. These periodic oscillations are quantitatively described in the framework of interference-enhanced Raman scattering and verify the 2D growth mode in this thickness range. The crystalline quality and the final thickness of the samples were determined by transmission electron microscopy.
Specific primary ionization induced by minimum ionizing electrons in CH4, C2H6, C3H8, i-C4H10, argon, dimethylether, triethylamine, and tetrakis(dimethylamino)ethylene
Authors: Malamud G. , Breskin A. , Chechik R. , Pansky A.
Published: July 2002

Specific primary ionization induced by minimum ionizing electrons has been measured in several gases and vapors. Charges deposited by β-electrons in a low-pressure gas were collected, amplified by a multistep gaseous electron multiplier, and counted. The high counting efficiency of the multiplier provided results of systematically higher values as compared to existing data. The respective values of the specific primary ionization in CH4, C2H6, C3H8, i-C4H10, argon, dimethylether, triethylamine, and tetrakis(dimethylamino)ethylene are: 0.034, 0.065, 0.095, 0.12, 0.03, 0.082, 0.195, and 0.370 clusters/cm Torr. The experimental method is presented and the results and their accuracy are discussed.
Resonant optical nonlinearity of fullerenes in free-standing polymethyl methacrylate films
Authors: Ji W. , Tang S. H. , Xu G. Q. , Chan H. S. O. , Ng S. C. , Ng W. W.
Published: July 2002

We have prepared films of polymethyl methacrylate in which fullerenes (a mixture of C60 and C70) are uniformly embedded. By performing both degenerate four-wave mixing experiments and irradiance-dependent transmission measurements with nanosecond laser pulses of 608-nm wavelength, the third-order nonlinear optical susceptibility of these films is determined to be of the order of 10−10 esu. This susceptibility is dominated by its imaginary part. Excited-state absorption is the main mechanism responsible for the observed nonlinear susceptibility.
Nondestructive measurement of minority-carrier lifetime and surface/interface recombination velocity in compound semiconductors
Authors: Wang Chih Hsin , Neugroschel Arnost
Published: July 2002

A new optical method is presented for measurement of the bulk lifetime τ and surface recombination velocity S in semiconductors. The method is based on measurement of the photoluminescence decay as a function of the self-absorption under different excitation conditions. Closed-form analytical expressions, accounting for the self-absorption, are derived for the photoluminescence decay for a semi-infinite sample with an arbitrary value of S. A comprehensive method based on the analytical expressions is devised for the nondestructive extraction of the parameters in bulk wafers. The technique is particularly suitable for characterization of the surface passivation of compound semiconductors with a direct band gap, and is demonstrated on a GaAs sample. Application of the technique for characterization of indirect band gap semiconductors is also discussed.
Excitons and impurity centers in thin wires and in porous silicon
Authors: Shik A.
Published: July 2002

Theoretical treatment is given to the problem of a Coulomb center in thin wire with the dielectric constant æ≫1. The model describes adequately impurity centers and excitions in porous Si. The shape of Coulomb potential φ(r) inside the wire disturbed drastically by image forces is analyzed. The exciton (or impurity) binding energy determined by the Schrödinger equation with φ(r) demonstrates a manifold increase with the decrease of the wire radius.
Properties of hydrogenated amorphous silicon prepared by alternatively repeating chemical-vapor deposition from disilane and hydrogen plasma treatment
Authors: Sakata I. , Yamanaka M. , Hayashi Y.
Published: July 2002

Experimental studies have been carried out to characterize hydrogenated amorphous silicon prepared by alternatively repeating chemical-vapor deposition (CVD) from disilane and hydrogen plasma (HP) treatment (referred to as HP treated CVD a-Si:H). It has been found that hydrogen plasma treatment induces structural relaxation of Si network in addition to the passivation of Si dangling bonds. Two types of defect states with different Gaussian energy distribution exist in this material, and this type of distribution has been found to be common in undoped a-Si:H prepared by a variety of methods. High film quality, i.e., a low defect density (<1016 cm−3) and a fairly long ambipolar diffusion length (0.12 μm), and significant reduction of light-induced changes have been simultaneously achieved in HP treated CVD a-Si:H. Steady state transport of electrons and holes under illumination are controlled by the states other than neutral Si dangling bond state (D0). The behavior of these states with long exposure to light is different from that of D0.
Formation mechanism of hydrogen-associated defect with an 11.9 mT doublet in electron spin resonance and red luminescence in 9SiO2:GeO2 fibers
Authors: Awazu Koichi , Muta Ken-ichi , Kawazoe Hiroshi
Published: July 2002

The nature and the origin of the 11.9 mT hyperfine doublets in electron spin resonance spectrum observed in 9SiO2:GeO2 fibers were examined. When fibers were heated in hydrogen atmosphere at 100 °C, the intensity of the doublet increased. When gamma-ray irradiated, the intensity drastically increased. The photoluminescence bands peaking at 1.91 eV (650 nm) and 1.83 eV (680 nm) excited with Ar ion laser operated at 488 nm were also observed in the treated fibers with the doublets. It should be noted that the relation between the photoluminescence and the doublets was found. The intensities of the photoluminescence peaking at 1.83 eV and the 11.9 mT ESR doublets have accumulation. The germyl radical was proposed as their structural model. The origin of the photoluminescence peaking at 1.91 eV was tentatively proposed as the small silicon particles with hydrogen impurities generated in the fibers.
The study of emitter thickness effect on the heterostructure emitter bipolar transistors
Authors: Chen H. R. , Lee C. P. , Chang C. Y. , Tsang J. S. , Tsai K. L.
Published: July 2002

AlGaAs/GaAs heterostructure emitter bipolar transistors were grown with emitter thicknesses varied from 300 to 900 Å and the emitter thickness effects on the current gain and offset voltage were studied. It was found that both the current gain and offset voltage are strongly dependent on the emitter thickness. The current gain decreases with increasing emitter thickness. For an emitter thickness smaller than 500 Å, the offset voltage decreases with increasing emitter thickness, but for an emitter thickness larger than 500 Å, the offset voltage stays at a nearly constant value. Offset voltage as low as 55 mV was obtained for an emitter thickness of 700 Å. This low offset voltage, mostly contributed by the geometric effect, indicates that the base-emitter junction is a homojunction. From the current gain and offset voltage considerations, the optimal emitter thickness was found to be about 300–500 Å.
Stabilization and growth interruption effects at ZnSxSe1−x/ZnSe quantum-well interfaces grown by metalorganic vapor-phase epitaxy
Authors: Schneider A. , Geyzers K. P. , Söllner J. , Heuken M.
Published: July 2002

ZnSxSe1−x/ZnSe quantum wells (QW) with and without growth interruption at the interface were grown using atmospheric pressure metalorganic vapor-phase epitaxy. It has been shown that growth interruptions have a major influence on the optical properties of the QW. An interruption of growth in ternary ZnSSe layers causes a quasi-QW photoluminescence (PL), explained by sulfur depletion of the layer. A stabilization is necessary to avoid a quasi-QW. High-quality ZnSSe/ZnSe QWs were achieved by suitable stabilization with H2S and DESe during the growth interruption at the interface, so a sulfur diffusion out of the barrier can be avoided. The QW thicknesses grown under these conditions were in agreement with the nominal well thicknesses, and for stabilized 1-nm QWs we obtain a PL blueshift of 158 meV for x=0.68 and 60 meV for x=0.4 compared to the ZnSe band-edge emission. Characteristic QW-PL transitions at room temperature were observed for unstabilized QW grown with a long interruption time, tp=90 s. The thermal activation energies of the QW excitons investigated by temperature-dependent PL show that with decreasing interruption time an increased binding energy is obtained. The binding energy shows no dependence on the stabilization so that other effects (e.g., interface roughness and sulfur diffusion) may play a role. For QW with a low sulfur content (x=0.4) in the barrier material, we find binding energies which fit to theoretical models. This was not achieved for QWs with x=0.68.
The gettering efficiency of a direct bonded interface
Authors: Kissinger G. , Morgenstern G. , Richter H.
Published: July 2002

In these studies the gettering efficiency of copper at a direct bonded interface is compared to the gettering efficiency of a bulk defect zone formed during a one step and three step annealing process of Czochralski silicon wafers. The gettering efficiency of a bulk defect zone is considerably higher than that of the interface. Thus intrinsic gettering is expected to be effective in gettering direct bonded p-n junctions. Without any intrinsic gettering zone, copper silicide readily precipitates at the bonded interface, forming complex star-shaped colonies. Their morphology changes into single colonies in {110} planes if the wafer pairs are prestressed during contacting at room temperature. In this case the gettering efficiency of the bonded interface increases in comparison to the bulk defect zone. Prestressing experiments have shown that the denuded zone width in tensile stressed regions of a wafer increases.
Hyperradiance from soliton oscillators synchronized by capacitive or inductive coupling
Authors: Gro/nbech-Jensen Niels , Blackburn James A.
Published: July 2002

The output power from coupled Josephson oscillators is investigated when the junctions are operated in their single fluxon mode. We demonstrate that both inductive and capacitive coupling mechanisms can give rise to hyperradiance when the power is coupled out through a boundary resistor. Analytical expressions are derived from adiabatic perturbation theory and excellent agreement is found between the analytical expression and numerical simulations.
Preparation of TlBa2Ca2Cu3O9±δ high T
c thin films by laser ablation in combination with thermal evaporation of thallium oxide
Authors: Piehler A. , Löw R. , Betz J. , Schönberger R. , Renk K. F.
Published: July 2002

TlBa2Ca2Cu3O9±δ high Tc thin films were prepared on MgO 〈100〉 surfaces by a combination of laser ablation from a stoichiometric Ba2Ca2Cu3Ox target and the thermal evaporation of thallium oxide. X-ray diffraction measurements showed that the films consisted of predominantly c axis oriented TlBa2Ca2Cu3O9±δ, and scanning electron microscopy revealed that the surfaces had a flat, platelike morphology. The ac inductive measurements indicated that the onset of superconductivity occurred at 117 K with a transition width (10%–90%) of ∼3 K. Zero resistivity was reached at 120 K. The critical current density was ∼3×104 A/cm2 at 110 K.
A photoluminescence study of CF4 reactive-ion-etched silicon: Various process conditions and magnetically enhanced etching
Authors: Henry A. , Monemar B. , Lindström J. L. , Oehrlein G. S. , Malinowski J. C.
Published: July 2002

The impact of reactive-ion-etching (RIE) on the near-surface crystal quality of Czochralski silicon has been studied by photoluminescence spectroscopy. The presence of carbon-related defects is investigated as a function of the pressure during CF4 RIE. The effects of adding hydrogen to the plasma as well as the time of treatment are studied and discussed in terms of defect formation and etch rate. Photoluminescence spectra of samples recorded after a magnetically enhanced reactive-ion-etching process are also presented. The introduction of defects depending on the self-bias voltage and the etch rate are investigated for different magnetic fields.
An edge dislocation near a macrocrack with a microcrack
Authors: Shiue Sham-Tsong , Zhang Tong-Yi , Lee Sanboh
Published: July 2002

The elastic interaction between an edge dislocation and a macrocrack with a microcrack has been investigated by means of the Muskhelishvili potential method. The stress and displacement fields were obtained. From the stress field, we derived the stress intensity factor at each crack tip and the image force on the dislocation. The effects of the distance between two cracks on the crack shielding and of the image force on the edge dislocation were examined. The effects of the microcrack length on the shielding of the macrocrack and of the image force of the dislocation were also considered. The dislocations in the microcrack play an important role in fracture. Two conditions, an edge dislocation emitting from the microcrack and originating from elsewhere, are discussed. We compare the mechanical behavior between edge and screw dislocations near the macrocrack with the microcrack. Newton’s third law is satisfied for this system. Two special cases are discussed.
Measurement of thermal switching of the magnetization of single domain particles (invited)
Authors: Lederman M. , Fredkin D. R. , O’Barr R. , Schultz S. , Ozaki M.
Published: July 2002

We present an experimental study of the thermally activated switching of the magnetization of individual isolated γ-Fe2O3 particles. These particles are prolate ellipsoids ∼3000 Å long and 650 Å wide. The measured angular dependence of the switching field, Hs(θ), is consistent at large angles with a uniform rotation, but as θ approaches zero, other modes of reversal appear possible, and most likely the mode of reversal is curling. By measuring the probability of reversal of the moment as a function of time and applied magnetic field at T∼300 K, we found that the switching was thermally assisted, but couldn’t be described by hopping over a single energy barrier. Our results indicate that the dynamics of switching are described by a complex path in the energy landscape.
Comparison of reproduce signal and noise of conventional and keepered CoCrTa/Cr thin film media
Authors: Sin Kyusik , Ding Juren , Glijer Pawel , Sivertsen John M. , Judy Jack H. , Zhu Jian-Gang
Published: July 2002

We studied keepered high coercivity CoCrTa/Cr thin film media with a Cr isolation layer between the CoCrTa storage and an overcoating of an isotropic NiFe soft magnetic layer. The influence of the thickness of the NiFe and Cr layers, and the effects of head bias current on the signal output and noise, were studied using a thin film head. The reproduced signal increased by 7.3 dB, but the signal-to-noise ratio decreased by 4 dB at a linear density of 2100 fr/mm (53.3 kfr/in.) with a 1000 Å thick NiFe keeper layer. The medium noise increased with increasing NiFe thickness and the signal output decreased with decreasing Cr thickness. A low output signal obtained with very thin Cr may be due to magnetic interactions between the keeper layer and magnetic media layer. It is observed that signal distortion and timing asymmetry of the output signals depend on the thickness of the keeper layer and the head bias current. The signal distortion increased and the timing asymmetry decreased as the head bias current was increased. These results may be associated with different permeability of the keeper under the poles of the thin film head due to the superposition of head bias and bit fields.
Polarization analysis of the magnetic excitations in Fe65Ni35 Invar
Authors: Lynn J. W. , Rosov N. , Acet M. , Bach H.
Published: July 2002

Triple-axis inelastic polarized neutron measurements have been performed as a function of temperature on a single crystal of the Invar alloy Fe65Ni35 to distinguish longitudinal from transverse magnetic excitations in the magnetically ordered phase. Well below the Curie temperature of 501 K the magnetic excitation spectrum is dominated by conventional transverse spin-wave excitations, which in fact follow the predictions of spin-wave theory very well. In particular, we find no evidence for propagating longitudinal excitations in this system, in sharp contrast to the behavior observed in the amorphous Invar Fe86B14 material as well as the non-Invar amorphous system Fe40Ni40P14B6.
Theory of ferromagnetic resonance relaxation in very small solids
Authors: Sokoloff J. B.
Published: July 2002

Two models for ferromagnetic resonance (FMR) relaxation in magnetic solids which are sufficiently small for the discreteness of the magnon and phonon modes to be important are studied. The models exhibit a transition as a function of the number of atoms in the solid from a regime in which energy loss occurs, as evidenced by the fact that the energy of the solid increases as a function of time, to a regime in which there is no energy loss, as evidenced by the fact that the energy does not increase as a function of time on the average. It is shown that this phenomenon is due to a transition to chaotic behavior of the solid. These results open up the interesting possibility that sufficiently small magnetic solids could exhibit FMR with practically no energy loss.
The effects of group IV B/V B/VI B additions on the magnetic properties of Sm2+δ Fe17 carbonitrides
Authors: Chen X. , Girt Er. , Altounian Z.
Published: July 2002

Curie temperature, anisotropy field, and lattice constants are measured for the Sm2+δ Fe17M0.4 (δ≤0.6) carbonitrides for M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W. It is found that minor additions of M reduce the Curie temperature, with the largest decrease of 28 K for M=Mo, and that the fluctuations in Curie temperature are not related to the changes in unit cell volumes. Compared with Sm2+δ Fe17NxCy, most of the M additives reduce the anisotropy field with the largest decrease of about 10 kOe for M=Zr. The V addition has almost no effect on the anisotropy field and the Ti addition gives a 4–5 kOe enhancement of the anisotropy field.
Aligned high anisotropy Pr(Fe,Co,Mo)12N film samples
Authors: Rani R. , Hegde H. , Navarathna A. , Cadieu F. J.
Published: July 2002

Highly aligned (002) textured sputtered films of Pr(Fe12−y−zCoyMoz)Nx, where y=0–2.5, and z=0.4–1.0 compounds have been successfully synthesized. Before nitriding, the films exhibited nonuniaxial soft magnetic properties with room temperature coercivities of 0.70 kOe. A profound change of magnetocrystalline anisotropy occurred on nitriding. X-ray diffraction traces showed that the ThMn12-type tetragonal structure was retained with a saturating increase in cell volume over the first 15 min of nitriding time at 750 K. The coercivity, however, reached a maximum for a nitriding time of 25 min at 750 K. Samples nitrided for times greater than ≊25 min were taken as fully nitrided with ≊1 N per ThMn12 formula unit. The samples slightly richer in Pr showed somewhat higher coercivity perpendicular to the film plane as compared to stochiometric Pr(Fe,Co,Mo)12N films. From high field measurements to 80 kOe, the maximum room temperature coercivity perpendicular to the film plane was 9.4 kOe, which rose to 22.0 kOe at 10 K. The 4πMs value at 80 kOe measured perpendicular to the film plane was 11.6±0.5 kG at 293 K, 12.5 kG at 10 K, for Pr1.04Fe10.36Co1.16Mo0.44N samples, and 10.5±0.5 kG at 293 K for Pr1.05Fe11.17Mo0.78N samples. After high field magnetization to 80 kOe, for measurements perpendicular to the film plane, the static energy product was 23.6 and 32.9 MG Oe at 293 and 10 K, respectively. The estimated anisotropy fields were 96 and 144 kOe at 293 and 10 K, respectively, for low cobalt film samples.
Numerical methods and measurement systems for nonlinear magnetic circuits (abstract)
Authors: Heitbrink Axel , Dieter Storzer Hans , Beyer Adalbert
Published: July 2002

In the past years an increasing interest in calculation methods of circuits containing magnetic nonlinearities could be observed. For this reason a new method was developed which makes it possible to calculate the steady state solution of such circuits by the help of an interactive cad program. The modular concept of the software allows to separate the circuit into nonlinear and linear subnetworks. When regarding nonlinear magnetic elements one can choose between several numerical models for the description of the hysteresis loops or an inbuilt realtime measurement system can be activated to get the dynamic hysteresis loops. The measurement system is also helpful for the parameter extraction for the numerical hysteresis models. A modified harmonic-balance algorithm and a set of iteration schemes is used for solving the network function. The combination of the realtime measurement system and modern numerical methods brings up a productive total concept for the exact calculation of nonlinear magnetic circuits. A special application class will be discussed which is given by earth-leakage circuit breakers. These networks contain a toroidal high permeable NiFe alloy and a relay as nonlinear elements (cells) and some resistors, inductors, and capacitors as linear elements. As input dc signals at the primary winding of the core any curveform must be regarded, especially 135° phasecutted pulses. These signals with extreme higher frequency components make it impossible to use numerical models for the description of the nonlinear behavior of the core and the relays. So for both elements the realtime measurement system must be used during the iteration process.

During each iteration step the actual magnetization current is sent to the measurement system, which measures the dynamic hysteresis loop at the probe. These values flow back into the iteration process. A graphic subsystem allows a look at the waveforms of all voltages and current when the iterations take place. One can determine how the steady state is reached, especially when one discusses different iteration methods. All parameters of interest, such as geometric data of the core, number of windings, and the linear elements can vary within the computer program and as a result all voltages and currents in the network stand can be used. With the help of this computer controlled measurement system and several iteration methods fast circuit design and optimization can be done.
Can the universal jump be observed in two-dimensional XY magnets?
Authors: Bramwell S. T. , Holdsworth P. C. W.
Published: July 2002

The universal jump in the spin-wave stiffness is a characteristic property of the Kosterlitz–Thouless–Berezinskii phase transition in an infinitely large 2D-XY magnet. It has recently been demonstrated [S. T. Bramwell and P. C. W. Holdsworth, J. Phys. Condens. Matt. 5, L53 (1993)] that finite-size rounding is much more important in two-dimensional than in three-dimensional systems. This is because the properties of the phase transition depend logarithmically, rather than algebraically, on the system size. Consequently, finite-size rounding will be dominant in any real, macroscopic 2D-XY magnet. How the rounded universal jump still exhibits observable universal properties is discussed.
Strong thermal fluctuation effects on the dynamics of Bloch walls (abstract)
Authors: Hartl M. , Garanin D. , Koetzler J.
Published: July 2002

Approaching the Curie temperature of the uniaxial Ba and Sr hexaferrites from below in zero magnetic field, a rapid decrease of the domain wall relaxation rate Γw is observed which attains a minimum at T*=0.99 Tc. Above T* the increase of the relaxation, Γw∼(Tc−T)−0.8, agrees with that observed previously on low-temperature ferromagnets1 and is assigned to the presence of linear domain walls. Accordingly, the speeding-up of Γw towards Tc is associated with the divergence of the correlation length of a 3-dimensional Ising-magnet determining the width of the linear wall. The temperature T* is found to be significantly larger than T*0 predicted by mean field work of Bulaevskii and Ginzburg2 for the second order transition from linear to Bloch domain walls, which is characterized by a continuously increasing transverse magnetization, mB ∼ (T*− T)βB, in the wall. Adjusting the temperature variation of the relaxation rate below T* to a recent kinetic theory for elliptic walls,3 one finds βB=0.10 to be close to the 2-dimensional Ising value. Along with the suppression of T* this constitutes the first signature of severe fluctuation effects in domain walls.4
Magnetic and crystallographic properties of Co-Cr-(Ta,Pt)/Cr films deposited by excimer laser ablation
Authors: Ishikawa A. , Tanahashi K. , Yahisa Y. , Hosoe Y. , Shiroishi Y.
Published: July 2002

The crystal structure and magnetic properties of Co-alloy films deposited by KrF excimer laser ablation were investigated. A pulsed laser beam with wavelength of 248 nm was focused onto the deposition targets which were fixed in the vacuum chamber. Cr underlayer and Co-alloy films were successively deposited at a rate of 0.012 nm/pulse. The film surface was microscopically smooth compared to the sputtered films. This may be due to the low shadowing effect during the laser deposition. The composition of the film was reproducibly controlled, though there was a slight difference between the composition of film and target material. The coercivities of Co-Cr-Pt/Cr films formed on the Si and Ni-P substrates at 250 °C were 130 and 220 Oe, which were about one-fifth of the coercivity of sputtered films. Crystallographic analyses showed that Cr underlayer had no crystal orientation, and Co-alloy film consisted of fine fcc-type crystal grains. Low coercivity of the laser-deposited film is probably due to the lack of hcp Co phase.
Relaxation in magnetic continua
Authors: Sobolev V. L. , Klik I. , Chang C. R. , Huang H. L.
Published: July 2002

Within a recently developed axiomatic formalism for construction of stochastic field equations of motion a new form of the equation for damped dynamics of magnetization in ferromagnets is proposed. It takes into account spatially inhomogeneous dissipation due to exchange interactions and replaces the customary dissipation constant (due to relativistic effects) by a dissipation operator. This operator is constructed here by comparison of the resultant phenomenological spin wave spectra with results of microscopic calculations. Dynamic description of relaxation of the modulus of magnetization is discussed.
Generalized equations for domain wall dynamics
Authors: Sobolev Vladimir L. , Huang Huei Li , Chen Shoan Chung
Published: July 2002

Generalized domain wall dynamic equations have been derived. The influence of the external magnetic field normal to the anisotropy axis on the critical parameters of the domain wall motion has been studied and illustrated.
Nucleation of the nanocrystalline phase in Fe73.5Cu1Nb3Si13.5B9 (abstract)
Authors: Ayers J. D. , Harris V. G. , Sprague J. A. , Elam W. T.
Published: July 2002

A new class of soft magnetic alloys derive their attractive properties from the fact that they contain a high volume fraction of iron rich, nanometer scale, DO3 structure precipitates. These precipitates are imbedded within, and separated from one another by, a portion of the amorphous phase from which they formed. In an effort to determine the mechanism by which the addition of modest amounts of Cu and Nb to Fe-Si-B amorphous alloys induce the formation of the metastable nanocrystalline structure, melt-spun samples of Fe73.5Cu1Nb3Si13.5B9, Fe76.5Cu1Si13.5B9, and Fe74.5Nb3Si13.5B9 were heat treated at 550 °C for times ranging from 2 to 60 min and studied by means of transmission electron microscopy (TEM), x-ray diffraction, and extended x-ray absorption fine structure (EXAFS) analysis. EXAFS spectra from the Cu absorption edge in the Fe73.5Cu1Nb3Si13.5B9 alloy show that tiny clusters of a Cu rich phase are present even in the amorphous, as-melt-spun ribbon, and that these clusters have close atomic packing. These clusters are found to coarsen and increase in number during the first few minutes of heat treatment. TEM observations show that a high proportion of the precipitates are nucleated within this same short time, supporting the belief that the clusters catalyze nucleation of the DO3 precipitates. Examination of the alloys without Cu or Nb indicate that the Nb is important both for stabilizing the amorphous phase component and for inducing formation of the Cu clusters.
A unified effective-field renormalization-group framework approach for the quenched diluted Ising models
Authors: de Albuquerque Douglas F. , Fittipaldi I. P.
Published: July 2002

A unified effective-field renormalization-group framework (EFRG) for both quenched bond- and site-diluted Ising models is herein developed by extending recent works. The method, as in the previous works, follows up the same strategy of the mean-field renormalization-group scheme (MFRG), and is achieved by introducing an alternative way for constructing classical effective-field equations of state, based on rigorous Ising spin identities. The concentration dependence of the critical temperature, Tc(p), and the critical concentrations of magnetic atoms, pc, at which the transition temperature goes to zero, are evaluated for several two- and three-dimensional lattice structures. The obtained values of Tc and pc and the resulting phase diagrams for both bond and site cases are much more accurate than those estimated by the standard MFRG approach. Although preserving the same level of simplicity as the MFRG, it is shown that the present EFRG method, even by considering its simplest size-cluster version, provides results that correctly distinguishes those lattices that have the same coordination number, but differ in dimensionality or geometry.
Random decorated antiferromagnetic ising model with mixed spins
Authors: Correia Vanêssa M. , dos Santos Roberto J. V.
Published: July 2002

A new approach to studying random quenched decorated models is introduced, which after an approximation on the decoration transformation equation makes use of the exact solution of the partition function of the host lattice. The method is applied to study the phase diagram of an antiferromagnetic Ising square lattice randomly decorated with spins S1=±1;0 with probability p or S2=±1 with probability 1−p, in the isotropic case, in which all the bonds are decorated. Several phase diagrams are obtained spanning the various parameter spaces of the model.
Activation volume of a pair of magnetostatically coupled particles
Authors: Lyberatos A.
Published: July 2002

The activation volume of an interacting pair of uniaxially anisotropic fine ferromagnetic particles is studied assuming a dipole approximation, for the case of the applied field parallel to the bond direction and the common anisotropy axis. Using an analytical treatment, it is shown that the activation volume depends primarily on activation energy and to a smaller extent on the location of the saddle point of the transition. The incoherent reversal by ‘‘fanning’’ and the collective thermal excitations of the magnetization result in a reduction of the activation volume. The dependence on coupling strength is also discussed. Numerical calculations using the Langevin formalism provide a correction to the theory that is significant in the limit of very short relaxation times.
Spin-dependent confinement in DMS-based heterostructures (invited)
Authors: Petrou A. , Fu L. P. , Yu W. Y. , Lee S. T. , Jonker B. T. , Warnock J.
Published: July 2002

We describe the results of a magneto-optical study of a class of semiconductor heterostructures in which one of the constituent layers is a diluted magnetic semiconductor (DMS). The large magnetic band splittings of DMS materials result in spin-dependent confining potentials for electrons and holes which can be changed externally by varying the applied magnetic field. The modifications in the band alignment result in changes, sometimes dramatic, of the optical properties in the vicinity of the band gap.
Effects of oxygen vacancies on magnetic properties of Ca-substituted yttrium iron garnet (abstract)
Authors: Song Y. J. , Bornfreund R. E. , Turpin G. B. , Wigen P. E.
Published: July 2002

Ca-substituted YIG samples, Y3−xCaxFe5O12−0.5x−z for 0.14≤x≤0.23 were annealed for 6–8 h at various temperatures between 600 °C and 1000 °C in N2 atmosphere. The magnetization barely shows any change by annealing under 900 °C but by annealing between 900 °C and 1000 °C a significant change over the temperature range from 4.2 K to the Curie temperature is observed. The lattice constant is also observed to change. The magnetization and the lattice constant of the samples did not change within experimental error by annealing in O2 atmosphere at 1000 °C. For the sample with x=0.23, annealing at 1000 °C in N2 resulted in a decrease of 4πM at 4.2 K by 560 G and a decrease of the lattice constant by 0.031 Å. The Curie temperature also changes from 531 K in the as-grown sample to 508 K for this annealing condition. The change in 4πM for various annealing conditions is linear with the change in the lattice constant. For the samples tested, the oxygen vacancies, z, in the as-grown state shows z≳0 and that z did not change by annealing in O2 atmosphere. A microscopic molecular field model based on the Heisenberg Hamiltonian in which a singly charged oxygen vacancy, VȮ, mediates a ferromagnetic superexchange between the iron ions at adjacent sites is proposed. The model explains the temperature dependence of 4πM, the magnitude of superexchange constant mediated by VȮ and the variation of the Curie temperature with annealing conditions. This model also explains the positive slope of 4πM at low temperature for Ca-substituted YIG.
Recording medium properties and capacity bounds
Authors: O’Sullivan J. A. , Porter D. G. , Indeck R. S. , Muller M. W.
Published: July 2002

The effect of such medium properties as magnetic unit size, film thickness, and strength of anisotropy, dipolar, and exchange interactions on the storage capacity limit of a thin-film recording medium is investigated. Relationships are derived for media with different properties that have equivalent capacity. The physically realizable range of material properties and their effect on the capacity bound are explored. Results on edge effects for capacity bound computations are presented. The results indicate that media for which anisotropy is the dominant source of magnetic energy have the greatest potential for information storage.
Molecular dynamics of magnetic particulate dispersions
Authors: Deymier P. A. , Jung C. , Raghavan S.
Published: July 2002

A computational investigation using the method of molecular dynamics was undertaken to characterize the state of magnetic particle dispersions. The simulations revealed that the microstructure of spherical particulate dispersions consists of chainlike clusters resulting from magnetic dipole alignment. Acicular particles formed clusters such as dimers, chains, and rings. The effect of fluid viscosity on the dispersion quality and the response of the magnetic dispersions to an external DC magnetic field are also reported.
Studies into the use of waterborne coating formulations for the preparation of magnetic tape (abstract)
Authors: Barrom Stacy , Bray Ashley , Cheng Song , Elike John , Fan Hong , Lane Alan M. , Nikles David E.
Published: July 2002

The objective of this research project is to replace the organic solvents used in modern tape manufacture with water, thereby eliminating the potential for solvent emissions. This has led to a search to identify tape components compatible with a waterborne coating process. The pigments were either cobalt-modified γ-Fe2O3 or barium ferrite, with the majority of the research focused on cobalt-modified γ-Fe2O3 formulations. A combination of sodium polyphosphate and Surfynol CT-136, a pigment grinding aid, were used as dispersing agents. The binders included commercial water-dispersed polyurethanes and a commercial ethylene-vinylchloride copolymer emulsion. A commercial waterborne melamine-formaldehyde was used as a cross-linking agent. Addition of the ethylene-vinylchloride copolymer to the polyurethane increased the tensile strength and Young’s modulus of the unpigmented binder films. The melamine-formaldehyde cross-linker further enhanced the mechanical properties and increased the adhesion between the pigmented binder films and the polyester base film. In a 180° peel test, the adhesion easily exceeded the ITO specification for 8 mm helical scan magnetic tape.1 Rheological studies of the waterborne dispersions revealed that the viscosity was too low. Hydroxyethylcellulose, a water soluble polymer, was added as a thickener and this gave rise to a desirable thixotropic behavior in the dispersion. Waterborne dispersions were cast onto polyester base film, oriented in a 2000 G longitudinal magnetic field, and cured in a convection oven at 60 °C. Magnetic hysteresis loops showed a squareness of 0.875 and a switching field distribution of 0.324 for films containing cobalt-modified γ-Fe2O3.
The initial phases of epitaxy of fcc Fe/Cu(100): Supersurface and subsurface island formation
Authors: Healy S. D. , Heim K. R. , Yang Z. J. , Hembree G. G. , Drucker J. S. , Scheinfein M. R.
Published: July 2002

Nanometer transverse resolved real space observations of the initial phases of room-temperature heteroepitaxy of fcc Fe/Cu(100) indicate that vertical atomic site exchange occurs locally. The formation of two-dimensional supersurface and subsurface islands has been characterized by secondary and Auger electron imaging. The persistence of vertical site exchange during the deposition of the first two monolayers is not inconsistent with the lack of observed ferromagnetism for the room-temperature grown fcc Fe/Cu(100) at these coverages.
Origin of the fine structure of subsidiary absorption in tangentially magnetized YIG films
Authors: Pomyalov A. V. , Laulicht I.
Published: July 2002

Parametric excitation of spin waves (SWs) under conditions of perpendicular pumping was investigated in epitaxial films of yttrium iron garnet (YIG). The related microwave power absorption was measured at the frequency 9.1 GHz in the magnetic field range 1200–1600 Oe. A series of absorption peaks closely spaced on the magnetic field scale were observed near the threshold of first-order Suhl instability of spin waves. The peaks spaced by 0.2–0.7 Oe originate from the shallow ripple of the butterfly curve. This is the first observation of the fine structure at subsidiary absorption in YIG films. The peak spacing was measured as a function of dc magnetic field H, film thickness d, and in-plane sample dimensions L for square samples. In the field range 1300–1400 Oe the spacing did not depend on H and was inversely proportional to L in the range of 0.6–3 mm. No thickness dependence was found in the range d=3–45 μm. Explanation is given in terms of standing spin waves set in the film plane and related to the exchange part of SW dispersion curves in ferrite film.
Instability mechanism of collective spin wave oscillations in finite-size ferrite samples
Authors: Slavin A. N. , Srinivasan G. , Cordone S. S. , Cherepanov V. B.
Published: July 2002

A sample-size effect on spin wave auto-oscillations under parallel pumping is studied experimentally in tangentially magnetized rectangular samples of yttrium-iron garnet (YIG) films where one of the in-plane sizes is varied. A previously developed one-dimensional theoretical model of unstable collective oscillations in the system of parametrically excited spin waves is used to describe the mechanism of spin wave auto-oscillations observed in YIG films. Our results show that the one-dimensional model (that works well for YIG spheres) gives only qualitative description of the sample-size effect in YIG films and the incorporation of the second finite size (film thickness) in this model is needed to achieve quantitative agreement with experiment in the film geometry.
Self-channeling of magnetostatic waves in ferromagnetic film (abstract)
Authors: Boyle J. , Nikitov S. A. , Boardman A. D. , Moghadam A. , Booth K. M. , Booth J. G.
Published: July 2002

Low power magnetostatic wave (MSW) experiments reporting beam formation and propagation inside ferromagnetic films have been carried out .1,2 In this work, we report the first observation of self channeling of MSW and the formation of the spatial solitons of MSW for an in-plane magnetized YIG film and waves propagating perpendicularly to the direction of magnetization. The experiments were conducted using YIG film of 4.9 μm thickness magnetized in-plane by a permanent magnetic field of 1094 Oe. A cw signal was launched into the film through a microstrip antenna. A Brillouin scattering system consisting of a tandem Fabry–Perot interferometer has been used to observe the profile of a MSW beam. Self-channeling MSW beam propagation occurring as a result of an interplay between diffraction of the beam and self-modulation was observed for various magnitudes of input power and initial frequency. At low power levels the formed MSW beam broadens and weakens during the propagation. As the input power increases, the intensity of the peak power of the beam also increases and a narrow channel of the MSW beam is formed. The power threshold depends on the wavelength of MSW and the width of the excitation microstrip. A theory describing the spatial MSW self-channeling is developed. The necessary and sufficient conditions are analyzed and numerical calculations are provided. The experimental data are in a semi-quantitative agreement with theory.
Magnetization processes in Co–Cr submicron samples: Hall measurements and micromagnetic simulations
Authors: van Kooten M. , de Haan S. , Lodder J. C. , Popma Th. J. A.
Published: July 2002

The magnetization reversal of Co–Cr with perpendicular anisotropy is studied by anomalous Hall effect measurements and by micromagnetic simulations. In a measured hysteresis loop of a 300 nm×300 nm sample (consisting of approximately 40 columns of Co–Cr), discrete jumps are observed, together with a more or less continuous change in magnetization, the latter with both positive and negative sign. The continuous and discrete parts of the hysteresis loop are separated to extract micromagnetic information from the anomalous Hall measurement. The discrete jumps are attributed to parts of columns that switch irreversibly, the continuous magnetization change is attributed to reversible rotation, partly of volumes with in-plane anisotropy. These assumptions are checked by means of a micromagnetic model.
Magnetic properties of modified Ba-ferrite particles
Authors: Sadamura H. , Sugita N. , Maekawa M. , Nagai N.
Published: July 2002

Modification with magnetite on Ba-ferrite particles was carried out by heating alkaline suspension including Ba-ferrite particles and Fe2+ hydroxides as described by Kiyama et al. [J. Solid State Chem. 99, 329 (1992)] to get higher magnetization. As a result of x-ray powder diffraction measurements, reflection intensity of magnetoplumbite phase decreases and that of spinel phase increases with increase of Fe2+ quantity added, simultaneously. Coercive force goes down and magnetization increases as Fe2+ quantity added increases. The differential hysteresis loop of the mixed samples (Ba-ferrite particles and magnetite particles) has two peaks. And that of the modified samples has only one peak. From these results, it becomes clear that there exists strong magnetic interaction between Ba-ferrite core and magnetite shell. The measurements of temperature dependence of magnetization at the applied field of about 16.5 kA/m shows that the Hopkinson peak shifts toward higher temperature with increase of the modified amount. It also indicates the strong magnetic interaction between core and shell materials.
Study of the coercivity of Pr(Mo, Fe)12Nx
Authors: Pan Qi , Zhang Xiao-dong , Cheng Ben-pei , Yang Ying-chang
Published: July 2002

A study of the permanent magnetic properties of Pr(Mo, Fe)12Nx powders has been carried out by using a mechanical alloying technique. The variations of coercive force and remanence as a function of composition and crystallizing temperature have been investigated. A coercive force of up to 6.0 kOe was obtained. The temperature dependence of the coercive force is presented in comparison with that of Nd2Fe14B-type magnets.