bluuebunny/crossref_metadata_2025 huggingface dataset:
The virtual cathode microwave amplifier experiment
Authors: Fazio M. V. , Kinross-Wright J. , Haynes B. , Hoeberling R. F.
Published: July 2002

This paper describes the results of an experiment that tests a new high-power microwave amplifier concept that uses the virtual cathode phenomenon as an amplifier rather than as a free-running oscillator. The virtual cathode is surrounded by a cavity resonator that is driven by an input signal. The output from this virtual-cathode amplifier is frequency locked to the input signal, and exhibits amplification over at least a 10-dB dynamic range.
0.8 eV excitation of the quenched EL2* level in semi-insulating GaAs
Authors: Jiménez J. , Alvárez A. , Gonzalez M. A. , Bonnafé J.
Published: July 2002

Excitation with 0.8 eV of the optically bleached state of semi-insulating GaAs results in a recovery of the 1–1.3 eV photocurrent. This recovery is found to occur in the advanced stages of the photoquenching with 1.1-eV light, when both the extrinsic and the intrinsic photocurrent are quenched. Such an observation is related to the existence of different metastable configurations, which are characterized by different optical sensitivities.
Model for interpreting one-sphere permittivity measurements of biological cells
Authors: Sancho M. , Martinez G.
Published: July 2002

A rigorous treatment of the multipole interaction between dielectric spheres situated in the vertices of a tetragonal lattice is developed. In contrast to other existing models, our approach includes dielectric losses in both medium and particles. The results are applied to the interpretation of one-sphere permittivity measurements, important for the analysis of structural characteristics of biological cells.
Aluminum composition dependence of reactive ion etching of AlGaAs with CCl2F2:O2
Authors: Pearton S. J. , Hobson W. S. , Chakrabarti U. K. , Emerson A. B. , Lane E. , Jones K. S.
Published: July 2002

The etch rate and surface chemistry of AlxGa1−xAs after reactive ion etching (RIE) in CCl2F2:O2 was examined as a function of etch time (1–22 min), plasma power density (0.3–1.3 W cm−2), pressure (1–30 mTorr), gas composition (0%–80% O2), gas flow rate (10–50 sccm), sample temperature (50–350 °C), and Al composition (x=0.15–1). The etch rate is nonlinear with time, and decreases rapidly with increasing AlAs mole fraction. Essentially no temperature dependence of the etch rate is observed under our conditions, and there are no major differences in the surface chemistries of AlGaAs etched at different temperatures. The formation of a thin layer (50–90 Å) of AlF3 during the RIE treatment appears to control the etch rate, and the surface morphology becomes progressively smoother for increasing Al composition. No residual lattice disorder is detected by cross-sectional transmission electron microscopy under any of our conditions, although current-voltage measurements on Schottky barrier diodes fabricated after RIE show higher ideality factors and barrier heights than unetched control samples. Annealing at 500 °C for 30 s almost restores the initial electrical properties of the material.
Breakdown in p-n junction diodes made on polycrystalline silicon of large grain size
Authors: Natarajan K. , Ramkumar K. , Satyam M.
Published: July 2002

This communication describes the voltage-current characteristics in the breakdown region of p-n junctions made on polycrystalline silicon of large grain size. The observed soft breakdown characteristics have been explained by taking into account the effect of curvature of the junction near the grain boundaries.
The role of the split-off band in electron-hole energy exchange dynamics in selected III-V semiconductors
Authors: Sadra K. , Maziar C. M. , Streetman B. G.
Published: July 2002

We report a Monte-Carlo investigation of Coulomb electron-hole scattering processes involving the split-off band in selected III-V semiconductors in the context of steady-state high-field minority-electron transport. Due to the small value of the relevant hole overlap factors, as well as the relative values of the Γ-L energy separation and the split-off energy, such processes do not make a significant contribution to the minority electron energy loss rate in In0.53Ga0.47As. In InP, however, the ΓhΓs process accounts for a significant fraction of the total Γ-electron energy loss to holes.
Investigation of excess noise in ultrathin metal films deposited onto single-crystal silicon
Authors: Cho Nam-Ihn , Bene Robert W.
Published: July 2002

We present a silicide nucleation process in planar metal-silicon interfaces in terms of excess noise properties. The excess noise power spectral density has been measured in systems composed of ultrathin metal films deposited onto silicon substrates as a function of metal thickness. The metal films have been prepared by electron-beam evaporation and rf sputtering, and structural information of the films has been obtained by using transmission electron microscopy and diffraction. Surface resistance measurements for near-noble metal films deposited onto silicon generally show that the increase of metal thickness reduces the surface resistance of the film, which undergoes a transition from a semiconducting to a metallic state immediately prior to the onset of the first nucleation of the crystalline silicide. Results of excess noise measurements show that some of the noise parameters of the power spectral density for cobalt and nickel film on silicon have similar trends along the nucleation reaction path. A large gradual increase in noise magnitude has been observed in the prenucleation regime followed by a sudden drop below the fluctuation level for high-resistance films after the crystallization has occurred. The noise magnitude of power spectral density is assumed to be indicative of the structural fluctuations of the interfacial layer, and thus the instability of the amorphous interfacial structure gradually grows as the metal content of the layer increases. After the electronic transition point, the chemical structure of the interfacial layer changes to a more stable long-range-order silicide structure. The high-frequency exponent of the power spectral density varies from values near 3 at high film resistance to values near unity after the crystallization has occurred. The frequency exponent data may indicate that the interactions between amorphous cluster structures in the interfacial layer become stronger as the metal content of the film increases.
Preparation of a single-phase (Bi0.9Pb0.1)2Sr2Ca2Cu3Oy ceramic superconductor
Authors: Mei Yu , Green S. M. , Jiang C. , Luo H. L.
Published: July 2002

Through careful processing, single-phase material of the (Bi,Pb)-Sr-Ca-Cu oxide with Tc∼111 K has been synthesized. Powder x-ray diffraction patterns are employed to verify that repetitive grinding and sintering leads to the virtual disappearance of peaks corresponding to the other superconducting polytypoids of this complex system. The dimensions of the face-centered pseudotetragonal unit cell are determined to be 5.41×5.40×37.13 Å3. The Meissner and shielding effects shown by dc magnetic susceptibility measurement of the bulk and pulverized specimens at various stages of processing demonstrate that the proper high-Tc superconductor can be synthesized in single-phase form.
Effect of macroscopic stress on accelerated aging of GaInAsP channeled substrate buried heterostructure lasers
Authors: Swaminathan V. , Koszi L. A. , Focht M. W.
Published: July 2002

We report the results of the measurement of radius of curvature of 1.3 and 1.5 μm wavelength GaInAsP-InP channeled substrate buried heterostructure lasers. The objective of this investigation is to quantify the macroscopic stress present in the device and correlate it with device reliability. The change in dc threshold current (ΔIth) after an accelerated aging test was used as a measure to access device reliability, with high ΔIth indicating decreased reliability. Changes were made in the p-side metallization to bring about a change in either ΔIth or radius of curvature and they included two different contact widths and different thicknesses of the Au bonding pad. It is observed that no correlation between device curvature and ΔIth exists even though the modifications in the p metallization caused significant changes in both quantities. It is suggested that it is not the macroscopic device stress that is measured by the radius of curvature but localized stresses that may exist in the vicinity of the lasing active layer which would affect device reliability. It is surmised that the most important role of stress is its effect on the direction of defect migration with the principal driving force coming from the nonradiative electron-hole recombination occurring in the vicinity of the active layer.
Influence of stresses on the thermal parameters of chromium-doped GaAs
Authors: Jouglar J. , M’Ghaieth R. , Vuillermoz P. L. , Gauthier R. , Pinard P. , Poiblaud G.
Published: July 2002

We have produced dislocations by plastic deformation in a semi-insulating material (GaAs-Cr) in order to clarify the part of dislocations in the raw materials used later as substrates in device technology. We have correlated the inhomogeneous distribution of the dislocations revealed by optical microscopy to the thermal conductivity measurements, observed residual stresses after mechanical and thermal treatments by computer aided photoelastometry procedure, calculated and established the stress mapping showing that shear stresses are localized, and observed a relaxation of the material with the time at low temperature.
Fast linear electro-optic effect based on cholesteric liquid crystals
Authors: Patel J. S. , Lee Sin-Doo
Published: July 2002

Flexoelectric effect in cholesteric liquid crystals can be used to produce optical switching on a microsecond time scale. In a simple geometry, this effect produces a light modulation the amplitude of which is linear in field. The switching time from one optical state to the other is predicted to be independent of the magnitude of the electric field. We confirm this prediction experimentally in a room-temperature cholesteric liquid crystal.
Photoluminescence line shape of excitons in GaAs single-quantum wells with and without heterointerface ordering
Authors: Fujiwara K. , Kanamoto K. , Tsukada N. , Miyatake H. , Koyama H.
Published: July 2002

The photoluminescence line shape of excitons at low temperatures is investigated in GaAs single-quantum wells grown by molecular-beam epitaxy with and without intentional heterointerface ordering. From the study of the excitation density dependence in the range between 1015 and 1017 cm−3, it is found that the inhomogeneous linewidth is significantly affected by band-filling effects of intrinsic defect states spatially localized in the wider wells. In the sample with heterointerface ordering the band-filling effects are directly identified by observation of saturation of the localized excitonic emissions, which are split as a result of increased spatial coherence of the excitonic states.
Electromagnetic mechanism of magnetic nerve stimulation
Authors: Yamaguchi Masuhiro , Yamada Satoshi , Daimon Nobuo , Yamamoto Isao , Kawakami Tadashi , Takenaka Toshibumi
Published: July 2002

Rabbit sciatic nerves were stimulated by pulsed magnetic fields and nerve responses were analyzed on the basis of electromagnetic theory to understand the dominant factors in magnetic stimulation. Some conducting substance surrounding the nerve is required to induce the currents exciting it. The strength of a magnetic stimulus is evaluated by the rate of change in the vector potential at the nerve, dA/dt, which equals the magnitude of the induced electric field E. The minimum strength of dA/dt for exciting the nerve is 18 Wb/ms (18 V/m) in the agar with a conductivity of 0.6 S/m. The induced current density of 10 A/m2 is comparable to that used in the electric stimulation of peripheral nerves. The component of the vector potential parallel to the nerve is more effective in stimulating the nerve than the component perpendicular to it.
A new analysis of the results of thermally stimulated measurements in polymers
Authors: Crine Jean-Pierre
Published: July 2002

It is shown that the compensation law observed in thermally stimulated measurements (TSC and TSDC) between the activation energy E and the preexponential term τ0 in the Arrhenius equation for various polymers can be explained by a linear relation between the activation enthalpy ΔH‡ and entropy ΔS‡ of the process. When this relation is combined with the well-known rate theory, TSC or TSDC results with polymers indicate that both techniques yield very similar results. It is also deduced that they are especially sensitive to entropy changes in the polymers studied. This points out the non-negligible value of ΔS‡ in polymer relaxations, and this implies that the proper energy term describing a polymer relaxation is not E but a free-energy term ΔG. The relation between the Vogel–Tammann–Fulcher and rate theory equations is also made. The correlations between the ΔG, ΔH‡, and ΔS‡ values, and the polymer thermodynamic and morphological properties are also briefly discussed.
Plasma-induced fixed oxide charge
Authors: Bos Jan , Hendriks Menso
Published: July 2002

The buildup of a plasma-induced fixed oxide charge (Qf ) in thermally grown silicon dioxide on silicon was studied. Whereas samples located in the plasma showed a persistent instantaneous saturation behavior [Qf (sat) =1.3×1013 charges/cm2], those that were situated in a field-free region outside the plasma produced dose and oxide thickness dependent buildup curves. A distinction is made between nonpenetrating electron/ion bombardments and penetrating photon irradiation. In the first case, the Qf creation can be explained by hole transport through the oxide film under the influence of an internally generated electric field and successive hole trapping near the interface. In the second case, we have to deal with direct interaction with the SiO2/Si interface. Furthermore, thermal annealing was used to reduce and/or eliminate plasma-induced Qf. Activation energies of 0.51 and 0.58 eV were found for the annealing process.
Two-dimensional electron gas density calculation in Ga0.47In0.53As/Al0.48In0.52As, Ga0.47In0.53As/InP, and Ga0.47In0.53As/InP/Al0.48In0.52As heterostructures
Authors: Yoon K. S. , Stringfellow G. B. , Huber R. J.
Published: July 2002

The Ga0.47In0.53As/InP and Ga0.47In0.53As/Al0.48In0.52As systems have been investigated in terms of two-dimensional electron gas densities formed at these heterostructures in an effort to determine the optimum combination of III-V semiconductors for high electron mobility transistor (HEMT) structures. From this study Ga0.47In0.53As/Al0.48In0.52As is shown to be, in some ways, a more suitable material combination for a HEMT structure than Ga0.47In0.53As/InP. However, the choice between these two combinations of materials will be a tradeoff between ease of fabrication and potential performance. As a consequence, it is suggested that a new HEMT structure (Ga0.47In0.53As/InP/Al0.48In0.52As )consisting of a heavily doped n-type Al0.48In0.52As as the wide band-gap layer with undoped InP as the spacer layer on an undoped Ga0.47In0.53As layer might be preferable.
Surface plasmon microscopic characterization of external surfaces
Authors: Hickel Werner , Rothenhäusler Benno , Knoll Wolfgang
Published: July 2002

Surface plasmon microscopy (SPM) is used, for the first time, to image external samples with a surface relief structure. This is demonstrated in an experimental coupling setup which combines elements of both the Otto and the Kretschmann configuration. The contrast is shown to be generated between areas of different surface plasmon excitation efficiencies. It is shown that periodic surface structures also can be read-out in Fourier-space by surface plasmon diffraction. Finally, Fresnel calculations are presented that demonstrate the contrast improvement achievable by immersion SPM using a suitable liquid.
Quantum Hall effect of two-dimensional electron gas in AlyGa1−yAs/Ga1−xInxAs/GaAs pseudomorphic structures
Authors: Luo J. K. , Ohno H. , Matsuzaki K. , Hasegawa H.
Published: July 2002

The quantum Hall effect of electrons in AlGaAs/Ga1−x Inx As/GaAs pseudomorphic strained heterostructures is reported for the first time. Despite the large strain induced by the lattice mismatch, clear quantum Hall plateaus and Shubnikov–de Haas oscillation are observed, indicating the presence of a well behaved two-dimensional electron gas. Although the mobility of the sample is fairly low, a pronounced asymmetry in the spin splitting of the magnetoresistivity appears at high magnetic fields in contrast to the previous results on AlGaAs/GaAs structures where asymmetry appears only in the high-mobility samples and disappears when the mobility is lower. This indicates that the shape of the state density is not influenced significantly by the presence of the high concentration of the short-range cluster-scattering centers.
Formation of buried SiO2 layer by oxygen implanted into Si/Ge and Si/Si0.5Ge0.5 substrates
Authors: Tang Y. S. , Zhang Jingping , Hemment P. L. F. , Sealy B. J. , Liu Hengda , Castle J. E. , Newstead S. M. , Powell A. R. , Whall T. E. , Parker E. H. C.
Published: July 2002

The formation of buried SiO2 layer by high-dose oxygen implanted into Si/Ge and Si/Si0.5Ge0.5 heterostructures is studied by infrared transmission and x-ray photoelectron spectroscopy. The results show that the Ge in the implanted region has no influence on the bonding properties of oxygen, and there exists a critical annealing temperature of about 1250 °C for all the implanted oxygen to be bonded as SiO2.
Simultaneous observation of the smoke signals during the growth of bcc Fe on the Ag(100) surface
Authors: Suzuki Y. , Kikuchi H. , Katayama T. , Yoshida S.
Published: July 2002

Simultaneous observation of the surface magneto-optic Kerr effect (SMOKE) and reflection high-energy electron diffraction (RHEED) intensity oscillation was done during the epitaxial growth of bcc Fe on fcc Ag(100) surface. The SMOKE system using the high-frequency (100 kHz) ac method has a high sensitivity (10−4 deg) and is able to obtain an absolute value of the Kerr rotation angle (φK ). φK of Fe/Ag(100) was measured at room temperature under magnetic fields of 0 and 100 Oe. When film thickness (d) is thicker than five monatomic layers (ML’s), φK of the longitudinal Kerr effect at 100 Oe increases with increasing d. No longitudinal SMOKE signal, however, was observed below 5 ML, while a polar Kerr signal was observed only in the range of 4 ML<d<5 ML. This suggests that the magnetic easy axis changes from perpendicular to in-plane at 5 ML, and below 4 ML the film is hardly magnetized by 100 Oe because of its high coercivity. These properties are discussed in relation to the crystal growth mechanism.
Absolute value of the magnetic moment per atom in Ag/Fe/Ag(001) and Ag/Co/Ag(001) epitaxial sandwich structures
Authors: Bland J. A. C. , Johnson A. D. , Norris C. , Lauter H. J.
Published: July 2002

We present the results of an investigation of epitaxial Ag/bcc Fe/Ag(001) and Ag/fcc Co/Ag(001) sandwich structures in which we use polarized neutron reflection (PNR) as a direct probe of the absolute value of the magnetic moment per atom in order to test recent theoretical calculations for these systems. An 8-ML bcc 57Fe and a 2-ML fcc Co film were found to display planar anisotropy at 300 K from surface magneto-optic Kerr effect measurements, and were selected for PNR measurements in the temperature range 4–300 K with the films magnetically saturated in-plane. The flipping ratio observed for the 8-ML 57Fe film is consistent with a ferromagnetic moment in the range 0.9–1.1 Bohr magnetons, which is significantly reduced from the bulk value, in contrast with recent theoretical predictions of a strongly enhanced ferromagnetic moment for this epitaxial system. No temperature dependence of the magnetization is detected, confirming the anticipated bulklike behavior. For the 2-ML fcc Co film, the observed flipping ratio is consistent with a ferromagnetic moment in the range 1.8–2.2 Bohr magnetons, which is significantly enhanced from the bulk hcp value, in agreement with recent theoretical calculations. No temperature dependence of the Co magnetization was detected.
The effect of chemisorption on the magnetic behavior of metastable layers of fcc Co(001) and Fe(001) (abstract)
Authors: Kief M. T. , Mankey G. J. , Willis R. F.
Published: July 2002

We have investigated the effect of chemisorption on the magnetic properties of metastable layers of fcc Co(001) and fcc Fe(001) epitaxed to Cu(001) substrates. The Curie temperature has been observed to fall rapidly with decreasing film thickness below 10 monolayers,1 so that the low-temperature chemisorptive properties can be studied. We have observed the binding energies of various adsorbates using thermal desorption kinetics as a function of magnetic film thickness. The magnetic behavior of the films was monitored in situ using the surface magneto-optic Kerr effect (SMOKE) method,2 and changes in the electronic states at the Fermi level were monitored with high-resolution photoelectron spectroscopy. The question of the basis for a so-called ‘‘magnetocatalytic effect’’3 is addressed in the light of these results.
Relaxation dynamics of metastable antiferromagnetic states
Authors: Gregg J. F. , Kob W. , Lord J. S. , Morris I. D. , Pfeffer J. Z. , Schilling R. , Wells M. R. , Wolf W. P.
Published: July 2002

Using microwave ultrasonic interferometry we have studied the relaxation dynamics of an order-order transition in an Ising antiferromagnet. The system is prepared in one antiferromagnetic phase and relaxes to its thermodynamically inequivalent time-reversed conjugate, driven by an induced staggered field. The time-reversed phases induce equal and opposite changes in the velocity of transverse polarized ultrasonic waves and this is exploited by the interferometer to monitor the volume fractions of the conjugate phase during the relaxation process. The results indicate that the relaxation is dominated in the initial stages by domain growth of the stable phase as opposed to nucleation-dominated growth. This process can be closely fitted by a time-cubed growth law for the domains of stable phase. The final stages are well approximated by exponential behavior.
Rotational properties of vector phenomenological models of magnetic media
Authors: Vajda Ferenc , Oti John , Della Torre Edward
Published: July 2002

A modular computer package was developed to obtain the behavior of both the reversible and the irreversible components of the magnetization of media models as an applied field’s strength is varied and rotated. This package was designed to evaluate vector models, and consists of a preprocessor, a solver, and a post-processor. Two solvers have been implemented, namely the vector-product model and the vector-moving model. This package, due its structure which is capable of comparing the properties of different models, as well as analyzing experimental data, is intended to be used to find an optimum set of model parameters. Since the modules communicate through files, one could directly drive a computer-controlled vibrating sample magnetometer from this package, instead of using a model solver. Thus, one can compare experimental and computed data directly with the same commands. The post-processor graphics allows the user to view and compare several outputs.
Preparation of TbFeCo thin films with large Kerr rotation angle by facing targets sputtering
Authors: Ito H. , Yamaguchi M. , Naoe M.
Published: July 2002

The TbFeCo films were prepared at argon gas pressure PAr of 0.8 and 3.6 mTorr by using the facing targets sputtering (FTS) apparatus. The films showed the polar Kerr rotation angle θk as large as 0.43° at a wavelength of 830 nm. In polar Kerr hysteresis loops, the films with large θk showed the rectangular loops in which the gradient of magnetization at an applied field of coercivity was extremely large. Moreover, the films deposited at PAr of 0.8 mTorr showed the strong wavelength dependence of θk compared with the ones deposited at PAr of 3.6 mTorr.
The effect of Tb and Gd composition on the corrosion resistance of sputtered GdTbFeCo thin films
Authors: Yan B. D. , Warren G. W. , Kim M. H. , Barnard J. A.
Published: July 2002

A series of GdxTby(Fe92Co8)100-x-y alloy thin films (about 200 nm in thickness) on glass substrates was prepared by dc magnetron sputtering using composite targets of pure Fe, Tb, Gd, and Co. Film composition was changed by varying the area percent of Tb and Gd in the target from 0% to 24%. In order to study the individual and combined effects of Gd and Tb, alloys of GdFeCo, TbFeCo, and GdTbFeCo were compared to each other and to an Fe92Co8 alloy. These comparisons were made on the basis of standard electrochemical tests, including anodic polarization as well as measurement of open circuit potentials and potential decay, in deaerated 0.15-M NaH2PO4 buffer solution (pH=4) at 25 °C. Results indicate that additions of Gd and Tb decrease the corrosion current of the Fe92Co8 alloy by a factor of 10 or more. Both Gd and Tb show very similar effects on the corrosion behavior of FeCo. Results suggest that a total of about 24% Gd and/or Tb may represent an optimum level for improving the corrosion resistance of GdTbFeCo magneto-optical thin films.
Optimum laser power and pulse length for recording of magneto-optical media
Authors: Otter E. L. , Finkelstein B. I. , Madison M. R. , McDaniel T. W.
Published: July 2002

This paper discusses the relationship between optimum laser power and pulse length to obtain the maximum signal amplitude in pulse position modulation magneto-optical (MO) recording. The American National Standards Institute standard (X3B11) for 89-mm disks contains an expression for the maximum allowable optical-pulse power incident at the surface of the recording disk as a function of laser-pulse length: Pw=C[(1/tw)+(1/(tw)1/2], where C=75, Pw is the power in mW, and tw is the laser-pulse duration in nanoseconds. This expression was derived empirically, with C a media characteristic (set to 75 in the standard). To evaluate the adequacy of this empirical equation, disks from five different MO disk suppliers were evaluated for write performance. Having examined different metrics to determine optimum write power for a given laser-pulse length, the power producing the maximum signal amplitude for 1.56-μm mark spacings was selected as the performance criterion. Signal amplitude maximizes because of the competition between mark size and intersymbol interference. The value of the constant in the equation was determined for all media tested. The purpose of this paper is twofold. First, we report the results of our experiments confirming the equation across a broad range of laser powers, pulse lengths, and media types. Second, we modeled the thermal properties of the media and the optical read-back process to explain the empirical relationship among signal amplitude, write power, and pulse length.
Magnetic theory of very small devices (invited)
Authors: Slonczewski J. C.
Published: July 2002

A principal theme of this report is that the exchange-stiffness parameter replaces magnetic anisotropy as the principal core-material parameter (besides MS) governing the sensitivity of a read head when the cross-sectional dimensions of the core are less than 1 μm. A cylindrical magnetic tube within the core of a read head would provide micromagnetic advantages. Under certain conditions the unstable exchange energy stored in a magnetic vortex interacts with the stabilizing effect of self-demagnetization to amplify the stored flux by drawing fringing flux from the underlayer and shield. Reading characteristics which are valid, from the micromagnetic standpoint, down to storage-cell areas as small as (40 nm)2 are predicted. For sensitive reading, the core length must be near 100 (D/1 μm)2 μm, where D is the outer diameter of the tube.
Phenomenological criteria for ferromagnetism and spin-density waves in Ce compounds and other narrow-band metals (abstract)
Authors: De Long L. E. , Huber J. G. , Bedell K.
Published: July 2002

Recent data suggest that ferromagnetic Ce compounds are only found for a restricted range of Ce-Ce distance1: 3.65 Å≤d≤3.83 Å. This hypothesis will be compared to the ‘‘R-γ*’’ correlation,2 which states that magnetic order can only occur when the electronic heat capacity coefficient γ*≥γ*m ≊3.5×104 erg cm−3 K2. Additional analysis suggests that, as a function of increasing γ*, weak ferromagnetism first occurs as γ* initially surpasses γ*m, yielding to antiferromagnetism at higher γ*. Materials such as URu2Si2 constitute evidence that a third region of spin-density-wave order preceeds that of ferromagnetism for γ*≊γ*m. Spin-density-wave order also competes with antiferromagnetism and coexists with heavy fermion superconductivity in the case of CeCu2Si2 and UPt3 (and possibly UBe13) in a narrow region about γ*≊1×105 erg cm−3 K2. The temporary stability of ferromagnetism may be explained by the wave-vector dependence of the electronic self-energy that is responsible for heavy fermion behavior.
Neutron depolarization in reentrant spin glasses Ni1−xMnx
Authors: Mirebeau I. , Itoh S. , Mitsuda S. , Watanabe F. , Endoh Y. , Hennion M. , Calmettes P.
Published: July 2002

Polarization analysis of the transmitted beam was performed using a pulsed neutron source in a reentrant spin-glass (RSG) system Ni1−xMnx, for a concentration range (0.23≤x≤0.28) encompassing the critical concentration. The same samples were previously used to determine the magnetic phase diagram by magnetization measurements. The polarization P was measured as a function of temperature (15≤T≤300 K), neutron wavelength (3≤λ≤10 Å) and applied field (7 Oe≤H≤125 Oe). Unlike other RSG systems, P measured in a 7 Oe field decreases at all temperatures below the Curie temperature even for the samples close to the critical concentration. The wavelength dependence of the polarization indicates the existence of magnetic domains whose size δ decreases with increasing x from a few tenths of micrometers at x=0.23 to a few thousands of angströms at x=0.25. No depolarization occurs for x>0.25. As shown for x=0.23, the irreversibilities of the polarization, which depends on the cooling conditions, indicate that the domain arrangement freezes at low temperature. Clearly, in NiMn, the drop of the susceptibility at low temperature does not result from a decrease of the size or magnetization of the domains but from a slowing down of their mobility.
Evolution of the magnetic structure in TbxY1−xAg (abstract)
Authors: Gotaas J. A. , Said M. R. , Kouvel J. S. , Brun T. O.
Published: July 2002

As nonmagnetic yttrium is substituted for terbium, TbAg changes from a conventional antiferromagnet into a complex magnetic system with components of both commensurate and incommensurate spin structures and bulk magnetic properties typical of spin-glass behavior for x≤0.5. We have carried out neutron diffraction experiments on polycrystalline samples with x=0.15, 0.5, and 1.0 to complement our earlier studies for x=0.3.1 For x=0.5, the system orders at T≊58 K into the mixed structure observed for x=0.3, but with a much sharper (1/2 1/2 0) peak. On cooling below 40 K the incommensurate peaks almost disappear, remaining only as low-intensity shoulders of the commensurate peak at the lowest temperature measured (T=4 K). Hysteresis is observed in the relative intensity of the two components of the structure between 45 and 55 K. For x=0.15, the system orders at T≊18 K into the two-component structure, but with an approximately constant intensity ratio and no evidence of hysteresis, similar to the behavior found for x=0.3. Thus, as the Tb concentration decreases, the relative stability of the commensurate and incommensurate structures appears to change. This balance between competing magnetic structures will be related to the spin-glass properties observed in this system.
The relaxation process of the uniaxial anisotropy of Co-Zr amorphous films (abstract)
Authors: Imagawa T. , Mitsuoka K. , Narishige S. , Sugita Y.
Published: July 2002

Thin film heads made of Co-based amorphous films with high saturation magnetization were reported to have superior capabilities for high-density magnetic recording.1 However, it was found that the uniaxial anisotropy is changed during head fabrication processes. In this study, we investigate the relaxation process of the uniaxial anisotropy of sputtered Co-Zr amorphous films. 1-μm-thick Co-Zr films of 1.4 T were prepared by rf sputtering. Uniaxial anisotropy was induced by the magnetic field of 50 Oe during deposition. After deposition, annealing was performed in the dc field of 500 Oe applied parallel to the easy axis of the films for stabilizing the uniaxial anisotropy (preannealing). The preannealing temperature was changed from 200 to 350 °C. The films were annealed again in a field along the hard axis to reduce the uniaxial anisotropy. Decrease of the anisotropy field was discussed based on Richter-type relaxation,2 where τ2 is the maximum and τ1 is the minimum relaxation time, respectively. Without preannealing, the bandwidth ln(τ2/τ1) is broad at higher annealing temperatures. With preannealing, the bandwidth becomes narrower and is almost constant for various preannealing temperatures. These results show that the relaxation processes with shorter relaxation times are all suppressed by preannealing. The average activation energy is 2.0 eV without preannealing. It increases with preannealing temperatures and reaches 2.9 eV at 350 °C. From these results, it is clear that preannealing at higher temperatures is more effective in stabilizing the uniaxial anisotropy in the CoZr amorphous films.
Pair correlations within dispersions of strongly interacting BaFe particles
Authors: Mayo P. I. , Bradbury A. , Chantrell R. W.
Published: July 2002

Barium hexaferrite has widely been reported as a particulate medium eminently suitable for high-density perpendicular recording. In this paper we report the results of Monte Carlo simulations of strongly interacting barium hexaferrite platelets, in a precoated, dynamic dispersion. Magnetization, light transmission, and particle correlation characteristics are investigated as a function of external applied fields up to 1000 Oe. As a result of magnetostatic particle interactions, these platelets are revealed to stack readily in zero external applied field, and show well-defined anisotropic stacking in the presence of a field.
Demagnetizing influence on the hysteresis loop of a perpendicular medium with columnar structure
Authors: Chang Ching-Ray
Published: July 2002

We propose a simple three-dimensional particulate model with average interaction field to calculate hysteresis behavior of a perpendicular medium with columnar structure. The influence of the thickness of thin film, gap between the columns, and direction of growth, Δω50, on the hysteresis loops are investigated. From the simulation results, we can conclude that the coercivity critically depends on the thickness. The coercivity of the in-plane loop is quite sensitive to the direction of growth, while the perpendicular loop is not. If the columns are not perfectly aligned, the in-plane loop will open. The larger Δω50 results in the larger opening. The only significant contribution from the variation of the gap between the columns is to change the anomalous shearing of the hysteresis loop. Our model was used to calculate minor loops. For the perpendicular direction, the minor loops are always quite flat and merge to the major loop, while for the in-plane loops they are asymmetrical to the origin.
Zero-point photon-electron field-field interaction as the origin of the high-temperature superconductivity of oxide (abstract)
Authors: Iida S. , Mitsui T. , Abe K.
Published: July 2002

The new frames in physics has united the physics of materials and a new terminology of supercorresponding c- and q-number systems is introduced for the classical and quantal physics. In the q-number system, the electron field as represented by the Schrödinger or Dirac wave function is regarded as the electron itself, which interacts with the zero-point photon field and gives the mass renormalization. The new frames can analyze this subject without the divergence difficulty. The resultant decrease in the self-energy of the electron of about 1 eV is proposed to be dependent on the environmental crystal situation, and may become cooperative to generate coherence to the conduction electron-photon coupled system. Our basic c-number equation includes the vector potential of the interacting photon field, the microscopic electronic electric current, the frequency of the considered photon field, the original plasma frequency of the conduction electrons, and an equivalent frequency and the resonance frequency of the bound-state electrons. Placing reasonable values for these constants, we get a detailed k-ω diagram for the photon-electron field complex. By the action of the bound-state electrons, the original plasma frequency of about 4 eV is separated into two, with the lower value of about 1.7 eV (2.6×1015 s−1). This branch crosses the free-photon mode near the same frequency, opening a sharp narrow window for the zero-point free-photon field. The Fermi frequency of the conduction electrons is about the same with the wavelength of about 10 Å, which may fit one of the crystal dimensions. The zero-point photon-electron field complex at this frequency may couple to the electron field at the Fermi level through the induced zero-point ripple electron field. The Heisenberg uncertainty, ΔEΔt≥ℏ, may operate to the window for generating the longer time-space coherence. We call this mechanism CRCG (crystallo-radiative coherence generator).
Superconductivity of Sb-doped BiPbSrCaCuO compounds (abstract)
Authors: Zhang Yu , Muhammed M. , Nogués J. , Rao K. V.
Published: July 2002

Extensive studies on Pb-doped Bi-Ca-Cu-O superconductors have seldom produced a single-phase Bi-based superconductor from a magnetic point of view without the 80-K phase in it, even though zero-resistance has been commonly observed around and above 100 K. This is mainly because of the additional intergranular phases controlled by calcium and copper diffusion which appears to be extremely sensitive to even minute traces of impurities. Hence, recent reports on the observation of superconductivity above 130 K in Sb-doped Bi-based alloys is of interest. We have prepared a number of Bi-based superconducting compounds containing Pb and/or Sb using the computerized coprecipitation method (which gives powders of submicrometer size particles) as well as a solid-state diffusion technique. The role of excess Ca on the superconducting properties have also been studied. Effects of various short- and long-term heat treatment conditions have also been investigated. The final pelletized samples have been characterized by x-ray, electrical, and magnetic property measurements. In most cases, Tc (R=0) of a sample can be easily enhanced to 105–110 K by modifying the annealing conditions. However, we have not yet been able to verify the claims of Tc(R=0) over 130 K although a small portion with Tc(onset) over 130 K does appear sometimes. Samples without Sb have to be annealed in air over 60 h to reach high Tc(R=0) than 100 K while those with Sb substitution need a maximum of 24 h to achieve the higher Tc. ac susceptibility studies over four decades of field amplitudes (3 mOe–30 Oe) show, below the diamagnetic onset temperature at 110 K, a knee around 100 K, which is perhaps due to intergranular low-field penetration. In any case in these samples it is possible to completely suppress the 80-K phase. From dc magnetic data we find that in these Sb-doped samples Jc(transport) can be varied from 350 to 1500 A/cm2 at 4.2 K, and is weakly field dependent as compared to those without Sb-doping. Systematic studies will be presented.
Strong low-field H
c2 anisotropy in Tl2Ba2Ca2Cu3Ox
Authors: Claus H. , Crabtree G. W. , Vandervoort K. G. , Liu J. Z. , Lam D. , Bukowski E. D. , Ginsberg D. M.
Published: July 2002

Low-field dc magnetization measurements in fields between 0.5 and 40 G are reported on a single crystal of nominal composition Tl2Ba2Ca2Cu3O10. It is observed that Hc2, determined from the diamagnetic onset temperature, is quite anisotropic. Extrapolating Hc2 to zero field yields zero field superconductive transition temperatures of Tc=118 K for H parallel to the c axis and Tc=111 K for H perpendicular to the c axis. This apparent inconsistency is thought to be due to the variation in stacking sequence of the Cu-O sheets.
Attempts to prepare Bi-based superconductors on a carbon fiber substrate
Authors: Al-Sharif A. , O’Shea M. J. , Thomas S. , Sherwood P. M. A.
Published: July 2002

We have studied the properties of thin films prepared by laser ablation of a Bi2Sr2CaCu2O8−δ target on carbon fiber substrates. Excimer laser radiation was used to prepare the films. Films were deposited on polyacrylonitrile-based carbon fibers. X-ray photoelectron spectroscopy (XPS) was used to detect the presence of the elements in the film. The structure of the film was studied using x-ray diffraction (XRD). Resistance measurements indicated that the films were not superconducting down to 4.5 K, but the XPS and XRD data showed that films could be formed on the fibers. We find evidence of oxidation of the carbon fiber surface by the superconductor material.
Fe Mössbauer effect in YxPr1−xBa2(Cu0.98Fe0.02)3O7
Authors: Rubinstein M. , Swartzendruber L. J. , Bennett L. H. , Ritter J. J. , Atzmony U.
Published: July 2002

5 7Fe Mössbauer effect and magnetic-susceptibility measurements were performed on Y0.2Pr0.8Ba2(Cu0.98Fe0.02)3O7−δ and Y0.8Pr0.2Ba2(Cu0.98Fe0.02)3O7−δ, where δ≊0. The insulating 80% Pr compound showed susceptibility anomalies and a large hyperfine field distribution at low temperatures in addition to a hyperfine field spectrum for Fe on the Cu(2) sites. The superconducting 20% Pr compound displayed none of these effects at any temperature. After examining the alternatives, it is postulated that magnetic ordering of the Pr ions and an enhanced rare-earth transition-metal interaction due to f-electron admixture is responsible for these observations.
U(Sn,Pb)3: A specific heat study of the magnetic-nonmagnetic transition
Authors: Yuen T. , Gao Y. , Perez I. , Crow J. E.
Published: July 2002

USn3 is a highly enhanced paramagnet and close to a magnetic instability. Whereas UPb3 has an antiferromagnetic transition at 31 K and the 5f electrons appear to be more localized. Both USn3 and UPb3 crystallize in the Cu3Au structure and complete pseudobinary alloys can be prepared. The temperature dependence of the specific heat C(T) for 1.5≤T≤50 K and magnetic susceptibility χ(T) for 1.8≤T≤ 400 K has been measured on U(Sn1−xPbx)3 for 0≤x≤1.0. Both χ(T) and C(T) data indicate that the substitution of Sn for Pb in UPb3 (i.e., decreasing x) causes the Neel temperature TN to increase initially to 38 K for x≊0.7 then decrease with TN→0 for x=0.07. The electronlike contribution to C(T) as T→0 remains high in the magnetically ordered state with the electronic specific heat coefficient γ for UPb3 ≊173 mJ/mol K2. The magnetic entropy removal below TN for UPb3 is ∼2−3 J/mol K, i.e., is smaller than expected for a U3+ or U4+ free ions and even for U3+ with a doublet ground state.
Pressure and magnetic field dependence of the antiferromagnetism of PrFe4P12
Authors: Kuric M. V. , Guertin R. P. , Torikachvili M. S. , Maple M. B. , Foner S.
Published: July 2002

The compound PrFe4P12, which has the cubic LaFe4P12 modified CoAs3 -type structure, orders antiferromagnetically, with TN=6.2 K and with several field-induced magnetic transitions for T<TN, up to 5 T for T≪TN. Heat capacity measurements show the transition is a bulk effect, with magnetic entropy consistent with a magnetic doublet or triplet crystal-field ground state. dc magnetization measurements, M(H), at T=4.2 K up to 23 T reveal no further field-induced transitions, and M(H) attains only 54% of the Pr3+ free-ion moment at highest fields. Pulsed field measurements of dM/dH at 4.2 K up to 45 T fail to resolve further transitions. The transition to the antiferromagnetic state is accompanied by a very sharp Suezaki–Mori-type electrical resistivity transition due to critical scattering of electrons by spin fluctuations. Hydrostatic pressure decreases TN, (dTN/dP=−0.11 K/kbar), in sharp contrast to the behavior for the isomorphic semiconducting ferromagnet, UFe4P12 (TC=3.15 K; dTC/dP=+0.26 K/kbar). The field-induced magnetic transitions are also sharply reduced with pressure. The pressure dependence of the electrical resistivity showed a decrease in TN, with a concomitant decrease in the size of the Suezaki–Mori anomaly. The results are discussed in terms of crystal-field splittings of the Pr ion and possible hybridization effects.
Magnetic viscosity studies in hard magnetic materials
Authors: Singleton E. W. , Hadjipanayis G. C.
Published: July 2002

The magnetic viscosity behavior has been studied in several hard magnets with different magnetization reversal mechanisms including barium ferrite powders, Cu-Mn-Al, ferrite magnets, Nd-Fe-B, and SmCo5, Sm2(Co,Fe,Cu,Zr)17. The measurements were made with a vibrating sample magnetometer for times up to 60 s and a SQUID magnetometer for longer times in the range of 60–2300 s. For most of the samples the magnetization was found to vary logarithmically with time. The field and temperature dependence of the magnetic viscosity coefficient S was studied. Here, S was found to vary with the applied field and it usually peaked around the coercive field Hc. The measured values of Smax at 10 K range from 0.004 to 1.853 emu/g for Cu-Mn-Al and Sm2(Co,Fe,Cu,Zr)17, respectively. The magnetic viscosity coefficient was used together with the magnetic susceptibility to determine the activation volume.
Magnetic hardening of Pr-Co-B sintered magnets (abstract)
Authors: Christodoulou C. N. , Wallace W. E. , Massalski T. B.
Published: July 2002

It is well known that the Pr2Co14B intermetallic compound has a high magnetocrystalline anisotropy (100 kOe at 294 K). This value is higher than that of the Pr2Fe14B compound (70 kOe at 294 K). Hard magnets, using the rapid-quenching technique, have been prepared for both the Co- and Fe-based materials. In the case of the conventional powder-metallurgy technique, only the Fe-based magnets have been successfully prepared. So far, there is no report of achieving any significant value of coercivity in the Pr-Co-B system. In this paper we report preliminary results on Pr-Co-B sintered magnets with room-temperature intrinsic coersivity, Hci, equal to 5.2 kOe and an energy product, (BH)max equal to 10.3 MGOe. At liquid-nitrogen temperature (77 K) Hci is greater than 18 kOe (which was the maximum available magnetic field) and (BH)max is equal to 12.5 MGOe. These values of (BH)max are twice those reported for the rapidly quenched magnets.1 The Curie temperature of the magnet is 988 K (corresponding to the presence of the Pr2Co14B phase). The microstructure of the magnets with high Hci is markedly different from that of the magnets with low Hci. The major difference is the lower concentration of voids and the well-defined shape of the grains. The grain size is between 5 and 30 μm. The major phases present in the magnet are Pr2Co14B, PrCo4B, and a minor amount of PrCo5. The metallurgical characteristics and phase-diagram features of the Pr-Co-B system seem to be substantially different from those of the Pr-Fe-B system, where the major phases present are the Pr2Fe14B, PrFe4B4, and the Pr-rich phase.