bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic properties across the metal-insulator transition (invited)
Authors: Sachdev Subir , Bhatt R. N. , Paalanen M. A.
Published: July 2002

We review recent theoretical and experimental work on the metal to insulator transition in doped semiconductors. The spin excitations on the insulating side of the transition can be described in terms of a spin- 1/2 Heisenberg antiferromagnet with the spins randomly located in space. Numerical analysis of this Hamiltonian has led to a fairly complete understanding of the static spin susceptibility and the electron spin resonance spectrum. The physics on the metallic side far from the transition is also fairly well understood in terms of a recently developed theory of the disordered Fermi liquid. The physics near the transition point is, however, still not clear. All the experimental evidence indicates the presence of local electronic moments on the metallic side of the transition. Recent theoretical work and open problems in the description of such a metallic phase are briefly discussed.
Magnetic behavior of both superconducting thin films and their deposition targets
Authors: Moorjani K. , Bohandy J. , Adrian F. J. , Kim B. F. , Atzmony U. , Shull R. D. , Chiang C. K. , Swartzendruber L. J. , Bennett L. H.
Published: July 2002

The superconducting properties of the bulk oxides La1.85Sr0.15CuO4−x and Ba2YCu3O7−y were investigated through their magnetic behavior by vibrating-sample magnetometry and complex ac susceptibility, and by dc resistivity and microwave response. These oxides were then used as targets in the preparation of thin films using a laser-ablation technique. The superconducting properties of the films were established through their microwave response. The M-H loops at low temperatures clearly establish the type-II behavior of the Ba-Y-Cu-O materials. The real part of the ac susceptibility showed the superconducting transition. Simultaneously, the imaginary part of the ac susceptibility showed the existence of nonsuperconducting portions in the samples which have a low enough resistivity to carry a significant amount of current. The novel method of microwave response was used to detect the superconductivity for both the bulk oxides and the thin films.
Effect of vacuum annealing on the superconducting transition temperature of the Y-Ba-Cu-O system
Authors: Wang Dajian , Wu Guobin , Huang Shengtao , Tang Chenghuan , Cei Wanqiu , Dan Peng
Published: July 2002

The effect of vacuum annealing on the superconducting transition temperature Tc of the ceramic superconductor Y-Ba-Cu-O system with a perovskite-type structure has been investigated. For fixed annealing time, the resistivity increase and Tc decrease become larger for higher annealing temperature, and the crystallographic lattice parameters decrease slightly with increasing annealing temperature. This may be understood as the result of the introduction of oxygen vacancies in the basal plane of the CuO6 octahedra. Therefore, it seems to be quite important to reduce the density of oxygen vacancies in the present system.
Electronic structure, charge transfer excitations, and high-temperature superconducting oxides (invited)
Authors: Freeman A. J. , Yu Jaejun , Massidda S. , Fu C. L. , Xu J.-H.
Published: July 2002

We present high-precision results on the electronic band structure and properties of YBa2Cu3O7−δ, YB2Cu3O6, GdBa2Cu3O7−δ, and La2−xMxCuO4 as obtained from highly precise state-of-the-art local density calculations. The results obtained demonstrate the close relation of the band structure to the structural arrangements of the constituent atoms and provide an integrated chemical and physical picture of the interactions and their possible relation to superconductivity. The ionic character of the Y is proven by similar detailed highly precise local density calculations for high TC GdBa2Cu3O7, and explains the coexistence of magnetism and superconductivity in the high TC rare-earth superconductors. Surprising features are the low density of states (DOS) at EF, especially for δ≥0.1 which is lower per Cu atom than that in La2−xSrxCuO4—in agreement with experiment and a relatively large magnetic Stoner factor. Strong indications are demonstrated for the inadequacy of a conventional phonon mechanism for obtaining the higher TC. Charge transfer excitations of occupied Cu1−O dpπ antibonding orbitals into their empty Cu1-O dpσ antibonding orbital partners, result in poorly screened ‘‘Cu3+-Cu4+’’ -like charge fluctuations. These charge transfer excitations (excitons) thus lead to strong polarization effects in these poorly screened (highly ionic) materials and induce attractive interactions among the 2D electrons. Thus, these interactions via exchange of excitons enhance the electron pairing and serve to enhance the TC proposed for the quasi-2D superconductors.
One-dimensional Hubbard model with first- and second-neighbor hopping
Authors: Kaplan T. A.
Published: July 2002

The one-dimensional Hubbard model with first- and second-neighbor hopping, t1 and t2, with t2≊t1/(2)1/2 and large repulsion U, is proposed as a testing ground of Anderson’s idea that the existence of an resonating valence bond ground state in the half-filled-band case will show superconductivity when a finite concentration of holes is introduced. Interestingly, our preliminary calculations in the U=∞ limit for small chains have revealed a new mechanism for enhancement of singlet pairing: We find that for t2<0 the ground state is a singlet, and, for t2<0 and ‖t2/t1‖≪1, the nearest-neighbor extended-singlet-pairing correlation function becomes enhanced appreciably as the temperature decreases below t2.
Magnetic susceptibility of rapidly solidified YBa2Cu3O7−x superconductors
Authors: McHenry M. E. , McKittrick J. , Sasayama S. , Kwapong V. , O’Handley R. C. , Kalonji G.
Published: July 2002

The superconducting YBa2Cu3O7−x phase has been produced through a novel rapid solidification processing route yielding a high-quality granular superconducting phase. Features in the magnetic susceptibility as measured at various fields can be attributed to important transitions in the superconducting behavior of these materials. At low fields (∼20 Oe) anomalies in the susceptibility arise from decoupling of the grains in these polycrystalline materials. Susceptibility response at higher fields (∼100 Oe) is representative of randomly oriented superconducting grains which show local superconducting current densities comparable to those of single crystals. At even higher fields features consistent with Muller’s superconducting glass state model are observed.
Schottky junctions on CuInSe2 films
Authors: Shih I. , Qiu C. X. , Qiu S. N. , Huang J. F.
Published: July 2002

Schottky junctions have been fabricated by evaporating aluminum on electrodeposited p-type polycrystalline CuInSe2 thin films. The devices showed a rectification effect with an ideality factor of about 2 in the low-forward-voltage region. Results of carrier concentration obtained from C-V measurements of the Schottky devices were found to be consistent with those for CdS/CuInSe2 heterojunctions fabricated on the same substrates. The apparent diffusion potential and carrier concentration of the Al/CuInSe2 Schottky junctions were both found to increase after a short heat treatment at 200 °C in air.
Wavelength tuning and switching of a coupled distributed feedback and Fabry–Perot cavity laser
Authors: Wu M. C. , Boenke M. M. , Werner M. , Schiffmann F. , Lo Y. H. , Wang S.
Published: July 2002

We present a theoretical analysis of a newly demonstrated semiconductor laser with coupled distributed feedback and Fabry–Perot (DFB-FP) cavities and show that three modes of operation are possible for such a laser. In mode-switched DFB mode, the wavelength can be switched between longitudinal modes on either side of the stopband. In coupled-cavity laser mode, there are successive mode hops inside the stopband. Finally, in continuously tunable distributed Bragg reflector mode, a wide wavelength tuning range (4.8 Å) without mode hopping can be obtained. The analysis is general enough to be applied to any laser with a periodic waveguide section, and provides an understanding of the mechanisms and the limits of wavelength tuning in such lasers. This type of laser has very important applications in coherent optical communications.
Model potential calculation of the thermal donor energy spectrum in silicon
Authors: Chen C. S. , Schroder D. K.
Published: July 2002

The two-parameter model potential originally proposed by Ning and Sah [Phys. Rev. B 4, 3468 (1971)] for calculating the ground-state energies of group V and group VI impurities in silicon is extended to the variational calculation of the thermal donor ionization energies. In the multivalley effective mass approximation, the theoretical results are in excellent agreement with the reported experimental data. This provides additional evidence for the assumption that thermal donors consist of five to thirteen oxygen atoms, as first proposed by Ourmazd, Schröter, and Bourret [J. Appl. Phys. 56, 1670 (1984)].
Photoreceptor charging mechanism by conductive particle rubbing and application to a novel electrophotographic printing technology
Authors: Tetsutani Nobuji , Hoshino Yasushi
Published: July 2002

A photoreceptor charging method by rubbing with magnetic conductive particles has been reported. In this paper, the charging mechanism of particle charging is further discussed. The different mechanisms involved in particle charging and corona charging are investigated in a-Se. It is shown that captured charges for particle charging exist near the surface from a comparison of the relative sensitivities between the two charging methods. The trap depth of trapped charges (holes) is roughly to be estimated between 0.8 and 0.9 eV. It is also verified that the particle charging method can be used to charge insulative film. As an application of the described particle charging method, a novel electrophotographic printing technology is proposed.
The nature of a 2050–2150-cm−1 infrared band in neutron-transmutation-doped silicon grown by the floating-zone method in a hydrogen atmosphere
Authors: Meng Xiang-ti , Qin Guo-gang , Du Yong-chang , Zhang Yu-feng
Published: July 2002

A strong, wide infrared absorption band in 2050–2150-cm−1 range has been found in neutron-transmutation-doped silicon grown by the floating-zone method in a hydrogen atmosphere and annealed at 650 °C. This band is due to the stretching vibration of the Si–H bond, formed as hydrogen atoms diffuse, aggregate, and saturate silicon dangling bonds in the neutron-irradiation disordered region. This absorption band is similar to that of hydrogenated amorphous silicon in the 1900–2150-cm−1 range except that its vibration frequency is somewhat higher. The disordered region defect and its amorphous-siliconlike property could be observed because of the incorporation of hydrogen. The hydrogen concentration of the floating-zone melted silicon grown in a hydrogen atmosphere used in this study is estimated to be (1–2)×1017 cm−3 on the basis of the strength of this wide infrared absorption band. A possible reason for this vibration frequency shift is discussed.
Secondary ion scattering in dark field ion microscopy
Authors: Bernardo Dan N. , Ausserer Walter A. , Ling Yong-Chien , Morrison George H.
Published: July 2002

The formation of dark field ion images by scattered secondary ions is proposed. It is shown that this model can account for various dark field observations, and that it can be used in the determination of surface feature inclinations. Its use in the estimation of topography-induced bright field intensity contributions is also demonstrated.
Residual strain gradients in a fully stabilized zirconia sample
Authors: Hwang Bing , Houska C. R. , Ice G. E. , Habenschuss A.
Published: July 2002

Polished and severely ground fully stabilized zirconia samples are examined using primarily x-ray diffraction (XRD). The XRD (111) profile reflections from both samples were broadened asymmetrically compared to that of an annealed sample. The asymmetry results from a d-spacing gradient extending from the free surface into undisturbed bulk material. There are two possible origins of this depth gradient, i.e., variations in residual strain or chemical composition. The latter is eliminated by means of x-ray photoelectron spectroscopy which did not reveal a chemical gradient. d-spacing profiles for both samples are obtained nondestructively using a trial and error fitting procedure. A maximum compressive strain of ∼4% is obtained at the surface of the ground sample which decreases gradually to zero at greater depths. The overall zone is ∼1–2 μm. A similar but smaller compressive zone is found in the polished sample which is followed by a zone of tension. The maximum compressive strain at the surface is ∼5% and the overall zone of residual strain is ∼0.1 μm.
Spin-density wave in spin glasses (C
uMn) and other dilute alloys (YGd)
Authors: Mydosh J.A.
Published: July 2002

Spin glasses have been of particular interest in magnetism for the past 15 years. Their present theoretical description, based upon a simple physical model, requires a fundamentally new type of statistical mechanics for its solution. Most experimental interpretations have attempted to follow this approach, although its physical meaning and direct comparison are partially obscured and somewhat strained. Almost 30 years ago Overhauser proposed a spin-density-wave (SDW) model which endeavored to explain the ‘‘antiferromagnetic’’ behavior in dilute alloys such as CuMn. In the intervening period the spin-glass phenomenon has mainly evolved unconnected with the SDW approach. However, in recent times a series of neutron-scattering experiments has renewed the connection and introduced the distinction between a ‘‘long-range’’ SDW as found in YGd and the ‘‘short-range’’ SDW of the spin glasses, e.g., CuMn. It is the purpose of this paper to review the available evidence for a SDW mechanism in these dilute alloys relying not only on neutron-scattering data, but also on macroscopic measurements such as specific heat, magnetization, and susceptibility. Here clear-cut differences arise between the long-range and short-range SDWs which are related to the glassy dynamics of the latter systems.
Evidence for particle interaction effects in Nd-Fe-B ribbons
Authors: Pinkerton F. E.
Published: July 2002

A model is developed to describe the magnetization behavior of randomly oriented uniaxial particles, in which it is assumed that the individual Nd2Fe14B crystallites do not interact. The model agrees qualitatively with the observed magnetization behavior of melt-spun Nd-Fe-B ribbons, but significant quantitative violations of the model provide evidence for substantial particle interaction effects in the ribbons. This corroborates earlier transmission electron microscope and magnetization studies in which indications of such effects were reported. A qualitative discussion of the role of particle interactions in the magnetization behavior is presented.
Coupling of guided modes in thin films with surface corrugation
Authors: Seshadri S. R.
Published: July 2002

Two guided modes in a grounded, thin, dielectric film waveguide having a periodic corrugation of the interface between the film and the cover can interact selectively in the neighborhood of a particular frequency. Five different theories exist for the investigation of this interaction. The first three are small amplitude theories which assume that the amplitude of the corrugation relative to the thickness of the film is small and the third and the fourth theories are large amplitude theories which do not make such an assumption. In the small amplitude theories, solutions are sought as a perturbation of ideal normal modes and boundary conditions are applied on the average flat surface. The first small amplitude theory makes use of quasioptical considerations and the other two small amplitude theories employ wave-theoretical techniques. All three theories lead to identical coupled mode equations governing the interaction. The first large amplitude theory is based on an expansion in terms of local normal modes and is applicable for a grating with a smooth profile. The second large amplitude theory is applicable for a lamellar grating and it makes use of a model of a repetitively mismatched transmission line. The various intrinsic dephasing mechanisms are explained. The large amplitude theories are able to account for the dephasing effects due to the finite (nonvanishing) depth and the finite (noninfinite) length of the corrugations. Some aspects of passive devices and some theoretical considerations of active devices using corrugated waveguides are reviewed. The transmission line model is able to take into account the end effects. For the mode of operation considered, there are no end effects and the overall contribution of the vertical discontinuities vanish for the lamellar grating.
Analysis of free electron laser performance utilizing the National Bureau of Standards’ cw mictrotron
Authors: Tang Cha-Mei , Sprangle Phillip , Penner Samuel , Maruyama Xavier K.
Published: July 2002

The National Bureau of Standards’ (NBS) cw racetrack microtron (RTM) will be utilized as a driver for a free electron laser (FEL) oscillator. The NBS RTM possesses many exceptional properties of value for the FEL: (i) cw operation; (ii) energy from 20–185 MeV; (iii) small energy spread and emittance; (iv) excellent energy stability; and (v) high average power. The 1D FEL gain formula predicts that the FEL would oscillate at the fundamental approximately from 0.25–10 μm when upgrading the peak current to ≥2 A. In this paper, we present 3D self-consistent numerical results including several realistic effects, such as emittance, betatron oscillations, diffraction, and refraction. The results indicate that the design value of the transverse emittance is small enough that it does not degrade the FEL performance for intermediate to long wavelengths, and only slightly degrades the performance at the shortest wavelength under consideration. Due to the good emittance, the current density is high enough that focusing, or guiding, begins to manifest itself for wavelengths >2.0 μm.
Influence of substrate misorientation on defect and impurity incorporation in GaAs/AlGaAs heterostructures grown by molecular-beam epitaxy
Authors: Radulescu D. C. , Wicks G. W. , Schaff W. J. , Calawa A. R. , Eastman L. F.
Published: July 2002

GaAs/AlGaAs heterostructures have been grown by molecular-beam epitaxy on GaAs substrates intentionally oriented (tilted) a few degrees (0–6.5) off the (001) plane towards either (111)A, (111)B, or (011). We observe that the 4-K photoluminescence and low-field electron transport properties of these structures may be functions of the substrate tilt angle and tilt direction depending on the concentration of impurities incorporated during growth. A substrate tilt during molecular-beam epitaxy is observed to have the largest effect on these properties when the background impurity concentration in the molecular-beam epitaxial machine is high. This supports our contention that the observed changes in material characteristics are due to differences in the incorporation of defects and impurities. The incorporation of defects and impurities are reduced by using substrates tilted toward (111)A in comparison to nominally flat (001) substrates or substrates tilted toward (111)B.
Models of second-order effects in metal-oxide-semiconductor field-effect transistors for computer applications
Authors: Benumof Reuben , Zoutendyk John , Coss James
Published: July 2002

Second-order effects in metal-oxide-semiconductor field-effect transistors (MOSFETs) are important for devices with dimensions of 2 μm or less. The short and narrow channel effects and drain-induced barrier lowering primarily affect threshold voltage, but formulas for drain current must also take these effects into account. In addition, the drain current is sensitive to channel length modulation due to pinch-off or velocity saturation and is diminished by electron mobility degradation due to normal and lateral electric fields in the channel. A model of a MOSFET including these considerations and emphasizing charge conservation is discussed.
Reverse sequence of formation of titanium nitrides by nitrogen implantation
Authors: Zhou X. , Dong H. K. , Li H. D. , Liu B. X.
Published: July 2002

Room-temperature implantation was conducted for the thin titanium films by 80-keV nitrogen ions. It was found that TiN began to appear at a dose around 2×1017 N/cm2, and the titanium film converted entirely into TiN after 1×1018 N/cm2 implantation. Surprisingly, Ti2N, which has a lower N/Ti ratio than TiN, was only detected at an even higher implantation dose, e.g., as high as 2×1018 N/cm2. This reverse sequence of titanium nitride formation was attributed to the structural compatibility between the matrix and new phase being formed. Viewed in this light, a shearing mechanism is proposed, which can explain the titanium nitride formation, and is also applicable to other metal nitrogen systems.
Reliability of planar InGaAs/InP photodiodes passivated with boro-phospho-silicate glass
Authors: Martinelli Ramon U. , Enstrom Ronald E.
Published: July 2002

Planar InP/In0.53Ga0.47As photodiodes with boro-phospho-silicate glass (BPSG) layers as a Zn-diffusion mask and as an InP surface p-n junction passivant exhibit stable, reverse-bias leakage-current characteristics at 220 °C for more than 104 h. Based on these results, estimates of the time required for the leakage current at 70 °C to saturate at 10 nA is, in the worst case considered, about 33 years. As a passivant and diffusion mask, BPSG compares favorably with silicon nitride.
Temperature and pulse shape dependence of the switching behavior in ferroelectric liquid crystals
Authors: Patel J. S. , Goodby J. W.
Published: July 2002

In this study we have examined the effect of temperature dependence of switching using a simple probe pulse waveform. The effect of more complex waveforms shows that the area law is observed for switching, but the magnitude of the area depends on the shape of the pulse employed. Switch and latch behavior is found all across the smectic C* range, although the switching time increases as the temperatures decrease. Symmetry in switching was also observed across the entire smectic C* range. The decrease in the switching speed is primarily due to the increase in the viscosity of the material, which for the most part behaves in an Arrhenius fashion. An unexpected and unusual phase transition around 3 °C was found which is most likely of the smectic-C*–to–smectic-I* type. This unusual behavior of the phase transition may be due to the complexity of the mixture producing this phase.
Microstructural investigation of the high-T
c superconductor YBa2Cu3Ox with a scanning tunneling microscope
Authors: Laiho R. , Heikkilä L. , Snellman H.
Published: July 2002

Surface structures of the high-Tc superconductor YBa2Cu3Ox have been investigated with a scanning tunneling microscope. Four typical features are revealed, e.g., (i) flat area of the size up to 0.2×0.2 μm2, (ii) step structures with the height of the step from tens to hundreds of angstroms, (iii) regular arrays of subgrains having the width down to 100 Å, and (iv) a complex formed by these features. The sharp surface corrugations suggest strong local fluctuations in the elastic and electronic properties of this material.
Time and temperature influence on surface index change in K+-Na+ ion exchanged optical waveguides
Authors: De Bernardi C. , Malvicino C. , Morasca S. , Morra M.
Published: July 2002

An accurate analysis of optical waveguides made by Na-K ion exchange in glass is presented. A strict correlation between surface index change, guide birefringence, surface stress, and relaxation as a function of time and temperature is demonstrated.
Discrimination between conduction and polarization processes in an ac electric field
Authors: Doi Akira
Published: July 2002

A comprehensive description of the current response of any medium to an ac electric field can be made by using complex notations for conduction and polarization terms in the Maxwell equation and by discriminating each of them. Our formalism was applied to complex admittance Y*(=G−jB) of a glass and to several optical properties of metals. For the former, it was found that the conductance G is the sum of contributions from dc conduction and the dielectric loss factor while the susceptance B is due to the dielectric constant. By specifying each contribution to the observed admittance, the origin of ac conductivity in glass was clarified more profoundly. For the latter, the optical properties of metals can be dealt with more precisely by discriminating contributions from the conduction by free electrons and from the polarization by bound electrons.
A photoacoustic study of photoinjection processes in double-layered organic photoconductors
Authors: Kanemitsu Yoshihiko , Imamura Shunji
Published: July 2002

Photoinjection efficiencies of holes from the carrier generation layer (CGL) into the carrier transport layer (CTL) in double-layered organic photoconductors were measured by using a photoacoustic technique. The photoinjection efficiency depends on the concentration of transporting molecules doped in the polymer of the CTL. At low molecular concentrations, the injection efficiency is mainly limited by the low ability to transport carriers in the CTL. The concentration dependence of the photoinjection efficiency is found to be similar to that of hopping transport processes in the CTL. At high molecular concentrations, the photoinjection efficiency is limited by the free-carrier generation process in the CGL.
A Mössbauer study of fine iron particles (invited)
Authors: Qiu Z. Q. , Du Y. W. , Tang H. , Walker J. C.
Published: July 2002

Mössbauer spectroscopic measurements have been carried out on uniform-size Fe particles over a size range from approximately 150 to 400 Å. The particles were produced by evaporation into a nitrogen atmosphere. We have discovered that the thin oxide coating on these particles results in a significantly altered surface-phonon spectrum as well as increased coercivity and a larger magnetic hyperfine field for the Fe core. These results correlate well with studies on thin epitaxial films.
Probing molecular cages in polymeric gels using paramagnetic ions: Internal motion of cupric ion in a cage
Authors: Suryanarayana Darbha
Published: July 2002

The effect of high-energy irradiation on the aqueous solutions of certain water soluble polymers, namely, poly(vinyl alcohol) and poly(ethylene oxide), brings about physical and chemical changes. The cross links or networks formed in the polymer chains cause gel formation. These gels are considered to be made up of cubic cells or molecular cages with size varying between 0.6 and 1.5 nm. Such cages have been recently probed using the paramagnetic ions of hydrated cupric as well as vanadyl ions and were studied by electron paramagnetic resonance (EPR) and pulsed EPR techniques. The time-dependent EPR spectra of Cu2+ ions in the cages are analyzed in this study using specific motional models, employing the modified Bloch equations. Computer simulations show that the spin probe’s motion is governed by the segmental motion of the polymer chains.
A NMR study of R2(TM)14B compounds with R=Sm, Gd, or Lu and TM=Fe or Co
Authors: Erdmann K. , Rosenberg M. , Buschow K. H. J.
Published: July 2002

Nuclear-magnetic-resonance spectroscopy was used to study hyperfine fields at Fe and B sites in R2(TM)14B compounds with R=Sm, Gd, or Lu and TM=Fe or Co. The transferred hyperfine fields at B sites reflect changes in the magnetic structure of these compounds and are discussed in terms of dipolar fields
Field cooling and demagnetizing field influence on the magnetic structure observed in reentrant spin glasses
Authors: Mirebeau I. , Hennion M. , Lequien S. , Hippert F.
Published: July 2002

In reentrant spin glasses, small-angle neutron scattering measurements performed at low temperature in an applied field show a magnetic structure, i.e., a peak of the scattering I(q) at a finite qmax value. This structure has been studied as a function of applied field, cooling conditions, and sample geometry. The qmax value is found proportional to (Hint)1/2 where Hint is the internal field. In FeMnPBAl, the structure is insensitive to the field applied during the cooling process Hcool whereas in NiMn qmax increases with Hcool. This difference between the two systems is reminiscent of that observed in magnetization measurements which show that the memory effects induced by field cooling are stable in NiMn and unstable in FeMnPBAl.
Phase diagram for the Bethe lattice spin glass (abstract)
Authors: Carlson J. M. , Chayes J. T. , Chayes L. , Sethna J. P. , Thouless D. J.
Published: July 2002

We present the results of rigorous calculations for the ±J Ising spin-glass model on the Bethe lattice. The phase diagram for varying temperature and fraction of ferromagnetic bonds is derived near the paramagnetic phase boundary. In addition to the spin-glass and paramagnetic phases, we find a nontrivial ferromagnetic phase and a magnetized spin-glass phase, characterized by diverging Edwards–Anderson susceptibility. The recursion relation for the distribution of single-site magnetizations is studied as a dynamical system on an appropriate function space, the bulk thermodynamics is described by the attractors of the recursion relation, and the phase transitions correspond to bifurcations in the dynamics. Using bifurcation theory, we establish the existence of a stable distribution of single-site magnetizations near the paramagnetic phase boundary. At least in single-site properties, the existence proof precludes chaos, and infinite hierarchy of transitions, and other conceivable bizarre possibilities. While our phase diagram is very similar to the phase diagram for the Sherrington–Kirkpatrick model, the Bethe lattice provides a useful description of the mean-field behavior of spin glasses because the interactions are short range, the analysis is much more straightforward, and the results have been made completely rigorous.
Magnetism and superconductivity in La2−xSrxCuO4−y (invited)
Authors: Jérome D. , Kang W. , Parkin S. S. P.
Published: July 2002

We report the results of resistivity and magnetic susceptibility measurements of La2−xSrxCuO4−y for 0≤x≤0.20. The various features (superconductivity, spin-density wave state, and localization) are consistent with a model of weakly correlated two-dimensional electrons in which van Hove singularities of the band structure near the Fermi level play a determinant role. We also emphasize that the electron localization observed near half-filling of the conduction band at x≂0.04 can be attributed to the effect of correlations boosted by electron Umklapp scattering in a nearly nested two-dimensional Fermi surface. We recall that similar effects have been observed in half-filled band organic conductors and superconductors.
Training of Barkhausen emission in nickel and iron
Authors: Weinstock H. , Erber T.
Published: July 2002

Barkhausen emission from annealed nickel and iron was measured with a SQUID gradiometer that could detect magnetization jumps above a noise level of 3×10−8 emu. The equipment was installed in a three-axis 12.7-m-diam Braunbeck coil system at NASA’s Goddard Space Flight Center. This arrangement provided an overall cancellation of the geomagnetic field to ±10−6 Oe, and a field homogeneity of 0.001% over a spherical volume of 24.4 m3. Auxiliary windings in the Braunbeck coils were used to generate uniform magnetizing fields at sweep rates as slow as 0.81 μOe/s. The nickel measurements were carried out in field ranges that interpolated between those used in previous training experiments with iron and gadolinium samples. In all cases, repeated field sweeps eliminated or diminished the Barkhausen signals in a systematic way paralleling the Kaiser effect in acoustic emission. These results can be related to the training behavior predicted by a general hysteresis theory.
Domain shapes of field modulation recorded TbFeCo and GbTbFeCo magneto-optical disks (abstract)
Authors: Okamine Shigenori , Nakao Takeshi , Maeda Takeshi , Takahashi Masahiko , Ohta Norio
Published: July 2002

Magneto-optical recording by magnetic field modulation is of particular interest, since direct overwriting of information can be performed as in conventional magnetic recording.1,2 We used a floating magnetic head to apply a modulated magnetic field (±100–±600 Oe) at 5 MHz. We report here the shape of the recorded domains by this technique on TbFeCo and GdTbFeCo 5 1/4 in. disks rotating at 1800 rpm. Both TbFeCo and GdTbFeCo films had a Curie temperature of 200 °C and a compensation temperature of 60 °C. The written domains were observed by a polarized microscope. For TbFeCo disks, regular chevron-type domains were formed according to a cooling tail of a continuous laser beam. However, irregular shape domains were observed in GdTbFeCo disks, that is, the outline of a domain was not smooth, which caused a large recording noise and decreased a signal-to-noise ratio. We discuss the relation between domain regularity and magnetic parameters (anisotropy, wall energy, etc.).
Magnetovolume in fcc ferro- and antiferromagnetic 3d-metal alloys
Authors: Acet M. , Zähres H. , Stamm W. , Wassermann E. F.
Published: July 2002

Thermal expansion measurements as a function of temperature and magnetic field were carried out on Fe-Ni-Cr and Fe-Ni-Mn alloys. The magnetic contribution to the thermal expansion was determined with respect to the paramagnetic reference samples in the Fe-Ni-Cr and Fe-Ni-Mn series. We observe a general behavior that alloys which order ferromagnetically show a positive magnetovolume effect which persists to temperatures much higher than the Curie temperature, whereas in the case of antiferromagnetic ordering the magnetovolume effect is positive below temperatures slightly higher than the Néel temperature, and negative above it.
The dynamics of longitudinal spin fluctuations
Authors: Balucani U. , Vaia R. , Federighi A. , Tognetti V.
Published: July 2002

The dynamical processes underlying the longitudinal spin fluctuation spectra in Heisenberg systems are investigated in two distinct physical regimes. Very recent polarized-neutron data in the one-dimensional ferromagnet CsNiF3 have supported our previous prediction of a regime in which longitudinal spectra are dominated by a coupling between spin and exchange energy densities. A quantum-mechanical analysis is found to give results in good quantitative agreement with the experimental line shapes. The second regime investigated deals with much smaller (and less easily accessible) wave vectors. Here, a new hydrodynamic mode (‘‘second magnon’’) may in principle be supported at not too high temperatures. The observability of this feature in real systems (in particular, EuO) is examined.
Zero point motion and magnetic field effects on the spin configuration in a Heisenberg rhombohedral antiferromagnet. Application to solid oxygen
Authors: Rastelli E. , Tassi A.
Published: July 2002

The ground state of the Heisenberg rhombohedral antiferromagnet in classical approximation exhibits infinite degeneracy corresponding to infinite unequivalent helices. We call degenerate helix (DH) such a spin configuration. In absence of anisotropy long-range order (LRO) is lost even in 3D with a possible algebraic decay of the correlation function due to a low-energy catastrophe related to ‘‘soft lines’’ of the magnon energy which vanishes for all wave vectors falling on the ‘‘degeneration line’’ ℒQ, the locus of the Q wave vectors characterizing the infinite helixes of the ground state. We have studied the effect of the zero point motion which removes the infinite degeneracy of the ground state so that quantum disorder sets up LRO. We have also studied the effect of an external magnetic field. We find that in classical approximation a magnetic field parallel to the c axis does not remove the infinite degeneracy of the ground state but it affects the spin-wave energy spectrum what could be observed in an inelastic scattering experiment. On the contrary, dramatic changes are expected when a magnetic field is applied perpendicular to the c axis because competition arises with the zero point motion effect so that the magnetic field in this configuration should act as a device to tune the DH phase. Possible application to solid oxygen is briefly analyzed.
Ferromagnetic and spin-wave resonance in multilayer films (abstract)
Authors: Soohoo R. F.
Published: July 2002

Whereas ferromagnetic and spin-wave resonance in single-layered films has been studied for a number of years,1 the study of such resonances in multilayer films is still at its infancy. Before the multilayer films can be used in possible applications, it is necessary to understand their superlattice properties. We have used FMR techniques to determine the basic parameters of these films, both theoretically and experimentally. Theoretically, we have calculated the FMR spectra for superlattices of single-crystal, polycrystal, and amorphous samples. Experimentally, we have studied ferromagnetic and spin-wave resonances in Fe/SiO2/Fe/SiO2 . . . , Fe/Co/Fe/Co . . . , NiFe/SiO2/NiFe/SiO2 . . . , and NiFe/Co/NiFe/Co . . . samples. It was found theoretically and experimentally that for applied fields parallel and perpendicular to the film plane, the internal fields in various layers may differ. This could result in widely separated resonances for the various layers. In the case of ferromagnet/insulator multilayered samples, the FMR spectra depended critically on the insulator thickness and its degree of perfection. Diffusion of ferromagnetic atoms through pinholes in the insulating layer provided exchange coupling between ferromagnetic layers and the FMR spectra appeared similar to samples without insulating layers.
A study of the thermal switching behavior in GdTbFe magneto-optic films using two laser beams
Authors: Greidanus F. J. A. M. , Godlieb W. F. , Scholte P. M. L. O.
Published: July 2002

An understanding of the thermal switching behavior of thin magneto-optic films is of primary importance for memory applications. We studied the formation of large domains induced by locally heating a GdTbFe layer with a krypton laser (753 nm) in a static magnetic field. The switching of the perpendicular magnetization was monitored in a small area of the larger krypton-laser-irradiated area by measuring the change in Kerr effect with a semiconductor laser (820 nm). It is observed that the switching is delayed with respect to the start of the heating pulse. There are strong indications that the delayed magnetization reversal starts simultaneously over the entire area of the heated spot and is followed by a small domain expansion.
Critical behavior of weakly diluted frustrated antiferromagnets
Authors: Fernandez Julio F.
Published: July 2002

We study the effect of small amounts of bond or site dilution on some frustrated antiferromagnets (AF) in zero external field. We argue that random field (RF) like effects are generated by any small amounts of dilution in a large class of Ising AFs, and that these effects (1) are not covered by the criterion of Harris, (2) obliterate AF order in these systems for d<2 (and for d=2 quite likely), and (3) produce a reduced crossover temperature to impure critical behavior given by the tc∼(δJ)2/φ and φ≊γ (instead of φ=α prescribed by the criterion of Harris for random ferromagnets) for all fcc AF and some bcc AF. Our transfer matrix results for long strips support our arguments.
Tricritical point in dilute Ising antiferromagnets in magnetic field
Authors: Azaria P. , Diep H. T. , Galam Serge
Published: July 2002

We show by Monte Carlo simulation that the inclusion of next-nearest-neighbor (nnn) interactions, which are always present in real systems, results in the existence of a tricritical point in the temperature-field plane. The location of this point depends on the dilution concentration p and on the nnn interaction strength. These results are discussed.
Long-range magnetic reconstruction of ferromagnetic films with diffused interfaces
Authors: Cullen J. R. , Hathaway K. B. , Coey J. M. D.
Published: July 2002

Two models of the magnetic structure of a ferromagnetic film deposited on a substrate are developed, with a view to explaining the reduced moment of Fe on GaAs. In the first, spin canting induced by antiferromagnetic exchange in the interface layer falls off rapidly in the interior of the ferromagnet, the characteristic distance being the size of the reconstructed magnetic unit cell in the interface. In the second, random anisotropy in the interface layer induces weakly pinned fluctuations of the magnetization which penetrate to a distance of order the domain wall width, approximately 100 Å.
Monolayer ferromagnetism of Fe(110) on W(110) studied by Mössbauer spectroscopy
Authors: Przybylski M. , Gradmann U.
Published: July 2002

Fe(110) films in the monolayer regime were prepared at 300 K in UHV on W(110). Pseudomorphic structure and layer-by-layer growth were established using LEED, AES, and conversion electron Mössbauer spectroscopy (CEMS). For the pseudomorphic monolayer, ferromagnetic order was established using CEMS below a monolayer Curie temperature TC (1)=(210±10) K=0.20TC(∞) for a free monolayer and TC(1)=(296±1) K=0.28TC(∞) for a Ag-coated monolayer, respectively. Magnetic hyperfine fields Bhf(T) could be measured for T≥90 K. Ground-state values could be determined by extrapolation to Bhf(0)=(10±1) T for the free and Bhf(0)=(11.9±0.3) T for the Ag-coated monolayer, respectively. A critical region 270<T<300 K was analyzed to some detail for the Ag-coated monolayer. In this region, the CEMS spectrum contains a magnetic and a nonmagnetic component. Transition to the paramagnetic phase occurs by transition of intensity from the magnetic to the nonmagnetic line, whereas Bhf of the magnetic component remains finite with approach to TC. Beyond the true monolayer, double layer and monolayer components can be distinguished in the CEMS spectra.
Structural and magnetic properties of Au/Pd/Au sandwiches: A total energy all-electron approach
Authors: Hong Soon C. , Fu C. L. , Freeman A. J.
Published: July 2002

The possibility of magnetism induced in Pd when sandwiched between Au layers, first proposed and studied experimentally by Brodsky and Freeman, is investigated by means of highly precise all-electron total energy local spin-density calculations employing the thin-film full-potential linearized augmented plane wave (FLAPW) method. Total energy local density calculations are employed on a Au/Pd/Au sandwich with three Pd layers to obtain the structural properties and five Pd layers to obtain the magnetic properties of an Au/Pd/Au sandwich. Detailed results are obtained with spin polarized calculations for the Pd lattice constant stretched taking into account (i) the misfit between the lattice constants of Au and Pd and (ii) proximity effects in a self-consistent way. The magnetic moment of the center Pd layer is found to be 0.02μB which is consistent with the experiment of Brodsky. A negligible magnetic moment for the interface Pd layers is obtained.
Basis of strong change of hybridization-induced magnetic ordering between CeSb and CeTe
Authors: Kioussis N. , Cooper B. R. , Wills J. M.
Published: July 2002

A sharp change in the nature of the magnetic ordering has been observed on going from CeSb to CeTe, both of which have NaC1 structures with a small decrease in lattice parameter. This is an interesting example of the way in which hybridization of partially delocalized f electrons with band electrons gives rise to highly unusual magnetic properties which show great chemical sensitivity. In the present paper we apply our previous ab initio treatment of hybridization-induced effects to investigate this striking change in magnetic behavior. We have performed self-consistent warped muffin-tin LMTO band calculations treating the Ce 4f states as resonance states that are constrained to be localized. Compared to CeSb, the anion-derived p bands in CeTe sink well below the Fermi energy, thus strongly changing the band-f hybridization. We have calculated the hybridization dressing of the crystal-field levels and the anisotropic two-ion exchange interaction and compared them with those calculated for CeSb and with experiment. A strong decrease in the two-ion interaction explains the drastic change in observed magnetic behavior between CeSb and CeTe.
Magneto-optical properties of PtMnSb films prepared by facing targets sputtering
Authors: Naoe M. , Kitamura N. , Shoji M. , Nagai A.
Published: July 2002

The magneto-optical properties of PtMnSb Heusler alloy films prepared by a facing targets sputtering (FTS) system have been studied. The specimen films were deposited onto glass substrates heated between 20 and 350 °C. The films exhibiting diffraction peaks of Clb-PtMnSb Heusler alloy were obtained at temperatures above 150 °C. The saturation magnetization 4πMs increased with increasing substrate temperature and the maximum value of magnetization was 5.8 kG, which is lower than PtMnSb bulk magnetization. The polar Kerr rotation measured with a He-Ne laser was evaluated at 1.9° on the glass side when an external dc magnetic field of 7 kOe was applied in a perpendicular direction.
Magnetic phase transition in Y0.97Dy0.03
Authors: Gotaas J. A. , Rhyne J. J. , Wenger L. E. , Mydosh J. A.
Published: July 2002

Neutron diffraction has been used to study the long-range helical magnetic order in a single crystal of the dilute alloy Y0.97Dy0.03. In zero applied magnetic field, the propagation vector is along the c axis and of magnitude 0.28(2π/c), with the magnetic moments lying in the basal plane of the hexagonal lattice. The Dy develops the full free-ion moment and exhibits a temperature dependence similar to that of the Brillouin function for J=15/2. At T=6 K, a magnetic field applied in the basal plane causes almost no shift in the value of the modulation wave vector and as the magnetic field is increased, the peak scattered intensity of the magnetic satellite drops continuously, with the helical phase being completely suppressed in a field of 3 T. On reducing the field, there is hysteresis in the peak intensity, with only 79% of the original intensity being recovered at H=0. This residual intensity grows slowly with time, increasing to approximately 82% of the original value over a period of 1 h. This behavior differs from that of the Y(Gd) system, which shows no hysteresis and displays a sharp transition between helical and sinusoidal phases, the difference presumably being due to the presence of crystal field anisotropy in the case of Dy.
Spin-polarized photoelectron diffraction: A new probe of short-range magnetic order (invited) (abstract)
Authors: Fadley C. S. , Sinkovic B. , Hermsmeier B. D. , Osterwalder J.
Published: July 2002

It has recently been pointed out theoretically and subsequently observed experimentally that core-level multiplet splittings can be used to yield a spin-polarized form of photoelectron diffraction (SPPD). This internally referenced source of electrons that are highly polarized with both orientations of spin thus eliminates the need for an external spin detector, permits studying both ferromagnetic and antiferromagnetic specimens, and can, in principle, detect short-range magnetic or for temperatures above the Curie or Néel temperature. In the first measurements of this type on the antiferromagnet KMnF3, an abrupt loss of short-range order at a transition temperature considerably above the bulk Néel temperature of the material was observed. More recent theoretical diffraction calculations have pointed out several ways in which SPPD should permit deriving unique information on short-range spin-order structures at and near the surfaces of magnetic materials. New experimental results have also been obtained for the antiferromagnet MnO. This paper will discuss these new developments, will review the advantages and disadvantages of SPPD, and also will consider some interesting future directions of investigation.