bluuebunny/crossref_metadata_2025 huggingface dataset:
Polysilicon films of high photoresponse, obtained by vacuum annealing of aluminum capped hydrogenated amorphous silicon
Authors: Sridhar Nagarajan , Chung D. D. L. , Anderson W. A. , Coleman J.
Published: July 2002

Hydrogenated amorphous silicon (a-Si:H) films having a thickness of 7 μm were deposited on molybdenum by dc glow discharge and then capped with a 0.1-μm-thick aluminum (Al) layer by thermal evaporation. Subsequent vacuum annealing at 500–725 °C resulted in the formation of crystalline Si, as observed by Raman scattering and x-ray diffraction. This was in contrast to the uncapped a-Si:H films which were still amorphous at the same annealing temperatures, except at ≳700 °C. That the Al capped films were crystalline caused a ten-fold increase in the dark conductivity in comparison to the uncapped film annealed at the same temperature. The capped films annealed at 500 °C showed a photoresponse (the ratio of the photoconductivity to dark conductivity) of 30, a photoconductivity of 2×10−4 (Ω cm)−1, and a carrier diffusion length of 5.3 μm—values much higher than those of the uncapped films (heated or not). This was due to a large grain size combined with the retention of hydrogen, which passivated the grain boundaries. On the other hand, a significant loss of hydrogen from the capped film at 580 °C, as observed by in situ evolved gas analysis, resulted in the diffusion length reducing to 1.8 μm and ten-fold decreases of the photoconductivity and photoresponse. All three parameters increased slightly with a further increase in the annealing temperature from 600 to 680 °C, due to the increase in the grain size. At 695 °C, further hydrogen evolution caused a sharp decrease in the values of these parameters.
Formation of shallow donor in fluorine-implanted silicon
Authors: Holm B. , Nielsen K. Bonde
Published: July 2002

A shallow fluorine-related donor is reported. The donor has been revealed by capacitance–voltage profiling applied to furnace-annealed, fluorine-implanted silicon samples. The implantations were carried out at high energy, in the range 5–7 MeV. The assignment of the donor structure to fluorine results from a detailed examination of the effective space-charge distribution around the implantation range (≊4 μm) in comparison with data obtained for similar oxygen-implanted samples. The depth-integrated density of the donors formed after annealing at 700 °C is ≊3×1010 cm−2 corresponding to ≊1% of the implantation dose. The density increases by more than one order of magnitude for samples annealed only at 600 °C. In addition to the shallow donor level (or levels) a fluorine-specific level at Ec−Et≊0.15 eV has been assigned by application of deep-level transient spectroscopy.
Influence of interfacial copper on the room temperature oxidation of silicon
Authors: Alford T. L. , Jaquez E. J. , Theodore N. David , Russell S. W. , Diale M. , Adams D. , Anders Simone
Published: July 2002

Thick (∼1.3 μm) oxide films were grown by room-temperature oxidation of silicon after low-energy copper-ion implantation. The structural properties of the silicon dioxide layer and the implanted silicon were characterized by Rutherford backscattering spectrometry and transmission-electron microscopy. During the room temperature oxidation a portion of the implanted copper resided on the surface and a portion moved with the advancing Si/SiO2 interface. This study revealed that the oxide growth rate was dependent on the amount of Cu present at the moving interface. The surface copper is essential for the dissociation of oxygen at the surface, and it is this oxygen that participates in the oxidation process. The resulting oxide formed was approximately stoichiometric silicon dioxide.
Steady-state photoconductivity in poly(2,5-dimethoxy-p-phenylene vinylene)
Authors: Narayan K. S. , Gautam K. S.
Published: July 2002

We report a study of the steady-state photoconducting properties in the conjugated polymer poly(2,5-dimethoxy-p-phenylene vinylene) using the sandwich and surface cell configurations. The photocurrent spectral response in both the configurations is nearly identical with the onset of the photocurrent around the band edge. The rise in the photocurrent is more gradual than the increase in absorption with respect to the photon energy and the peaks in the photocurrent responses are separated from the absorption maxima by nearly 1 eV. The chopper frequency, electric field, and the intensity dependence of the photocurrent are remarkably different for the two configurations. We speculate on the mechanisms leading to such differences.
Ultrafast optical switching of second-harmonic generation at the C60 single-crystal surface
Authors: Kuhnke K. , Becker R. , Berger H. , Kern K.
Published: July 2002

The switching of second-harmonic generation (SHG) at a C60 single-crystal surface has been observed in a pump-and-probe experiment. The SHG signal from a picosecond 1.17 eV laser pulse is suppressed by one order of magnitude upon illuminating the crystal surface with a 3.49 eV pump pulse. The nonlinear optical response is faster than 45 ps and persists for longer than 20 ns. SHG suppression to 1/e occurs at pump densities as low as 2.8 μJ/cm2. We suggest that nonlocalized excited electronic states determine the change in the nonlinear optical behavior.
Magnetic and nonmagnetic glass transitions in the Blume–Emery–Griffiths model with competing biquadratic interactions (abstract)
Authors: Snowman Daniel P. , McKay Susan R.
Published: July 2002

The Blume–Emery–Griffiths model, a spin-1 Ising model with bilinear (J), biquadratic (K), and crystal field (Δ) terms, provides a general system for the analysis of both density and magnetic fluctuations. Using a Monte Carlo hard-spin mean-field approach, we have calculated the phase diagrams resulting from a random bimodal distribution of biquadratic interactions, Kij=±K, with equal probabilities. The quenched disorder in K competes with the propagation of long-range magnetic order and leads to two types of glassy ordered phases, magnetic and nonmagnetic. The variance in the site density over the lattice serves as an order parameter for the transition to these glassy phases. We compare the structure and overlaps of their low lying energy states with those of the three-dimensional Ising spin glass, determined using the same method. The magnetic glass exhibits a much more gradual loss of overlap with its low-temperature state than the spin glass. The form of the overlap versus temperature curve in the vicinity of the critical temperature for the magnetic glass suggests that the system is fluctuating between a pair of ordered states, with one obtained from the other by flipping all spins, rather than sampling many different states.
Topological coupling in magnetic multilayer films
Authors: Zhang J. , White R. M.
Published: July 2002

The role of topological coupling, the magnetostatic coupling associated with interfacial roughness in magnetic sandwich/multilayer structures, was studied both analytically and experimentally. We report the first controlled measurements of such topological coupling. A coupling field of the order of 10 Oe between magnetic layers was observed in Co/Cu/Co sandwich structures fabricated with an artificially controlled in-phase roughness of variable amplitude with a fixed period of 3.5 μm. The coupling field was compared with the value predicted by a first-order approximation model and correlated with the geometry of the structure. Scale invariance of the coupling field allows the results to be extended to ultrathin giant magnetoresistance/spin valve structures.
Influence of sputter gas and sputter pressure on the structure and magnetic properties of Co–Pt–Cr thin films (abstract)
Authors: Roshko A. , Dulcie L. L. , Nguyen T. , Yogi T.
Published: July 2002

The magnetic properties of high density recording media have been shown to be very sensitive to the film microstructure. We have investigated the influences of different sputter gases (Ar, Xe, Kr) and different sputter pressures (3 to 24 mTorr) on the microstructures and magnetic properties of CoPtCr/Cr thin films. The magnetic properties of the films (coercive field, Hc, coercive squareness, S*, and remanant moment, Mr) were determined using a vibrating sample magnetometer. The microstructures were examined by transmission electron microscopy and x-ray diffraction. For all three gases studied the microstructures and properties of the CoPtCr films were found to change with sputter pressure in similar manners. Films deposited at the lowest pressure consisted of well connected, equiaxed grains. With only a slight increase in pressure, the grains formed chains separated by small gaps. As the gas pressure increased further, the chains became better defined and the gaps between them widened. The coercive squareness and coercive field correlate with the film microstructures. S* decreased with increasing sputter gas pressure, and Hc first increased and then decreased. For all three sputter gases, Hc reached a maximum around 12 mTorr.

Likewise, S* decreased quite slowly with increasing pressure up to 12 mTorr and then exhibited a sharp drop between 12 and 18 mTorr. From these results it appears that the sputter pressure, rather than the mass of the sputter gas, is most important for the microstructural development of the film. Likewise, the values of S* were found to be independent of the sputter gas. However, in spite of the similar microstructures, films deposited with Ar had higher values of Hc than films deposited with Xe or Kr, for all sputter pressures examined. The role of the sputter gas mass will be discussed.
Thresholds of spin wave envelope soliton formation in magnetic films with dissipation (abstract)
Authors: Slavin A. N.
Published: July 2002

The nonlinear mechanism responsible for the spin wave envelope soliton formation in magnetic films from input pulses of finite duration is the four-wave process with conservation law ω(k0)+ω(k0)=ω(k0+κ)+ω(k0−κ), κ≪k0. This process is also responsible for modulation instability of CW wave in a nonlinear dispersive medium and can only take place if the nonlinearity N and the dispersion D in the medium have opposite signs. In a dissipative medium this process has a finite amplitude threshold determined by the dissipation parameter γ and the effective ‘‘detuning’’ Δω proportional to the dispersion D and the square of the modulation wave number κ, ‖φ‖th2=[γ2+(Δω)2/4]/‖NΔω‖, hereafter referred to as Eq. (1), where ‖φ‖ is the dimensionless wave amplitude. For the process of modulational instability of a CW wave Δω=Dκ2. For the process of soliton formation from a finite input pulse (of the duration T) Δω=Dκ2n, where κn=(2n−1)π/vT, n is the number of solitons formed, and v is the group velocity of spin waves. Equation (1) gives a good description of the recent measurements of thresholds of spin wave envelope soliton formation in YIG films presented in Ref. . In particular, describes correctly the shape of the threshold curve, positions of threshold minima for the formation of one and two solitons, and explains the finite intercept on the threshold power dependence on the inverse square of the input pulse duration that was not explained in Ref. .
Highly sensitive magneto-optic transverse Kerr effect measurement system for the detection of perpendicular anisotropy and magnetic phases in thin films
Authors: Corrales J. A. , Rivas M. , F.-Calleja J. , Iglesias I. , Contreras M. C.
Published: July 2002

Transverse biased initial susceptibility (TBIS) measurements were made using a magneto-optical Kerr effect system on samples that exhibited an in-plane and a perpendicular anisotropy. A change in the slope of both the inverse transverse susceptibility and the hysteresis loop is observed at a certain value of the applied in-plane field Hs. At values of the applied field higher than Hs the magnetization lies in the plane of the film; at values lower than Hs an out-of-plane component appears. By performing TBIS measurements we can also detect different micromagnetic phases magnetically uncoupled with different values of the anisotropy field. The occurrence of a downwards curvature in the inverse susceptibility when the dc field is applied along the hard axis together with the multiple minima when the dc field is applied along the easy axis can be explained by the existence of weakly coupled micromagnetic phases.
Colossal magnetoresistivity in manganese-based perovskites (invited) (abstract)
Authors: Ramesh R. , Venkatesan T. , Ogale S. B. , Greene R. L. , Bhagat S. M.
Published: July 2002

Magnetoresistivity values of the order of 106% (and in some cases even higher) have been obtained in epitaxial AxB1−xMnO3−y (A=La,Nd; B=Ca,Sr,Ba) thin films grown by pulsed laser deposition. Ferromagnetic resonance experiments suggest a granular-type behavior with conducting ferromagnetic regions (Rcond<10 mΩ cm) in a less conducting matrix (Rinsulazting≳100.Rcond). Ion channeling experiments over a range of temperatures clearly reveal the existence of structural distortion at the peak resistivity temperature TP. Systematic studies of samples prepared under a variety of oxygenation conditions show that the resistivity above TP can be modeled with a single functional form: Rcond≊eΔ/kT, where Δ, the activation energy, is of the order of 50–200 meV. This suggests that these different samples represent the same basic material in a semiconducting matrix, with differing volume fractions of the two components which depends on the processing conditions. These ‘‘colossal’’ values of MR have been obtained at temperatures lower than room temperature and at fields of the order of a few Teslas, both of which are impediments to the development of viable MR sensor and nonvolatile storage technologies.

We are therefore addressing the critical scientific and technological issues through a variety of materials integration approaches. Using structural chemistry and lattice matching as fundamental guiding principles, we are growing epitaxial heterostructure superlattices consisting of the CMR oxides interleaved with magnetic perovskites such as La–Sr–Co–O (metallic ferromagnet), rare earth–Fe–O (ferromagnetic insulator). We are also exploring the possibility of using the semiconducting properties of these materials in an all-perovskite field effect transistor device. In this presentation, we will describe our progress to date on these studies to enhance the field and temperature dependence of the MR properties and explore new device architectures that utilize the inherently novel properties of these materials.
Perpendicular spin valve behavior in a microstructured Co/Cu-Cu oxide/Co trilayer
Authors: Matsuyama K. , Asada H. , Matsuguma I. , Saeki T. , Taniguchi K.
Published: July 2002

In this article we report a perpendicular spin valve behavior in a microstructured noncoupling magnetic trilayer system of Co/Cu-Cu oxide/Co. The Co thin films having markedly different coercivity Hc were prepared with a vacuum evaporation with electron beam (Hc= 20–30 Oe) and rf sputtering (Hc= 90–260 Oe), which enables the selective switching of the magnetization in the base and counter Co layers. The film surface of the Cu was oxidized by the rf sputter etching, which results in the increase of the resistance for the perpendicular current and enables measurements of the perpendicular spin valve behavior at room temperature. The additional spacer layer of sputtered SiO2 with a contact hole of 5 μm diameter was fabricated with photolithographic method between the bottom Co and the intermediate Cu layer, which defined the current path perpendicular to the film plane. The measured magnetoresistance ratio was 0.85% (dR=7.6 mΩ, R=0.89 Ω) at room temperature.
All refractory light-sensitive superconductive junctions
Authors: Granata C. , Russo M. , Testa G.
Published: July 2002

A procedure for fabricating all refractory superconducting tunnel junctions with a semiconducting barrier has been developed. The process is based on the deposition of a four-layer Nb–Al–CdS–Nb structure without breaking the vacuum. The photoconducting properties of the barrier produce a light-sensitive behavior of the junctions. Preliminary measurements have shown the occurrence of light induced quasiparticle tunneling.
On the origin of magnetoresistance in ferromagnetic perovskite structures
Authors: Zhang Shufeng , Yang Ze
Published: July 2002

In the presence of a nonuniform distribution of spin interactions in ferromagnetic perovskite materials, thermally activated magnetic domains appear. These domains are different from ordinary static ferromagnetic domains, rather their structures depend on temperature, magnetic field, and details of the distribution of magnetic interactions. Due to the half-metallic nature of these materials, magnetic domain boundaries contribute significantly to their electrical resistivity. Magnetoresistive effects are discussed within this framework.
Trapping mechanism to account for persistent photoeffects in heavily doped GaAs/AlGaAs multiple quantum wells
Authors: Gannon Andrew , Donnelly David , Covington Billy
Published: July 2002

We have investigated trapping mechanisms responsible for the persistent photoeffect in heavily doped GaAs/Al0.3Ga0.7As multiple-quantum-well structures. The study was performed using infrared-absorption techniques to study the intersubband transitions of the wells as a function of secondary illumination. The frequency of the secondary illumination was varied by the use of filters on the secondary source. The peak energies of the intersubband transitions can be modeled using the nonparabolic anisotropic envelope-function approximation, which accounts for many-body effects due to the high doping level. The red shift in the intersubband transition energy observed on secondary illumination indicates a decrease in the carrier concentration in the wells due to trapping in the barrier. We have found a decrease in the strength of the persistent photoeffect when the energy of the photons used in the secondary illumination is below the band gap of Si (1.172 eV). This observation is consistent with optically activated traps whose activation energy is ≥1.172 eV. These data, along with temperature recovery data, make the most probable candidate for the trap the DX center.
Film and interface morphology of CaF2 grown on Si(111) at low temperature
Authors: Wollschläger Joachim , Meier Andreas
Published: July 2002

We have applied profile analysis of low energy electron diffraction spots during deposition of CaF2 on Si(111)7×7 at 300 °C to study the morphology of the growing film. With increasing CaF2 coverage the nonfundamental spots of the 7×7 diffraction pattern fade out very efficiently. The exponential decay of the first order reconstruction spot shows that the transformed areas surrounding the CaF2 molecules overlap statistically. Initially the specular spot shows intensity oscillations dying out depositing 4–5 ml CaF2. We conclude from the damped oscillations that first the film grows in the multilayer growth mode followed by forming three-dimensional islands on top of it.
Critical properties of the spin Peierls transition in CuGeO3
Authors: Lumsden M. D. , Gaulin B. D. , Dabkowska H.
Published: July 2002

The spin-Peierls transition in CuGeO3 has been studied using x-ray diffraction techniques. The order parameter of the (1/2,5,1/2) superlattice reflection has been measured with a temperature stability of ±5 mK. Careful analysis has been performed fitting the order parameter to both an ordinary power law in the reduced temperature as well as to a power law with the first correction to scaling term included in an attempt to determine a consistent value of the critical exponent β. We obtain a value of β of 0.36±0.03 at a transition temperature of 14.06±0.02 K. This value of β is consistent with conventional three-dimensional Heisenberg or XY behavior and clearly inconsistent with the tricritical values reported previously.
Micromagnetic studies of inhomogeneous CoPtCr bicrystal thin-film media
Authors: Peng Qingzhi , Bertram H. Neal , Doerner Mary
Published: July 2002

The effects of medium inhomogeneities in microstructure and exchange configuration are studied by using micromagnetic simulation. Two types of media are studied: one has a uniform bicrystal structure and another has a partial bicrystal structure with 20% random nonbicrystal sites. The comparison of magnetic (M-H loops) and recording properties (transition profiles and noise) for these two types of media are made under two cases with homogeneous or inhomogeneous intergranular exchange coupling. It is found that the introduction of random nonbicrystal sites reduces the medium noise in the case with homogeneous intergranular exchange, while medium noise is barely affected in the case with inhomogeneous intergranular exchange.
Ferromagnetic resonance of sputtered Co/Mn multilayers
Authors: Spoddig D. , Schreiber F. , Pflaum J. , Pelzl J. , Wang Q. , Zabel H.
Published: July 2002

Co/Mn multilayers were prepared by rf sputtering onto single-crystal MgO(001) and Al2O3(112̄0) substrates resulting in, respectively, (001)and (111)-oriented layers. The structure was thoroughly analyzed by x-ray scattering in various geometries. For the magnetic investigations, ferromagnetic resonance (FMR) was applied. Complementary measurements employed the magneto-optical Kerr effect and Faraday balance magnetometry. Results for the in-plane anisotropy, the surface anisotropy, the magnetization, and the FMR linewidth are presented. The comparison with other Co-based multilayer system indicates that both the structural and the magnetic properties of the Co/Mn system are more complicated than in the case of systems with a presumably weaker electronic interaction at the interface.
Uniaxial and planar magnetic anisotropy of thin transition-metal films (abstract)
Authors: Lorenz R. , Hafner J.
Published: July 2002

We present a novel approach to the calculation of the spin structures and magnetic anisotropies in crystals and thin films. Our technique is based on self-consistent real-space recursion calculations on a tight-binding linear muffin-tin orbital (TB LMTO) Hubbard Hamiltonian including spin–orbit coupling and allowing for arbitary orientations of the local spin-quantization axes. It allows one to scan the magnetic energy continuously as a function of the orientation of the magnetic moment and thus to avoid the computational problems that plague other techniques. Applications are presented for bulk iron and for thin Fe overlayers on Cu (111) and Cu (100) substrates. For monolayer coverage, we predict a perpendicular direction of the magnetic moment for Fe/Cu(100) an in-plane orientation of the spins for Fe/Cu(111), with anisotropies of the order of 1–2 meV. For Fe/Cu(100) we present a detailed investigation of the variation of the spin structures and of the change from perpedicular to in-plane anisotropy with increasing thickness of the Fe films.
Investigation of magnetic coupling in sputtered epitaxial Fe/Cr and Co/Cu wedged structures
Authors: Bian X. , Hardner H. T. , Parkin S. S. P.
Published: July 2002

We explore the magnetic interlayer exchange coupling in single-crystalline Fe/Cr and Co/Cu wedged multilayers prepared by magnetron sputtering on two orientations of MgO—(100) and (110). Structural examination shows that high quality epitaxial films are obtained in both systems using seeded epitaxy growth techniques. Oscillatory interlayer coupling is observed in Cr-wedged samples grown on both MgO(100) and MgO(110) with similarly long oscillation periods of ∼18 Å. These results are comparable to earlier work for molecular beam epitaxy and sputter-deposited epitaxial Fe/Cr structures. Long-period oscillatory coupling is also observed in sputtered Co/Cu(110) for the Cu-wedged structures. Studies of the dependence of magnetic coupling on the thickness of the magnetic layers for fixed Cr layer thickness provides no evidence of any nonmonotonic dependence.
Anomalous compressibility and magnetovolume effects in Ce3(FeTi)29
Authors: Kamarád J. , Arnold Z. , Morellon L. , Algarabel P. A. , Ibarra M. R. , Fuerst C. D.
Published: July 2002

In this contribution we present the results of direct measurements of the compressibility, in a wide temperature range, and studies of the linear thermal expansion under hydrostatic pressure using a modified strain gauge method in Ce3(FeTi)29. A large magnetic contribution (κm=0.13 Mbar−1) to the isothermal compressibility was observed in the ferromagnetic state in comparison with the compressibility in the paramagnetic state induced by pressure at room temperature. The pronounced lambda-shaped anomalies in the thermal expansion coefficient (negative values of α) and in the compressibility of the Ce3(FeTi)29 compound were simultaneously observed in the vicinity of TC.
Magnetic properties and magnetostriction of twin-free single-crystal Tb0.27Dy0.73(Fe1−xAlx)2
Authors: Wang Jinghua , Wu Guangheng , Zhao Xuegen , Jia Kechang , Zhan Wenshan
Published: July 2002

Magnetization curves, Curie temperature, and lattice constants of twin-free [111]-oriented single-crystal Tb0.27Dy0.73(Fe1−xAlx)2 (x=0, 0.1) were investigated. It was observed that with the substitution of Al for Fe, Curie temperature and magnetocrystalline anisotropy decrease, and lattice constants increase. It is shown from the curves of magnetic moment versus temperature that the anisotropy compensation temperature of Tb0.27Dy0.73(Fe0.9Al0.1)2 is 280 K, which is higher than 260 K of Tb0.27Dy0.73Fe2. Besides, the saturation magnetostriction λ111 of Tb0.27Dy0.73(Fe0.9Al0.1)2 was measured by using strain gauge method. The result indicates that λ111 increases from 1.01×10−3 to 1.54×10−3 as the compressive stress is increased from 0 to 12 MPa, and that the maximal D33(dλ/dH) is about 1.0. It is the first time to present the experimental data of the twin-free [111]-oriented single crystal of Tb0.27Dy0.73(Fe0.9Al0.1)2.
Three-dimensional modeling techniques applied to regions with small susceptibility variations (abstract)
Authors: Hill-Cottingham R. J. , Eastham J. F. , Young I. R. , Hajnal J. V.
Published: July 2002

The article considers the modeling of regions where the susceptibility is 0.01–10 ppm different from the majority of the space of interest. These calculations are of use in MR imaging, for example, in one possible application, the susceptibility of an haemorrhagic deposit could be adduced. Two methods are considered. Firstly, the use of finite element modeling using double precision arithmetic in conjunction with carefully set convergence limits. Secondly, a method which considers the induced polarity in the region of enhanced susceptibility, and calculates the difference field. The work is validated initially by calculating analytically the field perturbation produced by a sphere of susceptibility of 1×10−8 and comparing the results with the two methods given above. It is shown that excellent agreement is obtained. As a further confirmation the field produced by a short cylindrical object was measured experimentally using a MR scanner and again the applicability of both modeling techniques was confirmed.
Response to an electric field of Nd0.7Sr0.3MnO3 films in a MOSFET configuration (abstract)
Authors: Ogale S. B. , Talyansky V. , Xiong G. C. , Sharma R. P. , Ramesh R. , Greene R. L. , Venkatesan T.
Published: July 2002

The effect of an electric field on GMR oxide films was studied in a MOSFET configuration where the gate dielectric was a layer of SrTiO3 epitaxially grown on an underlying layer of the manganate which served as the source/drain. The response of the manganate channel was studied for different gate voltages. The following significant features were observed. The peak resistance temperature shifted to lower temperature for both polarities of the field. The resistance change varied quadratically with the field indicating the dominance of strain or polarization effects. In dynamical studies of the system using the gate voltage as an excitation the system showed anomalous slowing down near the peak of the resistivity. These results are understood on the basis of a stress effect on the film due to electro-elastic effects in the SrTiO3 layer, which introduces a tensile stress in the manganate layer upon the application of a gate voltage. The anomalous slowing down of the system near the ferromagnetic phase transition suggests a strong coupling between the spins, transport and structural distortions in the system.
Electro-optic switching in spin-coated ferroelectric mesomorphic polymer films and its analysis
Authors: Uto Sadahito , Moritake Hiroshi , Ozaki Masanori , Yoshino Katsumi , Skarp Kent , Helgee Bertil
Published: July 2002

Thin films of a chiral side-chain polymer with a ferroelectric phase were deposited on a substrate by spin coating. Electro-optic switching in the films were observed by applying an external field using in-plane electrodes. Characteristics of the switching are presented, together with an analysis of the switching dynamics that takes into account the inhomogenous field distribution in the active area.
Analysis of oxide–nitride–oxide/Si(111) interfaces by optical second harmonic generation and secondary ion mass spectroscopy
Authors: Watanabe Kohji , Kawata Masato , Hasegawa Eiji , Hirayama Hiroyuki
Published: July 2002

We use optical second harmonic generation and secondary ion mass spectroscopy (SIMS) to study oxide–nitride–oxide (ONO)/Si(111) interfaces. We explain the relationship between the second harmonic (SH) intensity from the interfaces and the SIMS depth profiles. The intensity from the wet oxide/Si(111) interface decreases as the temperature during wet oxidation increases, because the number of interface dangling bonds is reduced as the wet temperature is increased. With subsequent nitridation, this SH intensity of the oxidized interface decreases. This is because hydrogen and nitrogen terminate the interface dangling bonds. However, the SH intensity increases again with reoxidation, because the dangling bonds grow again at the interface. The SH intensity from annealed nitridation of the oxide/Si(111) interface remains constant, although the number of hydrogen atoms decreases and the number of nitrogen atoms slightly increases. We propose two reasons for this. One is that most of the desorbed hydrogen atoms do not terminate the dangling bonds at the interface. The other is that interface dangling bonds grow temporarily due to desorbed hydrogen; however, nitrogen piled up near the interface terminates them again.
The trap emptying process of low temperature thermoluminescence of 6H–SiC single crystals
Authors: Stiasny Th. , Helbig R.
Published: July 2002

Thermoluminescence (TL) and thermally stimulated conductivity (TSC) of SiC n- and p-type single crystals were measured in the temperature region 12–300 K. To describe the underlying process, we assumed the classical model consisting of one species of trap and recombination center, respectively. The calculated glow curve can be fitted to the experimental one with good conformity. The analysis of the glow curve main peaks leads to the thermal activation energy of the emptied traps. For nitrogen (N) doped n-type crystals, we obtain thermal activation energies of 80 meV or 140 meV for highly compensated samples. Aluminum (Al) doped p-type samples show an activation energy of 220 meV, boron (B) containing p-type samples one of 330 meV. This corresponds to the well known ionization energies of the shallow doping (N on cubic and hexagonal lattice site for n type and Al, B for p type). To obtain the energy necessary for a charge transfer of the impurity, we studied TL as a function of the excitation wavelength. Here, an excitation energy between 1.65 and 1.9 eV turned out to be necessary to recharge the recombination center. The recombination center responsible for the TL normally acts as compensation in thermal equilibrium. For p-type samples, we can distinguish between the omnipresent N and a second impurity as compensation, which is energetically deeper than 240/300 meV. The chemical nature of these centers is still unknown.
Solution of magnetic field and eddy current problem induced by rotating magnetic poles (abstract)
Authors: Liu Z. J. , Low T. S.
Published: July 2002

The magnetic field and eddy current problems induced by rotating permanent magnet poles occur in electromagnetic dampers, magnetic couplings, and many other devices. Whereas numerical techniques, for example, finite element methods can be exploited to study various features of these problems, such as heat generation and drag torque development, etc., the analytical solution is always of interest to the designers since it helps them to gain the insight into the interdependence of the parameters involved and provides an efficient tool for designing. Some of the previous work showed that the solution of the eddy current problem due to the linearly moving magnet poles can give satisfactory approximation for the eddy current problem due to rotating fields. However, in many practical cases, especially when the number of magnet poles is small, there is significant effect of flux focusing due to the geometry. The above approximation can therefore lead to marked errors in the theoretical predictions of the device performance. Bernot et al. recently described an analytical solution in a polar coordinate system where the radial field is excited by a time-varying source. A discussion of an analytical solution of the magnetic field and eddy current problems induced by moving magnet poles in radial field machines will be given in this article. The theoretical predictions obtained from this method is compared with the results obtained from finite element calculations. The validity of the method is also checked by the comparison of the theoretical predictions and the measurements from a test machine. It is shown that the introduced solution leads to a significant improvement in the air gap field prediction as compared with the results obtained from the analytical solution that models the eddy current problems induced by linearly moving magnet poles.
Simulations of highly anisotropic Co–Cr–Ta thin films
Authors: Dean C. , Chantrell R. W. , Suzuki H. , Kodama N. , Bissell P. R.
Published: July 2002

Co–Cr–Ta thin film media produced by transfer deposition are known to be highly anisotropic. Earlier experimental work shows these films achieve the maximum theoretically possible δI in the parallel direction but small deviation from zero transverse to the sputtering direction. A thin film micromagnetic model has been extended to simulate this highly anisotropic behavior. It was found that anisotropic exchange coupling was insufficient to account for the anisotropic behavior. Good agreement with experiment has been obtained using an approach which modifies the exchange coupling and the magnetostatic interaction. The large values of δI are ascribed to difficulties in the production of the ac demagnetized state.
Spin glass behavior of amorphous Fe5(InTe4)3
Authors: Zhang Jian H. , Williams Damon , Tao Cuihong , O’Connor Charles J.
Published: July 2002

The amorphous phases of Cr5(InTe4)3 and Fe5(InTe4)3 have been prepared via the precipitation reaction of the Zintl polyanion InTe−54 with Cr3+ and Fe3+ in aqueous solution, respectively. Their magnetic and conducting properties have been examined as a function of temperature. The high-temperature susceptibility of Cr5(InTe4)3 follows the Curie–Weiss law with θ=−30.2 K and C=1.65 emu/mole Cr atom. Cr5(InTe4)3 has a conductivity of 1.07×10−3 Ω−1 cm−1 at 300 K and is a semiconductor with Ea=0.20 eV. Fe5(InTe4)3 exhibits relatively high conductivity due to partial delocalization of electrons. Its conductivity is 7.41 Ω−1 cm−1 at 300 K, and shows little temperature dependence. Fe5(InTe4)3 obeys the Curie–Weiss law at high temperatures with θ=−26.3 K and C=1.60 emu/mole Fe atom, and undergoes a transition to a spin glass state at 6.0 K. The spin glass state has been characterized by an analysis of the field and temperature dependence of the thermal remanent magnetization and isothermal remanent magnetization. The hysteresis loops measured at 2.0 K under field-cooling and zero-field-cooling conditions reveal the presence of a small unidirectional exchange interaction.
Magnetoelastic energy in domains separated by 90° walls
Authors: Arrott A. S.
Published: July 2002

An iron whisker with a current along its axis has four domains in a frame about a central core. The domains in the frame are magnetized perpendicular to the axis. The central core is magnetized along the axis. The magnetoelastic energy and its corresponding effective field, found analytically using the calculus of variations, account for the experimental observation that for each current, below a critical current, there is a narrow range of fields for which there are two stable sizes for the central core.
Flux trapping and levitation forces in directionally solidified superconducting YBa2Cu3O7 ingots
Authors: Mora J. , Carrera M. , Granados X. , Fontcuberta J. , Piñol S. , Obradors X.
Published: July 2002

YBa2Cu3O7 cylinders with 10%–20% of Y2BaCuO5 and 1% CeO2 additions have been directionally solidified under a temperature gradient. It is shown that a steady growth regime of domains, typically 1 cm in diameter, is established after polynucleation at the bottom of the cylinders on the substrate interface. The length of the region where a steady growth proceeds is limited by the liquid loss, which induces an enrichment in unreacted Y2BaCuO5 in the upper part of the cylinder and a polycrystalline structure. The vertical and lateral magnetic levitation forces and flux trapping profiles have been measured and a direct correlation with the size and location of the domains has been found.
Theoretical interpretation of optical conductivity of YbCu4Ag,Au
Authors: Continenza A. , Monachesi P. , Galli M. , Marabelli F. , Bauer E.
Published: July 2002

We focus on YbCu4Ag and YbCu4Au and interpret the optical conductivity, measured from 2 meV to 6 eV at 10 and 300 K, in terms of self-consistent electronic structure calculations. Most of the relevant spectral structures are interpreted as interband transitions from hybridized f–d states below and at the Fermi level at high symmetry points. In particular, the large shoulder at about 2.4 and 2 eV in the spectrum of YbCu4Au and YbCu4Ag, respectively, can be interpreted in terms of a transition Γ4→Γ3 involving mostly d states. We also compare the electronic structure of YbCu4Ag with that of LuCu4Ag, to ascertain the influence of the f states on the conduction bands and carrier density at EF. This allows us to draw some qualitative considerations on the f-band hybridization in these compounds.
Force gradient mapping of domain wall structures in magnetite (abstract)
Authors: Proksch Roger , Schäffer Tilman E. , Foss Sheryl
Published: July 2002

We have used a magnetic force microscope (MFM) in a new imaging mode to study domain walls in magnetite. The oscillation amplitude and phase of a vibrating cantilever were recorded versus cantilever tip-sample separation at each point in an x-y raster scan of a conventional MFM image. Amplitude and phase changes of the MFM cantilever depend on the topographic, interferometric, damping, and magnetic force gradients effects the cantilever experiences. For small scan areas, the magnetic force gradient acting between the tip and sample could be separated from the other interactions. This allowed quantitative extraction of the magnetic force gradients into a three-dimensional dataset called a force gradient map (FGM). FGMs were made over a number of samples; the work reported here focuses on domain wall structures in single crystal magnetite. The results of previous conventional MFM images and FGM images will be compared. One result was that the apparent resolution of the MFM tip was strongly dependent on the oscillation amplitude of the cantilever tip, implying that this must be considered when modelling MFM response. The data also suggest that FGMs may provide more sensitive tests of micromagnetic models than conventional MFM imaging of magnetite. This is for two reasons: (i) as mentioned above, FGMs allow the magnetic signal to be separated from the topographic, damping and interferometric effects and (ii) because FGMs sample the interactions between the tip and sample at a variety of well defined separations, it is possible to identify critical micromagnetic structural length scales.
Sensitive detection of magnetic field distribution using scanning interference electron microscope (invited) (abstract)
Authors: Yajima Y. , Takahashi Y. , Kuroda K. , Sugita Y.
Published: July 2002

We have implemented interference imaging mode as well as Lorentz imaging mode on a scanning transmission electron microscope (HITACHI HF-2000 with a scanning option; 200 kV accelerating voltage). In the interference mode, a pair of mutually coherent fine probes scan simultaneously across the region of magnetic induction. By monitoring the relative phase change occurring in the probe pair, the variation of the magnetic flux bound by the probe trajectories can be sensed by the universal unit of e/h (−‖e‖: electron charge and h: Planck’s constant). The combined use of this imaging mode with the Lorentz mode which is based on the detection of the probe deflection by Lorentz force facilitates detailed analyses of fringing field around magnetic materials and magnetic induction inside magnetic films. The microscope described above is particularly suited for the study of magnetic storage devices whose microscopic magnetic properties are drawing much attention recently. We show here the results of its application to the observation of field distributions near the MR (magnetoresistive) heads and induction distributions associated with high density longitudinal and perpendicular recording media.
Magnetic properties of DyCo10−xNixSi2 compounds
Authors: Zhao Z. G. , Tang N. , Brück E. , Buschow K. H. J. , de Boer F. R.
Published: July 2002

The temperature dependence of the magnetization of DyCo10−xNixSi2 compounds with x=0, 2, 4, and 6 was measured in fields up to 5.0 T and in the temperature range from 4.2 to 300 K in a SQUID magnetometer. Compensation temperatures were observed for the compounds with x=4 and 6. The values of the R-T exchange interaction obtained by fitting the M-T curves on the basis of two-sublattice molecular-field theory are compared with the values obtained from high-field magnetization measurements on powder particles free to be oriented by the applied field. In all four compounds magnetic transitions are found in the ac-susceptibility measurements which were carried out between 4.2 K and room temperature. The types of magnetic anisotropy were investigated by magnetization measurements on samples that were magnetically aligned at room temperature.
Metal-insulator transition in the presence of excitonic correlation
Authors: Continentino Mucio A. , Japiassú Gloria M. , Troper Amós
Published: July 2002

We study the possibility of an excitonic transition in a two-band model and show that a true phase transition does not occur in the presence of hybridization since the one-body mixing term acts as a conjugate field to the order parameter of the excitonic phase. We suggest an alternative interpretation for recent experiments on rare earth semiconductors based on a metal-insulator transition associated with the opening or closing of a hybridization gap at a critical pressure.
Transport, magnetic, and 119Sn Mössbauer studies on magnetically ordered valence fluctuating compound SmRuSn3
Authors: Mazumdar Chandan , Hossain Z. , Nagarajan R. , Godart C. , Dhar S. K. , Gupta L. C. , Padalia B. D. , Vijayaraghavan R.
Published: July 2002

SmRuSn3 is a unique compound among the known Sm-based valence fluctuation (VF) systems. Its crystallographic structure has two inequivalent Sm sites and Sm ions at only one of them are in VF state while the Sm ions in the other site orders magnetically. Our 119Mössbauer studies show a quadrupolar splitting at the Sn site, consistent with the noncubic symmetry of the Sn site. A broadening of the Mössbauer spectrum is seen due to magnetic ordering of the material. The transferred hyperfine field at Sn site at 4.2 K is small.
Search for hybridization gap in the electronic density of states in CeNiSb
Authors: Menon Latika , Dhar S. K. , Malik S. K. , Yelon W. B.
Published: July 2002

The compounds CeRhSb and CeNiSn show hybridization gap in the electronic density of states. It is of interest to look for a similar gap in a related compound, CeNiSb. We present here the results of the structural, magnetic, and transport studies on CeNiSb. Rietveld analysis of the room-temperature neutron diffraction data reveals that this compound crystallizes in the hexagonal structure (space group P63/mmc). Magnetization measurements show that this compound is ferromagnetically ordered with a transition temperature of ∼4 K. The magnetic susceptibility of CeNiSb follows Curie–Weiss behavior between 10 and 300 K with effective paramagnetic moment close to that of Ce3+ ion. The heat capacity of CeNiSb shows a rise at about 4 K due to the ferromagnetic ordering and a moderately large value of γ is obtained. Its electrical resistivity shows a weak shallow minima at 15 K which is a characteristic feature of Kondo lattice systems. However, no rise in the resistivity at low temperatures, as seen in CeRhSb and CeNiSn, is observed in CeNiSb.
Effect of sputter gas on the physical and magnetic microstructure of Co/Cu multilayers
Authors: Donnet D. M. , Tsutsumi K. , de Haan P. , Lodder J. C.
Published: July 2002

The physical structure of Co/Cu multilayers, sputtered in different gases (Ar, Kr, and Xe) together with the domain structures that these films support have been investigated using electron microscopy in an attempt to explain the differences in their measured magnetoresistance (MR). Both planar and cross-sectional analyses were undertaken. Due to only partial antiferromagnetic coupling submicron domain structures were observed by Lorentz microscopy in all multilayers. The complex nature of these domain structures made classification difficult, although small magnetic field application allowed wall motion and nucleation to be observed. All films were polycrystalline in nature, although average grains sizes differed. However, smoother interfaces together with less well defined crystal boundaries were observed in the Kr and Xe sputtered films. This trend did not correlate with giant MR (GMR) measurements as the Xe sputtered films had the lowest GMR value of the three.