bluuebunny/crossref_metadata_2025 huggingface dataset:
Doping profile control and two-dimensional electron gas formation by Si diffusion into III-V compounds
Authors: Matsushita Shigeharu , Terada Satoshi , Fujii Emi , Inoue Daijiro , Matsumura Kohji , Harada Yasoo
Published: July 2002

The doping characteristics of Si-diffused III-V compounds have been investigated using fully dielectric diffusion sources consisting of undoped SiOx/SiN double-layered films prepared by plasma-enhanced chemical-vapor deposition. It is demonstrated that the rectangular-shaped carrier concentration profiles resulting from the Si diffusion in GaAs can be controlled by varying the deposition parameters of the SiOx/SiN films, i.e., the N2O flow rate in the SiOx deposition, the SiOx film thickness, and the NH3 flow rate in the SiN deposition. It is explained that the profile variations are determined by the quantity of an excess of Si in the SiOx bottom film and the outdiffusion rate of Ga and/or As from GaAs. Furthermore, the Si diffusion experiments are carried out in AlxGa1−xAs, and the formation of a two-dimensional electron gas with a Hall mobility of 5060 cm2/V s at 300 K and 97 500 cm2/V s at 77 K is achieved by diffusing Si into an undoped GaAs/Al0.22Ga0.78As heterostructure.
Coercivity and magnetization process versus dipolar coupling in Ni80Fe20/Cu/Co/Cu spin valves
Authors: Kergoat R. , Miltat J. , Valet T. , Jerome R.
Published: July 2002

The elaboration of four different sample types, namely, isolated NiFe or Co layers in their Cu environment and full, exchange decoupled, [Cu/NiFe/Cu/Co/Cu] periods with either the Co or NiFe layers in the vicinity of the free surface has allowed for a detailed study of the magnetization process of the constitutive layers of one period of a ‘‘hard–soft’’-type spin valve. The domain distribution and magnetization fluctuations within domains have been monitored by Kerr microscopy. The experiments point to a strong influence of dipolar coupling on the properties of the soft permalloy layer, with a dramatic increase of the NiFe film coercive field with respect to the corresponding quantity in an isolated NiFe layer. The macromagnetization fluctuations detected in the NiFe layer appear most likely linked to fluctuations of low amplitude occurring in the Co layer in the vicinity of the NiFe coercive field.
Magnetic properties of FexCu1−x granular alloy films
Authors: Chubing Peng , Haiying Chen , Guozhong Li , Daosheng Dai
Published: July 2002

The magnetic properties of the FexCu1−x granular system, as functions of iron particle size, were studied. Using high resolution transmission electron microscopy, iron particle size was determined to be ranged from 1–4 nm in as-deposited samples as x=0.06–0.34. Magnetic measurements revealed that samples exhibit superparamagnetic relaxation below 300 K as x≤0.22. At 1.5 K, the average magnetic moment per each iron atom is reduced as the average number N of iron atoms in a particle is less than 450, and approaches the value of bulk iron as N≥600. Moreover, the temperature dependence of magnetization for Fe–Cu granular alloys was found to obey the Bloch’s T3/2 law below 300 K as x≳0.22. We suggest that spin wave excitations of long wavelength occur due to the weak exchange coupling among iron particles as x≳0.22. This behavior was confirmed by the ferromagnetic resonance study.
Galvanomagnetic properties of quasi-one-dimensional superconductors
Authors: Arutyunov K. Yu. , Danilova N. P. , Nikolaeva A. A.
Published: July 2002

The transport properties of thin single-crystalline tin and indium filaments in glass cover were studied. The length of the samples was much greater than the quasiparticle relaxation length. The width of the resistive transition and the electron free path of the samples studied were comparable with the corresponding values of the perfect whiskers. The observed step-like current-voltage characteristic could be described by the model of noninteracting phase-slip centers.
Direct measurements of magnetostrictive process in amorphous wires using a scanning tunneling microscope (abstract)
Authors: Costa J. L. , Nogués J. , Rao K. V.
Published: July 2002

We demonstrate a new and versatile method to measure, on a nanometric scale directly, the magnetostrictive properties during the magnetization process to saturation, using a modified scanning tunneling microscope (STM). Both positive and negative magnetostrictive amorphous as-quenched wires have been studied. The studied samples are single pieces of amorphous wires typically 125 μm in diameter and 10–15 mm long. The magnetostriction data are then correlated with the longitudinal magnetization process, measured by a conventional induction technique and by SQUID magnetometry. The longitudinal magnetization process measurements have been performed in the same wires used in the STM studies. The field dependence of the magnetostriction helps to discern the operative magnetization process in the wires. In a 12-mm-long Co-based amorphous wire we observe a continuous rotation of the magnetization from zero field to saturation. The field dependence of the magnetization is a linear process reaching a value of about 5100 G at 8 Oe applied field. The magnetostrictive process for the same wire measured with a STM shows a continuous shrinking of the samples as a function of the applied field, reaching a value of −280 Å for 8 Oe applied field. The obtained δl/l‖sat, −2.3×10−6, agrees well with reported values of the saturation magnetostriction constant for this wire. In a 15-mm-long Fe-based amorphous wire we observe a more complicated field dependence of both the magnetization and the magnetostriction processes from zero-field to saturation.

The longitudinal magnetization saturates at about 14000 G for 60 Oe applied field. The magnetostrictive process for the same wire measured with STM shows an elongation onset at about 1 Oe, pointing unambiguously to a 180° domain wall movement as the operative magnetization process below this field value. The field dependence of the magnetostriction saturates at about +5000 Å for 60 Oe applied field. The obtained δl/l‖sat, about +3.3×10−5, is again consistent with reported values of the saturation magnetostriction constant. Thus the STM approach to determine λ, on an Å length scale, gives us a distinct great advantage in studying the magnetization process towards saturation.
Magnetic susceptibility studies in Gd2CuO4 below 300 K
Authors: Mira J. , Castro J. , Rivas J. , Baldomir D. , Vázquez-Vázquez C. , Mahía J. , López-Quintela A. , Fiorani D. , Caciuffo R. , Rinaldi D. , Jones T. , Oseroff S. B.
Published: July 2002

Measurements of the real (χ′) part of the ac magnetic susceptibility have been performed on two polycrystalline Gd2CuO4 samples sintered at different temperatures, 850 °C and 1080 °C in the temperature range 10<T(K)<300. Two well-defined maxima have been observed in the temperature dependence of χ′. One, located at about 20 K, that is frequency independent. The second is at about 280 K for the sample annealed at 1080 °C, and it is frequency independent. Instead, for the sample annealed at 850 °C the maximum appears at about 210 K, and it is frequency dependent. These results suggest the existence of magnetic domains, with their coherence lengths being a function of the thermal treatment.
Heterodyne microwave mixing in a superconducting YBa2Cu3O7−x coplanar waveguide circuit containing a single engineered grain boundary junction
Authors: Seed R. G. , Vittoria C. , Widom A.
Published: July 2002

The purpose of this work was to utilize the nonlinear current-voltage properties of induced grain boundaries in high temperature superconducting YBa2Cu3O7−x thin films to fabricate a planar microwave mixer. The experiment involved constructing a coplanar waveguide microwave circuit, the center conductor of which had a constriction patterned in it containing a single high angle grain boundary, thus forming a weak link junction. Analysis was provided by use of the resistively shunted junction model with excess current.
Studies of Mössbauer spectrum on Sm2(Fe,Ga)17C1.5 alloy
Authors: Chen Hong , Xu Zu-xiong , Ma Ru-zhang , Pan Shu-ming , Shen Bao-gen , Xue De-yan , Ni Qiang
Published: July 2002

X-ray-diffraction patterns and Mössbauer spectra of Sm2(Fe,Ga)17C1.5 alloys have been studied. The results of x-ray-diffraction patterns and Mössbauer spectra indicate that little α-Fe and carbonides appear in Sm2(Fe,Ga)17C1.5 alloys. The addition of a few Ga atoms show that the thermal stability in Sm2(Fe,Ga)17C1.5 alloys is getting better. Structural analyses indicate C atoms as interstitial atoms occupying the 9d site. Ga atoms seem to substitute partially for Fe atoms and occupy preferentially the 18h site.
Light scattering from spin waves in MnF2
Authors: Cottam M. G. , Gnezdilov V. P. , Labbé H. J. , Lockwood D. J.
Published: July 2002

Measurements of the weak one-magnon light scattering in the antiferromagnet MnF2, including Stokes and anti-Stokes scattering, as a function of temperature are reported. From the theoretical analysis of the temperature and polarization dependences of the integrated Raman intensities, values are deduced for the ratio of quadratic and linear magneto-optical coupling coefficients.
Nonlinear self-localized surface spin waves in ferromagnets (abstract)
Authors: Boardman Alan , Bespyatykh Yurij , Dikshtein Igor , Nikitov Sergey
Published: July 2002

It is well known that in linear theory, surface spin waves (SW) do not exist in a pure exchange-coupled ferromagnet for the case of free spins at the surface. However, in these approximations of the linear theory the plane volume spin waves (VW), propagating along the surface of a ferromagnet, satisfies not only the Landau–Lifshitz equation for the magnetization motion, but the boundary conditions for the free surface spins. Such VW can be unstable and can be transformed into SW under small changes of a magnetic medium, e.g., if the surface spins are partly pinned. In the present work a new type of self-localized SW in the ferromagnet has been considered. The existence of such waves is conditioned entirely by the nonlinear properties of a ferromagnet. The penetration length of such SW is proportional to 1/A, where A is a maximum of the magnetization amplitude on the surface of the crystal. The dispersion equations have been obtained for pure exchange and dipole-exchange nonlinear SW. In the latter case the influence of the second harmonic generation on the wave propagation at the fundamental frequency was studied. The conditions when the SW excites the VW, carrying the energy into the volume of the crystal, are derived. The nonlinear Schrödinger equation for the SW envelope amplitude was derived and its solitonic solutions are obtained. The estimations of threshold values for the wave numbers of the propagating waves are provided.
A Mössbauer effect study on the acicular cobalt ferrite particles
Authors: Na J. G. , Han D. H. , Zhao J. G. , Luo H. L.
Published: July 2002

The causes of the coercivity enhancement and the coercivity instability of acicular cobalt ferrite particles containing 0–9.2 wt % Fe2+ ions under the influence of magnetic field and temperature were investigated by means of transverse magnetic field annealing, torque, and Mössbauer experiments. The coercivity enhancement and the instability of Co modified iron oxide particles are closely related to the uniaxial magnetic anisotropy. A Fe2+-CO2+ pair model was proposed to explain the uniaxial magnetic anisotropy of Co modified iron particles. To verify the nature of the Fe2+-Co2+ pair, the probability of one or more Co ions appearing in the neighborhood of a B site Fe2+ ion was calculated using the chemical composition, the Mössbauer parameters, Pi(Fe+2) and Pi(Fe+3) and discussed conditions for forming the pair between the nearest-neighbor Co2+ and Fe2+ ion.
X-ray photoelectron spectroscopy and Mössbauer study of Ho(Fe1−xMnx)2 compounds
Authors: Tang Y. J. , Feng Y. B. , Luo H. L. , Pan S. M.
Published: July 2002

Cubic Laves compounds Ho(Fe1−xMnx)2 (x=0, 0.1, 0.2, 0.3) were investigated by XPS and Mössbauer measurements. It was found that the binding energy (BE) of compounds obtained by XPS remains almost the same for all the compounds and no chemical shift (δ) of the core-electron binding energy was found, which implies that no charge transfer takes place from Mn atoms to Fe atoms due to Mn substitution. Mössbauer study has shown that the average hyperfine fields and the deduced Fe moment decrease with increasing Mn content. It is considered that the variation of isomer shift (IS) due to Mn substitution obtained by Mössbauer measurement is mainly due to the size effect since the isomer shift (δ) for the compounds are almost linearly dependent on the volume; electron transfers seem to be of minor importance.
Neutron diffraction from the vortex lattice in the heavy fermion superconductor UPt3 (invited) (abstract)
Authors: Kleiman R. N. , Aeppli G. , Bishop D. J. , Broholm C. , Bucher E. , Stüchelli N. , Yaron U. , Clausen K. N. , Howard B. , Mortensen K. , Pedersen J. S.
Published: July 2002

The heavy fermion superconductor UPt3 is thought to have a d-wave pairing ground state. The principal experimental evidence for this consists of the anisotropy of the power-law behavior observed in transverse ultrasound and μ+ SR measurements. The observation of a complex phase diagram in the superconducting state in ultrasound, torsional oscillator, and specific heat measurements may be a further indication of an unconventional pairing state. Theoretical investigations suggest the possibility of vortex lattices that are unconventional in their symmetry, their quantization, or the structure of their composite vortex cores. Transitions between such exotic vortex lattices are in principle allowed and could explain the observed features at H≊0.6 Hc2 (for H∥ĉ) and H≊0.3Hc2 (for H⊥ĉ). Neutron diffraction is an ideal bulk probe of the microscopic properties of the vortex lattice. We have studied the vortex lattice with H⊥ĉ and T≊50 mK in the field range 0.75<H<10 kG. The structure of the vortex lattice and the quantization of the vortices, in addition to the London penetration depth, λL, the coherence length, ξ, and the effective mass anisotropy are all well determined by our measurements. The lattice is oblique hexagonal with conventional quantization. Its anisotropy can be explained by considering a combination of Fermi surface and gap anisotropy. However, the lattice does not appear to change near the transition between superconducting phases identified by other techniques.
Small angle neutron scattering from the vortex lattice in 2H-NbSe2 (invited) (abstract)
Authors: Gammel P. L. , Yaron U. , Huse D. A. , Kleiman R. N. , Batlogg B. , Oglesby C. S. , Bucher E. , Bishop D. J. , Mason T. E. , Mortensen K.
Published: July 2002

We report on small angle neutron scattering studies of the flux-line lattice in single crystal 2H-NbSe2. For fields inclined with respect to the c axis, we find distortions and form factors consistent with Ginzburg–Landau corrections to the London equations with a mass anisotropy Γ=10.1±0.9. The flux lattice orientation, however, remains pinned to the crystal lattice for all tilts studied, in disagreement with the orientation defined by anisotropic London theory. For fields below 2 kG parallel to the c axis, the peaks are no longer resolution limited. The correlation lengths extracted are history dependent, and show that the lattice is annealed when a current greater than the critical current is applied. This occurs both when a direct transport current is used, or an induced current in a zero field cooled experiment. The annealing is seen in both the transverse and longitudinal correlation lengths, and calls into question the relationship between the Larkin–Ovchinnikov correlation length and the measured critical currents in this system.
Kerr microscopy observation of carbon diffusion profiles in Sm2Fe17Cx
Authors: Zawadzki J. , Wendhausen P. A. P. , Gebel B. , Handstein A. , Eckert D. , Müller K.-H.
Published: July 2002

The effect of carbon diffusion from CH4 gas into coarse Sm2Fe17 powder on the magnetic domain structure (MDS) was studied at room temperature by means of the magneto-optical polar Kerr effect. The diffusion occurs initially along preferential paths and then through the bulk. A gradual variation in the MDS through incompletely carburized grains was observed, which reflects local changes in spontaneous magnetization and magnetic anisotropy. This suggests the existence of smooth ‘‘bathtublike’’ carbon concentration profiles. An acceleration of the carburization process caused by cracking of grains during the diffusion was observed. The diffusivity R of carbon in Sm2Fe17 at 450 °C was estimated to be 2.5×10−16 m2/s. For Sm2Fe17C2.2 the domain-wall energy density is γ=3.1×10−2 J/m2, the domain-wall thickness δ=3.3 nm, the single-domain particle size Dc≊0.3 μm, and the exchange constant A=8.1×10−12 J/m.
FMR doublet in two-layer iron garnet films (abstract)
Authors: Grishin A. M. , Dellalov V. S. , Nikolayev E. I. , Shkar V. F. , Yampolskii S. V.
Published: July 2002

Multilayer epitaxial iron garnet films have been investigated very intensively in the recent past. The dipole and exchange couplings of spins lying in different layers results in the existence of new types of oscillations in these structures and give rise to the characteristic resonance properties of them. Resonance microwave absorption in two-layer iron garnet films, of which one layer is in a saturated state and the other is in a demagnetized state, is investigated. The films were prepared by the epitaxial method on a gallium-gadolinium substrate with (111) orientation. The first layer on the substrate was the doped iron yttrium garnet with the easy plane magnetization. The second easy axis layer was the bubble domain layer. The resonant field of the first layer FMR line versus the external in-plane magnetic field was investigated. It was determined that the FMR line is doubled when the external field is applied along the [112]-type axes. The lines of the FMR doublet merge into a single line and the resonance intensity is doubled if the magnetic field is oriented in the [110] directions. It was established that the FMR line splitting is conditioned by the layer exchange interaction, the cubic anisotropy, and the domain dissipative field. The magnetic and anisotropy of the resonance fields agrees with the model of an isolated layer magnetized by the domains of the neighboring layer.
Structural and magnetic properties of Co/Ag multilayers
Authors: van Alphen E. A. M. , van Heijden P. A. A. , de Jonge W. J. M.
Published: July 2002

The structural properties and growth of sputtered Co/Ag multilayers were studied with nuclear magnetic resonance. For samples with a nominal Co thickness of less than 10 Å the Co grows in three-dimensional islands. If the nominal Co thickness is larger than 10-Å continuous Co layers are formed. The relation of the structural properties with magnetization and magnetoresistance is discussed.
Hybrid NiFeCo-Ag/Cu multilayers: Giant magnetoresistance, structure, and magnetic studies
Authors: Jarratt J. D. , Barnard J. A.
Published: July 2002

Giant magnetoresistance (GMR), crystal structure, and magnetic properties have been investigated in sputtered Ni66Fe16Co18-Ag/Cu hybrid granular multilayer thin films. High angle x-ray diffraction (HXRD) was used to reveal the overall film structure and growth texture and low angle XRD was used to investigate the periodicity and flatness of the multilayer structures. Hysteresis loops for the as-deposited Ag-rich films show superparamagnetic behavior (and conventional granular GMR) which does not saturate in 14 kOe. Very NiFeCo-rich films are magnetically soft and exhibit induced in-plane uniaxial anisotropy.
Phase transition in a system of interacting triads
Authors: Diep H. T. , Loison D.
Published: July 2002

The present work is motivated by the controversy on the nature of the phase transition on the Heisenberg stacked triangular antiferromagnet (STA). In particular, the renormalization group with 4−ε expansion suggests a new universality class, while the renormalization group with 2+ε expansion using a nonlinear σ (NLS) model shows that the transition, if not mean-field tricritical or first order, is of the known O(4) universality class. In order to verify this conjecture, we study here an equivalent system obtained from the STA by imposing the local rigidity as has been used in the NLS model. The results show that none of the scenarios predicted by the NLS model is found. The critical exponent ν=0.48±0.05 is quite different from the original STA without local rigidity, indicating that the local rigidity changes the nature of the transition. It is also different from that of O(4). It means that successive transformations used to buildup the NLS model from the original STA may lead to the O(4) universality class.
Study of the spin glass transition of amorphous FeZr alloys using small angle neutron scattering
Authors: Mergia K. , Messoloras S. , Nicolaides G. , Niarchos D. , Stewart R. J.
Published: July 2002

SANS experiments of an amorphous Fe90Zr10 alloy were carried out in the temperature region 20–300 K with and without the application of a magnetic field. The scattering versus temperature for different Q values (zero field cooled magnetization) shows two transitions at Tc=240 K and Tf=40 K. However, both transitions are not observed in magnetic field (2.6 kG) cooling down curves. By switching off the magnetic field and warming up the sample only the transition at Tf=40 K is observed. Application of a magnetic field at different temperatures in a zero field cooled sample reduces the scattering; the scattering returns to its initial value after switching off the field.
Understanding the great range of magnetic ordering behavior in correlated f-electron systems
Authors: Sheng Q. G. , Cooper B. R.
Published: July 2002

The magnetic ordering behavior of correlated f-electron systems varies widely. With regard to the value of the ordered moments, there are systems of saturated moment (e.g., CeSb), of moment somewhat reduced from the saturated value (e.g., UTe), of very small moment (e.g., UPt3) and of no moment at all (e.g., CeCu2Si2). We show that such wide diversity in magnetic ordering is a manifestation of the competition between (1) hybridization and exchange interaction and (2) localization and itinerancy. By analyzing these effects, we develop a theory which organizes the diverse magnetic behavior into a unified picture describable through one model Hamiltonian. An important feature of this analysis is that we recognize and treat the effect of band-f Coulomb exchange simultaneously with that of band-f hybridization. Rather than adopting the standard analysis using the ‘‘Kondo resonance’’–‘‘Kondo compensation’’ concept, the development of this theory offers a new approach to treat the correlated f-electron state. The present theory naturally leads to a nonmagnetic singlet ‘‘Kondo state’’ which is one of the possible states, along with other magnetic states which the system could be in, when the conditions determining the state of the system favor that choice. The f orbital motions and spin-orbital coupling are given full consideration in the theory.
Microstructure and magnetic properties of mechanically alloyed anisotropic Nd-Fe-B
Authors: Wecker J. , Cerva H. , Kuhrt C. , Schnitzke K. , Schultz L.
Published: July 2002

Mechanically alloyed Nd-Fe-B powders with additions of Dy, Co, and Ga were hot compacted to magnetically isotropic materials and then deformed by die upsetting to obtain anisotropic magnets. Magnets can be processed either with a medium coercivity Hci of 7.8 kA/cm but with a high-energy product (BH)max of 330 kJ/m3 or with a coercivity of 21.2 kA/cm and a lower (BH)max of 192 kJ/m3. Plane-view transmission electron microscopy (TEM) pictures show nearly equiaxed Nd2Fe14B grains. In cross section, the grains display the typical platelet form known from MQ 3-type magnets with a length of about 300 nm and a height of 100 nm. Characteristic for mechanically alloyed materials are up to 500 nm large globular NdOx particles distributed regularly throughout the microstructure which inhibit the anisotropic grain growth. In addition 5–20 nm small NdOx precipitates are found within some Nd2Fe14B grains. X-ray diffraction studies also indicate that the degree of texturing after die upsetting is smaller for mechanically alloyed than for melt-spun materials. High-resolution TEM experiments did not show any grain boundary phase usually found in melt-spun materials; however, this needs to be confirmed.
Generation of highly uniform fields with permanent magnets (invited)
Authors: Abele M. G.
Published: July 2002

A review of a design methodology of permanent magnets composed of magnetic materials with quasilinear demagnetization characteristics is presented. The paper focuses on the design of efficient permanent magnets for applications that require strong and highly uniform fields. The compensation of the field distortion caused by design constraints and fabrication tolerances is one of the aspects of magnet design discussed in this paper. A design procedure based on active filter structures that restore a required degree of field uniformity is reviewed.
Mössbauer effect investigation of the pentagonal approximant phase in the Fe-Nb system
Authors: Dunlap R. A. , Kyriakidis J. , Yewondwossen M.
Published: July 2002

The formation of the pentagonal Frank–Kasper phase (μ phase) in the Fe50+xNb50−x system near x=0 has been investigated. The effects of composition and heat treatment on the phase diagram of this system have been studied. Only the as-cast x=0 alloy was found to consist of a pure μ phase. X-ray diffraction patterns indicate the existence of a phase with a high degree of structural order but considerable chemical disorder. A Mössbauer spectrum of the μ-phase material exhibits a room-temperature quadrupole split doublet with Δ=0.209 mm/s and an isomer shift δ=−0.289 mm/s relative to α-Fe. A shell model analysis of the spectrum indicates a structure with a high degree of order. Results are discussed in terms of measurements on related icosahedral and decagonal phase materials.
Nitriding studies of aligned high anisotropy ThMn12-type NdFe11Co1−yMoyN film samples (abstract)
Authors: Navarathna A. , Samarasekara P. , Hegde H. , Rani R. , Cadieu F. J.
Published: July 2002

Highly aligned film samples of ThMn12-type NdFe12−y−zCoyMoz, y+z≊1, have been directly crystallized onto heated substrates by rf sputtering. The deposited films have then been nitrided at different nitriding pressures and temperatures to determine the optimum conditions to maximize the coercivity. The films have generally been (002) textured so that the easy axis of magnetization of the nitrided films is perpendicular to the film surface. For low Co concentrations it has been possible to enhance the saturation flux densities, the coercivity, and the anisotropy field of the nitrided samples. Maximum room temperature coercivities and anisotropies of 11.3 and 145 kOe were obtained for a ThMn12-type Nd8.9Fe80.3Co6.0Mo4.8N (002) textured film sample. At lower temperatures the coercivity and anisotropy rose smoothly to 29.5 and ≊200 kOe respectively by 10 K. Based on nitriding studies on similar films, all nitride sites should be occupied at ≊1 N per ThMn12 formula unit for the high coercivity films. Previously NdFe11Co0.5Mo0.5N films were reported with a room temperature BHmax=46.3 MGOe at 293 K and 59.6 MGOe at 10 K. In general, films that have been made to exhibit a gradient in any one component have exhibited lower energy products due to either a reduced coercivity or to shouldered hysteresis loops.
Effect of amorphous carbon layers on the growth of diamond in dual-frequency plasma
Authors: Reinke P. , Klemberg-Sapieha J. E. , Martinu L.
Published: July 2002

In the present work we study the growth of diamond in a dual-mode microwave/radio frequency plasma. We investigate the effect of the thickness of predeposited hydrogenated amorphous carbon (a-C:H) films and of ion bombardment on the nucleation process and on the crystal quality. The deposits are characterized by x-ray photoelectron spectroscopy (XPS) and by scanning electron microscopy. The XPS spectra of the C(1s) carbon peak and of the plasmon features confirm the presence of an amorphous, carbonaceous phase and of silicon carbide on the surface. Radio frequency biasing during the initial stage of diamond growth leads to a lower crystal quality, but to a higher nucleation density (ND). Without biasing, good quality, predominantly (100) oriented diamond crystals are obtained on a Si(100) surface. The ND values are found to increase with the thickness of the predeposited a-C:H layer. Evolution of the nucleus size distributions indicates that the a-C:H film contributes to the carbon supply, enhancing the nucleation efficiency and shortening the incubation time of seed crystals. Before a continuous layer is formed, the growth of crystals is determined by the interaction with the gas phase as well as by the amount of carbon available on the surface.
Surface photovoltage measurements on pyrite (100) cleavage planes: Evidence for electronic bulk defects
Authors: Bronold M. , Pettenkofer C. , Jaegermann W.
Published: July 2002

Temperature dependent contactless surface photovoltage measurements by photoelectron spectroscopy have been performed on cleaved (100) surfaces of pyrite (FeS2) single crystals. The results have been fitted by thermionic emission, recombination, and tunneling models for the majority carrier transport to the surface. Neither of them is able to explain the small photovoltages consistently. By calculating electronic defect levels due to the sulfur deficiency of pyrite a high number of defect states in the band gap is obtained. As a consequence a nonuniform depletion layer is expected with a part of the band bending potential falling off at a very small distance near the surface. The small photovoltages can be explained by a tunneling of majority carriers through the narrow barrier and by recombination losses due to the defects.
Method of magnetic domain modeling on a moving magneto-optical disk
Authors: Jeng Tzuan-Ren , Chao Shiuh
Published: July 2002

A method is proposed to simulate the formation process and the shape of a magnetic domain on a moving magneto-optical disk. It is assumed that the direction of the force acting on the domain wall is perpendicular to the isothermal line of the critical temperature. Slicing lines perpendicular are taken at regular intervals to this isotherm, and by applying the one-dimensional bubble model the wall position on each slicing line is determined. Connecting the wall positions in each time frame the evolution of the domain shape in the formation process is obtained. By applying this method to Tb23Fe77 thin film, it is found that the domain wall in Tb23Fe77 thin film on a moving disk was first pinned in the cooling region which was the leading region of laser exposure on the disk, then the pinned wall expanded toward the heating region, and a closed domain was formed after the laser was turned off.
Nearly cut-off modes caused by diffusion in lithium niobate
Authors: Kwiatkowski Stephen L. , Mickelson Alan R.
Published: July 2002

We have observed more extraordinary polarized guided modes near cut-off in titanium in-diffused lithium niobate planar waveguides than are predicted by titanium in-diffusion models. We also observe these nearly cut-off modes in lithium niobate specimens fabricated without titanium under a variety of diffusion atmospheres. We found more guided modes, with higher effective index in these plain lithium niobate specimens fabricated in wet oxygen than in specimens fabricated in either air, dry oxygen, dry argon, or wet argon. Values of the effective index of these nearly cut-off modes found in plain lithium niobate planar waveguides do not agree with those calculated using the lithium out-diffusion model for either vacuum-diffused or oxygen-diffused lithium niobate waveguides. We found good agreement between measured values of the effective mode index in specimens fabricated in either dry oxygen or air and calculated values found using an empirical diffusion model of a step index profile whose depth increases as the square root of diffusion time.
Longitudinal phonon modes in a ZnSe/ZnSxSe1−x lattice-mismatched superlattice
Authors: Xia Hua , Jiang S. S. , Zhang Wei , Zhang X. K. , Guan Z. P. , Fan X. W.
Published: July 2002

Longitudinal acoustic and optical phonon modes of a ZnSe/ZnSxSe1−x (x≂0.20) lattice-mismatched superlattice, prepared with atmospheric metal organic chemical vapor deposition method, have been investigated by light scattering measurements. Despite a lattice mismatch as large as 1% between the alternating layers, the measured longitudinal elastic constants are in agreement with the calculated values of an unstrained effective medium model. Furthermore, a correlative study was made by fitting the spectra to a spatial correlation model, which reproduces line shapes of the observed confined longitudinal-optical modes without incorporating the strain effects. The results demonstrate that a combination of Brillouin and Raman spectroscopy provides a good method to determine accurately the elastic constants and strain information of the lattice-mismatched superlattices and heterostructures.
Oscillations in the frequencies and coupling strengths of two-dimensional plasmons induced by grating couplers
Authors: Tyson R. E. , Bangert D. E. , Hughes H. P.
Published: July 2002

Direct optical coupling to the plasmon modes of a two-dimensional electron gas in a stratified semiconductor system requires an overlaid grating coupler. Plasmons can be excited with wave vectors equal to integer multiples of the grating wave vector, and the periodic screening by the grating splits these into two modes, symmetric and antisymmetric with respect to the grating profile. We present calculations of the dispersions and optical coupling strengths for several orders of both modes in a typical structure, and show that the splitting and coupling strengths of the higher order modes oscillate with varying grating mark fraction. These effects are discussed in terms of the oscillating field strengths and charge density profiles.
Temperature effects on heavily doped polycrystalline silicon
Authors: Deen M. J. , Naem A. A. , Chee L. Y.
Published: July 2002

The effects of operating temperature and current density on the resistivity of low-pressure chemical-vapor-deposition polycrystalline silicon heavily doped with different impurities, phosphorous (3.1×1020 cm−3), arsenic (4.6×1020 cm−3), or boron (3.1×1020 cm−3), were studied. The resistivity of the films was measured over a wide range of temperatures (15–195 °C) and current levels (1–20 mA). The arsenic-doped polycrystalline silicon results agree with the widely used thermionic emission model; however, unexpected results were obtained for the phosphorous- and boron-doped samples where the resistivity increases with temperature and current density. For the phosphorous and boron-doped materials, an empirical model based on carrier mobility that can predict the resistivity of the polycrystalline silicon over a wide range of operating temperatures and current densities has been developed; the agreement between the model predictions and the experimental data is good.
Granular magnetoresistance ribbon material suitable for isotropic magnetic field transducers
Authors: Li M. , Zhao X. G. , Zhao T. Y. , Zhao J. G. , Shen B. G. , Gu Y. S. , Chai W. P. , Mai Z. H. , Mei L. M.
Published: July 2002

Magnetic granular CoCu alloy ribbons were prepared by the melt-spun method, and the structural, magnetotransport, magnetic properties, and the effects of annealing on these properties were studied by x-ray diffraction, vibrating sample magnetometer, and magnetoresistance (MR) measurements at room temperature. Giant magnetoresistance was observed over a broad range of Co concentrations with a maximum MR ratio of 7.8% (at x=0.20 and 500 °C annealing) under a magnetic field of 20 kOe at room temperature. It is found that the relationship between MR and magnetic field H depended strongly on the annealing temperature and a linear MR-H relation could be obtained at an optimum annealing condition. The MR ratio did not depend on the direction between H and electric current I. These features are very suitable for making isotropic magnetic field transducers.
Theoretical and practical aspects of collimated sputtering
Authors: Dew S. K.
Published: July 2002

Aspects and implications of collimated sputtering have been studied through experiments and simulations. Dependences and scaling relationships of via bottom coverage, deposition efficiency, thickness and coverage uniformity, and collimator lifetime have been examined. Geometrically, collimator effectiveness, transmission efficiency, and target erosion-related nonuniformity scale principally with collimator cell aspect ratio. Ripple variations in thickness and coverage depend not only on aspect ratio, but also on the ratio of cell depth to the collimator-substrate distance. Collimator lifetime depends on cell size, aspect ratio, and on the positions of the collimator and substrate in front of the target. Such geometrical factors are complicated by scattering by the sputter gas which scales according to the product of pressure and the appropriate distance.
Critical scattering of electromagnetic waves on spin fluctuations in nonsaturated magnetic films under acoustic pump
Authors: Dikshtein I. E. , Kryshtal R. G. , Medved A. V.
Published: July 2002

The theoretical and experimental investigation of the microwave (MW) combination scattering on spin fluctuations (SF) in the yttrium iron garnet film under acoustic pump were pursued. MW frequency f≂3.5–4.1 GHz was well above spin wave spectrum at a preassigned magnetic field. At the external magnetic field range H0≂5–40 Oe one (or more) intensity peaks of MW scattering were observed. The positions of these peaks do not depend on MW and surface acoustic wave (SAW) amplitudes and frequencies and depend heavily on the magnetic field directions in a film plane. Because these phenomena were closely associated with the domain walls (DW) reorientation or at the domain structure transformations, e.g., at the flexural instability of DW, one may speculate that critical scattering of MW on SAW and SF was observed. Cross section for such scattering was calculated. In nonsaturated magnetic films the phenomena of MW scattering on SAW and spin fluctuations at high amplitudes of the MW and SAW were investigated as well. In the area of external magnetic fields H0≥250 Oe a stepped dependence of intensity of scattering MW was observed. There is no appropriate explanation of this effect now.
Discussion of the magnetic dichroism in the x-ray resonance scattering
Authors: Rennert Peter
Published: July 2002

The cross section for the x-ray resonance scattering is evaluated at the K edge for a transition metal like Ni. The Green function is calculated analytically from a model system. The differences in the spectra measured with different polarization can be explained from these results. The examples show that the shape of magnetic dichroism is very sensitive to the spin polarized p part of the Green function.
A theory of electroded thin thermopiezoelectric plates subject to large driving voltages
Authors: Yang J. S. , Batra R. C.
Published: July 2002

We first reduce existing equations for nonlinear thermoelectroelasticity to the case of large electric fields, small strains, and infinitesimal temperature variations. These equations are then specialized to the case of thin plates with completely electroded major surfaces, and it is shown that in this case the charge equation of electrostatics is satisfied trivially to the lowest order. An example is given to illustrate the effect of heat conduction on the response of a thin electroded plate subjected to time harmonic voltages on the two major surfaces.
Ohmic contacts on diamond by B ion implantation and TiC-Au and TaSi2-Au metallization
Authors: Das K. , Venkatesan V. , Humphreys T. P.
Published: July 2002

Low resistance ohmic contacts have been formed on diamond films using bilayer structures of TiC/Au or TaSi2/Au. The initial films of TiC or TaSi2 were sputter-deposited from preformed targets. The overlying Au film was deposited by e-beam evaporation. Contact resistivities of the order of 10−5 Ω cm2 were obtained for the as-deposited layers. It was observed that the contact resistivity can only be marginally modified upon vacuum annealing at a temperature of 850 °C. In the more conventional approach of forming ohmic contacts using Ti/Au, a post-deposition anneal at a temperature of 850 °C is essential in order to obtain low contact resistivities. However, during this anneal procedure, a substantial amount of O is taken up by the Ti layer resulting in the formation of mixed oxide phases of Ti in addition to the interfacial TiC. In contrast, it has been demonstrated that the oxide formation can be avoided by sputter depositing the initial layer of either TiC or TaSi2 instead of elemental Ti. Moreover, in the case of the TaSi2/Au contact, the Au film is most probably lost due to a eutectic formation with Si. In this respect, the annealed TiC/Au contact appears satisfactory.
Influence of In on Si local vibrational modes in InxGa1−xAs (0≤x≤0.12)
Authors: Alvarez A. L. , Calle F. , Wagner J. , Sacedón A. , Maier M. , Fernández de Avila S. , Lourenço M. A. , Calleja E. , Muñoz E.
Published: July 2002

Local vibrational modes (LVM) of Si in substitutional sites have been observed by resonant Raman spectroscopy in highly doped (≥8×1018 cm−3) InxGa1−xAs layers, either relaxed or under strain, on [100] GaAs substrates. The peak frequency ωLVM of the Si on Ga site (SiGa) LVM in unstrained samples shifts to lower values with increasing In content. For x≤0.10 this shift is clearly higher than expected from a linear interpolation between the measured values in the binaries. The comparison between the SiGa peak frequency measured in both a full strained layer and a relaxed layer with similar composition provides a rough determination of the deformation potentials for the SiGaLVM in these layers: q/ω2LVM=−2.7±1 and p/ω2LVM=−2.5±1. As the In content becomes higher the width of the SiGa peak increases much more than that of the GaAs-like longitudinal optical-phonon peak, revealing the splitting due to the loss of local symmetry introduced by the In. New calibration factors for the Si-defect concentrations have been deduced, which allow estimation of the solubility limit for the Si incorporation in substitutional positions, which ranges from 2.3×1019 to 2.6×1019 cm−3 for the layers at the growth conditions used. The analysis of the integrated intensity of the LVM Raman peaks indicates that the degree of electrical compensation is clearly reduced for increasing In up to x≤0.05, due to both an increase of the solubility limit for Si in these layers and a saturation or slight reduction of the SiAs-related defect concentrations. This conclusion is also supported by Hall and plasmon measurements.
Helicoidal, magnetic vortex in a current-carrying superconductor in a longitudinal magnetic field: New exact solution (abstract)
Authors: Genenko Yuri A.
Published: July 2002

A resistive state in a type-II superconductor (SC) is conditioned by the Abrikosov magnetic vortex motion in them. In absence of external magnetic field the entry of vortex rings of a transport current self field determines the resistivity oneset. Being applied along the current-carrying SC cylinder the magnetic field does not affect the entry of self-field vortex loops, because of force-free geometry. In a latter case other vortex configurations, more likewise a field line pattern, seem to face less edge barrier against vortex entry and determine the resistivity onset. In this work an exact solution for helicoidal magnetic vortex is found in a London approximation, similar to magnetic helicoidal configurations known in magnetism. The Gibbs free energy of the current-carrying SC cylinder in a parallel magnetic field is constructed and the edge barrier problem of irreversible entry of helicoid into the SC sample is solved. An optimal parameter of helicoid is chosen by minimization of critical SC parameters (current or field) for vortex entry. The phase diagram of resistive state in coordinates current field is evaluated. An essential difference of magnetic behavior between thin (of radius R<1, London penetration depth) and thick (R≫1) samples is shown. The latter exhibit the field-dependent critical current Jcr due to the helicoid entry in almost all of the field region H<Hc, the thermodynamic critical field, while for R<1 Jcr, almost field-independent, is determined by the vortex ring entry.
Ground state of antiferromagnetic systems in a magnetic field and in the presence of surfaces
Authors: Trallori L. , Politi P. , Rettori A. , Pini M. G. , Villain J.
Published: July 2002

We determine the ground state of semi-infinite uniaxial antiferromagnets and films, in terms of a two-dimensional area-preserving map, where the surfaces are introduced as appropriate boundary conditions. For the film, the ground state is calculated in a very rapid and accurate way for any value of N, the number of planes. For the semi-infinite system, we show that the so-called surface spin-flop state does not exist, while a non-homogeneous ground state is found in the bulk spin-flop phase.
Correlation decay in low-dimensional spin glasses
Authors: Kocharian A. N. , Sogomonian A. S.
Published: July 2002

A new method to investigate the two-point correlation function decay in a finite size random spin system and at finite temperatures is proposed. It is applicable to systems with cyclic boundary conditions and in any local or uniform magnetic field. The efficiency of this method is presented in the case of an isotropic XY spin glass model with Gaussian bond distribution in external field. Using unitary U(1) gauge transformation and operator inequalities we have obtained explicit upper bounds for generalized spin-glass susceptibility κ(T,R) that describes off-diagonal transverse magnetization in two dimensional random XY spin lattices at finite temperatures. It is found that the slowest possible decay for susceptibility in random XY spin glass is of the Kosterlitz–Thouless-type with power-law decay in 2D, and the exponential type in 1D, at low temperatures. These upper bounds rule out the possibility of the corresponding magnetic ordering in 1D and 2D isotropic XY spin glasses at finite temperatures.
Impact ionization in GaAs metal–semiconductor field-effect transistors with a lightly doped drain structure and an Al0.2Ga0.8As/GaAs heterobuffer layer
Authors: Haruyama Junzi , Katano Humiaki
Published: July 2002

Kink effects (abrupt increases in drain current) were observed with an abrupt increase of gate current (IG), substrate current (Isub), substrate potential (Vsub), and photoemission intensity (Iphoto) in GaAs metal–semiconductor field-effect transistors (MESFETs) with a lightly doped drain (LDD) structure and an Al0.2Ga0.8As/GaAs heterobuffer layer. The kink drain voltage (VKD) increases as substrate temperature increases. Since impact ionization has a negative coefficient for increase of temperature, the increase of VKD indicates that impact ionization occurs at VKD. VKD also increases as VG becomes more negative and as the ratio of the dose of the silicon ions is reduced in the LDD region. Thus it is confirmed that impact ionization occurs at the drain side along the channel current path because those changes of VG and dose ratio reduce the electric field at the drain-side channel. In addition, impact ionization coefficients were calculated from IG and Isub based on the method proposed by Hui et al. Both the coefficients were exponentially proportional to the negative inverse of the electric field, although the magnitude of Isub was approximately 250× greater than that of IG. On the other hand, it was found that Iphoto was linearly proportional to Vsub. It suggests that Iphoto depends on the number of holes which overflow the n-GaAs channel/i-GaAs buffer interface potential barrier into the channel layer. These behaviors of IG, Isub, Iphoto, and Vsub are attributed to this FET structure which has a thin channel layer with a high donor density and a heterobuffer layer.