摘要:

Superconducting YBa2Cu3O7−xthin films were obtained under high vacuum (10−5Torr) on substrates of polycrystalline Al2O3sapphire, SrTiO3, and Si, having zero resistance at 81, 85, 87, and 79 K, respectively. A N2laser of 3.5 J cm−2energy density was used for the evaporation. The substrates were heated by a cw single‐mode CO2laser and the annealing was performed by the same laser in O2atmosphere. Local planar superconducting regions were obtained by focusing the radiation of the cw CO2laser upon the films. The films were investigated by scanning electron microscope, x‐ray microanalysis, and x‐ray diffraction.

ISSN:0021-8979    

DOI:10.1063/1.345039

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The magnetic age hardening produced by decomposition of a &bgr;‐CoAl(B2) alloy has been studied by correlating the microstructure with the changes in magnetic properties during aging. A model based on coherent rotation has been developed to describe quantitatively the intrinsic coercivity of these fine‐particle ferromagnets as well as the temperature dependence of the coercivity over the temperature range from 77 to 757 K. The model incorporates both magnetocrystalline anisotropy constantsK1andK2in addition to the shape anisotropy of the elongated single‐domain particles of metastable HCP cobalt which precipitate from solid solution during heat treatment. The magnetic properties of the particles are shown to depend critically on the crystallography of the solid‐state transformation and particle morphology which were determined by transmission electron microscopy and diffraction.

ISSN:0021-8979    

DOI:10.1063/1.345040

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The interaction of domain walls with microstructural features in sintered Fe‐Nd‐B magnets has been investigated by Lorentz microscopy using a high‐voltage transmission electron microscope. Grain boundaries that are normal to the wall do not affect the progress of a moving wall, while those that are parallel to the wall interact strongly. The effect of the grain boundary Nd‐rich fcc phase on the movement of the wall appears to depend upon the thickness of the fcc phase. Inclusions inside the matrix phase that are within 1000 A˚ interact with the walls to pin them, while larger inclusions do not. It is suggested that a uniform distribution of inclusions with magnetic properties very different from that of the 2‐14‐1 phase be used to pin domain walls in order to improve the temperature dependence of the intrinsic coercivity and remanence.

ISSN:0021-8979    

DOI:10.1063/1.345041

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The effect of perpendicular compression on initial susceptibility of polycrystalline soft magnetic materials is investigated. In particular, it is shown that susceptibility magnitude does not decrease immediately with the stress applied. Consequently, a threshold is pointed out below which the domain‐wall susceptibility keeps constant and equal to the initial value. Investigations of this threshold, between room temperature and the Curie temperature (550 K), has allowed a tight connection with intrinsic parameters of the spontaneous domain‐wall arrangement to be established. In order to demonstrate this result, two types of materials have been prepared: perfect materials with domain‐wall configuration approaching ideal orientation and defective materials with randomly dispersed domain‐wall arrangement.

ISSN:0021-8979    

DOI:10.1063/1.345042

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Magnetic softness of polycrystalline Fe‐based alloy films is investigated. From calculation of magnetocrystalline anisotropy and magnetostriction assuming the magnetization is confined in a certain crystal plane of bcc Fe or Fe‐Si alloy, Fe alloy films with (111) orientation parallel to the film plane are expected to have good magnetic softness. Fe‐6.9 wt. % Si polycrystalline films with various crystallographic orientations such as (100), (110), or (111) oriented parallel to the film plane plane are deposited on underlayers such as MgO, ZnO, and ZnSe by a high‐rate sputtering system. Fe‐Si films on ZnSe underlayers with (111) orientation exhibit smallerHcand better thermal stability than those of films with other orientations. Magnetic softness and thermal stability are improved for the double‐layered films with those orientations. For multilayered Fe‐6.9 wt. % Si films on ZnSe underlayers, theHcof deposited films is about 0.5 Oe, and is stable against annealing up to 500 °C.

ISSN:0021-8979    

DOI:10.1063/1.345043

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The degradation of the pulsed surface flashover performance of 99.9% pure polycrystalline alumina in an insulator‐bridged vacuum gap, due to the mechanical grinding and finishing of the insulator is reported. The flashover characteristics of three different surface finishes—as‐fired (0.8 &mgr;m), a ground surface (0.25 &mgr;m), and a polished surface (0.05 &mgr;m)—were studied using time‐coordinated voltage, current, and luminosity diagnostics with a temporal resolution of ∼1.5 ns, for 0.5/15‐&mgr;s pulsed voltage excitations. The flashover strengths of the ground samples are lower than the as‐fired sample by about 50% while the strengths of the polished surfaces are intermediate between the two. These results contradict earlier reports which attribute the lower holdoff of smooth finishes to the enhanced secondary emission yield as compared to the rougher surface finishes. The lower flashover strengths for the ground surface as compared to the as‐fired surface are attributed by us to the surface defects consisting of intergranular and transgranular fracture features induced by the mechanical grinding operation. The improved performance of the polished surface over the ground surface is proposed to be due to the partial removal of the damage created during the grinding operations. The suggested increase in the defect density in the surface layers of ground and polished surfaces relative to the as‐fired finish is substantiated by characteristic defect‐dominated signatures in the luminosity‐current profiles. These results are in agreement with an earlier report describing the role of trapped carriers in the flashover process. It is concluded that the insulating properties of the surface as influenced by the microstructural features are strongly dependent on the precise mode of material removal during grinding and polishing operations and hence cannot simply be correlated to the gross surface topography in terms of surface roughness, particularly for hard and brittle ceramic materials.

ISSN:0021-8979    

DOI:10.1063/1.345044

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

This work aims at a systematic study of phase modulation in GaAs/AlGaAs double‐heterostructure waveguides with different doping profiles. Both theoretical (part I) and experimental (part II) aspects are investigated, leading to interesting new results. Phase modulation is the sum of a linear electro‐optic term, a quadratic electro‐optic term, and a free‐carrier term. The carrier term is shown to be the sum of a plasma term, an intervalence‐band term (for holes only), a band‐filling term, and a band‐shrinkage term, the latter being due to many‐body effects. A new analytic expression for the band‐filling term is derived which shows that the band‐filling effect does not depend on the carrier effective mass. We prove that the band‐shrinkage term is approximately half of the band‐filling term, but has opposite sign. The phase modulation is computed using an overlap integral between the optical intensity and the local refractive‐index difference. We also report an analytic expression for the modulation efficiency of ap‐i‐njunction double‐heterostructure modulator. This expression is very accurate and only requires knowledge of the depletion width and the guiding parameters.

ISSN:0021-8979    

DOI:10.1063/1.345045

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Phase modulation in GaAs/AlxGa1−xAs double heterostructures with different doping profiles is systematically investigated. Very good agreement between the experimental measurements and the theory developed in Part I of this paper is reported. By measuring the phase modulation for both transverse electric and transverse magnetic modes along the [110] and [11¯0] crystallographic directions, we are able to deduce accurate values for the linear electro‐optic coefficient. Values ofr41=−1.68×10−10cm/V at &lgr;=1.15 &mgr;m andr41=−1.72×10−10at &lgr;=1.09 &mgr;m are obtained with an estimated accuracy of ±5%. An accurate estimation of the carrier effect permits us to deduce the quadratic electro‐optic coefficient for GaAs. The values areR11=−2.0×10−16cm2/V2,R12=−1.7×10−16cm2/V2at &lgr;=1.15 &mgr;m, andR11=−2.9×10−16cm2/V2,R12=−2.4×10−16cm2/V2at &lgr;=1.09 &mgr;m with an estimated uncertainty of ±25% for all values. Contrast measurement of Fabry–Perot fringes enables us to evaluate the modulator losses. A value of 4.8 cm−1is reported for aP‐ndoped modulator (n=6×1017cm−3). Free‐carrier absorption is shown to be the dominant loss process in high‐quality structures.

ISSN:0021-8979    

DOI:10.1063/1.345046

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A detailed photoluminescence study of a deep broadband emission as a function of the dopant concentration is conducted in molecular‐beam epitaxy‐grown silicon‐doped Al0.3Ga0.7As single layers. The deep broadband which is correlated to the Si concentration consists of several emissions among which three are suggested to be due to different Si complexes. The effects of annealing the samples contributed immensely to this investigation. A model to explain the direct and indirect participations of SiAsin the formation of Si complexes during heat treatment is proposed. The photoluminescence is complemented by the Hall‐effect, capacitance, and secondary‐ion‐mass spectroscopy measurements.

ISSN:0021-8979    

DOI:10.1063/1.345047

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The cathodoluminescence spectra of microwave plasma‐deposited polycrystalline diamond films have been measured at liquid‐nitrogen temperatures over the spectral region of 230–800 nm. The diamond coatings had been deposited under several different deposition temperatures and reactant compositions. Measurements on natural type‐IIB diamond crystals were made for comparison. The intrinsic exciton emission bands which fall in the UV just below the band edge were observed, as well as several defect and impurity bands which extend throughout the visible part of the spectrum. SEM micrographs and Raman spectra were obtained for the same set of samples used for the cathodoluminescence measurements. It was found that the diamond‐related cathodoluminescence features were most intense in samples whose Raman spectra exhibited the most intense cubic diamond line at 1332 cm−1and the least intense graphitic band at about 1500 cm−1.

ISSN:0021-8979    

DOI:10.1063/1.345048

出版商:AIP    

年代:1990

数据来源: AIP