摘要:

We have investigated melt‐spun Pr‐Co alloys with the objective of optimizing their permanent magnet characteristics. Among a variety of elemental additives studied, carbon was found capable of significantly improving the properties, the coercivity in particular. For binary Pr‐Co systems optimum values of the remanence,Br=5.7 kG, intrinsic coercivity,Hci=5.8 kOe, and energy product, (BH)max=4.7 MG Oe, were obtained from the Pr16Co84composition. These values were enhanced toBr=5.8 kG,Hci=16.5 kOe, and (BH)max=7.4 MG Oe for Pr18Co76C6. In both cases the melt‐spun ribbons were principally composed of PrCo5. The 16.5 kOe coercivity of the carbon‐containing ribbons is the highest ever reported for a PrCo5‐based material. Two new rare earth‐cobalt phases were tentatively identified during the course of this work: PrCo7(hexagonal TbCu7structure) and PrCo2Cx(cubic MgCu2structure). Survey results for other melt‐spun, RCo5‐based alloys are also described.

ISSN:0021-8979    

DOI:10.1063/1.354355

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Magnetoresistance, antiferromagnetic coupling, and crystallographic orientation of Co/Cu superlattices with intentionally mixed interfaces have been studied as a function of the thickness of the mixed region. The antiferromagnetic coupling is weakened, and spin‐independent scattering of free electrons is enhanced with increasing thickness of the mixed region, although the morphology and the superlattice period remain unchanged. Saturation magnetoresistance is reduced from 27% to 4% as the result of the formation of a 0.15 nm mixed region at the interfaces. Moreover, the crystallographic orientation of Co/Cu superlattices is also found to be varied by formation of the mixed region. Giant magnetoresistance, antiferromagnetic coupling, and the crystallinity of Co/Cu superlattices are governed by the events in the thin region at the interfaces less than 1 monolayer.

ISSN:0021-8979    

DOI:10.1063/1.354356

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Insulating barium titanate films were successfully grown on Ti‐deposited silicon substrates using the hydrothermal method. The film thickness was 35 and 49 nm for films treated at 200 and 250 °C, respectively, in a 0.25 M Ba(OH)2solution for 8 h. The BaTiO3films did not reach the Ti/Si interface. X‐ray photoelectron spectroscopy revealed OH‐free and nearly carbon‐free films, which was corroborated using Auger electron spectroscopy (AES) depth analysis. AES revealed that the oxygen and barium concentrations are correlated throughout the film, and the existence of a diffuse BaTiO3/Ti interface. A discussion on the film growth mechanism is made using existing information on the subject.

ISSN:0021-8979    

DOI:10.1063/1.354357

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Hydrogenated amorphous carbon films were prepared from CH4, H2, and Ar mixtures by plasma enhanced chemical vapor deposition. Films with various physical properties resulting from various deposition conditions were utilized for this study. The varying deposition parameters included H2flow rates, Ar flow rates, total pressures, substrate temperatures, and power densities. A systematic study regarding the relationship between deposition conditions and the microstructures, optical, and thermal properties was conducted. Furthermore, how the optical and thermal properties related to the microstructures was analyzed. Fourier transform infrared spectroscopy was employed in this paper for determining the hydrogen concentration and the amounts of tetrahedral and trigonal bondings associated with C—H bond and their relative ratio while the optical properties were measured by optical spectrophotometer. Additionally, photothermal deflection spectroscopy was applied for the measurements of thermal diffusion length.

ISSN:0021-8979    

DOI:10.1063/1.354358

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The relationship between the magnitude of the piezoelectric field and the degree of lattice constant mismatch is investigated via low‐temperature photoluminescence measurements of the quantum‐confined Stark effect for a series of (111)B Al0.15Ga0.85As‐InyGa1−yAs pseudomorphic quantum well heterostructures. The experimental strain‐induced electric field values agree well with theoretical calculations for indium mole fractions in the range 0.037≤y≤0.09. In addition, an anomalous saturation of the photoluminescence transition energy is observed at values of applied voltage greater than that required to nullify the piezoelectric field, despite the indication from separate electroreflectance measurements that the net electric field within the quantum well reverses polarity under similar electrical biasing conditions.

ISSN:0021-8979    

DOI:10.1063/1.354359

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The effects on the reflectance spectra of zinc oxide powders, of heat treatment, and of mechanical grinding were investigated for both undoped and aluminum‐doped ZnO. A broad absorptance band at 390–400 nm was induced in the undoped powders both by heating in air and by grinding. From a comparison with electron paramagnetic resonance data from the literature, the band could be related to oxygen vacancies. It was found that aluminum doping suppresses the band formation induced by grinding; however, the doping does not suppress the band formation induced by heat treatment.

ISSN:0021-8979    

DOI:10.1063/1.354360

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Optical and electrical properties of thermally evaporated lithium borate (LiBO2) films have been studied. This work is especially aimed towards the application of such films as lithium ion conductors (IC) in electrochromic (EC) devices. LiBO2films exhibit a high degree of transmission (over 90%) in the solar spectral range and a lithium ion conductivity of the order of nearly 10−7S cm−1. Hence, such films have a good potential for use as ICs in EC devices.

ISSN:0021-8979    

DOI:10.1063/1.354361

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The doping‐dependent, near‐band‐edge optical‐absorption coefficient &agr;(h&ngr;) was deduced from optical transmission measurements inn‐type GaAs thin films. The selenium‐doped films were grown by metalorganic chemical‐vapor deposition and doped to produce room‐temperature electron concentrations from 1.3×1017to 3.8×1018cm−3. The transmission measurements covered photon energies between 1.35 and 1.7 eV and were performed on double heterostructures with the substrate removed by selective etching. The results show good qualitative agreement with previous studies and good quantitative agreement, except for the heavily doped samples. Forn0=3.8×1018cm−3, &agr;(1.42 eV) is approximately four times that reported by previous workers. Secondary‐ion‐mass spectrometry measurements on films grown under differing conditions demonstrate that &agr;(h&ngr;) is sensitive to electrically inactive dopants and supports the hypothesis that precipitates or compensation influenced previous measurements. These comprehensive results on high‐quality, uncompensated material should prove useful for fundamental studies of optical transitions inn‐type GaAs as well as for modeling optoelectronic devices.

ISSN:0021-8979    

DOI:10.1063/1.354336

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Frequency upconversion characteristics in Er3+‐ or Tm3+‐ion‐activated and Yb3+‐ion‐ sensitized luminescent materials have been investigated. The principal goal was to obtain high‐brightness, blue and green light sources under 980 nm emitting laser diode light excitation. Processes leading to the efficient upconversion have been investigated both by experimental and analytical methods. It has been shown that upconversion efficiencies are principally determined with Yb3+ion excited state lifetime, which is highly influenced by device optical confinement as well as material characteristics. The effect of the optical confinement has been analyzed quantitatively on the basis of a rate equation model. Criteria for realizing efficient infrared upconversion devices have been presented. A strong tendency toward temperature quenching has been observed especially in blue emitting materials. This temperature dependence has been shown to be principally due to increase of nonradiative decay in the Yb3+ion excited state.

ISSN:0021-8979    

DOI:10.1063/1.354337

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The experimental measurements of nonlinear modulated photoreflectance (MPR) phenomena in Si and GaAs wafers, in which the obvious second‐harmonic (2 f) MPR signals are detected in addition to the fundamental frequency (f) MPR signals when the pumping laser power is sufficiently strong, are reported. Meanwhile, a linear‐to‐quadratic transition in the curve of thefMPR signal versus the pumping laser power is observed in Si samples but not in GaAs samples. It is also found that the 2 fMPR signals are more sensitive to the ion‐implantation doses than thefMPR signals which are traditionally applied to monitor the ion implantation in semiconductors. It is found that the contribution of the photoinduced modulation of the surface electric field is the main source of the nonlinear MPR signals and the contribution is theoretically discussed in detail. A one‐dimensional theoretical model is established, and the theoretical results are in good agreement with the experimental data. The dependencies of thefand the 2 fMPR signals on the third‐order optical nonlinear susceptibility &khgr;(3)and the surface‐state densityNsare also discussed, and a method for measuring &khgr;(3)andN4of semiconductors is introduced.

ISSN:0021-8979    

DOI:10.1063/1.354338

出版商:AIP    

年代:1993

数据来源: AIP