摘要:

The effects of the spatial dependence of the effective mass in GaAs/AlxGa1−xAs parabolic quantum wells on several of their optical properties have been calculated. The largest effects are on the conduction‐band levels which are lowered with respect to those with constant effective mass. Effects on oscillator strengths, hole energy levels, and electron/hole overlaps are small.

ISSN:0021-8979    

DOI:10.1063/1.351401

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Field‐orientedY1Ba2Cu3Oysamples with various values of the transport critical current density,Jc, have been prepared with various heat treatments. TheJcincreased rapidly with an increase in relative sintered density of the oriented samples. The field‐oriented samples with higherJchad grains of larger sizes and better connections between the neighboring grains than the lowJcsamples. The maximum values ofJcat a magnetic field of 7 T were 370 A/cm2at 4.2 K and 24 A/cm2at 77 K. The hysteresis in theJc‐Hcurves at 4.2 K decreased with an increase inJcboth for an applied fieldH∥abplane and forH∥caxis. The critical currents of the samples with higherJcwere less degraded by aging in the air for 1 year than those of the lowJcsamples. The relation between the field‐cooledJc‐Hcurves and the zero‐field‐cooled ones for an oriented sample with lowJcis qualitatively interpreted by the effect of the intragrain currents on the intergrain fields.

ISSN:0021-8979    

DOI:10.1063/1.351372

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The determination of the critical current is usually based upon a voltage criterion that is used as an indication that the critical state has been reached in the superconducting regime. Resistive measurements of theI‐Vcharacteristics are carried out on patterned thin films of epitaxially grown andc‐axis‐oriented YBa2Cu3O7. Bridges with widths ranging between 1 and 100 &mgr;m and lengths between a few &mgr;m and 7 mm are characterized. The sensitivity of the measurement is better than 10−9V. The volume pinning force is evaluated for different voltage criteria. Although the absolute values of the volume pinning force change with the value of the criteria, the field and temperature scaling is not affected. They agree with the expectations of the flux‐line sheer model. The value obtained for the width of the weakly pinning channels is independent of the chosen criterion. Finally, model calculations on the basis of a modified flux‐line shear mechanism are carried out that are in good agreement with the experimentally measuredI‐Vcharacteristics.

ISSN:0021-8979    

DOI:10.1063/1.351298

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A technique is proposed for measuring the anisotropy energy of magneto‐optical media. the technique consists of rotating a sample in a fixed external magnetic field and monitoring the extraordinary Hall effect signal of the sample. From the Hall effect signal, the angle of rotation of the sample, and the saturation magnetization of the sample (which is measured separately by a vibrating sample magnetometer), we obtain the anisotropy energy as a function of the deviation of magnetization from the easy axis. The technique is applied to multilayered Co/Pd and Co/Pt films as well as to amorphous rare‐earth transition metal TbFeCo alloy films. The anisotropy constants thus obtained are compared with those obtained using other techniques such as torque magnetometry and Hall effect (or Kerr effect) measurements with applied field in the plane of the sample.   

ISSN:0021-8979    

DOI:10.1063/1.351299

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Equiaxed magnetite media with coercivity of up to 1560 Oe, saturation and residual magnetizations of up to 83 and 50 emu/g, respectively, and squareness ratio of up to 0.61 have been developed by the modification of Mn and Co, and by proper annealings. They were prepared by coprecipitation of Fe, Mn, and Co ions from an aqueous solution, by suitable alkali such as triethylamine, to adjust the pH, followed by calcination, reduction, oxidation, step‐annealings, and final reduction. Oxidation leads to a defect spinel ferrite structure, whose coercivity can be greatly enhanced by a step‐annealing treatment, and high coercivity can be retained after the final reduction to magnetite, whose coercivity can not be raised at all by the step‐annealing. These media have a temperature coefficient of coercivity as low as 0.20–0.33%/°C, hence are potential candidates for high density recording.  

ISSN:0021-8979    

DOI:10.1063/1.351300

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The (100) Cu(100 A˚)‐Ni(25 A˚) superlattice structure, grown on Cu/Si(100), that shows a reversed magnetic anisotropy between the in‐plane and normal‐to‐plane magnetizations, has been heated to 400 °C, and the magnetic and structural changes investigated. The periodic structure remains mostly intact up to a 30‐min heating at 300 °C, and partially so after heating at 400 °C. The reversed anisotropy, with a hysteresis loop seen for the field perpendicular to the film plane, but not for that parallel to the film plane, remains seen up to 400 °C. This is compared with two (100) Cu/Ni/Cu structures containing 50‐ and 500‐A˚ Ni, respectively, with two 1000‐A˚ Cu layers each. The former structure, with a completely reversed magnetic anisotropy, lost both the Ni layer and the magnetic characteristics after heating at 200 °C. The latter structure retained its magnetic properties up to 300 °C, and lost both the Ni layer and the magnetic characteristics after heating at 400 °C. The different reactions involved are discussed, including that with Si and mixing between Cu and Ni.

ISSN:0021-8979    

DOI:10.1063/1.351301

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The magnetic behavior of a chain ofNdisks are investigated. The critical fieldHcrifor the discontinuous jump of the magnetization and the coercive forceHcfor the fanning mechanism, and the uniaxial magnetic anisotropy constantKuare estimated as a function ofN, the aspect ratio of the disks, and the magnetocrystalline anisotropy constantK1. By comparing theMHloops calculated and the experiments, the values ofK1andNare obtained for an assembly with random orientation of Ba ferrite particles. This value ofK1agrees well with the experimental results obtained by Fujiwara [IEEE Trans. Magn.MAG‐23, 3125 (1987)].

ISSN:0021-8979    

DOI:10.1063/1.351302

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Magnetic‐property measurements of pyrolytic carbon prepared by the thermal chemical vapor deposition of adamantane were carried out using the Faraday balance method and the electron‐spin‐resonance (ESR) spectroscopy. Two kinds of samples under different preparation batches were employed to examine the batch dependence as well as the magnetic properties of each. The content of ferromagnetic metallic oxide as impurity in these samples has been confirmed to be low enough not to influence the present magnetic‐property measurements. Analysis of the magnetization curves observed in the magnetic field range 0–5 T has shown that these samples are basically superparamagnetic and, even, ferromagnetic at low temperature in one sample. The ESR measurements have also supported these aspects. The connection of the magnetic properties and batch dependence obtained here will be discussed in detail.

ISSN:0021-8979    

DOI:10.1063/1.351303

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The magnetic properties of thin film Pt/Co multilayers with perpendicular magnetic anisotropy depend on their preparation. Vapor‐deposited and Xe‐sputtered multilayers have the largest anisotropy energy and coercivities, attractive for magneto‐optical recording media. Films sputtered with a smaller mass gas (Ar) have less attractive magnetic properties. In this paper it is shown by high resolution electron microscopy that microstructural differences, related to the energetics of the deposition process, caused the differences in magnetic properties. Films prepared by Ar‐sputtering experienced energetic bombardment by reflected Ar neutrals and consequently had smooth, continuous Co and Pt layers, but only modest (111) fcc texture. Vapor‐deposited and Xe‐sputtered films, which experienced much less energetic bombardment, had superior (111) texture and sharp column boundaries, although Co–Pt layer interfaces were rougher. It was concluded that the superior (111) fcc texture was responsible for a large magnetic anisotropy energy. And both the larger anisotropy energy and the sharper column boundaries contributed to large coercivities.

ISSN:0021-8979    

DOI:10.1063/1.351304

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The electron paramagnetic resonance (EPR) in a single crystal of gadolinium–gallium–garnet has been studied with the aim of understanding the behavior of EPR in systems with large dipolar and small exchange fields, such as encountered in many of the rare‐earth based highTcsuperconducting compounds. Only one wide line, arising from Gd3+has been observed. Its shape at room temperature is analyzed using the Kubo–Toyabe [MagneticResonanceandRelaxation, edited by R. Blinc (North Holland, Amsterdam, 1967), p. 810] stochastic model for the line shape at static modulation. At lowT, the anisotropy in the resonance field is a purely shape effect.

ISSN:0021-8979    

DOI:10.1063/1.351305

出版商:AIP    

年代:1992

数据来源: AIP