摘要:

A new method for observing full domains in thin magnetic films by electron microscopy is described. Observations are made with standard transmission instruments utilizing an off‐centered objective aperture diaphragm as a knife edge. The method has the high‐resolution advantage that the microscope is focused on the specimen during domain observations. Limitations of the method and comparisons with the previously reported defocusing technique are presented. Applications to the interpretation of complex domain patterns and cross‐tie walls are demonstrated. The observations were made with electrostatic‐focusing microscopes, the AEG‐Zeiss and Tru¨b‐Ta¨uber instruments, which allow the use of full objective power without influencing the magnetization distribution of low coercive force films. A second method using a knife edge is proposed that would potentially permit a simple measurement of the detailed magnetization distribution of a domain wall in a thin film.

ISSN:0021-8979    

DOI:10.1063/1.1735428

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

The wave equation is derived for the propagation of longitudinal waves along a stretched filament of a highly elastic material. It is found that the tension in the filament reduces the effective modulus by twice the product of Poisson's ratio and the tensile traction, leaving the internal viscosity term unchanged. This result is illustrated by measurements of continuous 1 kcps wave propagation in natural rubber at 50°C, where the damping is small (tan &dgr;<0.1). An alternative derivation is given for the purely elastic case of zero damping without restriction upon the amplitude.The ``equilibrium'' or ``static'' Young's modulus is obtained for extensions up to about 600% from the slope of the equilibrium stress‐strain curve and used to predict the corresponding wave velocities from the wave equation for zero damping. The predicted velocities are slightly higher—by up to about 10%—than the measured velocities. It is shown that the deviations could arise from differences in rate of strain between the wave‐propagation and the stress‐strain measurements. At the higher extensions the rubber is very hysteretic for large deformations, and the Young's modulus governing the small‐amplitude wave propagation is shown to relate substantially to theloadingbranch of the stress‐strain curve.

ISSN:0021-8979    

DOI:10.1063/1.1735429

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

Energy balance, space charge, and thermionic equations have been combined to provide a description of the mechanism of the cathode in an ac arc, and the quantities required for solution determined experimentally. The experiments were carried out on barium‐activated electrodes in xenon at 1.4 atm and extended over a range of 25–75 amps rms. The work function was found to be 1.77 v at zero‐field and reduced to 1.67 v under Schottky effect of the space charge field. The work function did not change with current, but the Richardson‐DushmanAfell, as the current was raised, to a value of approximately 0.1 at the highest load. The cathode fall varied with phase angle and showed a maximum of 1.2 v at maximum current. The component of current at the cathode carried by ions reduced from 12% to 6% as the load increased.

ISSN:0021-8979    

DOI:10.1063/1.1735430

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

An investigation is described in which a high‐sensitivity analytical balance of standard design is used for determination of the surface density of that thorium layer which corresponds to maximum thermionic emission with the thorium‐on‐tungsten system. Following Brattain and Becker's definition of this value of surface density as one ``funit,'' thorium vapor is allowed to accumulate on a quartz plate until a weighable number offunits (about 600) has been deposited. The thorium flux infunits per minute is monitored periodically by means of a tungsten filament diode during the course of the deposition. Two polycrystalline specimens of ribbon from the same source have yielded the value of (4.2±0.2)1014atoms/cm2as the density for maximum emission. This is to be compared with a value of 5.0×1014atoms/cm2obtained in 1934 on theoretical grounds by Langmuir.

ISSN:0021-8979    

DOI:10.1063/1.1735431

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

Reverse bias transition capacitance measurements on alloy diodes of InSb at 78°K give values a factor of three times those calculated from normal diode theory. This result is in contrast to the reasonable agreement obtained for diodes of germanium and gallium arsenide in a comparable doping range. The experimental techniques are critically reviewed and an attempt made to assess the implications of the results.

ISSN:0021-8979    

DOI:10.1063/1.1735432

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

Large cesium chloride crystals were obtained from solution only upon addition of urea. These crystals, however, show NH4+impurity. Since CsCl undergoes a phase transition between its melting point and room temperature, normal growing methods from the melt could not be used, but a modified (``double‐run'') Bridgman method yielded good results.

ISSN:0021-8979    

DOI:10.1063/1.1735433

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

Formulas and graphs are given for the calculation of the propagation constants of an electromagnetic wave in weakly ionized, dry air at ionospheric temperatures in the presence of a magnetic field. Experimental studies of electron collision frequencies in nitrogen and oxygen are reviewed and used to obtain the magnitude and energy dependence of the electron collision frequency in air. The equations for the components of the conductivity tensor are developed taking into account the approximately linear dependence of the electron collision frequency on electron energy. Expressions derived on this assumption are found to be accurate except at low temperatures, high pressures, and low frequencies. The errors resulting from the use of an effective value for the energy independent collision frequency in the Appleton‐Hartree equations are evaluated. Procedures are given for the calculation of the propagation constants for electromagnetic waves propagated parallel to and perpendicular to the magnetic field. These results are then applied to the derivation of relations required to reanalyze the ionospheric collision frequency measurements reported by Kane. The use of an effective collision frequency is found to lead to errors comparable to the experimental errors in the ionospheric observations.

ISSN:0021-8979    

DOI:10.1063/1.1735434

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

This investigation was concerned with the effects of high temperature treatmentin vacuoor in rarefied oxygen atmospheres upon the effective electrical resistivity of dilute (copper‐rich) CuMn, CuFe, and CuCo rods and wires. The CuMn alloys studied most extensively contained between 1 and 12 at. % manganese while the CuFe and CuCo alloys contained between 0.1 and 0.3 at. % iron or cobalt. It was found that the solutes in the CuFe and CuCo wires and rods become rapidly oxidized (internally) during heat treatment for several hours at 990°C in pure oxygen pressures of 0.5 &mgr; or greater, while in 0.05 &mgr; O2this oxidation is absent and previously oxidized specimens are reduced. These observations are based on measurements of electrical resistivity, and in the case of CuFe, substantiated by measurements on the magnetization. Heat treatment of these CuFe and CuCo wires and rods near 1000°C for several hours eitherin vacuo(0.1 &mgr; air) or 5 &mgr; O2does not appreciably affect the cross‐sectionaluniformityof these specimens. In the case of CuFe alloys, the magnetic behavior demonstrates the virtually complete oxidation of solute to Fe3O4after prolonged anneal in an oxidizing atmosphere. The behavior of the CuMn alloys was found to be more complicated, partly because of the high solute concentrations involved. It was found that wires and rods of CuMn rapidly become nonuniform in solute concentration throughout their cross section when annealed near 1000°C for several hours either in a vacuum (0.1 &mgr; air) or in 5 &mgr; of pure oxygen; this nonuniformity of manganese concentration develops at a considerably higher rate in the 5 &mgr; O2atmosphere than in vacuum, and leads to erroneous values of the resistivity and to erroneous curves of resistivity vs temperature for CuMn wires or rods. On the other hand, annealing of such wires for several hours in vacuum at temperatures around 800°C does not appreciably disturb the cross‐sectional uniformity. Finally, it is emphasized that the ``carrier gas'' technique (involving a flow system with small concentrations of oxygen mixed with helium at atmospheric pressure, for example) is entirely unsatisfactory for experiments on internal oxidation, particularly if one wants to know the oxygen partial pressure.

ISSN:0021-8979    

DOI:10.1063/1.1735435

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

The gradual increase in the difference in temperatures of two Kanthal heating elements enables CdS crystals to be grown directly on the inside of a silica tube, remote from one another, instead of on a continuous polycrystalline CdS substrate as happens in most existing methods.

ISSN:0021-8979    

DOI:10.1063/1.1735436

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

A simple kinetic model is employed to investigate nonequilibrium evaporation from a liquid. Molecules are assumed to evaporate into a (one‐sided) Maxwellian velocity distribution at the liquid surface. Molecules reaching the surface from the vapor are assumed to form part of an ellipsoidal velocity distribution. Of the molecules approaching the liquid at the interface only the fraction &agr; condenses there; the remainder is taken to undergo specular reflection back into the vapor. By equating physical conditions at the surface with conditions in the vapor, one can relate the vapor pressure to that which would prevail at equilibrium, as a function of the surface accommodation coefficient &agr; and the mean velocityuof the vapor relative to the interface.

ISSN:0021-8979    

DOI:10.1063/1.1735437

出版商:AIP    

年代:1960

数据来源: AIP