Table of contents

60 Jahre lang besteht die Idee von der Unterteilung magnetischer Körper in Elementarbereiche. Sie ist in dieser Zeit mit wachsendem Tempo entwickelt aber bis heute noch nicht in allen Konsequenzen ausgeführt worden. Seit einigen Jahren konzentrieren sich die experimentellen und theoretischen Untersuchungen auf die Struktur der Grenzen zwischen den Elementarbereichen und auf typische Abweichungen vom einfachen Bild der homogen magnetisierten Bereiche. Der vorliegende Artikel soll gerade diese neue Richtung durch Beispiele veranschaulichen. Außerdem wird eine Übersicht gegeben über die Methoden der Bereichsbeobachtungen, die den Hauptanteil unserer heutigen Kenntnisse geliefert haben.

The concept that magnetic substances are subdivided into elementary domains has existed for 60 years. This concept has been developed with increasing rapidity during this time but the full details are not known even today. For several years the experimental and theoretical investigations have concentrated on the structure of the boundaries between the elementary domains and on deviations from the simple picture of uniformly magnetized domains. The aim of this article is to illustrate this new trend by examples. In addition a review is given of the methods of observing domains, which have contributed most to our present knowledge.

Ultra-high vacuum apparatus has been developed for the electron-beam evaporation of II-VI semiconducting compounds on to ionic substrates cleaved in vacuum and heated by an external infra-red lamp. Epitaxial films of cubic ZnS, free of wurtzite-structure grains and microtwins, were grown on NaCl at temperatures down to 225°C. Cleavage in an ultra-high vacuum system was essential in order to obtain films reproducibly at this temperature that were free of included grains. The effect on film structure of different types of contamination was studied.

Thin films of indium arsenide have been prepared by the vacuum evaporation of the elements on to heated glass substrates. The optical constants were obtained by analysis of the interference fringes shown in quantitative transmittance measurements made in the wavelength range 0.7-5.0 μm. They were found to be functions of the substrate temperature. The range of substrate temperatures and film thicknesses that was used was determined by the requirements of the preparation method and the necessity of producing a film with a specularly reflecting surface. The absorption coefficient of between 10⁴-10⁵ cm⁻¹ for the semiconductor edge was found to agree with single-crystal data for films prepared above 200°C but the refractive index decreased as the substrate temperature increased, from 4 at 3.5 μm for films produced at ambient temperature to 3 for those prepared at the highest temperature of 400°C.

The construction of a GaAs p-i-n diode in which the i region is formed by diffusion of chromium into n-type material is described. The diode exhibits a current-controlled negative resistance with a threshold voltage which may be reduced by illumination. The electrical and photoelectrical properties were found to be in qualitative agreement with theory.

Photoconductivity measurements in samples of melt-grown chromium-doped semi-insulating GaAs have shown an impurity peak response at 0·87 ev. Evidence is presented that the transition responsible for the peak is an electron transition from compensated chromium acceptor levels to the conduction band. Infra-red quenching experiments confirm that the chromium level, deduced to be approximately 0·79 ev from the conduction band, does indeed behave as a compensated deep acceptor level.

Hall-effect measurements have been performed on polycrystalline samples of dilute alloys of Zn-Cu, Zn-Ag and Zn-Al. Pole figure determinations indicate that effects due to texture changes on alloying are insignificant. The Hall coefficient varies with the average number of valence electrons per atom, n, in a manner qualitatively similar to that reported for noble metal alloys: there is a cusp at the pure metal composition (n=2) and maxima for n<2. However, unlike the noble metal alloys, the positions of the maxima do not correlate with the compositions at which the impurity scattering becomes equal to the phonon scattering.

The parameters describing the linear effect of an electric field on the X-band electron paramagnetic resonance frequency of the impurity Cr³⁺, charge compensated by Li⁺, in zinc tungstate have been measured. This allows an explanation of the anomalous behaviour of the resonance linewidth when the magnetic field is rotated in the magnetic zx plane. The linewidth in specimens of high Cr³⁺ concentration is almost entirely due to the internal electric fields arising from the charged impurities Cr³⁺ and Li⁺ acting via the linear electric field effect. The resulting linewidth arises from the equal and opposite shifts in the resonance frequency for the two non-equivalent sites for Cr³⁺ in the zinc tungstate lattice. The root-mean-square internal field has been measured and agreement found with the value calculated by assuming the Li⁺ and Cr³⁺ ions to be randomly distributed throughout the lattice. A specimen of lower Cr³⁺ concentration has been investigated, and an explanation of the complete behaviour of the linewidth in terms of a simple model is presented.

The results of breakdown voltage measurements in various rare-gas - alkali-metal mixtures at atmospheric pressure and elevated temperature are given, the diluent gases being helium, neon and argon, and the seed metals, caesium, potassium and sodium. The breakdown voltage is shown to increase linearly as the electrode spacing rises from 1 to 15 mm, and to show little systematic variation with changing temperature over the range 650-950°C. The breakdown voltage falls slowly as the seed pressure rises from very low values, passes through a broad minimum in the region of 1 torr, then rises sharply at high pressures.

For a given seed, the breakdown voltages with helium as diluent are considerably higher than those with argon as diluent, with neon occupying an intermediate position. This is attributed to the different electron collision cross sections of the three gases, the breakdown voltage apparently rising as the electronic mean free path falls. Sodium-seeded plasmas require much higher breakdown voltage values than those seeded with caesium or potassium, but little difference is shown to exist between the latter two seeds, caesium being only slightly superior. It is concluded that the most promising mixtures from the magnetohydrodynamic point of view are caesium and potassium in argon, as they have the lowest breakdown voltage values under all conditions.

Expressions are derived for the time-dependent components of the electron density in a plasma subjected to an amplitude-modulated electric vector by assuming the electron density to follow the time-dependent (at modulation frequency) instantaneous changes in the electron temperature. These expressions for the time-dependent components in the electron density have been used to investigate the various time-dependent components of the current density when another weak wave of frequency ω₁ is also propagating in the plasma; this expression for the current density is substituted in the appropriate wave equation and the solutions have been used to study the change in modulation of the strong wave (ω) and the cross modulation of the weak wave (ω₁) when they are reflected from a plasma-free-space interface, the wave being normally incident on it from the free space side. Some numerical calculations have been carried out for the dependence of change in modulation and cross modulation in the reflected wave on relevant parameters by assuming the electron density to be governed by Saha's equation corresponding to the electron temperature; these results are presented in the form of tables.

A study of negative charge behaviour in the insulating liquid hexane has shown the necessity of taking due account of the bulk liquid motion associated with carrier displacement. The time for charge to cross an electrically stressed gap was found to vary with the magnitude of charge injection. Mobility values, incorporating corrections for transit-time modification due to liquid motion, are deduced from measurements made at stresses up to 20 kv cm⁻¹. These values are lower than those recorded by previous investigators, and suggest that the charge carriers may be negative ions or polarons.

The viscosities of chloroform, acetone, methyl and ethyl alcohols have been measured over the temperature range 30-200°C by using an oscillating-disk viscometer. The data have been used to obtain information concerning the intermolecular potentials of these polar organic vapours.

Values of Young's modulus have been obtained for commercial 60-40 brass, 0.9% carbon steel, Vitreosil silica, super-purity aluminium and alumina by determination of the resonance frequencies in `free-free' transverse vibration and an attempt made to correct the results for shear and rotatory inertia. However, it was found that an empirical correction depending linearly on the stiffness ratio gives a more dependable result than the various suggested theoretical corrections.

The increase in width of both the angular distribution and the energy distribution of electrons scattered in the specimen-supporting membrane of an electron microscope as the membrane thickness is increased has been measured from the changing appearance of Fresnel fringes. Measurements were made of the scattering characteristics of electrons (initial energy 80 kev) transmitted through films of plastic, carbon, germanium and platinum of mass-thicknesses corresponding to between 0.32 and 7 elastic scattering acts. As far as could be observed, energy loss had no influence on the Fresnel fringes. The results show that the influence of a specimen-supporting membrane of ordinary thickness will not be serious in highly resolved electron microscope images, so far as scattering and chromatic aberration are concerned. It was found that more than 70 Fresnel fringes could be observed even with a conventional tungsten hairpin filament. With increasing amount of defocus the number of Fresnel fringes rises to a maximum, determined by the resolution of the photographic plate and by the coherence length of the electron beam.

The ion-optical properties of the radial electrostatic field analyser have been studied by several workers and appear mainly in the form of mathematical formulae. In this paper some of these properties are re-examined and are presented graphically over ranges of values of several parameters. This should be helpful when selecting values of these parameters for a design.

Solid-to-solid diffusion techniques are described whereby zinc is diffused from zinc-doped silica or silicon films on the surface of GaAs to give abrupt junctions. There is evidence that the presence of oxygen can affect the character of the junction.

Gallium arsenide layers with mobilities up to 7900 cm² v⁻¹ s⁻¹ at 295°K and 52 000 cm² v⁻¹ s⁻¹ at 78°K have been grown epitaxially from gallium melts, using arsenic trichloride as the source of arsenic. Controlled doping with selenium has also been carried out.

Electrical resistivity changes during the cure of epoxy resins are interpreted using a model of `hopping' conductivity. If the energy barrier against ionic movement between neighbouring hopping sites increases proportionally to the degree of cross-linking of the resin, then it is justifiable to use the rate of change of the logarithm of resistivity with time as a measure of the reaction rate. The limitations of the method are discussed in the light of the behaviour of real materials.

Controversy exists about whether the discharge channel existing after breakdown in alkali halide crystals marks the path of the electronic instability or results from secondary processes. A fast electro-optical shutter has been used to observe the pre-breakdown light emission from KCl single crystals, and it has been found that the discharge channel does lie in the path of the electronic instability and that the latter does not necessarily occur in the field direction. It has been also demonstrated that the effect of mechanical strain in the crystals is to suppress the development of breakdown in directions other than the field direction.

The magneto-Seebeck effect in bismuth is shown to be strongly dependent on the nature of the surfaces of the sample.

Alloys of the pseudo-binary alloy system SnTe-PbTe were prepared and their lattice parameters determined by powder x-ray diffraction. A graph of lattice parameter against PbTe concentration (at.%) was established, and showed a positive deviation from Vegard's Law.

The dependence of the spherical aberration coefficient C_s on object position z₀ and magnification M is studied using Glaser's bell-shaped field model. It is shown that C_s may be written in the forms

IMG1

where a_i and b_i are functions of the lens excitation k². a_i and b_i are tabulated for a range of values of k².

The characteristics of two types of d.c. electroluminescent powder phosphors in the ZnS (Mn, Cu) system are described. The first shows a quantum efficiency (photons/electron) of over 100% at a luminance of 0·5 ft lamberts. The second yields over 10³ ft lamberts with a quantum efficiency of 5% at room temperature. The power efficiencies are 0·33 and 0·05% respectively. Typical current-voltage-luminance relationships are shown.