R Ruud Metselaar

Light scattering by pores in polycrystalline materials: Transmission properties of alumina

The in‐line transmission of polycrystalline high‐density ceramics is discussed in terms of light scattering by pores. The Mie theory for light scattering is applied to calculate scattering coefficients using generalized parameters. Results are shown for scattering by spherical pores with uniform size and with a lognormal size distribution. Specific examples are given of the calculated transmission of alumina in the wavelength region 0.4–5 μm. Experimentally determined transmission curves of normally sintered and of hot‐pressed alumina are compared with calculated curves. The porosity, the position of the maximum, and the width of the lognormal distribution are treated as variables in the calculation. Good agreement with the experimental data is obtained.

Diffusion of oxygen vacancies in yttrium iron garnet investigated by dynamic conductivity measurements

Abstract In yttrium iron garnet, Y3Fe5O12, the oxygen vacancy concentration at high temperatures depends on the partial oxygen pressure. Due to the electron donating nature of the vacancies, changes in the oxygen vacancy concentration can be measured by electrical conductivity measurements. We discuss a dynamic method for studying the diffusion of oxygen vacancies by measurements of the time dependence of the electrical conduction after a change in the oxygen partial pressure has taken place. It is shown that the interpretation of the measurements is straightforward if the relative change in conductivity remains small (⩽ 10%). Measurements were performed on single crystals and on polycrystalline samples at temperatures 900–1400°C. The samples were made n-type by substitution with Si or p-type by substitution with Ca, Zn or Pb. The partial oxygen pressure was changed between 1 and 0.1 atm. For all samples the diffusion coefficient D of the oxygen vacancies can be represented by D = A exp (− Q kT ) , where A = 8400 cm2s−1 and Q = 2.90 eV. It is shown that the activation energy of 2.90 eV is due to the migration enthalpy of the vacancies only.

The stoichiometry and defect structure of yttrium iron garnet and the nature of the centres active in the photomagnetic effect

Abstract The nature of the donor centres which are involved in the photo-induced changes in permeability in yttrium iron garnet is investigated. Annealing experiments at different oxygen partial pressures produce changes in the lattice constant and in the photomagnetic effect. A description is given in terms of a defect model in which Fe 2+ is compensated by oxygen vacancies or cation interstitials.

Defects and the electronic properties of Y3Fe5O12

Abstract The electrical and the optical absorption properties of yttrium iron garnet (YIG) are investigated. The stoichiometry is studied by means of lattice constant measurements and the defect mechanism is discussed. From high-temperature conductivity measurements at various oxygen pressures the band-gap energy is determined to be 2.85 eV. The influence of extrinsic impurities (Pb and Si) on the electrical conductivity and the optical absorption is investigated on thin films grown by liquid phase epitaxy. On the basis of these results and optical work by other investigators a model for the band structure of YIG is proposed and discussed.

Light‐induced changes in permeability of n‐ and p‐type YIG films

The influence of irradiation with white light on the permeability at 77 ° of n‐ and p‐type yttrium iron garnet films has been investigated. The investigations were performed on epitaxialy grown films with varying concentrations of Pb and Si. The existence of an ireversible photomagnetic effect in both n‐ and p‐type YIG is demonstrated. Measurements of the spectral dependence of the photomagnetic effect in the wavelength region 1000–3 nm show a rapid increase of the sensitivity towards shorter wavelengths.

Light‐Induced Increase in Domain‐Wall Stiffness in Si‐Doped YIG

The initial susceptibility of Si‐doped YIG is reduced by irradiation with light. This effect is explained by a simple two‐center model, a strongly anisotropic center being formed from a less anisotropic one by light‐induced electron transfer. Domain walls are pinned by these strongly anisotropic centers, of density n. If n is small enough, the stiffness will be proportional to n, since by local wall deformations each center within the wall can be made to occupy a site of lowest energy. For larger n this is no longer possible: instead, the optimum wall position will be determined by the statistical fluctuations of the center distribution, and the stiffness will be proportional to n1/2.

Decomposition of garnet epitaxial layers caused by annealing

The changes in chemical composition which accompany the process of stress relief in garnet epitaxial layers during annealing have been studied. It is shown that the process involves the evaporation of PbO and formation of Fe2O3 crystals on the layer surface. The associated volume reduction of the garnet layer results in the observed decrease in the lattice‐misfit stress. Weight‐loss measurements with a thermobalance have been employed to monitor the evaporation of PbO, and a quantitative comparison with the amount of Fe2O3 observed by optical microscopy has been made. These results are interpreted in terms of a specific decomposition reaction and are found to be in numerical agreement with x‐ray diffraction measurements of the lattice changes.

Crystal defect mapping via impurity‐activated luminescence in gadolinium gallium garnet

Faceting and growth striations have been revealed by the photographic mapping of the Tb3+‐ or Eu3+‐activated luminescence in Czochralski‐grown gadolinium gallium garnet. Both fluorescence and thermoluminescence topographs have been prepared. For the case of Tb3+ luminescence, facet cores exhibit a contrast in thermoluminescence topographs which is the opposite of that in fluorescence.

LPE of InP and InGaAsP on InP substrates; A verification of the diffusion limited growth model

Abstract InP was grown on (001) and (111)B InP substrates by the supercooling and step-cooling technique and In 1- x Ga x As y P 1- y ( x ≈ 0.2 and y ≈ 0.5) was grown on (001), (111)A and (111)B InP substrates at 640-650δC by the step-cooling technique. Calculated growth rates assuming diffusion limited growth, using experimental phase diagram relations were compared with experimental data. Excellent agreement was found for the growth of InP on (001) and (111)B InP and for the growth of InGaAsP on (001) InP substrates. For the nucleation of InGaAsP on {111} faces a critical supersaturation of 4δC was observed. The criterion of constant composition for quaternary layers grown at constant temperature was verified using double crystal X-ray diffractometry. A constant composition was observed on the (001) and (111)A faces, in contrast to the (111)B face, where the growth seems to be dictated by surface kinetics.

Optical measurement of magnetic anisotropy in thin garnet films

This paper describes an improved optical method for measuring locally the cubic and uniaxial magnetic anisotropy fields in thin garnet films. The derivative of the in-plane component of the magnetization is measured, using a double modulation technique which combines polarization modulation with field modulation. A simple graphical method is devised to calculateHk andHu from the extrema in this derivative curve. The results of measurements on magnetic garnet films obtained by different methods are compared. Local measurements of the anisotropy induced by substrate facet strain are described.