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Two epitaxial thin film garnet compositions Gd3Sc.13Gal4.87012 and Y3Sc2.0A1.9lGa2.090012, have been developed and operated as otpical waveguides. The refractive index of the solid solution films is shown to be linear with respect to mole fraction of the end member garnets. Compositions of twelve additional solid solutions are proposed which will allow selection of film-substrate combinations from among a wide range of refractive index differences.

The nature and site occupancy of Fe4+centers in Ca-doped YIG films is investigated by studying the symmetry properties of the near-infrared absorption. We apply the recently proposed AVIA (angular variation of induced anisotropy) method: we measure, as the single- -crystal film is rotated relative to a magnetic field, the angular variation of the magnetic linear dichroism, which is the anisotropy property utilized in this case. The AVIA pattern thus obtained is compared with the theoretical angular variations appropriate to the symmetry of the garnet sites. The result we find is a uniform distribution of Fe4+over the a (octahedral) and d (tetrahedral) sublattices. This uniform distribution does not change, in spite of varying Fe4+concentration, after successive annealing treatments in oxidizing or reducing atmospheres. The structural changes through the annealing steps are monitored by X-ray diffraction and the lattice is found to expand, under reducing treatments, only as long as there are Fe4+centers. After all Fe4+ions have been reduced (namely at zero near-infrared absorption), rapid diffusion of oxygen anions is hindered, in spite of the large concentration of oxygen vacancies. All observed effects are explained by a single mechanism: electron deficiency (or holes), in Ca-doped YIG, results in a widespread distribution of Fe 4+ and, effectively, in the presence of O- oxygens.