Evolution of properties of epitaxial bismuth iron garnet films with increasing thickness

Abstract

Bismuth iron garnet (BIG) films of thicknesses from 470 to 2560 nm were prepared by pulsed laser deposition under identical deposition conditions. All films are epitaxial, bismuth deficient, and show rms surface roughnesses between 15 and 40 nm . X-ray coherence lengths decrease with increasing film thickness. Films below approximately 1 μm are free of cracks, thicker films possess a network of cracks. From fits of optical transmission spectra, real and imaginary parts of the refractive indices were found for wavelengths from 500 to 850 nm . The effects of thin film interference and surface roughness were included. With these data as input information, each of our experimental Faraday rotation spectra was described by a single diamagnetic line in visible light. The measured spectra could be reproduced and parameters of the magneto-optical transition were obtained. We observed a broadening of the transition with increasing film thickness and a red shift of the center frequency. This corresponds to our experimental observation that the wavelength of maximum Faraday rotation for BIG films in visible light shifts to longer wavelengths by almost 40 nm for a 2560-nm-thick film as compared to a 470-nm-thick film. As BIG is not thermodynamically stable, aging is a crucial question. We found that careful annealing in oxygen below the deposition temperature increases the angle of Faraday rotation, while film properties deteriorate during long annealing times at the deposition temperature.