Stress effects on the electron-spin-resonance spectra of Er in gold and silver thin films

Electron-spin-resonance measurements were carried out at liquid-helium temperatures on diluted Er in Au and Ag thin films (300-5000 Å) evaporated on NaCl and quartz substrates. The observed anisotropy of the $g$ value, linewidth, and line shape are explained by the mixing of excited crystalline field levels with the ${\Gamma }_7$ ground state: these admixtures are caused by the stresses induced by the difference between the thermal expansion coefficients of the substrate and the film. Lower limits for the tetragonal and trigonal second-order orbit-lattice coupling parameters can be estimated from the data. In Au their magnitude and sign agree with those predicted by the point-charge model, while in Ag the trigonal parameter does not. Qualitative information about the strain distribution across the film was obtained by employing the anisotropy of the $g$ value, the linewidth, and the line shape together with x rays and electron-microscopy analysis.