Interpretation of Moessbauer spectra in thin iron films

Mössbauer spectra for films condensed and analyzed in uhv are reported in the thickness range 1–42 Å and from 15–350 K. The main features of the spectra can be explained in terms of superparamagnetic fluctuations of the magnetization within a given crystallite. Films in the thickness range 15–42 Å, have a value of internal magnetic field which is slightly less than the 330 kG observed in bulk iron. As the films approach 12 Å, the hyperfine lines move inwards by about 16% when the temperature is raised from 15 to 350 K, but the lines are still well resolved. Assuming a simple uniaxial anisotropy model the magnetization is stable on a time scale of the Larmor period for particles >90 Å average size and the magnetic hyperfine spectrum collapses to a doublet for particle size <70 Å. We interpret this as arising from the complete collapse of the four excited state levels and the partial decrease of the ground state splitting corresponding to the superparamagnetic fluctuation time being limited by the frequency factor τ0 rather than the volume or temperature. The large separation of the doublet in the 0.4-Å film is explained in terms of an electric field gradient at the iron nuclei due to the glass substrate or some contamination of the substrate.