Increases in magnetic hyperfine field at the surface of ultra-thin epitaxial Fe films (invited)

Abstract

Previous investigations of ultra‐thin epitaxial Fe (110) films grown on Ag (111) have indicated that there is a measurable increase at 4.2 K in the magnetic hyperfine field compared with bulk value in iron atoms in the first three surface layers of the films. As these films were produced in ultra‐vacuum and then removed for study by transmission Mössbauer spectroscopy it was necessary to provide a protective overcoat to prevent oxidation of the surface. Because of this it was not possible to rule out interaction of the Fe surface with the overcoat layers as a cause of the increased surface hyperfine field. A series of new films have been made of isotopically pure ⁵⁶Fe. A thin probe layer of ⁵⁷Fe ranging in thickness from 4–10 Å was then deposited on the film surface and a variety of different overcoat layers subsequently chosen to protect these films. Overcoating materials included Ag, Au, Cu, Ge, MnF₂, and NaCl. While there were differences in the hyperfine fields and isomer shifts of the ⁵⁷Fe probe layer associated with different overcoat materials, the overall results suggest that the increased hyperfine field is related to an increased localization of the 3d electrons for Fe atoms at or near the surface. An observation of a decrease in 4s electron density indicated by the isomer shift data is consistent with this view and is in qualitative agreement with the theoretical work of Wang and Freeman.