Correlation between structure and magnetism in epitaxial Fe monolayers on GaAs(001)

Epitaxial ultrathin Fe films were grown at room temperature on 4×6 reconstructed GaAs(001) in the thickness range of 2–30 monolayer with and without an over-layer of Ag. The magnetic anisotropy density and the magnetization were studied using in situ Ferromagnetic resonance (FMR) and in situ   SQUID magnetometry, respectively. A strong correlation between the structure and magnetism has been found for both Fe/GaAs and Ag/Fe/GaAs systems. At room temperature ferromagnetism is observed at a critical thickness tc=2.8tc=2.8 monolayer equivalents (MLE) when the initially formed Fe clusters coalesce and form a quasi-continuous film. A magneto-elastic model is used to explain the measured anisotropy contributions. The large in-plane interface magnetic anisotropy (K2‖int) is related to the in-plane stress at the interface of the Fe and GaAs. The small volume contribution to the perpendicular uniaxial anisotropy (K2⊥v) can be quantitatively understood by considering the strain in the inner part of the film. This strain was determined by an IV-LEED experiment. Covering the Fe monolayers with an Ag cap increases the critical thickness for onset of ferromagnetism from 2.8 MLE for Fe/GaAs to 3.1 for 1 MLE Ag /Fe/GaAs at room temperature. It also suppresses the surface contribution to the perpendicular uniaxial anisotropy (K2⊥s) by about 20% and has only very small effects on K2‖int.