Magnetic anisotropy of Fe/GaAs(0 0 1) ultrathin films investigated by in situ ferromagnetic resonance

Uncapped epitaxial 5–20 monolayers (ML) of Fe grown by molecular beam epitaxy (MBE) on 4×64×6 surface reconstructed GaAs(0 0 1) at room temperature have been studied by in situ Ferromagnetic Resonance (FMR) in ultrahigh vacuum (UHV). A strong uniaxial anisotropy has been found for films with a thickness d⩽7ML. A combination of two and fourfold anisotropy was observed for 7 ML⩽d⩽15ML, and a bulk like magnetic anisotropy for d>15ML which is slightly modified by a surface/interface anisotropy contribution. From thickness dependent measurements we determine values for the volume and surface/interface anisotropy contributions, e.g., in-plane (K2∥s,eff=-8.9±0.4×10-5J/m2) and out-of-plane (K2⊥s,eff=1.17±0.1×10-3J/m2). The different anisotropy contributions are discussed in terms of current magneto-elastic models, but cannot be quantitatively explained. Especially the in-plane interface anisotropy can be understood only, when hybridization effects at the interface are considered.