Evolution of magnetic anisotropy in epitaxial Fe films by ferromagnetic resonance

Single-crystal Fe films of 4 to 33 monolayers (ML) on GaAs (100) prepared by molecular-beam epitaxy were studied by ferromagnetic resonance (FMR). The evolution of both in-plane and out-of-plane anisotropies was observed. The FMR data show a predominant in-plane uniaxial magnetic anisotropy in the films with the thickness t less than 12.7 ML, with an easy axis along the <1 (1) under bar0> direction. An in-plane fourfold anisotropy due to cubic magnetocrystalline anisotropy starts to appear and coexists with uniaxial magnetic anisotropy when t=8.4 ML, and increases with increasing film thickness. For t=33 ML, the cubic anisotropy constant K-1 reaches 28.2x10(4) erg/cm(3), which is 60% of the value for bulk bcc Fe. A strong perpendicular anisotropy in the ultrathin Fe films was observed. For t=4 ML, the perpendicular anisotropy constant is as high as 14.6x10(6) erg/cm(3). It decreases with increasing the thickness t and reaches about 2.1x10(6) erg/cm(3) for 33 ML, while a small fourth-order perpendicular anisotropy appears and increases with the same trend as the cubic anisotropy. (C) 2003 American Institute of Physics.