Magnetic anisotropy related to strain and thickness of ultrathin iron oxide films on MgO(001)

Abstract

Iron oxide films with different thicknesses (7.6–30 nm) were grown on clean MgO(001) substrates using reactive molecular beam epitaxy at 250 °C depositing Fe in a 5 × 10−5 mbar oxygen atmosphere. X-ray photoelectron spectra and low energy electron diffraction experiments indicate the stoichiometry and the surface structure of magnetite (Fe3O4). Film thicknesses and the lattice constants were analyzed ex situ by x-ray reflectometry and x-ray diffraction, respectively. These experiments reveal the single crystalline and epitactic state of the iron oxide films. However, the obtained vertical layer distances are too small to be strained magnetite and would rather suit to maghemite. Although Raman spectroscopy carried out to analyze the present iron oxide phase showed that the films might have slightly been oxidized in ambient conditions, a posteriori performed XPS measurements exclude a strong oxidation of the surface. Therefore we consider the presence of anti phase boundaries to explain the low vertical layer distances of the magnetite films. Further magnetooptic Kerr measurements were performed to investigate the magnetic properties. While the thinnest film shows a magnetic isotropic behavior, the thicker films exhibit a fourfold magnetic in-plane anisotropy. The magnetic easy axes are in the Fe3O4 directions. We propose that the magnetocrystalline anisotropy is too weak for very thin iron oxide films to form fourfold anisotropy related to the cubic crystal structure.