Growth and magnetic anisotropy of Fe films deposited on Si(111) using an ultrathin iron silicide template

Abstract

The growth and magnetic properties of thin Fe films deposited at room temperature on ultrathin iron silicide seed layer epitaxially grown on Si(111) single crystal were investigated as a function of Fe thickness ($0<{t}_{\mathrm{Fe}}<300$ monolayers). The growth mode and structure have been determined in situ by means of scanning tunneling microscopy, low energy electron diffraction, and x-ray photoelectron diffraction. The magnetic properties were characterized ex situ by conventional polar and longitudinal magneto-optical Kerr effect, transverse biased initial inverse susceptibility and torque (TBIIST) measurements, and superconducting quantum interference device magnetometry. Fe growth is of Volmer-Weber type (island growth) and the epitaxial film adopts the bcc $\ensuremath{\alpha}$-Fe structure. Onset of long-range ferromagnetic order occurs at 4.7 monolayers (ML), in the vicinity of the percolation threshold of the Fe islands. The Curie temperature increases continuously with Fe coverage, varying from 135 K for 4.7 ML to 260 K for 7.3 ML. Two different spin reorientation transitions have been observed versus Fe coverage. First, the magnetic easy axis rotates from normal to the film plane, for coverage below 6 ML, to in plane, for thickness above 7 ML. Then, in-plane magnetized films present tiny (anisotropy fields less than 4 Oe) uniaxial and sixfold magnetic anisotropies. From sixfold anisotropy the small higher order cubic anisotropy constant ${K}_{2}$ was measured precisely by TBIIST. It decreases monotonously with increasing coverage and changes its sign at approximately 20 ML, which in turn results in a switching of the sixfold anisotropy easy axis from ⟨1--10⟩ to ⟨1--21⟩ directions. It appears that TIBIIST magnetometry is a powerful method for a quantitative determination of the various contributions to in-plane magnetic anisotropies in ultrathin films.