Film thickness and temperature dependence of the magnetic properties of pulsed-laser-depositedFe3O4films on different substrates

Abstract

We report a systematic study of 〈100〉 epitaxial films of magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ of thickness 300–2000 Å, grown on single-crystal substrates (MgO, ${\mathrm{MgAl}}_2{\mathrm{O}}_4,$ and ${\mathrm{SrTiO}}_3$) using pulsed laser deposition. The magnetic state was explored by measuring magnetization, conversion-electron Mössbauer spectroscopy, and ferromagnetic resonance. At 300 K, films grown on ${\mathrm{MgAl}}_2{\mathrm{O}}_4$ and ${\mathrm{SrTiO}}_3$ are, more or less, normal for all thicknesses. That is, the in-plane anisotropy is fourfold. The films on MgO, however, show no in-plane anisotropy if the thickness is less than ∼800 Å. In contrast to some previous claims, the present films exhibit magnetic saturation at quite modest (⩽2 kOe) fields. We propose simple models to account for a variety of observations. In addition to the shape anisotropy, strains appear to play the major role in determining the variation of the magnetic behavior with thickness, substrate, and temperature.