Film thickness and temperature dependence of the magnetic properties of pulsed-laser-deposited Fe₃O₄ films on different substrates

We report a systematic study of <100> epitaxial films of magnetite (Fe₃O₄) of thickness 300-2000 Å, grown on single-crystal substrates (MgO, MgAl₂O₄, and SrTiO₃) 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 MgAl₂O₄ and SrTiO₃ 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.