Thickness dependence of anomalous magnetic behavior in epitaxial Fe3O4(111) thin films: Effect of density of antiphase boundaries

We study the magnetic behavior of Fe₃O₄(111) thin films with thicknesses between 50.3em0exnm and 500.3em0exnm. The films are epitaxially grown on α-Al₂O₃(0001) single crystals by atomic-oxygen-assisted molecular beam epitaxy. The Fe₃O₄(111) thin films exhibit high structural order with sharp interfaces and low roughness and exhibit a Verwey transition for thicknesses above 80.3em0exnm. However, the samples have magnetic properties that deviate from the bulk ones. The magnetic moment varies between 2.4μB for 5-nm-thick film and 3.2μB for 50-nm-thick film in a field of 1.20.3em0exT, which is lower than that of bulk samples (4.1μB∕Fe₃O₄ formula). Still the magnetic saturation is never reached, even in fields as large as 20.3em0exT. The thinner the film, the slower the approach to saturation. Structural analysis, performed using high-resolution transmission electron microscopy, reveals the presence of antiphase boundaries (APB's), the density of which decreases when the thickness increases. Using a model of ferromagnetic domains separated by antiferromagnetically sharp interfaces, we show that the slow approach to saturation observed in the films as a function of thickness is driven by the APB density.