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

Abstract

We study the magnetic behavior of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ thin films with thicknesses between $5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The films are epitaxially grown on $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ single crystals by atomic-oxygen-assisted molecular beam epitaxy. The ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ thin films exhibit high structural order with sharp interfaces and low roughness and exhibit a Verwey transition for thicknesses above $8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. However, the samples have magnetic properties that deviate from the bulk ones. The magnetic moment varies between $2.4{\ensuremath{\mu}}_{B}$ for $5\text{\ensuremath{-}}\mathrm{nm}$-thick film and $3.2{\ensuremath{\mu}}_{B}$ for $50\text{\ensuremath{-}}\mathrm{nm}$-thick film in a field of $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, which is lower than that of bulk samples ($4.1{\ensuremath{\mu}}_{B}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ formula). Still the magnetic saturation is never reached, even in fields as large as $2\phantom{\rule{0.3em}{0ex}}T$. 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.