Magnetic moment ofFe3O4films with thicknesses near the unit-cell size

Abstract

We perform a systematic study on the evolution of the magnetic spin moment (${m}_{s}$) of epitaxial [100]- and [111]-magnetite films of increasing thickness. The ultrathin films are characterized by low-energy electron diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). By employing sum rules on the XMCD spectra we obtain ${m}_{s}=3.6$ ${\ensuremath{\mu}}_{B}$/f.u. for samples of around 35 \AA{}. This is considered a bulk value and has been reported only for films more than 10 times thicker. Moreover, we show that even 10-\AA{}-thick magnetite already presents a significant magnetic moment. For both grown directions the moment increases similarly with the thickness. The ferromagnetic behavior for each iron ion site (${\mathrm{Fe}}_{\mathrm{octa}}^{2+}$, ${\mathrm{Fe}}_{\mathrm{octa}}^{3+}$, ${\mathrm{Fe}}_{\mathrm{tetra}}^{3+}$) of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ is measured by monitoring XMCD peaks. The deduced hysteresis curves (per ion, per site) exhibit a coercive field of 300 Oe. Our results show that both the ferrimagnetic order and the bulk moment value are preserved at room temperature around the thickness of 2 unit cells.