Magnetic Moment Of Fe3o4 Films With Thicknesses Near The Unit-Cell Size

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 mu(B)/f.u. for samples of around 35 angstrom. This is considered a bulk value and has been reported only for films more than 10 times thicker. Moreover, we show that even 10-angstrom-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 (Fe-octa(2+), Fe-octa(3+), Fe-tetra(3+)) of Fe3O4 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.