  We study the underlying chemical, electronic and magnetic properties of a
number of magnetite based thin films. The main focus is placed onto
NiO/Fe$_3$O$_4$(001) bilayers grown on MgO(001) and Nb-SrTiO$_3$(001)
substrates. We compare the results with those obtained on pure Fe$_3$O$_4$(001)
thin films. It is found that the magnetite layers are oxidized and Fe$^{3+}$
dominates at the surfaces due to maghemite ($\gamma$-Fe$_2$O$_3$) formation,
which decreases with increasing magnetite layer thickness. From a layer
thickness of around 20 nm on the cationic distribution is close to that of
stoichiometric Fe$_3$O$_4$. At the interface between NiO and Fe$_3$O$_4$ we
find the Ni to be in a divalent valence state, with unambiguous spectral
features in the Ni 2p core level x-ray photoelectron spectra typical for NiO.
The formation of a significant NiFe$_2$O$_4$ interlayer can be excluded by
means of XMCD. Magneto optical Kerr effect measurements reveal significant
higher coercive fields compared to magnetite thin films grown on MgO(001), and
a 45$^{\circ}$ rotated magnetic easy axis. We discuss the spin magnetic moments
of the magnetite layers and find that the moment increases with increasing thin
film thickness. At low thickness the NiO/Fe$_3$O$_4$ films grown on
Nb-SrTiO$_3$ exhibits a significantly decreased spin magnetic moments. A
thickness of 20 nm or above leads to spin magnetic moments close to that of
bulk magnetite.
