  Ferrites with (inverse) spinel structure display a large variety of
electronic and magnetic properties making some of them interesting for
potential applications in spintronics. We investigate the thermally induced
interdiffusion of Ni$^{2+}$ ions out of NiO into Fe$_3$O$_4$ ultrathin films
resulting in off-stoichiometric nickelferrite-like thin layers. We synthesized
epitaxial Fe$_3$O$_4$/NiO bilayers on Nb-doped SrTiO$_3$(001) substrates by
means of reactive molecular beam epitaxy. Subsequently, we performed an
annealing cycle comprising three steps at temperatures of 400\,$^{\circ}$C,
600\,$^{\circ}$C, and 800\,$^{\circ}$C under an oxygen background atmosphere.
We studied the changes of the chemical and electronic properties as result of
each annealing step with help of hard x-ray photoelectron spectroscopy and
found a rather homogenous distribution of Ni and Fe cations throughout the
entire film after the overall annealing cycle. For one sample we observed a
cationic distribution close to that of the spinel ferrite NiFe$_2$O$_4$.
Further evidence comes from low energy electron diffraction patterns indicating
a spinel type structure at the surface after annealing. Site and element
specific hysteresis loops performed by x-ray magnetic circular dichroism
uncovered the antiferrimagnetic alignment between the octahedral coordinated
Ni$^{2+}$ and Fe$^{3+}$ ions and the Fe$^{3+}$ in tetrahedral coordination. We
find a quite low coercive field of 0.02\,T, indicating a rather low defect
concentration within the thin ferrite films.
