  In this work thin magnetite films were deposited on SrTiO$_3$ via reactive
molecular beam epitaxy at different substrate temperatures. The growth process
was monitored in-situ during deposition by means of x-ray diffraction. While
the magnetite film grown at 400$^\circ$C shows a fully relaxed vertical lattice
constant already in the early growth stages, the film deposited at 270$^\circ$C
exhibits a strong vertical compressive strain and relaxes towards the bulk
value with increasing film thickness. Furthermore, a lateral tensile strain was
observed under these growth conditions although the inverse behavior is
expected due to the lattice mismatch of -7.5%. Additionally, the occupancy of
the A and B sublattices of magnetite with tetrahedral and octahedral sites was
investigated showing a lower occupancy of the A sites compared to an ideal
inverse spinel structure. The occupation of A sites decreases for a higher
growth temperature. Thus, we assume a relocation of the iron ions from
tetrahedral sites to octahedral vacancies forming a deficient rock salt
lattice.
