  We present time-resolved high energy x-ray diffraction (tr-HEXRD),
time-resolved hard x-ray photoelectron spectroscopy (tr-HAXPES) and
time-resolved grazing incidence small angle x-ray scattering (tr-GISAXS) data
of the reactive molecular beam epitaxy (RMBE) of $\mathrm{Fe_3O_4}$ ultrathin
films on various substrates. Reciprocal space maps are recorded during the
deposition of $\mathrm{Fe_3O_4}$ on $\mathrm{SrTiO_3(001)}$, MgO(001) and
NiO/MgO(001) in order to observe the temporal evolution of Bragg reflections
sensitive to the octahedral and tetrahedral sublattices of the inverse spinel
structure of $\mathrm{Fe_3O_4}$. A time delay between the appearance of rock
salt and spinel-exclusive reflections reveals that first, the iron oxide film
grows with $\mathrm{Fe_{1-\delta}O}$ rock salt structure with exclusive
occupation of octahedral lattice sites. When this film is 1.1$\,$nm thick, the
further growth of the iron oxide film proceeds in the inverse spinel structure,
with both octahedral and tetrahedral lattice sites being occupied. In addition,
iron oxide on $\mathrm{SrTiO_3(001)}$ initially grows with none of these
structures. Here, the formation of the rock salt structure starts when the film
is 1.5$\,$nm thick. This is confirmed by tr-HAXPES data obtained during growth
of iron oxide on $\mathrm{SrTiO_3(001)}$, which demonstrate an excess of
$\mathrm{Fe^{2+}}$ cations in growing films thinner than 3.2$\,$nm. This rock
salt phase only appears during growth and vanishes after the supply of the Fe
molecular beam is stopped. Thus, it can be concluded the rock salt structure of
the interlayer is a property of the dynamic growth process. The tr-GISAXS data
link these structural results to an island growth mode of the first 2-3$\,$nm
on both MgO(001) and $\mathrm{SrTiO_3(001)}$ substrates.
