  We present an investigation on the structural and magnetic properties of the
interfaces of $\mathrm{Fe_3O_4/MgO(001)}$ and $\mathrm{Fe_3O_4/NiO/MgO(001)}$
by extracting cation-selective magnetooptical depth profiles by means of x-ray
magnetic reflectivity (XRMR) in combination with charge-transfer multiplet
simulations of x-ray magnetic circular dichroism (XMCD) data. For
$\mathrm{Fe_3O_4/MgO(001)}$, the magnetooptical depth profiles at the
$\mathrm{Fe^{2+}_{oct}}$ and the $\mathrm{Fe^{3+}_{oct}}$ resonant energies
follow exactly the structural profile, while the magnetooptical depth profile
at the $\mathrm{Fe^{3+}_{tet}}$ resonance is offset by $3.2\pm1.3\,$\r{A} from
the interface, consistent with a B-site interface termination of
$\mathrm{Fe_3O_4}$ with fully intact magnetic order. In contrast, for
$\mathrm{Fe_3O_4/NiO(001)}$, the magnetooptical depth profiles at the
$\mathrm{Fe^{2+}_{oct}}$ and the $\mathrm{Ni^{2+}}$ resonances agree with the
structural profile, but the interface positions of the magnetooptical depth
profiles at the $\mathrm{Fe^{3+}_{oct}}$ and the $\mathrm{Fe^{3+}_{tet}}$
resonances are laterally shifted by $3.3\pm 1.4\,$\r{A} and $2.7\pm0.9\,$\r{A},
respectively, not consistent with a magnetically ordered stoichiometric
interface. This may be related to an intermixed $\mathrm{(Ni,Fe)O}$ layer at
the interface. The magnetooptical depth profiles at the Ni $L_3$ edge reveal
uncompensated magnetic moments throughout the NiO film.
