  Interfacial mixing of elements is a well-known phenomenon found in thin film
deposition. For thin-film magnetic heterostructures, interfacial compositional
inhomogeneities can have drastic effects on the resulting functionalities. As
such, care must be taken to characterize the compositional and magnetic
properties of thin films intended for device use. Recently, ferrimagnetic
Mn$_4$N thin films have drawn considerable interest due to exhibiting
perpendicular magnetic anisotropy, high domain-wall mobility, and good thermal
stability. In this study, we employed X-ray photoelectron spectroscopy (XPS)
and polarized neutron reflectometry (PNR) measurements to investigate the
interfaces of an epitaxially-grown MgO/Mn$_4$N/Pt trilayer deposited at 450
$^{\circ}$C. XPS revealed the thickness of elemental mixing regions of near 5
nm at both interfaces. Using PNR, we found that these interfaces exhibit
essentially zero net magnetization at room temperature. Despite the
high-temperature deposition at 450 $^{\circ}$C, the thickness of mixing regions
is comparable to those observed in magnetic films deposited at room
temperature. Micromagnetic simulations show that this interfacial mixing should
not deter the robust formation of small skyrmions, consistent with a recent
experiment. The results obtained are encouraging in terms of the potential of
integrating thermally stable Mn$_4$N into future spintronic devices.
