  The magneto-optic Kerr effect is a powerful tool for measuring magnetism in
thin films at microscopic scales, as was recently demonstrated by the major
role it played in the discovery of two-dimensional (2D) ferromagnetism in
monolayer CrI$_3$ and Cr$_2$Ge$_2$Te$_6$. These 2D magnets are often stacked
with other 2D materials in van der Waals heterostructures on a SiO$_2$/Si
substrate, giving rise to thin-film interference. This can strongly affect
magneto-optical measurements, but is often not taken into account in
experiments. Here, we show that thin-film interference can be used to engineer
the magneto-optical signals of 2D magnetic materials and optimize them for a
given experiment or setup. Using the transfer matrix method, we analyze the
magneto-optical signals from realistic systems composed of van der Waals
heterostructures on SiO$_2$/Si substrates, using CrI$_3$ as a prototypical 2D
magnet, and hexagonal boron nitride (hBN) to encapsulate this air-sensitive
layer. We observe a strong modulation of the Kerr rotation and ellipticity,
reaching several tens to hundreds of milliradians, as a function of the
illumination wavelength, and the thickness of the SiO$_2$ and layers composing
the van der Waals heterostructure. Similar results are obtained in
heterostructures composed by other 2D magnets, such as CrCl$_3$, CrBr$_3$ and
Cr$_2$Ge$_2$Te$_6$. Designing samples for the optimal trade-off between
magnitude of the magneto-optical signals and intensity of the reflected light
should result in a higher sensitivity and shorter measurement times. Therefore,
we expect that careful sample engineering, taking into account thin-film
interference effects, will further the knowledge of magnetization in
low-dimensional structures.
