  Magneto-optic Kerr effect can probe the process of magnetization reversal in
ferromagnetic thin films and thus be used as an alternative to magnetometry.
Kerr effect is wavelength-dependent and the Kerr rotation can reverse sign,
vanishing at particular wavelengths. We investigate epitaxial heterostructures
of ferromagnetic manganite, La$_{0.7}$Sr$_{0.3}$Mn$_{0.9}$Ru$_{0.1}$O$_3$, by
polar Kerr effect and magnetometry. The manganite layers are separated by or
interfaced with a layer of nickelate, NdNiO$_3$. Kerr rotation hysteresis loops
of trilayers, with two manganite layers of different thickness separated by a
nickelate layer, have intriguing humplike features, when measured with light of
400 nm wavelength. By investigating additional reference samples we disentangle
the contributions of the individual layers to the loops: we show that the humps
originate from the opposite sense of the Kerr rotation of the two different
ferromagnetic layers, combined with the additive behavior of the Kerr signal.
