Magneto-optical signal from $\mathrm{Co_2Mn}$-based Heusler thin films in MOKE and BLS

$\mathrm{Co_2Mn}$-based Heusler compounds offer a versatile, composition-tunable platform for magnonics and spintronics. Among them, the half-metallic $\mathrm{Co_2MnSi}$ is of particular interest for magnonics owing to its ultralow Gilbert damping, yet its weak magneto-optical response in the visible challenges optical probing such as Brillouin light scattering (BLS). We study the magneto-optical response of epitaxial $\mathrm{Co_2Mn}X$ films ($X = \{\mathrm{Al}_x\mathrm{Si}_{1-x}, \mathrm{Ga}_x\mathrm{Ge}_{1-x}, \mathrm{Sn}\}$) by magneto-optical Kerr effect (MOKE) spectroscopy and BLS. Angle-resolved MOKE resolves a significant, wavelength-dependent quadratic MOKE (QMOKE) only for $\mathrm{Co_2MnSi}$, whereas $\mathrm{Co_2MnAl}$, $\mathrm{Co_2MnGa}$ and $\mathrm{Co_2MnSn}$ respond dominantly linearly. Comparing BLS intensities of thermal magnons at two wavelengths, $\mathrm{Co_2MnSi}$ gives the weakest signal at $532\,\mathrm{nm}$ yet among the strongest at $457\,\mathrm{nm}$, tracking the spectral dependence of its Kerr angle. These results emphasise the relation between the two magneto-optical techniques, guiding the choice of probing wavelength for $\mathrm{Co_2Mn}$-based Heusler compounds.