(color online) (a) Oscillation frequencies as a function of probe wavelength for Co₂MnAl films with different thickness values and GaAs. The solid red lines show the fitting curves based on Eq. (3), where the sound velocities of GaAs and Co₂MnAl are evaluated to be m/s and m/s respectively. (b) The wavelength-dependent effective refractive index n and extinction coefficient k from ellipsometry measurement in a wavelength range of 600–800 nm for Co₂MnAl film.