  Brillouin light scattering in ferromagnetic materials usually involves one
magnon and two photons and their total angular momentum is conserved. Here, we
experimentally demonstrate the presence of a helicity-changing two-magnon
Brillouin light scattering in a ferromagetic crystal, which can be viewed as a
four-wave mixing process involving two magnons and two photons. Moreover, we
observe an unconventional helicity-changing one-magnon Brillouin light
scattering, which apparently infringes the conservation law of the angular
momentum. We show that the crystal angular momentum intervenes to compensate
the missing angular momentum in the latter scattering process.
