  The spectral distribution of parametrically excited dipole-exchange magnons
in an in-plane magnetized epitaxial film of yttrium-iron garnet was studied by
means of frequency- and wavevector-resolved Brillouin light scattering
spectroscopy. The experiment was performed in a parallel pumping geometry where
an exciting microwave magnetic field was parallel to the magnetizing field. It
was found that for both dipolar and exchange spectral areas parallel pumping
excites the lowest volume magnon modes propagating in the film plane
perpendicularly to the magnetization direction. In order to interpret the
experimental observations, we used a microscopic Heisenberg model that includes
exchange as well as dipole-dipole interactions to calculate the magnon spectrum
and construct the eigenstates. As proven in our calculations, the observed
magnons are characterized by having the highest possible ellipticity of
precession which suggests the lowest threshold of parametric generation.
Applying different pumping powers we observe modifications in the magnon
spectrum that are described theoretically by a softening of the spin stiffness.
