  Based on micromagnetic simulations and experimental observations of the
magnetization and lattice dynamics following the direct optical excitation of
the magnetic insulator Bi:YIG or indirect excitation via an optically opaque
Pt/Cu double layer, we disentangle the dynamical effects of magnetic anisotropy
and magnetoelastic coupling. The strain and temperature of the lattice are
quantified via modeling ultrafast x-ray diffraction data. Measurements of the
time-resolved magneto-optical Kerr effect agree well with the magnetization
dynamics simulated according to the excitation via two mechanisms: The
magneto-acoustic coupling to the experimentally verified strain dynamics and
the ultrafast temperature-induced transient change in the magnetic anisotropy.
The numerical modeling proves that for direct excitation both mechanisms drive
the fundamental mode with opposite phase. The relative ratio of standing
spin-wave amplitudes of higher order modes indicates that both mechanisms are
substantially active.
