  Identifying an efficient pathway to change the order parameter via a subtle
excitation of the coupled high-frequency mode is the ultimate goal of the field
of ultrafast phase transitions. This is an especially interesting research
direction in magnetism, where the coupling between spin and lattice excitations
is required for magnetization reversal. Despite several attempts however, the
switching between magnetic states via resonant pumping of phonon modes has not
yet been demonstrated. Here we show how an ultrafast resonant excitation of the
longitudinal optical phonon modes in magnetic garnet films switches
magnetization into a peculiar quadrupolar magnetic domain pattern,
unambiguously revealing the magneto-elastic mechanism of the switching. In
contrast, the excitation of strongly absorbing transverse phonon modes results
in thermal demagnetization effect only.
