  Magnetisation dynamics in exchange spring magnets have been studied using
simulations of the FePt/Fe bilayer system. The FePt hard layer exhibits strong
perpendicular magnetocrystalline anisotropy, while the soft (Fe) layer has
negligible magnetocrystalline anisotropy. The variation of the local spin
orientation in the Fe layer is determined by the competition of the exchange
coupling interaction with the hard layer and the magnetostatic energy which
favours in-plane magnetisation. Dynamics were studied by monitoring the
response of the Fe layer magnetisation after the abrupt application of a
magnetic field which causes the systems to realign via precessional motion.
This precessional motion allows us to obtain the frequency spectrum and hence
examine the dynamical magnetisation motion. Since the rotation of the spins in
the soft layer does not have a well defined magnetic anisotropy, the system
does not present the usual frequency field characteristics for a thin film.
Additionally we obtain multi-peaked resonance spectra for the application of
magnetic fields perpendicular to the film plane, though we discount the
existence of spin wave modes and propose that this arises due to variations in
the local effective field across the Fe layer. The dynamic response is only
considered in the Fe layer, with the FePt layer held fixed in the perpendicular
orientation.
