Magnetization dynamics in exchange-coupled spring systems with perpendicular anisotropy

Magnetization 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 in 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 favors in-plane magnetization. Dynamics were studied by monitoring the response of the Fe layer magnetization 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 magnetization 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 multipeaked 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.