Magnetisation dynamics in exchange coupled spring systems with perpendicular anisotropy

TL;DR

This study investigates the magnetisation dynamics of exchange spring FePt/Fe bilayer systems with perpendicular anisotropy using simulations, revealing complex resonance spectra influenced by local effective field variations.

Contribution

It provides new insights into the dynamic behavior of exchange spring magnets with perpendicular anisotropy, highlighting the origin of multi-peaked resonance spectra without spin wave modes.

Findings

Multi-peaked resonance spectra observed under perpendicular magnetic fields.

Variation in local effective field causes complex dynamical behavior.

FePt layer remains fixed while Fe layer dynamics are analyzed.

Abstract

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.