Advanced atomistic models for magnetisation dynamics

The next generation of ultra-high density storage technology will be based on heat-assisted magnetic recording that uses the highly anisotropic L10 phase of FePt. As the areal density increases, the grain size decreases and finite-size effects are becoming crucial. The damping mechanism controls the magnetisation dynamics and the writing speed of the information, hence its behavior for small FePt grain sizes needs to be studied. Using atomistic spin dynam- ics simulations, the variation of damping with temperature and system size is systematically analysed by employing ferromagnetic resonance calculations. The damping of FePt grains is enhanced with increased temperatures, but the linewidth of the system can decrease in the presence of size distributions due to the transition of small grains to the paramagnetic state. Switching is investigated within the heated dot limit, assuming the largest areal density possible in recording media and shows that the numerical calculation involving a dynamical switching of the media leads to larger bit-error rates due to thermal transitions over the energy barrier and subsequent smaller areal densities.
Atomistic spin dynamics simulations assume a fixed lattice of atoms, however, as the magnetic material is heated up, both magnetic and mechanical properties will be dynamically affected. To include the lattice contribution during magnetic relaxation (magnon-phonon interactions), a coupled spin-lattice dynamics model has been developed. The fundamental properties of the SLD framework, with application to BCC Fe, have been analysed. The existence of a direct channel of energy and angular momentum transfer between magnons and phonons increases the damping via spin-lattice coupling, which is important for the study of both magnetic insulators and metals. Finally, the magnetisation dynamics is studied under the effect of THz phonon excitation, showing that it is possible to switch the magnetisation via the direct excitation of various phonon modes.