A numerical implementation of the transition state theory (TST) is presented which can be used to calculate the attempt frequency $f_{0}$ of arbitrary shaped magnetic nanostructures. The micromagnetic equations are discretized using the finite elemen
t method. The climbing image nudged elastic band method is used to calculate the saddle point configuration, which is required for the calculation of $f_{0}$. Excellent agreement of the implemented numerical model and analytical solutions is obtained for single domain particles. The developed method is applied to compare $f_{0}$ for single phase and graded media grains of advanced recording media. $f_{0}$ is predicted to be comparable if the maximum anisotropy is the same in these two media types.
The potential of exchange spring bilayers and graded media is reviewed. An analytical model for the optimization of graded media gives an optimal value of the magnetic polarization of Js = 0.8 T. The optimum design allows for thermally stable grains
with grain diameters in the order of 3.3 nm, which supports ultra high density up to 5 to 10 Tbit per inch2. The switching field distribution is significantly reduced in bilayer media and graded media compared to single phase media. For the graded media the switching field distribution is reduced by about a factor of two. For bilayer media the minimum switching field distribution is obtained for soft layer anisotropies about one fifth of the hard layer anisotropy. The influence of precessional switching on the reversal time and the reversal field is investigated in detail for magnetic bilayers. Exchange spring bilayers can be reversed with field pulses of 20 ps.
