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 J s = 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 -10 Tbit/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 1/5 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.

A Monte Carlo approach and a modified nudged elastic band method are used to study the dynamic coercivity of interacting particle arrays in particular perpendicular recording media and exchange spring bilayers. Monte Carlo simulations are performed to study the effect of the interactions on the dynamic coercivity of interacting particle arrays. It is shown that the interactions in magnetic recording media only slightly influence the dynamic coercivity. The reliability of energy barrier measurements based upon Sharrock's equation for frequency-dependent coercivity data is investigated using a modified nudged elastic band method. It is shown that the extrapolated energy barrier at zero field may deviate from the correct one by up to 18% if the conventional exponent n = 1.5 is assumed. Our micromagnetic simulations furthermore indicate that the accuracy of the extrapolated energy barrier can be improved by about a factor of 3 upon measuring the dynamic coercivity at an angle of 45°and using the exponent n as an additional fit parameter.

A numerical implementation of the transition state theory (TST) is presented which can be used to calculate the attempt frequency f0 of arbitrary shaped magnetic nanostructures. The micromagnetic equations are discretized using the finite element method. The climbing image nudged elastic band method is used to calculate the saddle point configuration, which is required for the calculation of f0. Excellent agreement of the implemented numerical model and analytical solutions is obtained for single domain particles. The developed method is applied to compare f0 for single phase and graded media grains of advanced recording media. f0 is predicted to be comparable if the maximum anisotropy is the same in these two media types.

We report a finite temperature micromagnetic method (FTM) that allows for the calculation of the coercive field of arbitrary shaped magnetic nanostructures at time scales of nanoseconds to years.Instead of directly solving the Landau-Lifshitz-Gilbert equation, the coercive field is obtained without any free parameter by solving a non linear equation, which arises from the transition state theory. The method is applicable to magnetic structures where coercivity is determined by one thermally activated reversal or nucleation process. The method shows excellent agreement with experimentally obtained coercive fields of magnetic nanostructures and provides a deeper understanding of the mechanism of coercivity.

We performed micromagnetic simulations in order to investigate the effect of intergranular exchange coupling on the magnetic properties of advanced magnetic recording structures. We found that the coercive field of granular recording media decreases with increasing intergrain exchange coupling ( int ). We observed this decay even for perfect films without switching field distributions and soft magnetic inclusions. A mean field exchange field of about 0 ex = 0 34 T leads to the same thermal stability of the grains at the transition and in the center of a bit. A larger value of int lowers the thermal stability of the grains close to the transitions. Micromagnetic simulations of coupled granular continuous (CGC) media indicate that the top layer in CGC media is not continuous. The simulations suggest that the top layer is granular with a relatively weak intergrain exchange coupling, int .