  Ensuring a permanent increase of magnetic storage densities is one of the
main challenges in magnetic recording. Conventional approaches based on single
phase grains are not suitable to achieve this goal, because their grain volume
is limited due to the superparamagnetic limit. Grains with graded anisotropy
are the most promising candidates to overcome this limit, providing magnetic
memory bits with small volumes, low coercivity and high thermal stability at
the same time. Combining micromagnetic simulations with forward flux sampling
(FFS), a computational method for rare events that has been recently applied to
the magnetic nanostructures, we have determined thermal escape rates and
attempt frequencies of a graded media grain and two single phase grains of the
same geometry. We find that graded anisotropy can increase the thermal
stability of a grain by 12 orders of magnitudes from tens of milliseconds to
centuries without changing the coercive field.
