  We report two effects that lead to a reduction of the switching field
distribution in exchange spring media. The first effect relies on a subtle
mechanism of the interplay between exchange coupling between soft and hard
layers and anisotropy that allows significant reduction of the switching field
distribution in exchange spring media. This effect reduces the switching field
distribution by about 30% compared to single-phase media. A second effect is
that due to the improved thermal stability of exchange spring media over
single-phase media, thermal fluctuation leads to reduced fundamental transition
jitter. The influence of this overall improved switching field distribution on
the transition jitter in granular recording and the bit error rate in
bit-patterned magnetic recording is discussed. The transition jitter in
granular recording for a distribution of K1 values of 3% in the hard layer,
taking into account thermal fluctuations during recording, is estimated to be a
= 0.78 nm, which is similar to the best reported calculated jitter in optimized
heat-assisted recording media.
