  We investigate the switching field distribution and the resulting bit error
rate of exchange coupled ferri-/ferromagnetic bilayer island arrays by
micromagnetic simulations. Using islands with varying microstructure and
anisotropic properties, the intrinsic switching field distribution is computed.
The dipolar contribution to the switching field distribution is obtained
separately by using a model of a triangular patterned island array resembling
$1.4\,\mathrm{Tb/in}^2$ bit patterned media. Both contributions are computed
for different thickness of the soft exchange coupled ferrimagnet and also for
ferromagnetic single phase FePt islands. A bit patterned media with a bilayer
structure of FeGd($5\,\mathrm{nm}$)/FePt($5\,\mathrm{nm}$) shows a bit error
rate of $10^{-4}$ with a write field of $1.16\,\mathrm{T}$.
