  Nanopatterned FePt nano-dots often exhibit low coercivity and a broad
switching field distribution, which could arise due to edge damage during the
patterning process causing a reduction in the $L1_{0}$ ordering required for a
high magnetocrystalline anisotropy. Using an atomistic spin model, we study the
magnetization reversal behavior of $L1_{0}$ FePt nanodots with soft magnetic
edges. We show that reversal is initiated by nucleation for the whole range of
edge widths studied. For narrow soft edges the individual nucleation events
dominate reversal; for wider edges, multiple nucleation at the edge creates a
circular domain wall at the interface which precedes complete reversal. Our
simulations compare well with available analytical theories. The increased edge
width further reduces and saturates the required nucleation field. The
nucleation field and the activation volume manipulate the thermally induced
switching field distribution. By control of the properties of dot edges using
proper patterning methods, it should be possible to realize exchange spring bit
patterned media without additional soft layers.
