Micromagnetic simulations are performed to investigate the reversal process
and the thermal stability of a grain of a typical perpendicular recording
media (Co-Cr). The integration of the LLG equation yields that the reversal
process changes slowly and steadily from coherent rotation to nucleation w
ith increasing column length. The region between homogeneous rotation and n
ucleation becomes smaller and is shifted to smaller column lengths if the d
amping constant is reduced from alpha = 1 to alpha = 0.02. In the weakly da
mped case very fast switching modes exist if the switching field is only sl
ightly larger than the coercive field. In this small regime the switching t
ime increases with higher switching fields. Using solutions of LLG simulati
ons, energy barriers between the two stable states at zero field are estima
ted. For column lengths larger than 30 nm the energy barrier for inhomogene
ous reversal processes are smaller than for coherent rotation.