Reversal modes, thermal stability and exchange length in perpendicular recording media

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 with increasing column length. The region between homogeneous rotation and nucleation becomes smaller and is shifted to smaller column lengths if the damping constant is reduced from alpha = 1 to alpha = 0.02. In the weakly damped case very fast switching modes exist if the switching field is only slightly larger than the coercive field. In this small regime the switching time increases with higher switching fields. Using solutions of LLG simulations, energy barriers between the two stable states at zero field are estimated. For column lengths larger than 30 nm the energy barrier for inhomogeneous reversal processes are smaller than for coherent rotation.