The physical volume of thermal switching unit, the activation volume for perpendicular recording media, has been investigated using the Wohlfarth equation based on micromagnetic simulation. It was found that the activation volume obtained from the Wohlfarth equation Vac-Wohlfarth exhibits significant field dependence, which is very different from direct observation of thermal switching process. This shows that Vac-Wohlfarth is invalid for perpendicular recording media. The reason for that was interpreted in terms of the narrow dispersion of the energy barrier for thermal switching, originating from the high degree of easy axes orientation along the direction perpendicular to the film plane. [ABSTRACT FROM AUTHOR]