  The magnetic susceptibility of self-organized two-dimensional Co nanodots on
Au(111) has been measured as a function of their size in the 2-7~nm diameter
range. We show that the activation energy for the thermal reversal displays a
power law behavior with the dot volume. Atomic scale simulations based on the
Heisenberg hamiltonian show that this behavior is due to a deviation from the
macrospin model for dot size as small as 3~nm in diameter. This discrepancy is
attributed to finite temperature effects through the thermal excitation of
spin-wave modes inside the particles
