  We investigate the thermal activated magnetisation reversal in a single
ferromagnetic nanoparticle with uniaxial anisotropy using Monte Carlo
simulations. The aim of this work is to reproduce the reversal magnetisation by
uniform rotation at very low temperature in the high energy barrier hypothesis,
that is to realize the N\'eel-Brown model. For this purpose we have considered
a simple cubic nanoparticle where each site is occupied by a classical
Heisenberg spin. The Hamiltonian is the sum of an exchange interaction term, a
single-ion anisotropy term and a Zeeman interaction term. Our numerical data of
the thermal variation of the switching field are compared to an approximated
expression and previous experimental results on Co nanoparticles.
