  We compute mean waiting times between thermally-activated magnetization
reversals in a nanodisk with parameters similar to a free CoFeB layer used in
magnetic random access memories. By combining Langer's theory and forward flux
sampling simulations, we show that the Arrhenius prefactor can take values up
to 10$^{21}$ Hz, orders of magnitude beyond the value of 10$^{9}$ Hz typically
assumed, and varies drastically as a function of material parameters. We show
that the prefactor behaves like an exponential of the activation energy, which
highlights a case of the Meyer-Neldel compensation rule. This suggests that
modeling information retention times with a barrier-independent prefactor in
such magnetic storage elements is not justified.
