  Magnetic relaxation in molecular magnets under a sweeping field is studied by
taking into account local stray fields. It is found that the randomness of
local stray field leads to a distribution of the relaxation rate which
subsequently makes the relaxation deviate from the exponential law as predicted
by the Landau-Zener model such that the Landau-Zener method needs to be revised
to deduce an exact tunneling splitting. The tunneling splitting and
distribution width of local stray fields are derived from the experimental data
for molecular magnets Fe8.
