  The decay of a heavy neutral scalar particle into fermions and into charged
scalars are analyzed when in the presence of an external magnetic field and
finite temperature. Working in the one-loop approximation for the study of
these decay channels, it is shown that the magnetic field leads in general to a
suppression of the decay width whenever the kinematic constrain depends
explicitly on the magnetic field. Our results are also compared with common
approximations found in the literature, e.g., when the magnitude of the
external magnetic field is smaller than the decaying product particle masses,
i.e., in the weak field approximation, and in the opposite case, i.e., in the
strong field approximation. Possible applications of our results are discussed.
