  A study of the d-dimensional classical Heisenberg ferromagnetic model in the
presence of a magnetic field is performed within the two-time Green function's
framework in classical statistical physics. We extend the well known quantum
Callen method to derive analytically a new formula for magnetization. Although
this formula is valid for any dimensionality, we focus on one- and three-
dimensional models and compare the predictions with those arising from a
different expression suggested many years ago in the context of the classical
spectral density method. Both frameworks give results in good agreement with
the exact numerical transfer-matrix data for the one-dimensional case and with
the exact high-temperature-series results for the three-dimensional one. In
particular, for the ferromagnetic chain, the zero-field susceptibility results
are found to be consistent with the exact analytical ones obtained by M.E.
Fisher. However, the formula derived in the present paper provides more
accurate predictions in a wide range of temperatures of experimental and
numerical interest.
