  The phase diagram of the Heisenberg ferromagnetic model in the presence of a
magnetic random field (we have used bimodal distribution) of spin S=1/2
(quantum case) and $S=\infty $ (classical case) on a simple cubic lattice is
studied within the framework of the effective-field theory in finite cluster
(we have chosen N=2 spins). Integrating out the part of order parameter
(equation of state), we obtained an effective Landau expansion for the free
energy written in terms of the order parameter $\Psi (m)$. Using Maxwell
construction we have obtained the phase diagram in the $T-H$ plane for all
interval of field. The first-order transition temperature is calculated by the
discontinuity of the magnetization at $T_{c}^{\ast}(H)$, on the other hand in
the continuous transition the magnetization is null at $T=T_{c}(H)$. At null
temperature (T=0) we have found the \textbf{coexistence} field $H_{c}=3.23J$
that is independent of spin value. The transition temperature $T_{c}(H)$ for
the classical case ($S=\infty $), in the $T-H$ plane, is larger than the
quantum case (S=1/2).
