  Phase transitions in spinor Bose gases with ferromagnetic (FM) couplings are
studied via mean-field theory. We show that an infinitesimal value of the
coupling can induce a FM phase transition at a finite temperature always above
the critical temperature of Bose-Einstein condensation. This contrasts sharply
with the case of Fermi gases, in which the Stoner coupling $I_s$ can not lead
to a FM phase transition unless it is larger than a threshold value $I_0$. The
FM coupling also increases the critical temperatures of both the ferromagnetic
transition and the Bose-Einstein condensation.
