  We compute the phase diagram of a biased graphene bilayer. The existence of a
ferromagnetic phase is discussed with respect both to carrier density and
temperature. We find that the ferromagnetic transition is first order, lowering
the value of $U$ relatively to the usual Stoner criterion. We show that in the
ferromagnetic phase the two planes have unequal magnetization and that the
electronic density is hole like in one plane and electron like in the other.
