First order ferromagnetic phase transition in the low electronic density   regime of a biased graphene bilayer
  The phase diagram of a biased graphene bilayer is computed and the existence of a ferromagnetic phase is discussed both in the critical on-site interaction $U_{c}$ versus doping density and versus temperature. 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. We give evidence for a \emph{first-order} phase transition between paramagnetic and ferromagnetic phases induced by doping at zero temperature. 
T. Stauber, Eduardo V. Castro, N. A. P. Silva, and N. M. R. Peres