  We present a theoretical investigation of the magnetic properties exhibited
by twisted bilayer graphene systems with small twist angles, where the
appearance of flat minibands strongly enhances electron-electron interaction
effects. We consider a tight-binding Hamiltonian combined with a Hubbard
mean-field interaction term and employ a real-space recursion technique to
self-consistently calculate the system's local density of states. The $O({\cal
N})$ efficiency of the recursion method makes it possible to address
superlattices of very large size by means of a full real-space analysis. Our
procedure is validated by comparison with literature momentum-space
calculations that find a magnetic phase in charge-neutral twisted bilayer
graphene. We use our method to investigate the properties of small-angle
twisted bilayer graphene at three-quarters filling of the conduction miniband.
Our calculations indicate the emergence of a ferromagnetic phase that can be
understood in terms of the Stoner mechanism, in line with recent experimental
observations.
