  We show that a chiral nonlinear sigma model coupled to degenerate neutrons
exhibits a ferromagnetic phase at high density. The magnetization is due to the
axial anomaly acting on the parallel layers of neutral pion domain walls
spontaneously formed at high density. The emergent magnetic field would reach
the QCD scale ~ 10^19 [G], which suggests that the quantum anomaly can be a
microscopic origin of the magnetars (highly magnetized neutron stars).
