Flat-band ferromagnetism proposed for an organic polymer crystal

Motivated from the flat-band ferromagnetism conceived theoretically for a single chain of five membered rings (polyaminotriazole) by Arita {\it et al.}, [Phys. Rev. Lett. {\bf 88}, 127202 (2002)], we have studied whether the magnetism can indeed occur as a bulk, i.e., in a three-dimensional crystal of the polymer, by means of the spin density functional calculation. We find that the intra-chain ferromagnetism is robust against crystallization as far as the flat band is made half-filled. We have further investigated the actual crystal doped with various compounds, where HF$_2$ is shown to put the system close to the bulk ferromagnetism while stronger anions such as BF$_4$ or PF$_6$ should be promising.