Magnetic ordering in fullerene charge-transfer complexes

We have determined the ground states of the charge-transfer (CT) complexes in which the energy levels of the highest occupied molecular orbital (HOMO) of donors and the lowest unoccupied MO (LUMO) of acceptors are closely located, and examined some fullerene complexes consisting of C${}_{60},$ C${}_{70},$ tetrakis(dimethylamino)ethylene (TDAE), and 1,1’,3,3’-tetramethyl-${\Delta }^{{2,2}^{\prime }}$-bi(imidazolidine) (TMBI). The observed magnetic properties of TDAE-C${}_{60},$ TMBI-C${}_{60},$ and TDAE-C${}_{70}$ can be accounted for by employing realistic parameters. The effective Hamiltonian including up to the fourth-order perturbation has also been derived in the fourfold degenerate model space. The effective Hamiltonian can plausibly reproduce the magnetic phase diagram obtained by the variational treatment of TDAE-C${}_{60}.$ It has been shown that the third and the fourth processes contribute to the stabilization of the antiferromagnetic state.