Origin of ferromagnetic exchange interactions in a fullerene-organic compound

Organic ferromagnets, which exhibit exchange interactions between unpaired electrons in pi-orbitals, are rare(1-6), and the origin of ferromagnetism i n these compounds has so far remained unexplained. Tetrakis(dimethylamino)e thylene-fullerene[60] (TDAE-C-60) shows a transition to a ferromagnetic sta te with fully saturated s = 1/2 molecular spins at the relatively high Curi e temperature (for organic materials) of 16 K (ref. 4). It has been suggest ed(7-9) that the orientations of the C-60 molecules may be important for fe rromagnetism in this material, but in the absence of structural data at low temperatures there has been little progress towards understanding these mi croscopic interactions. Here we report the results of a comparative structu ral study of two different magnetic forms of TDAE-C-60 crystals at low temp eratures, correlating the structural properties-in particular, the intermol ecular orientations-with the magnetic properties. We rnd that both ferromag netism and spin-glasslike ordering are possible in this material, and depen d on the orientational state of C-60 molecules. This resolves the apparent contradictions posed by different macroscopic measurements(4,10-14), and op ens the way to a microscopic understanding of pi-electron ferromagnetic exc hange interactions in organic materials.