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 in these compounds has so far remained unexplained. Tetrakis(dimethylamino)ethylene-fullerene[60] (TDAE-C-60) shows a transition to a ferromagnetic state with fully saturated s = 1/2 molecular spins at the relatively high Curie temperature (for organic materials) of 16 K (ref. 4). It has been suggested(7-9) that the orientations of the C-60 molecules may be important for ferromagnetism in this material, but in the absence of structural data at low temperatures there has been little progress towards understanding these microscopic interactions. Here we report the results of a comparative structural study of two different magnetic forms of TDAE-C-60 crystals at low temperatures, correlating the structural properties-in particular, the intermolecular orientations-with the magnetic properties. We rnd that both ferromagnetism and spin-glasslike ordering are possible in this material, and depend on the orientational state of C-60 molecules. This resolves the apparent contradictions posed by different macroscopic measurements(4,10-14), and opens the way to a microscopic understanding of pi-electron ferromagnetic exchange interactions in organic materials.