C60 molecular configurations leading to ferromagnetic exchange interactions in TDAE-C60

The charge-transfer salt tetrakis(demethylamino)ethylene-fullerene [C60] or TDAE-C60 is a rare exception among pure organic crystalline systems because it shows a transition to a ferromagnetic (FM) state with fully saturated s=1/2 molecular spins at a respectable Tc=16 K. In spite of extensive experimental and theoretical work in the last ten years, the origin of the ferromagnetism in TDAE-C60 has remained mysterious. To resolve this problem we have performed a comparative structural study of two different magnetic forms of TDAE-C60 crystals, one being magnetic and the other nonmagnetic, at low temperatures, fully correlating the structural properties - and particularly the inter-molecular orientations-with the magnetic properties. We have identified the relative orientations of C60 molecules along the c-axis as the primary variable controlling the ferromagnetic order parameter and have shown that both FM and low-temperature spin-glass-like ordering are possible in this material, depending on the orientat...