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 T_c=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 orientational state of C₆₀ molecules. Thus we have resolved the apparent contradictions posed by different macroscopic measurements and have opened the way to a microscopic understanding of p-electron FM exchange interactions.

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Дата начала обработки статистических данных - 27 января 2016 г.