C_(60) Molecular COnfigurations Leading to Ferromagnetic Exchange Interactions in TDAE~*C_(60)

摘要

The charge-transfer salt tetrabis(dimethylamino)ethylene-fullerene (C_(60)), or TDAE~*C_(60), 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 over the past ten years, the origin of the ferromagnetism in TDAE~*C_(60) has remained a mystery. To resolve this problem, we performed a comparative structural study of two different magnetic forms of TDAE~*C_(60) crystals, one of which is magnetic and the other is nonmagnetic at low temperatures, and fully correlated their structural properties (particularly, the intermolecular orientations) with their magnetic properties. We identified the relative orientation of C_(60) molecules along the c axis as the primary variable that controls the ferromagnetic order parameter and showed that both FM and low-temperature spin-glass-like ordering are possible in this material, depending on the orientational state of the C_(60) molecules. Thus, we resolved the apparent contradictions intrinsic to different macroscopic measurements and opened a path to a microscopic understanding of p-electron FM exchange interactions.