Ferromagnetic and merohedral ordering in powder and crystalline tetrakis (dimethylamino) ethylene (TDAE)-C60

A.c. susceptometry and electron spin resonance (ESR) measurements on crystals and powders show direct evidence of coupling between spin and molecular merohedral degrees of freedom in tetrakis(dimethylamino)ethylene (TDAE)-C60 (16 < Tc < 26 K). Ordering the spins using an external field in the magnetic phase is shown to imprint a merohedral order. Conversely, merohedrally ordering the C60 molecules at high temperature influences the low-temperature magnetic transition temperature. The merohedral disorder gives rise to a distribution of π-electron exchange interactions between spins on neighbouring C60 molecules, suggesting a microscopic origin for the observed spin-glass-like behaviour in the magnetic state. ESR measurements on single crystals of TDAE-C60 show a frequency shift which is maximum when the absolute value of applied field ¦Hext¦ is along the crystallographic a axis. On the basis of the present data we suggest the material to be a ferromagnet, with the magnetic axis along a and with the dipole-dipole interactions being responsible for the large ESR line shifts and line broadening in the magnetic phase. New IR reflectivity data suggest a low-frequency excitation unobserved to date, whose origin could be a Drude peak, indicating a metallic ground state or a possible low-frequency charge density wave (CDW) excitation.