NMR study of orientational ordering in C60 and the magnetic transition in TDAE-C60

The nature of the orientational ordering transition at Tc = 256 K in C60 as well as the microscopic nature of the magnetic ordering transition at TM 16.1 K in tetrakis (dimethylamino) ethylene-C60 (TDAE-C60) have been studied by 13C and proton NMR. Whereas the currently accepted model of the structural phase transition in C60 assumes that we deal with isotropic rotational diffusion of the C60 molecules above Tc and with uniaxial jump reorientations between two nearly degenerate orientations - separated by 60 -below Tc, our data clearly show that this is not true: The reorientational motion below Tc in C60 is quasi-isotropic rather than uniaxial and C60 is significantly more disordered than assumed so far. The temperature dependence of the paramagnetic respectively ferromagnetic shifts of the two proton NMR lines in TDAE-C60 demonstrate the existence of a magnetic ordering transition in this system around 15 K. (orig.)