Sup 13]C NMR of molecular configurations leading to ferromagnetic exchange in TDAE-C[sub 60

Abstract

The temperature dependence of the 13C NMR spectra of a 40% 13C enriched well annealed TDAE-C60 sample has been measured at a Larmor frequency vL=95.57 MHz in a field of 9 T between 300 K and 7 K. Three transitions are found: the motional transition around 150 K, the ferromagnetic transition which occurs in a field of 9 T at Tc=24 K, and a transition to an inhomogeneous ferromagnetic phase around 9 K. The 13C spectra are dominated by the Fermi contact interaction between the unpaired electron and the 13C nuclei as well as by the electron-nuclear dipolar interactions. The temperature dependence of the observed lineshapes can be understood by a superposition of uniaxial rotations of the C60− ions around their 3-fold axes plus a flipping of the axes of rotation. The flipping starts to freeze out below the motional transition at 150 K whereas uniaxial rotational jumps seem to persist down to the lowest temperature studied. The results are compatible with the C60− orientational disorder proposed by Narymbetov e...