Proton NMR in a TDAE-C-60 single crystal

The temperature dependence of the methyl proton NMR shift of a TDAE-C-60 single crystal follows a Curie-Weiss law above T-C with a positive Curie temperature T-C approximate to 12 K as expected for the case of a ferromagnetic transition. Below T-C the shifts are proportional to the magnetization and saturate below 10 K. The internal field at the TDAE protons amounts at 6 K to 600 G and exceeds by an order of magnitude the internal field determined by electron-spin resonance at the C-60(-) ions. The result shows that in addition to the direct C-60(-)-C-60(-) interactions which seem to be of antiferromagnetic nature, a part of the exchange coupling between the C-60(-) ions is mediated by the TDAE(+) groups. It is this indirect coupling which might give rise to the Dzyaloshinsky-Moriya term in the spin Hamiltonian which is responsible for weak ferromagnetism in this system.