The temperature dependence of the methyl proton NMR shift of a TDAE-${\mathrm{C}}_{60}$ single crystal follows a Curie-Weiss law above $T_C$ with a positive Curie temperature $T_C\approx 12$ K as expected for the case of a ferro-magnetic 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 ${\mathrm{C}}_{60}^{-}$ ions. The result shows that in addition to the direct ${\mathrm{C}}_{60}^{-}$-${\mathrm{C}}_{60}^{-}$ interactions which seem to be of antiferromagnetic nature, a part of the exchange coupling between the ${\mathrm{C}}_{60}^{-}$ ions is mediated by the TDA${\mathrm{E}}^{+}$ 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.

• Received 12 September 1995

DOI:https://doi.org/10.1103/PhysRevB.53.14028