Singlet-triplet energy gap in the non-ferromagnetic

The ESR intensity of the $\alpha'$ modification of TDAE-$\rm C_{60}$ vanishes below 20 K in contrast to the α modification where the ESR intensity tremendously increases due to the ferromagnetic transition. The proton NMR spectra of the $\alpha'$ modification show a single temperature independent NMR line close to the Larmor frequency. Both the ESR and the NMR results can be understood by the existence of S=0 spin pairing in the ground state of the non-ferromagnetic $\alpha'$ modification of TDEA-$\rm C_{60}$ at low temperatures

Transition to inhomogeneous ferromagnetic state in

\chem{{}^{13}C} NMR spectra of a 40% \chem{{}^{13}C}-enriched well-annealed \chem{TDAE}-\chem{C_{60}} powder sample show a huge increase in the \chem{{}^{13}C} NMR linewidth, accompanied by an increase in \chem{{}^{13}C} spin-lattice relaxation time T1, on cooling below T'=10 \un{K}. On the other hand, the \chem{TDAE} methyl proton NMR spectra of a well-annealed \chem{TDAE}-\chem{C_{60}} single crystal at 6.34\un{T} show, in the ferromagnetic phase between T_\ab{c}=16 \un{K} and T'=10 \un{K}, the presence of two broad inhomogeneous lines with their centers shifted by 2.5\un{MHz} and 0.5\un{MHz}, respectively, from the proton Larmor frequency. The intensity of the strongly shifted proton NMR line, which dominates the \chem{{}^{1}H} spectra between T_\ab{c} and T' in the ferromagnetic phase, continuously decreases with decreasing temperature and becomes suddenly very small below T'. This indicates a dramatic decrease of the unpaired spin density at one half of the \chem{TDAE} proton sites and could result from the near disappearance of one of the two alternating ferromagnetic \chem{C_{60}^{-}} orientations below T' as suggested by the structural model of Narymbetov et al. involving increased \chem{C_{60}^{-}} orientational disorder at low temperatures