Evidence for S=0-type spin pairing in the nonferromagnetic modification ofTDAE-C-60

The X-band electron spin resonance (ESR) spectra, the methyl-proton H-1 nuc lear magnetic resonance spectra, and the proton spin-lattice relaxation rat e T-1(-1) of the nonferromagnetic modification of tetrakis-dimethylamino-et hilene (TDAE)-C-60 have been studied between room temperature and 4 K. The results are compared with those previously obtained for the ferromagnetic m odification of TDAE-C-60. The nonferromagnetic modification of TDAE-C-60 sh ows only a single proton line and an X-band ESR intensity that vanishes bel ow 10 K. The temperature dependencies of the proton spin-lattice relaxation rate and the ESR intensity are characteristic for a system with a S= 0 gro und state and a nonzero energy gap between the ground state and the first e xcited triplet state. The singlet-triplet energy gap is estimated to be of the order of 15 K at T=5 K. The nonferromagnetic modification seems to be m etastable and spontaneously transforms to a ferromagnetic modification at r oom temperature or above. [S0163-1829(99)01507-6].