High field ESR study of TDAE-C60 powder at different Larmor frequencies

SQUID magnetization measurements at low magnetic fields were performed on a freshly prepared and an aged sample of TDAE-C-60. The ferromagnetic phase transition temperature T-c was 16 K in the freshly prepared sample, confirming previous results, whereas T-c = 5 K was measured for the aged sample. ESR spectra of the same samples were recorded between room temperature and 5 K at Larmor frequencies of about 110, 180, 280, 380 GHz. For both samples below 60 K the spectrum shows a selective downshift of the resonant fields which is more pronounced in the lower part of the spectrum profile. The results were qualitatively explained by the theory developed to account for the one-dimensional short-range order effects between spins with inequivalent g tensors. Specifically, the observed negative field-shifts have been attributed to short-range correlation effects induced by anisotropic exchange and dipolar interaction fluctuations between inequivalent spins on TDAE(+) and C-60(-) centers.