High-field ESR in TDAE-C-60

The ESR spectra of TDAE-C-60 single crystals have been measured between 30 MHz and 245 GHz both above and below T-c= 16 K. No separate TDAE(+) ESR sig nal has been seen even at high fields suggesting that the TDAE(+) and C-60( -) spins are either strongly exchange coupled or that spin cancellation and charge separation of the TDAE ions take place. The nonlinear variation of the resonance frequency versus resonance field relation characteristic of f erromagnetic resonance has been observed only in the radio frequency but no t in the microwave region. No second "hard" mode characteristic for a spin- canted "weak" ferromagnet could be observed between 0 and 100 kG. The addit ional fine structure seen in the high-field ESR spectra below T-c is due to standing spin-wave resonance. It allows for the determination of the excha nge coupling constant J = 60 K. The exchange field H-E =448 kG is thus much larger than the anisotropy field H-K= 29 G determined from the ferromagnet ic resonance data. The temperature dependence of the magnetization determin ed from the spin-wave resonance data can be indeed described by the Bloch T -3/2 law as expected for an isotropic or nearly isotropic Heisenberg ferrom agnet. [S0163-1829(98)02445-X].