FERROMAGNETIC-RESONANCE IN TDAE-C-60

A nonlinear variation of the electron resonance frequency with resonan ce field characteristic of ferromagnetic resonance has been observed i n a TDAE-C-60 in the radio-frequency region. The results not only defi nitely prove the existence of long range order below Te but also show that well annealed TDAE-C-60 behaves as an easy axis three dimensional Heisenberg ferromagnet with an exceptionally small anisotropy field H -K = 29 G. The easy axis coincides with the crystal c direction which is the direction of closest approach of the C-60(-) ions.