Ferromagnetic Resonance in TDAE-C60

Abstract

A nonlinear variation of the electron resonance frequency with resonance field characteristic of ferromagnetic resonance has been observed in a TDAE- ${\mathrm{C}}_{60}$ in the radio-frequency region. The results not only definitely prove the existence of long range order below $T_C$ but also show that well annealed TDAE- ${\mathrm{C}}_{60}$ behaves as an easy axis three dimensional Heisenberg ferromagnet with an exceptionally small anisotropy field $H_K=29\mathrm{G}$. The easy axis coincides with the crystal $c$ direction which is the direction of closest approach of the ${\mathrm{C}}_{60}^{-}$ ions.