The ESR spectra of a ${\mathrm{TDAE}}^{+}-C_{60}^{-}$ single crystal show the existence of antiferromagnetic (AFM) correlations between the unpaired spins on neighboring $C_{60}^{-}$ ions along the c axis and spin canting which leads, below $T_c$, to weak ferromagnetism along a direction perpendicular to the c axis. This suggests that a Dzyaloshinsky-Moriya type mechanism is responsible for ferromagnetic ordering rather than itinerant ferromagnetism. The incomplete orientational ordering of the $C_{60}^{-}$ ions leads to a distribution of exchange coupling constants resulting in spin-glass type behavior which coexists with long range ferromagnetic ordering below $T_c$. The observation of AFM correlations may help to understand why the same basic molecule ${\mathrm{C}}_{60}$ can support such different phenomena as ferromagnetism (in TDAE-${\mathrm{C}}_{60}$) and superconductivity (in, e.g., ${\mathrm{K}}_3$${\mathrm{C}}_{60}$).

• Received 14 August 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.523