Antiferromagnetic Resonance in the Linear Chain Conducting Polymers ${\mathrm{RbC}}_{60}$ and ${\mathrm{CsC}}_{60}$

Antiferromagnetic resonance has been observed in powders of the conducting alkali fulleride linear polymers, ${\mathrm{RbC}}_{60}$ and ${\mathrm{CsC}}_{60}$, at high frequencies (75, 150, and 225 GHz). This is proof for an antiferromagnetically ordered ground state and shows that these systems are not spin glasses. The sublattice magnetization is independent of applied magnetic field up to at least 8 T. Magnetic fluctuations are observed between 35 and 50 K. Comparison with the spin-density-wave system $(\mathrm{TMTSeF}{)}_2{\mathrm{PF}}_6$ clearly shows that these polymers are also quasi-1D spin-density-wave systems with 3D ordering at low temperatures, as suggested previously.