Is the ground state of AC60 (A = Rb, Cs) antiferromagnetic?

Electron paramagnetic resonance measurements in the metallic and the low-temperature orthorhombic phases of RbC₆₀ and CsC₆₀ powder samples and a RbC₆₀ powder sample aligned by uniaxial pressure have been performed at 34 and 94 GHz. A detailed analysis of the low-temperature signal of all samples allows the assignment of several line components to paramagnetic defects, leaving one broad line component unaccounted for. Although the behavior of this component gives strong indications of magnetic correlations, we do not find typical characteristics of antiferromagnetic resonance that have been reported by other groups at higher fields. We ascribe this to a high degree of structural disorder in the samples which inhibits the development of a long-range antiferromagnetic order. Therefore, we interpret the signal behavior as being due to magnetically coupled spin clusters.