Low-temperature bistability in the magnetic properties of solvent-including lanthanide metallofullerene crystals

Single crystals of the endohedral La@C₈₂(CS₂)1.5 have been investigated by ESR, SQUID, and optical absorption measurements. We find that the crystal is a localized paramagnet at room temperature with evidence of strong antiferromagnetic interactions (θ~-130 K). ESR spectroscopy reveals evidence of a phase transition at 150 K, that we assign to be an orientational ordering transition of the fullerene molecules. Furthermore, at low temperatures we find a magnetic bistability that can be controlled by the cooling rate. Investigations of Ce@C₈₂(CS₂)1.5 reveal evidence of similar bistability and hence we postulate that it is a common phenomena in these endohedrals.