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

Single crystals of the endohedral $\mathrm{La}@{\mathrm{C}}_{82}({\mathrm{CS}}_2{)}_{1.5}$ 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 $\left(\theta \sim -130\hspace{0.5em}\mathrm{K}\right).\mathrm{}$ 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 $\mathrm{Ce}@{\mathrm{C}}_{82}({\mathrm{CS}}_2{)}_{1.5}$ reveal evidence of similar bistability and hence we postulate that it is a common phenomena in these endohedrals.