Magnetization of a large single C60 crystal was measured as functions
of applied field and temperature. At T = 260 K, a discontinuity in sus
ceptibility was observed. The relative change in total moment is 1.2%,
independent of applied field to within experimental accuracy. This di
scontinuity is attributed to the change in the intramolecular geometry
at the orientational order-disorder transition. This is the first exp
erimental demonstration that the cooperative phase transition affects
the magnetic susceptibility of a single molecule.