  We present a magnetic study of the Gd$_3$N@C$_{80}$ molecule, consisting of a
Gd-trimer via a Nitrogen atom, encapsulated in a C$_{80}$ cage. This molecular
system can be an efficient contrast agent for Magnetic Resonance Imaging (MRI)
applications. We used a low-temperature technique able to detect small magnetic
signals by placing the sample in the vicinity of an on-chip SQUID. The
technique implemented at NHMFL has the particularity to operate in high
magnetic fields of up to 7 T. The Gd$_3$N@C$_{80}$ shows a paramagnetic
behavior and we find a spin transition of the Gd$_3$N structure at 1.2 K. We
perform quantum mechanical simulations, which indicate that one of the Gd ions
changes from a $^8S_{7/2}$ state ($L=0, S=7/2$) to a $^7F_{6}$ state ($L=S=3,
J=6$), likely due to a charge transfer between the C$_{80}$ cage and the ion.
