We present a magnetic study of the Gd3N@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 the National High Magnetic Field Laboratory has the particularity of being able to operate in high magnetic fields of up to 7 T. The Gd3N@C80 shows a paramagnetic behavior and we find a spin transition of the Gd3N structure at 1.2 K. We perform quantum mechanical simulations, which indicate that one of the Gd ions changes from a S-8(7/2) state (L-0, S-7/2) to a F-7(6) state (L-S-3, J-6), likely due to a charge transfer between the C-80 cage and the ion. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3536514]