Nanoparticles for use as magnetic resonance imaging contrast agents are described. The nanoparticles are made up of a polymeric support and a manganese-oxo or manganese-iron-oxo cluster having magnetic properties suitable of a contrast agent. The manganese-oxo clusters may be Mn-12 clusters, which have known characteristics of a single molecule magnet. The polymer support may form a core particle which is coated by the clusters, or the clusters may be dispersed within the polymeric agent.