Superparamagnetic nanoclusters may be used in imaging in biomedicine and in mapping of petroleum reservoirs, by generating either ultrasonic or acoustic signals with oscillating magnetic motion. For a given magnetization per weight of iron oxide, nanoclusters with diameters from 20 to 100 nm experience a much larger magnetic force than that of the primary sub-10-nm primary particles. Aqueous dispersions of 0.1 wt % superparamagnetic iron oxide nanoclusters were stabilized with citric acid on the particle surface, with a high loading of 90% iron oxide. The dispersions were stable for months even with high salt concentrations up to 4 wt % at a pH of 6 and 8 based on the hydrodynamic diameter from dynamic light scattering. The citrate ligands provide electrostatic repulsion, as characterized by the ζ potential. The small size of the clusters, superparamagnetic properties, and high salt tolerance are highly beneficial in various applications including the mapping of petroleum reservoirs with magnetomotive tec...