Colloidal iron nanoparticles with a core size of 10.6 nm were examined using magnetic force microscopy. Surprisingly, single nanoparticles were more prominently visible in magnetic force gradient images than clusters. A simple qualitative model is proposed to explain this observation, speculating that the local field produced by a cluster of particles may be sufficient to align their moments in the plane of the cluster, even though the particles are superparamagnetic. An alternative possibility of spin glass formation within clusters is also considered. Calculations performed with nanoparticles represented as single dipoles appear to match experimental data quite well.
Pedreschi, F., Sturm, JM., O'Mahony, JD., & Flipse, C. F. J. (2003). Magnetic force microscopy and simulations of colloidal iron nanoparticles. Journal of Applied Physics, 94(5), 3446-3450. https://doi.org/10.1063/1.1593219