Figure 1.

Illustration of the critical size effect of superparamagnetic nanoparticles. As particle size is reduced below the critical size, superparamagnetic (SPM, solid red line) nanoparticles are only magnetized in response to an externally applied magnetic field. In comparison, ferromagnetic (FM, dashed lines) nanoparticles are permanently magnetized. H represents the applied magnetic field strength, and M is the measured magnetization. Reproduced with permission from ²². Copyright 2010, IOP Publishing.