Figure 1. A schematic showing the coercivity (H_c) behaviour of a magnetic particle as a function of its diameter (D).

As a particle size decreases, the domain wall disappears, resulting in an increase in H_c until the particle size reaches D_S. When the particle size is further decreased, thermal agitation energy overcomes magnetic anisotropy energy and, as a result, the particle enters the superparamagnetic regime, D_P.