Figure 9.

Frequency-dependent AC susceptibility spectra of liquid dispersions (“liquid”) and poly(acrylic acid) hydrogels (“gel”) incorporating: (a) 1.6 wt% of magnetite (S) nanoparticles, (b) 2 wt% of magnetite (L) nanoparticles, and (c) 2 wt% of cobalt ferrite nanoparticles. After immobilization of the nanoparticles in a gel, the rapid Néel relaxation of the small particles continues (a), except for a small fraction of larger particles. The magnetic relaxation of the larger magnetite particles (b) and cobalt ferrite (c) is almost fully quenched after incorporation in hydrogel because no Néel relaxation occurs and the particles are immobilized inside the polymer network.