Self-Heating Property of Magnetite Nanoparticles Dispersed in Solution

The magnetic characterization of polyethyleneimine (PEI)-coated magnetite (Fe₃O₄) nanoparticles having diameters of 20-30 nm that were dispersed in a solution was performed, and their self-heating property was investigated. The hydrodynamic diameter of PEI-coated Fe₃O₄ nanoparticles was approximately 155 nm on an average. To investigate the self-heating property, the increase in the temperature of a sample heated by an applied ac magnetic field was measured, and the specific loss power (SLP) was calculated. The effect of magnetic relaxation loss was estimated by determining the dependence of the SLP on the frequency of an applied magnetic field. For the magnetic characterization of the nanoparticles, dc and ac magnetization curves of the sample were measured and compared with each other to elucidate the heating mechanism. Uncoated Fe₃O₄ nanoparticles having diameters of 20-30 nm exhibited ferromagnetic characteristics in dry condition. However, the PEI-coated Fe₃O₄ nanoparticles dispersed in a solution did not exhibit hysteresis of the dc magnetization curve because the particles could easily rotate with the changing magnetic field. The magnetization curve measured under an ac magnetic field had a large area compared to the dc magnetization curve. The results indicate that Brownian relaxation is dominant during magnetization reversal.