ROLE OF NÉEL AND BROWNIAN RELAXATION MECHANISMS FOR WATER-BASED <font>Fe₃O₄</font> NANOPARTICLE FERROFLUIDS IN HYPERTHERMIA

Abstract

Hydrophilic Fe 3 O 4 nanoparticles with a mean diameter size of 15 nm were synthesized by means of the direct reduction of FeCl 3 and FeCl 2 in an aqueous solution. The heating properties of Fe 3 O 4 nanoparticles under an alternating magnetic field were investigated in a frequency range of 50–500 kHz and a magnitude range of 3.1–5.0 kA/m. To distinguish the roles of Néel and Brownian relaxation mechanisms, polydimethylsiloxane (PDMS) and water were employed as a medium, in which PDMS plays a significant role of eliminating the effect of Brownian relaxation. It is experimentally and theoretically confirmed that, for the Fe 3 O 4 /water system, high-specific absorption rates are due to a combination of both mechanisms, with an enhanced contribution due to Néel relaxation with increasing frequency and magnitude of the alternating electromagnetic field. The contribution efficiency of Néel relaxation increases from 36–56% by increasing the electromagnetic field frequency from 50–300 kHz, and then retains a saturated value of ~ 56% at 300–500 kHz. Moreover, a linear increase of the contribution efficiency of Néel relaxation at 300 kHz was observed from 55–65% by increasing the magnitude of electromagnetic field at 3.1–5.0 kA/m. The current research can be widely-expanded to explain the electromagnetic heating effect of other ferrofluid systems.