The magnetization dynamics involved in applying an alternating field are composed of a superposition of Néel and Brownian relaxations. To evaluate the mechanisms of magnetic relaxations, it is necessary to individually evaluate the Néel and Brownian regimes. In this study, by applying a fast responding pulse field, the two-step magnetization response of magnetic nanoparticles dispersed in a fluid in the Brownian regime occurred after the Néel regime. We isolated Néel and Brownian relaxations from an experimentally observed superposition relaxation system by fitting the theoretical calculation to the measured time evolution response of the magnetization, which was in agreement with the susceptibility that was measured through applying an alternating magnetic field. The dependence of Néel and Brownian relaxation dominancy in the superposition system on relaxation times was clearly observed. In particular, the effect of dipole interactions on Néel and Brownian relaxation times were confirmed by changing the particle concentration in a magnetic fluid. Elucidating the transition process from Néel to Brownian relaxation, which is influenced by the proportion between Néel and Brownian relaxation times and the effect of dipole interactions on the individual relaxation processes, allows us to develop optimal magnetic nanoparticles for a variety of biomedical applications.

Self-heating temperature rising characteristics, cytotoxicity, and magnetic properties of $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles have been investigated to confirm the effectiveness as an in vivo hyperthermia agent in biomedicine. $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles showed both superparamagnetic and ferrimagnetic behaviors depending on particle sizes. The quantitative cytotoxicity test verified that both uncoated and chitosan-coated $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles had noncytotoxicity. The solid state $35\mathrm{nm}$ size $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles first exhibited a maximum self-heating temperature of $44.2°\mathrm{C}$ at $H_{0}f=5.1\times {10}^8\mathrm{A}{\mathrm{m}}^{-1}{\mathrm{s}}^{-1}$ . The physical nature of the self-heating was primarily thought to be due to the magnetic hysteresis loss, Neel rotations, and Brownian rotations of $35\mathrm{nm}$ size $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles.

Magnetic particle dipole interaction was revealed as a crucial physical parameter to be considered in optimizing the ac magnetically induced heating characteristics of magnetic nanoparticles. The ac heating temperature of soft $M{\text{Fe}}_2{\text{O}}_4$ $\left(M=\text{Mg},\text{Ni}\right)$ nanoparticles was remarkably increased from 17.6 to $94.7°\text{C}$ $\left({\text{MgFe}}_2{\text{O}}_4\right)$ and from 13.1 to $103.1°\text{C}$ $\left({\text{NiFe}}_2{\text{O}}_4\right)$ by increasing the particle dipole interaction energy at fixed ac magnetic field of 140 Oe and frequency of 110 kHz. The increase in “magnetic hysteresis loss” that resulted from the particle dipole interaction was the main physical reason for the significant improvement of ac heating characteristics.