The magnetic relaxation of magnetic nanoparticles (MNPs) has been used as a potential heating agent for magnetic hyperthermia treatment (MHT). This requires an understanding of the heating mechanism of MNPs, such as Néel relaxation; however, few studies about magnetic relaxation using a low-frequency AC magnetic field have been reported. This study attempts to clarify the correlation between the dominance of Néel relaxation in low-frequency AC fields and the magnetic properties. Nanoparticles of Ni0.8Zn0.2Fe2O4 coated with poly(ethylene glycol) (PEG) were synthesized in various sizes (d = 12, 15, and 19 nm), and were subjected to structural analysis, PEG modification, and magnetic measurements. The PEG400 coating results in a hydrodynamic diameter ten times smaller than that of our previous sample. The heat generation experiment was conducted on samples suspended in solvents of different viscosities in the presence of an AC field (h = 3.2 kAm -1 , f = 90 kHz). The specific absorption rate (SAR) as a function of the viscosity of the 15-nm NP sample is consistent with the theoretically calculated value in cases where the Néel relaxation is dominant. Therefore, we conclude that the Néel relaxation dominates the heating mechanism of the 15nm sample. Rather than being fully superparamagnetic, this sample was partly superparamagnetic and slightly ferromagnetic, with the dominance of the Néel relaxation to a certain degree affected by spin blocking. Detailed analysis of the magnetic relaxation is crucial to improve the heating efficiency of MNPs for MHT.

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Co1‐xMgxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) nanoparticles embedded in amorphous SiO2 with particle sizes of approximately 4.5 nm were prepared using an original wet chemical method. We performed spin‐echo magnetic resonance imaging measurements for Co1‐xMgxFe2O4 nanoparticles using a 0.3‐T MRI system. The particles exhibited a significant T2 shortening effect compared with that of agarose, depending on the composition. All particles exhibited effective relaxivity, R2, and a significant contrast was observed in the phantom image. Furthermore, thiol group was modified to enable particles to bind specifically to maleimide proteins. These particles are expected to be potential theranostic agents.

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