Study of Heating Efficiency as a Function of Concentration, Size, and Applied Field in γ-Fe₂O₃ Nanoparticles

The specific absorption rate (SAR) of γ-Fe 2 O 3 nanoparticles (NPs) under an alternating magnetic field has been investigated as a function of size, concentration, coating, liquid carrier, and frequency and amplitude of the applied magnetic field. The NPs have been synthesized by coprecipitation method with sizes ranging from 6 to 14 nm with low polydispersity (0.2) and high crystallinity degrees. The small NPs size (6–14 nm) and the value of the maximum applied field (<7.5 kA/m) allow the use of the linear response theory for the analysis of the experimental SARs values. Under this condition, Neel–Brown relaxation times of about 10 –7 s are obtained from SAR field frequency dependence. The NPs have been immobilized in agar to investigate the heating mechanisms, i.e., inversion of the magnetic moments inside the monodomain volume or particle rotation. The results suggest that there is a critical size of around 12 nm for obtaining the most effective heating in viscous media. Furthermore, the surface modification by aminopropylsilane coating does not affect the heating efficiency, making these NPs good candidates for hyperthermia treatment as well as model samples for standardization of hyperthermia apparatus.