Influence of the Polarizing Magnetic Field and Volume Fraction of Nanoparticles in a Ferrofluid on the Specific Absorption Rate (SAR) in the Microwave Range

For the study, we used four kerosene-based ferrofluid samples containing magnetite nanoparticles stabilized with oleic acid. Starting from the initial sample (A0), the other three samples were obtained by dilution with kerosene. The complex magnetic permeability measurements were performed in the microwave region (0.5–6) GHz, for different H values of the polarizing magnetic field, between (0–115) kA/m. These measurements revealed the ferromagnetic resonance phenomenon for each sample, allowing the determination of the anisotropy field (HA) and the effective anisotropy constant (Keff) of nanoparticles, depending on the volume fraction of particles (φ). At the same time, the measurements allowed the determination of the specific magnetic loss power (pm), effective heating rate (HReff), intrinsic loss power (ILP), and specific absorption rate (SAR) as functions of the frequency (f) and magnetic field (H), of all investigated samples, using newly proposed equations for their calculation. For the first time, this study evaluates the maximum limit of the applied polarizing magnetic field (Hmax ≈ 80 kA/m) and the minimum limit volume fraction of nanoparticles (φmin ≈ 3.5%) at which microwave heating of the ferrofluid remains efficient. At the same time, the results obtained show that the temperature increase of the ferrofluid samples, upon interaction with a microwave field, can be controlled by varying both H and φ, pointing to possible applications in magnetic hyperthermia.