Effect of viscosity on the AC magnetization of magnetic nanoparticles under different AC excitation fields

For in vivo applications of magnetic nanoparticles (MNPs), environment viscosity is one of the most important parameters that determines their AC magnetization. In this study, we investigate the effect of viscosity on the AC magnetization of MNPs under different AC excitation field conditions. We show that the AC M - H curve and harmonic magnetization spectrum strongly depend on the viscosity for a small excitation field intensity and low excitation frequency, while they are insensitive to the viscosity for a large excitation field intensity and high excitation frequency. We then show that the difference in the AC magnetization between these cases can be qualitatively explained by taking the field dependent Brownian f(B)(H-ac) and Neel f(N)(H-ac) frequencies into account. The frequencies are obtained from numerical simulations based on the Fokker-Planck equations. Finally, we show that an excitation field with a relatively large intensity and a frequency f that satisfies f(B)(H-ac) << f <= f(N)(H-ac) is suitable for the magnetic fluid hyperthermia (MFH) application.