Effect of initial susceptibility and relaxation dynamics on radio frequency alternating magnetic field induced heating in superparamagnetic nanoparticle dispersions

[Display omitted] •SAR increased with initial susceptibility of the MNP.•Relaxation dynamics alone is not sufficient to account SAR.•Initial susceptibility decreased with increasing sample concentration.•SAR decreased with enhancement of dipolar interaction.•Neel relaxation dynamics was largely unaffected by medium viscosity. Radiofrequency alternating magnetic field (RFAMF) induced heating of superparamagnetic nanoparticle dispersions (magnetic nanofluids) have attracted wide attention due to its superior potential for various industrial and bio-medical applications. Here, we probe the role of initial susceptibility and relaxation dynamics on RFAMF induced heating efficiency of magnetic nanofluids containing oleic acid capped single domain Fe3O4 magnetic nanoparticles (MNP) with nearly similar saturation magnetization values. Our results suggest that the initial susceptibility of MNP plays a major role in RFAMF induced heating efficiency and the variation of MNP size dependant relaxation dynamics alone is not sufficient to account for the field induced heating efficiency. Calculations on quasi-static hysteresis loops revealed a decrease in initial susceptibility with increasing MNP loading due to enhanced dipolar interaction, which resulted in the observed decrease in heating efficiency with increasing MNP concentration. The effect of relaxation dynamics on SAR, probed by varying the base fluid viscosity, showed that the heating efficiency is largely unaffected with increasing medium viscosity for magnetic nanofluids with Neel dominated relaxation modes, whereas heating efficiency decreased by ∼70% for magnetic nanofluids with Brownian dominant relaxation modes. This study attempts to decouple the individual contributions of initial susceptibility and relaxation dynamics and the results obtained are beneficial for designing magnetic nanofluids with enhanced magneto-thermal conversion efficiency.