The role of Ag on dynamics of superspins in MnFe2−x Ag x O4 nanoparticles

The effect of dipole–dipole and exchange interactions on dynamics of superspins in a system of MnFe2−x Ag x O4 (where x = 0, 0.1, 0.2, 0.3, and 0.6) nanoparticles has been studied by Ac magnetic susceptibility measurements. Average crystallite size of samples was estimated to be ~7 to ~4 nm, with respect to the doping level of x = 0–0.6. It was found that the nanoparticles are superparamagnetic at room temperature with almost zero coercivity. Saturation magnetization of samples showed a remarkable reduction by increasing non-magnetic Ag doping level. By decreasing the temperature, a transition to frustrated superspin glass state was observed in all samples. Freezing temperatures of superspins were decreased by increasing the Ag content, as a result of decreasing size of crystallites, magnetization of nanoparticles, and consequently weakening of dipole–dipole interactions. The estimated values of zυ, τ 0, and T 0, using critical slowing down model, justify the observed variation of freezing temperatures. Furthermore it was realized that sensitivity of samples to the variation of applied frequency, an important parameter in hyperthermia based therapy, is affected by magnetic interactions between nanoparticles.