Polaronic Relaxation and Variable-Range-Hopping Conductivity in Fe3O4 Nanoparticles

Abstract

We report the polaronic relaxation and Variable Range Hopping (VRH) conductivity in 100nm average particle size cold pressed Fe3O4 nanoparticles. The crystal structure was studied using XRD showing spinel structure with (311) intense peak. The AC transport in the cold pressed powder was studied from 10 K to room temperature and from 100 Hz to 5.5 MHz. It was observed that the solid couples capacitively with the perturbing electric field initially and as the frequency increases it becomes more inductive. The impedance data was modelled using modified Havriliak–Negami relaxation model. It was observed that the cold pressing introduces an asymmetric broadening in the relaxation process. The change in impedance with temperature showed a drastic drop at around 120 K due to Verwey transition in real as well as imaginary part. The change in DC resistance with temperature indicated VRH mechanism of charge transport in the solid with a metal insulator transition at around 120 K. The relaxation times calculated indicated that the hopping process has an attempt frequency of the order of 30 microseconds. The activation energy change across the phase transition with relaxation time and resistance indicated that the charge transport is coherent. The activation energy of ∼0.05eV above Varway transition indicated that the transport is small polaronic in nature.