Interface controlled electrical and magnetic properties in Fe-Fe₃O₄-silica gel nanocomposites

Abstract

Iron nanoparticles with a shell of Fe₃O₄ phase with a total diameter of 5.3 nm have been grown within a silica gel matrix in the percolative configuration by suitable reduction followed by oxidation treatments. dc electrical resistivity measurements were carried out in the temperature range 80-300 K. The resistivity of the nanocomposites was found to be about 7 orders of magnitude lower than that of the reference gel. The electrical conduction has been explained on the basis of a small polaron hopping mechanism. The activation energy in the case of the composites was calculated from experimental data to be about one-fifth that for the reference sample. An interfacial amorphous phase is believed to cause such reduction in resistivity. The effective dielectric constant of this phase was estimated to be about four times that of the reference glass. Magnetization measurements on these specimens were carried out in the temperature range 5-300 K both in zero field cooled and field cooled states. A peak in the magnetization at ~120 K was ascribed to an order-disorder (Verwey) transition. Another peak at ~55 K was explained as arising due to a spin glass like disorder at the interface between the ferromagnetic iron ores and the ferrimagnetic Fe₃O₄ shell. A loop shift was observed as a result of the spin freezing below this temperature.