Magnetic nanoparticles with enhanced λ-Fe₂O₃ to α-Fe₂O₃ phase transition temperature

Abstract

We report a simple method for producing magnetic nanoparticles with enhanced maghemite (λ-Fe₂O₃) to haematite (α-Fe₂O₃) phase transition temperature. By controlling the properties of the solvent media, we have been able to tune the particle size from 2.3 to 6.5 nm. Nanoparticles with higher transition temperatures have been achieved by using NaOH as an alkali during the co-precipitation. The maghemite to haematite phase transition was complete at a temperature below 600°C for nanoparticles prepared using ammonia, whereas the phase transition was not complete until 750°C for the samples prepared with NaOH. The increase in average particle size after heat treatment at 600°C is attributed to coalescence of particles by solid state diffusion, where the system reduces its free energy by reducing the surface area. The final particle diameters of the haematite after the heat treatment were 35.4, 31.38 and 26.85 nm respectively for nanoparticles of initial diameters 5, 7 and 9.8 nm. Our studies on the effect of the initial particle size on the transition temperature show that the transition temperature decreases with decreasing particle size due to the reduced activation energy of the system.