Enhancement in maghemite to hematite phase transition temperature with very low fraction of Co (II) doping

We study the effect of cobalt (II) ion doping in Fe₃O₄ on crystal structure, magnetic properties and phase transition temperature under air and vacuum annealing. Magnetite nanoparticles are prepared by co-precipitation method with ammonium hydroxide as an alkali. Our results show that the γ- Fe₂O₃ to α-Fe₂O₃ phase transition temperature increases with Co²⁺ doping. For 0.1 fraction of Co₂+ metal ion doping in magnetite, a 100 °C enhancement in the γ-Fe₂O₃ to α-Fe₂O₃ phase transition temperature is observed in air annealed samples. The enhanced phase transition is attributed to the increased activation energy in presence of Co²⁺. The room temperature magnetization measurements for cobalt fraction x=0.1, shows an Ms value of 70.5 emu/g in as synthesized sample, which is reduced on air annealing at 500 °C, due to the weakening of A-B exchange interaction because of thermal fluctuations. The DSC results corroborate the XRD data of the increase in γ-Fe₂O₃ to α-Fe₂O₃ phase transition temperature with cobalt fraction. The enthalpy change decreases from 90 to 71 Jg^-1 as the cobalt content increased from 0 to 0.1, which clearly indicates that the degree of conversion from maghemite to hematite decreases with cobalt content. These results suggest that a very small percentage of Co²⁺ metal ion doping can dramatically enhance the thermal stability of magnetic nanoparticles, which is useful for their utility in high temperature applications.