Facile approach to suppressγ-Fe₂O₃toα-Fe₂O₃phase transition beyond 600 °C in Fe₃O₄nanoparticles

Magnetic iron oxide nanoparticles on a zeolite template have been synthesized using wet chemical approach. The average particle size initially decreases from 8.5 to 6 nm (increasing zeolite concentration from 0 to 75 mg) but increases to 11 nm for higher zeolite concentration (100 mg). Room temperature magnetization curves show an initial decrease in saturation magnetization from 62 to 42 emu per gram due to decrease in particle size as well as increase in contribution from nonmagnetic zeolite template. Further increase in zeolite concentration to 100 mg results in a significant increase in saturation magnetization from 42 to 51 emu per gram. Calorimetric studies show a continuous enhancement in γ -Fe 2 O 3 to α -Fe 2 O 3 phase transition temperature from 590 to 715 °C by increasing the zeolite concentration from 0 to 75 gm. The exothermic peak corresponding to the γ -Fe 2 O 3 to α -Fe 2 O 3 phase transition has been completely suppressed for nanoparticles prepared in presence of 100 mg of zeolite. Mössbauer spectra of as-synthesized nanoparticles show an increase of superparamagnetic components from 7 to 36% corresponding to increase in zeolite concentration from 0 to 100 mg. Mössbauer spectra of pure Fe 3 O 4 nanoparticles annealed at 500 °C shows formation of pure α -Fe 2 O 3 phase and Mössbauer spectra of particles prepared in presence of 25 mg shows only 18% of α -Fe 2 O 3 phase after annealing at 550 °C. Further increase in zeolite concentration to 50 and 75 mg (annealed at 550 °C) leads to pure γ -Fe 2 O 3 . Annealing of Fe 3 O 4 nanoparticles prepared in the presence of 100 mg of zeolite at 650 °C shows formation of only 8% α -Fe 2 O 3 phase. Our results show an easy and effective method to enhance the thermal stability of magnetic iron oxide nanoparticles making it suitable for high temperature applications.