Cation distributions and magnetism of Al-substituted CoFe2O4 - NiFe2O4 solid solutions synthesized by sol-gel auto-combustion method

Abstract Aluminium substituted cobalt-nickel ferrite nanoparticles were synthesized by citrate gel auto-combustion method followed by annealing at 1000 °C for 1 h in air. Scanning electron micrographs of all the samples show crystalline particles of irregular morphology with a small variation in particle sizes (~ 110–160 nm). From the analysis of the X-ray diffraction results we observed that the unit cell parameter decreases linearly with increase in aluminium concentration due to the smaller ionic radius of the Al 3+ ions substituting the other cations such as Co 2+ , Ni 2+ and Fe 3+ ions in the compounds. The room temperature Mossbauer spectra of the samples show Zeeman split sextet patterns corresponding to the tetrahedral (Th) and octahedral (Oh) interstitial iron (Fe 3+ ) cations. The observed magnetic hyperfine field (B hf ) decreases with increase in Al-concentration due to the distribution of diamagnetic Al 3+ in the environment of 57 Fe probe atoms. The saturation magnetization measured by Vibrating Sample Magnetometer (VSM) shows a similar trend like that of B hf . The distributions of the cations obtained from the Rietveld refinement and Mossbauer spectroscopy results indicate an increase in Fe 3+ (Th)/Fe 3+ (Oh) occupancy-ratio on increasing Al 3+ concentration, and Ni 2+ cations prefer the octahedral site, whereas Co 2+ and Al 3+ ions redistribute themselves in tetrahedral and octahedral sites, in the ratio 2:3.