Synthesis and Characterization of Al, Y, Co and Cr Substituted Ultrafine Powder of NiFe2O4

The ultrafine spinel NiFe_1.9M_0.1O₄ powders (M=Al, Y, Cr, Fe) were synthesized by thermal decomposition of appropriate mixtures of complex compounds with acetylacetone - (2,4 pentadione) ligands ([M(AA)_x]) at 500 °C. Samples were annealed in the temperature range 600-1000 °C and the crystallization process was followed by XRD. The largest crystallite size and the highest crystallization rate are observed for NiFe_1.9Al_0.1O₄, while the opposite was found for NiFe_1.9Y_0.1O₄. Strain value in NiFe_1.9Y_0.1O₄ changes sign at temperatures between 700-800 °C, while for NiFe_1.9M_0.1O₄ (M=Al, Cr) it has a positive sign and reaches the maximum value at 700 °C. The crystal structure of the samples annealed at 1000 °C was refined using the Rietveld profile method. Cation substitution in Ni-ferrite changes its structural and microstructural parameters, and can also significantly influence other physical properties.