Metal-insulator type transition in aluminium and chromium co-substituted nickel ferrites

Polycrystalline Al and Cr co-substituted disordered spinel series NiAlxCrxFe2-2xO4 with varying Al-Cr content (0.1 less than or equal to x less than or equal to 0.9) have been prepared in the pellet form and studied by X-ray diffraction, magnetization and electrical resistivity measurements. The lattice constants are determined and the applicability of Vegard's law has been tested. Samples with x greater than or equal to 0.5 exhibit a relatively sharp cusp in the electrical resistivity vs, temperature curve at T = T-max identified as metal-insulator transition showing metallic behaviour for T < T-max and insulating behaviour for T > T-max. Metallic conductivity and ferrimagnetism occur in these x = 0.5 to 0.9 samples mainly due to Cr spins and 3d transition metal electron correlation. (C) 1999 Elsevier Science B.V. All rights reserved.