Influence of Al-substitution on structural, electrical and magnetic properties of Mn-Zn ferrites nanopowders prepared via the sol-gel auto-combustion method

A series of nano-sized Al-substituted manganese zinc ferrites, Mn0.8Zn0.2Fe2-xAlxO4 (x = 0.0-1.0), with a cubic spinel crystal structure were prepared through the citrate sal-gel auto-combustion method. Thermogravimetric measurements revealed that the optimum calcination temperature for complete formation of ferrites is 300 degrees C. At higher calcination temperatures X-ray diffraction indicated the presence of Fe2O3 and Mn2O3 secondary phases, attributed to the ferrites decomposition. The average crystallite size, calculated using the Debye-Scherrer equation, lies in the range 5-38 nm. The gradual decrease in lattice parameter with increasing Al-content can be attributed to the smaller ionic radii of the Al3+ ion. The TEM image showed large agglomerates of strongly connected cubic aggregates. FT-IR spectra exhibited two prominent absorption bands, nu(1) and nu(2), characteristic for the spinet structure. VSM measurements indicated a gradual decrease in the saturation magnetization with increasing Al-content, until it vanished at x = 0.8, followed by a slight increase. The reduction in coercivity with increasing Al-content can be ascribed to the effect of magneto-crystalline anisotropy. Based on XRD, FT-IR and VSM studies, an appropriate cation distribution for the present investigated system was suggested. The temperature dependence of the AC-conductivity as a function of the applied frequency showed semiconducting behavior with a transformation in the conduction mechanism from the ferrimagnetic to the paramagnetic state for ferrites with x <= 0.4. (C) 2013 Elsevier Ltd. All rights reserved.