Structural, magnetic and electrical properties of cobalt ferrites prepared by the sol–gel route

Nanosized cobalt ferrites having the general formula CoFe2−xAlxO4 (for x = 0.00, 0.25, 0.50) have been synthesized by the sol–gel route. The effect of Al3+ ions on structural, Curie temperature, DC electrical resistivity and dielecltric properties are presented in this paper. From the analysis of powder X-ray diffraction patterns, the nanocrystallite size was calculated by the most intense peak (3 1 1) using Scherrer formula. The crystallite size decreases with increase in aluminium concentartion. The lattice parameter ‘a’ also decreased with increase in aluminum concentration. The magnetic and electrical properties have been studied as a function of temperature. The Curie temperature was determined from AC magnetic susceptibility measurement. It is observed that Curie temperature decreases and DC electrical resistivity increases with increasing concentration of Al3+ ions. The observed variations in DC electrical resistivity have been explained by Verwey's hopping mechanism. The activation energy was found to increase with increasing Al3+ ions. The variations of dielectric constant for all the samples have been studied as a function of frequency in the range 500 Hz to 1 MHz at room temperature. The room temperature dielectric constant decreases with increase of trivalent Al3+ ions. The observed variation in dielectric constant has been explained on the basis of space charge polarization.