Structural, magnetic and transport properties of spinel type Li0.5-x/2 Cux Fe2.5-x/2O4

Abstract

Li0.5-x/2CuxFe2.5-x/2O4 ferrites were sintered at three different temperatures 1200, 1100 and 1000oC, in air for 3 hours, where x in the range of 0.0 to1.0. Samples were also sintered at 800oC for x = 0.9 and 1.0. Samples were prepared by solid state reaction technique. X-ray diffraction technique was used to study the structure of above mentioned ferrites in the range of 0.0 ≤ x ≤ 1.0. For the sintering temperature of 1000oC, Li0.5-x/2CuxFe2.5-x/2O4 undergoes cubic to tetragonal transformation for higher Cu content. However, this tetragonal deformation could not be observed for the sintering temperatures of 1100 and 1200oC in the x-ray diffraction pattern. Composition dependence of lattice parameter has been observed and it does not follow the Vegard’s law in higher Cu content because of structural change at higher Cu content for higher sintering temperature. The saturation magnetization has been measured by a home built Vibrating Sample Magnetometer (VSM) at room temperature. The measurement shows that the saturation magnetization decreases with the increase of Cu content. This is due to the fact that the decrease of A-B interaction is observed to be weakened with increasing of Cu2+ ions. Using the values of saturation magnetization, the experimental magnetic moment (Bohr magneton) has been calculated for different samples. The thermal variation of the initial permeability μ i was measured to determine the Curie temperature TC, for the toroid shape samples. Curie temperatures of Li0.5-x/2CuxFe2.5-x/2O4 ferrites were found to decrease with the increase of Cu2+ content due to the reduction of A-B interaction. Curie temperature reduces linearly up to x = 0.5 for sintering temperature of 1100oC and x = 0.7 for sintering temperature of 1200oC, respectively, and nonlinearity at higher Cu content decreases with the increase of sintering temperature. The real part of the complex initial permeability μ i for frequency up to f = 13 MHz have been measured. It is observed that the initial permeability is higher for higher Cu content. The B-H loops were measured for the Li0.5-x/2CuxFe2.5-x/2O4 ferrites at constant frequency f = 1200 Hz at room temperature. Coercivity and hysteresis loss are decreased with the increase of Cu content due to increased of magnetic softness at the Cu-rich end. The transport property, in particular, electrical resistivity has been investigated as function of temperature. It is observed that ferrites are of semiconductor nature and their resistivity decreases with increasing of temperatures. At room temperature, the resistivity increases up to x = 0.3, but it decreases for x > 0.3, which can be attributed to the decrease of Fe3+ and Li+ concentration with increase of Cu content, but Fe2+ ions are increased. This indicates that Fe+2 ions are present as traces in the samples of x > 0.3. Activation energy has been calculated for different compositions and it is increased with increase of Cu content. The thermal variation of dielectric constant was measured for different composition. It is observed that the dielectric constant increases slowly in the beginning and after that dielectric constant increases rapidly for different samples. As the temperature is increased, thermal energy provided to the sample plays a role to decrease the relaxation time and hence there is an increase in dielectric constant. Dielectric constant has been observed as a function of frequency and it decreases with increase of frequency.