الفهرس | Only 14 pages are availabe for public view |
Abstract Polycrystalline samples of the chemical formula Li0.5Crx Fe2.5−xO4 (x = 1.15, 1.20, 1.25, 1.30 and 1.35) were prepared by using the standard ceramic method. X-ray diffraction analysis confirmed the formation of a single phase of a face centered cubic spinel structure for all prepared samples. The lattice parameter of each sample was accurately determined by using Nelson-Riley function. The lattice parameter and bulk density decreased, while the x-ray density and porosity increased with increasing Cr content. Magnetic measurements were carried out by using the electromagnetic induction method in the temperature range from −100 to 350 ºC. The temperature variation for all samples exhibited the N type anomalous dependence in which a compensation point appears. The magnetization below the compensation temperatures decreased, while that above them increased with the increase in the Cr content. Meanwhile, both compensation and Curie temperatures manifested a decreasing trend. The magnetic results were explained using Néel two-sublattice model of ferrimagnetism. Electrical and dielectric measurements were performed by using the two probe method in the temperature range from −70 to 390 ºC and frequency range from 102 to 106 Hz. All specimens showed semiconducting nature. The conduction mechanism changed once at the compensation temperature, where the activation energy increased, and another at Curie temperature. The values of both the dielectric constant and dielectric loss in addition to their dependency on temperature and the frequency increased with increasing temperature. The increase in the dielectric constant was accompanied by the appearance of a hump. Such a hump shifted towards higher temperatures with increasing the frequency as well as it grew from nothing to a peak as the Cr concentration increased. The temperature variation of the frequency exponent factor revealed that the conduction mechanism around the compensation temperature in all compositions is attributed to the polaron hopping Fe2+↔Fe3+. Also, it showed that the polaron effective mass increases at the compensation temperature and decreases with increasing Cr content. The composition dependence of the dc resistivity and dc activation energy exhibited maxima at x = 1.25, while that of the total resistivity and total activation energy, in the paramagnetic region, exhibited an increasing trend. Meanwhile, the total activation energy in the ferrimagnetic state, below and above compensation, decreased with increasing the Cr concentration. It is the same compositional dependence of both the dielectric constant and dielectric loss. The total resistivity, dielectric constant and dielectric loss showed the well known dispersion of ferrites. The resistivity dispersion decreased and shifted towards higher frequencies with increasing temperature. The electrical and dielectric results were explained in light of Koops model and the excess formation of Fe3+ clusters ith increasing Cr content. Also, it was proposed that the spin polaron is formed on B sublattice at the compensation temperature. |