Effect of Al3+ substitution on the electrical and magnetic properties of Li-base ferrites

Aluminum substituted lithium ferrites with chemical formula Li-0.6(Ni Co)(0.2)AlxFe2.2-xO4 (x = 0.0,0.15,0.3,0.45,0.6) have been synthesized by sol-gel method and sintered at 900 degrees C for 5 h. The samples were characterized by XRD, electrical, dielectric and VSM measurements. X-ray diffraction studies confirmed that all the samples exhibit single phase cubic spinel structure with space group Fd3m. The crystallite size, bond lengths, x-ray density and lattice parameters decrease with the increase of Al3+ concentration. Dielectric properties such as dielectric constant (epsilon'), complex dielectric constant (epsilon ''), tangent loss (tan delta) and ac conductivity (sigma(ac)) of all the samples were measured and evaluated as a function of frequency at room temperature. The variation of dielectric properties; epsilon', epsilon '', tan and sigma(ac) versus frequency reveals that the dispersion is due to Maxwell-Wagner type of interfacial polarization in general and the hopping of charge between Fe2+ and Fe3+. The Al-doped nano-crystalline lithium ferrite samples exhibit a very large value of dielectric constant of the order of 10(4). The room temperature resistivity and activation energy increases with the increase of Al3+ concentration. The temperature dependent resistivity of all samples decreases with increase of temperature indicating the semiconducting behavior. The room temperature DC resistivity increases from 1.65 x 10(10) to 2.23 x 10(11) ohm-cm. The small value of magnetization may be due to canting superparamagnetic spins at the surface of the samples. The decrease in the magnitude of magnetization value may also be due to the presence of Al3+ paramagnetic ions with spin-down orientation instead of Fe3+ cations with spin-up orientation in the octahedral sites. The squareness ratio (Mr/Ms) is less than 1 which indicates single domain behavior. The increase in coercivity is attributed to lattice defects and decrease is associated with the superparamagnetic behavior.