Enhanced magnetic properties at low temperature of Mn substituted Ni-Zn mixed ferrite doped with Gd ions for magnetoresistive applications

Mn substituted Ni-Zn mixed ferrites doped with Gd3+ ions belonging to the series Mn0.2Ni0.6Zn0.2GdxFe2-xO4 (x = 0.00, 0.02, 0.04, 0.06) have been synthesized by sol-gel method. The effect of Gd3+ substitution on the structural, cationic distribution, magnetic and dielectric properties has been investigated in detail. X-Ray analysis reveals formation of single phase spinel cubic ferrite nanoparticles with no secondary phases. The average crystallite size estimated using Scherrer's relation lie in the 11-13 nm range with the size decreasing with increase in Gd ion content. Lattice constants calculated from proposed cation distribution are in close agreement with reported values. Surface morphology studies using SEM and HRTEM show spherical shaped nanoparticles which are slightly agglomerated. EDX analysis confirms the expected stoichiometry. The valence state of materials was characterized by XPS. The dielectric variations with frequency and temperature were analyzed using a precision impedance analyzer. The relative permittivity and dielectric loss show an increasing trend with Gd concentration up to x = 0.04 and then decrease with further increase in doping. Increasing the temperature increases the relative permittivity. Measurements of magnetic parameters using VSM were performed at room temperature and at 77K. M - H loops recorded have shown that the synthesized ferrite nanoparticles exhibit soft ferrimagnetism. The saturation magnetization and coercivity decrease with increase in Gd doping. The samples possess high magnetization and coercivity at low temperatures. A remarkable difference in the magnetic moments estimated from measured magnetic parameters and from the cation distribution over A and B sites on the basis of Neel's two sublattice model of ferrimagnetism was noted in all the samples. Higher Y-K angles obtained imply their potential use in synthesis of magnetoresistive (MR) materials.