Structural and electrical properties of Dy3+ substituted NiFe2O4 ceramics prepared from powders derived by combustion method

Dysprosium (Dy) substituted nickel ferrite (NiDyxFe2-xO4) powders with varying Dy content (x=0.0, 0.025, 0.05, 0.075, 0.1, 0.2) have been prepared by combustion method using DL-alanine fuel. Sintering characteristics of the powders and electrical properties of ceramics have been studied. Effective substitution of Dy3+ for Fe3+ is seen up to x=0.075 yielding improved properties, and a higher Dy content (x >= 0.1) leads to partial substitution, disturbed stoichiometry, and diffusion of Dy to the grain boundaries and segregation as a secondary phase. Increasing Dy content reduces the crystallite size, powder particle size, and grain size in sintered ceramics, and the changing microstructural evolution is better resolved with back scattered electron imaging and compositional analysis. Raman spectroscopy confirms inverse spinel structure formation and substantiates the presence of secondary phase evidenced through X-ray diffraction and electron microscopy. A marginal increase in the electrical resistivity (rho(dc)) and magnetization are observed due to effectual substitution of Dy3+ for Fe3+ at the octahedral sites up to x=0.075. For x >= 0.1, the increasing influence of highly resistive DyFeO3 secondary phase' at the inter-granular boundaries leads to a rapid increase in resistivity and reduction in dielectric losses, and the magnetization is reduced due to the anti-ferromagnetic nature of the secondary phase (DyFeO3). Dense ceramics with high resistivity (similar to 10(9) Omega cm), low dielectric loss (tan delta similar to 0.002) at 1 MHz, and high magnetization (50.07 emu/g) are obtained for an optimum Dy content of x=0.075. Dielectric response, complex impedance, and electrical modulus spectroscopy in the frequency range (10(-2)-10(6) Hz) reflect the changes in the microstructure, and suggests a non-Debye type relaxation.