Investigation of structural, dielectric and magnetic properties of Ho substituted nanostructured lithium ferrites synthesized via auto-citric combustion route

A comprehensive study on structural, spectral, magnetic and dielectric properties of holmium substituted (Ho3+) lithium ferrites; Li1.2Co0.4HoxFe2-xO4 (x = 0.00-0.15) synthesized via auto-citric combustion route has been carried out. The annealing temperature essential for complete decomposition of precipitates and spinel phase formation was found around 950 degrees C which was estimated from thermo-gravimetric analysis (TGA). The development of face centered cubic (FCC) spinel structure was confirmed from X-ray diffraction (XRD) analysis. However, the traces of an ortho phase (HoFeO3) were appeared for x >= 0.06 indicating the solubility limit of Ho3+ ions. The lattice constant 'a' increased from 8.351 to 8.360 angstrom upon Ho addition up to x = 0.06 and it decreased thereafter from 8360 to 8.352 angstrom. This decrease in lattice constant was attributed to the secondary phase formation revealing segregation of Ho3+ ions at grain boundaries. The porosity was found to decrease from 44 to 23% by the inclusion of Ho3+ ions. Fourier transform infrared spectra (FTIR) exhibited tetrahedral (v(1)) and octahedral (v(2)) intrinsic vibrational bands at 627 cm(-1) and 479 cm(-1) respectively and these bands revealed a shift with increasing Ho concentration. The variations in force constants (K-0 and K-t) were calculated and discussed with the tetrahedral (r(A)) and octahedral (r(B)) bond length trends. The dielectric parameters have been investigated as a function of composition and frequency. The systematic addition of Ho gradually increased the ac conductivity from 8.97 x 10(3) to 5.56 x 10(4) (Omega-cm)(-1) up to x = 0.12 and it subsequently decreased for x = 0.15. The complex impedance examination (Cole -Cole plots) confirmed a grain -interior phenomenon, effectively contributing to the dielectric properties. The coercivity was enhanced while the saturation magnetization (Ms) was lessened by the incorporation of Ho3+ ions. The maximum value of coercivity (706 Oe) was obtained for Li1.2Co0.4Ho0.12Fe1.88O4. The improved coercivity may suggest their utilization in perpendicular recording media applications. (C) 2017 Elsevier B.V. All rights reserved.