Dielectric and magnetic properties of sol-gel-derived lead iron niobate ceramics.

In this work, we report the synthesis of sol-gel-derived lead iron niobate [Pb_1.10(Fe_0.5Nb_0.5)O₃] (PFN) powders and sintered ceramics. The PFN powders were calcined at (T_a), 973, 1073, 1173, 1273, and 1373 K for 3 h in air. As envisaged from x-ray-diffraction analyses, PFN powder calcined at 1173 K was crystallized into pure monoclinic perovskite phase whereas powders calcined at all other temperatures had varied amounts of retained pyrochlore (Pb₃Nb₄O₁₃) phase coexisted with dominant monoclinic perovskite phase. The PFN pellet (prepared using the phase pure powder calcined at 1173 K) sintered at 1373 K for 4 h in air also had a minute quantity of retained pyrochlore phase coexisting with desired perovskite phase. From the temperature dependence of measured capacitance and loss tangent at different frequencies, the ferroelectric to paraelectric phase-transition temperature of PFN ceramics was observed at T_c≈370 K. The diffused nature of this transition and high dielectric constant of PFN is related to the cation disorder at the B site of A(B_I⁺³B_II⁺⁵)O₃ lattice. For PFN powders, calcined at different temperatures, the temperature dependence of the magnetic susceptibility (χ) was measured in a temperature range of 2–360 K, whereas the magnetic hysteresis loops and electron magnetic resonance (EMR) spectra were measured at room temperature. Room-temperature ferromagnetism is observed in all the calcined powder samples and it was found that the magnetization increases with the increase in calcination temperature (T_a). The symmetric EMR line shape with g≈2.01 observed in all calcined samples was identified to be due to Fe³⁺ ions. It is suggested that the observed weak ferromagnetism, which increases with an increase in T_a, may be due to canting of the Fe³⁺ spins. These observations suggest PFN to be a very attractive single-phase ferroelectric/ferromagnetic material for room-temperature applications. [ABSTRACT FROM AUTHOR]