Role of aluminum ions on the dielectric and conducting properties of multiferroic Tb1−xAlxMnO3: Study at high temperatures

Abstract

Dielectric and conducting properties of Tb1−xAlxMnO3 (x = 0, 0.05) synthesized by the solid–state reaction method have been investigated. The Al ion has the same valence as substituted Tb but is nonmagnetic and its small size gives rise to microstructural strain which affects the multiferroic properties of the parent compound. Samples were characterized by means of complex impedance spectroscopy (CIS) in the frequency range from 40 Hz to 5 MHz, at temperatures above room temperature. The conductivity curves for the two samples are well fitted by the Jonscher law σ(ω) = σdc + Aωn. Results of the fitting procedure indicate that for the studied samples, the hopping motion involves localized hopping without the species leaving the neighbors. Frequency dependence of the dielectric constant (ε″) and tangent loss (tan δ) display a dispersive behavior at low frequencies that can be explained on the basis of the Maxwell–Wagner model and Koop's theory. The relaxation dynamics of charge carriers has been studied by means of the electric modulus formalism. In turn, the variation of the imaginary part of the complex impedance, Z″, shows a peak at a relaxation angular frequency (ωr) related to the relaxation time (τ) by τ = 1/ωr. The complex impedance spectra (Nyquist plots) show well-defined semicircles which are strongly dependent on the Al-doping level and temperature. The complex impedance data have been modeled using electrical equivalent circuits.