Non-overlapping small polaron tunneling conduction coupled dielectric relaxation in weak ferromagnetic NiAl2O4

Abstract

The inverse spinel nickel aluminate micro-particles was successfully synthesized via solid-state synthesis techniques. The single phase cubic structure with space group Fd3m of the as-prepared sample was entrenched from Rietveld refinement of x-ray diffraction pattern. In addition, the photoluminescence (PL) and FTIR spectra were also performed to give strong evidence of its pure phase formation. The narrow hysteresis m–h loop and UV-DRS spectra at room temperature demonstrate its weak ferromagnetic and semiconducting nature with saturation magnetization 64.96 × 10−3 emu gm−1 and direct optical band gap 2.03 eV respectively. The high-resolution FESEM micrographs and EDS elemental analysis exhibit its grain growth in µm range (217 µm) and presence of elemental compound Ni, Al and O respectively. The electrical transport properties were accomplished by complex impedance spectroscopy as a function of frequency (100 Hz–1 MHz) with the evolution of high temperature (300 °C–500 °C). The Nyquist plots (Z″ versus Z′) were well fitted with an equivalent circuit model (QR) (QR) consisting of a series combination of intra and inter-granular contribution. Furthermore, the imaginary modulus spectra were also fitted with Kohlrausch–Williams–Watts (KWW) function, which represents two thermally activated peaks of grain and grain boundary effects. The low-frequency dispersive ac conductivity was elucidated using the following equation: . The increasing nature of temperature dependent frequency exponent (n) suggests the quantum mechanical tunneling: non-overlapping small polaron tunneling (NSPT) concept for conduction mechanism. In low-frequency region, the lattice and charge carrier polarization simultaneously contribute in dielectric permittivity.