Role of phase transition in the dielectric and magnetic properties of Na containing NiO

Abstract

In this study, we systematically investigated the phase transition (cubic (90°) → rhombohedral (60.01°)) and its role in the electronic structure, dielectric, and magnetic behavior of Ni 1−x Na x O (0.02≤x≤ 0.2). X-ray photoelectron spectroscopy indicated non-local screening of the divalent Ni by oxygen and monovalent sodium ions together with giant multiplet splitting. At 310 K, the longitudinal optical phonon mode (500 cm −1 ) dominated over all the vibrational excitations, but for the dilute dispersion of Na, the transverse optical phonon mode (455 cm −1 ) exhibited the highest intensity among all the modes. Even at a very low level (x∼ 0.02) of Na substitution, the intensity of the 2-magnon mode (1530 cm −1 ) was significantly suppressed. The onset of the structural transition and antiferromagnetic Néel temperature (T N ) shifted to a high temperature region with moderate Na substitution, which has significant implications for the dependence of the relative dielectric permittivity (ε R (T)) and magnetic susceptibility (χ mag (T)) on the temperature. The dependence of the ac-resistivity ρ ac (T, f) on the temperature and frequency followed Mott's variable range-hopping mechanism for charge carriers between the localized states, where the activation energies were highly dependent on the spin (S = 1/2) of the type-II antiferromagnetic system. The magnetization isotherms (M–H) were analyzed using a modified Langevin function M=M o L([Formula presented])+χ a H and we extracted T N for the Ni 1−x Na x O system. © 2019 Elsevier Ltd