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 Ni1−xNaxO (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 (TN) shifted to a high temperature region with moderate Na substitution, which has significant implications for the dependence of the relative dielectric permittivity (εR(T)) and magnetic susceptibility (χmag(T)) on the temperature. The dependence of the ac-resistivity ρac(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=MoL(μPHkBT)+χaH and we extracted TN for the Ni1−xNaxO system.