Magnetic exchange interactions and dielectric studies of Zn_1–x Ni_xO–NiO composites

We report a detailed study on the nature of magnetic exchange interactions and dielectric properties of ${Zn}_{1-x}$Ni_xO–NiO composites. The magnetic exchange constants $\left|J_1\right|$ and $\left|J_2\right|$ for the two sublattices have been evaluated using both molecular-field approximation ($J_1\sim 0.15$ meV and $J_2\sim 1.15$ meV for $x=1$) and Green's-function theory ($J_1\sim 0.78$ meV and $J_2\sim 1.77$ meV for $x=1$). Magnitudes of these exchange constants determined from the Green's-function analysis are consistent with the values obtained from the spin-wave dispersion curves ($J_1\sim 0.137$ meV and $J_2\sim 1.90$ meV for $x=1$) and Raman scattering ($J_2\sim 1.98$ meV). Moreover, the carrier activation energy ${ϵ}_h$ (∼59.9 meV) and nearest-neighbor exchange coupling parameters $J_{ij}$ (∼1.61 meV) calculated from the temperature dependence of ac resistivity ${\rho }_{ac}\left(T\right)$ across the antiferromagnetic Néel temperature (T _N) of NiO using a small-polaron model involving spin-dependent activation energies are compared with the theoretical estimations from the Green's-function theory. In addition, the role of oxygen stoichiometry in the temperature variation of $J_{ij}$, ${ϵ}_R$, ρ _ac and the unit-cell volume V _C have been discussed. (paper)