Magnetic exchange interactions and dielectric studies of Zn1-x NixO-NiO composites

We report a detailed study on the nature of magnetic exchange interactions and dielectric properties of Zn1-xNixO-NiO composites. The magnetic exchange constants vertical bar J(1)vertical bar and vertical bar J(2)vertical bar for the two sublattices have been evaluated using both molecular-field approximation (J(1) similar to 0.15 meV and J(2) similar to 1.15 meV for x = 1) and Green's-function theory (J(1) similar to 0.78 meV and J(2) similar to 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) similar to 0.137 meV and J(2) similar to 1.90 meV for x = 1) and Raman scattering (J(2) similar to 1.98 meV). Moreover, the carrier activation energy is an element of(h) (similar to 59.9 meV) and nearest-neighbor exchange coupling parameters J(ij) (similar to 1.61 meV) calculated from the temperature dependence of ac resistivity rho(ac)(T) across the antiferromagnetic Neel 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), is an element of(R), rho(ac) and the unit-cell volume V-C have been discussed.