Localized Charge Carrier Transport Properties 
of Zn1−xNixO/NiO Two-Phase Composites
We report the localized charge carrier transport of two-phase composite Zn1−xNixO/NiO (0 ≤ x ≤ 1) using the temperature dependence of ac-resistivity ρac(T) across the Néel temperature TN (= 523 K) of nickel oxide. Our results provide strong evidence to the variable range hopping of charge carriers between the localized states through a mechanism involving spin-dependent activation energies. The temperature variation of carrier hopping energy εh(T) and nearest-neighbor exchange-coupling parameter Jij(T) evaluated from the small poleron model exhibits a well-defined anomaly across TN. For all the composite systems, the average exchange-coupling parameter (Jij)AVG nearly equals to 70 meV which is slightly greater than the 60-meV exciton binding energy of pure zinc oxide. The magnitudes of εh (∼0.17 eV) and Jij (∼11 meV) of pure NiO synthesized under oxygen-rich conditions are consistent with the previously reported theoretical estimation based on Green’s function analysis. A systematic correlation between the oxygen stoichiometry and, εh(T) and Jij(T) is discussed.