Effects of annealing on ferromagnetism of Ni-doped ZnO powders

The structural and magnetic properties of Ni-doped ZnO powders Zn1−xNixO ( x = 0 to 0.15) prepared via sol–gel route and post-annealed in hydrogen atmospheres have been systematically investigated by X-ray diffraction, X-ray photoelectron spectroscopy and magnetization measurements. Interestingly, these as-synthesized powders were found to reveal room temperature ferromagnetism (RT-FM), where the magnetization increases with Ni doping at low Ni content of x ≤ 0.05 and drops steeply at x > 0.05 . The decrease of magnetization at high Ni concentration is attributed to the segregated NiO nano-grain which results in enlarging of the unit cell lattice, and consequently weakening the ferromagnetic coupling between neighboring Ni2+ ions. After hydrogen treatment at 400 °C, the lattice parameters of wurtzite ZnO increase due to large lattice relaxation caused by H interstitials, while the magnitude of the saturation magnetization remains almost unchanged for the H2-annealed powders of Zn1−xNixO:H at x ≤ 0.05 . In contrast, for phase segregated powders of Zn1−xNixO:H at high Ni content of x = 0.10 and 0.15, a huge enhancement of saturation magnetization was observed. This is mainly ascribed to the precipitation of metal Ni cluster. Our data demonstrate a clear correlation between the magnetization and the structural parameters of Ni-doped ZnO, suggesting that the observed RT-FM can be ascribed to the exchange interaction between free carriers and the localized Ni2+ ions occupied in the Zn sites.