Structural, optical, electrical and magnetic properties of sol-gel derived Zn1–x–yCoxAlyO films

A detailed study has been presented of optical, electrical, structural and magnetic properties of Zn1–xCoxO (0 < x ≤ 0.25) films co-doped with 1 at. % of Al. The polycrystalline films have been synthesized on Corning glass 7059 substrates by the sol-gel technique using spin coating. Highly preferential c-axis oriented films have been obtained at the annealing temperature of 600 °C. The lattice constant d of c-axis wurtzite Zn1–x–yCoxAlyO obeys Vegard’s law for (0 < x ≤ 0.25). The inclusion of Al in ZnO is highly beneficial for the magnetism in ZnO because it enhances the free electron density. Optical spectra measurements reveal that band gap energy exhibits a blue shift upon increasing Co concentration. A positive magnetoresistance for Co doped ZnO and negative magnetoresistance for ZnO without cobalt at 77 K has been observed. The ferromagnetic behaviour has been confirmed by measurements using superconducting quantum interference device. The coercive field and the remanence increase with increase in Co content.