Structural and magnetic characterization of mixed valence Co(II, III)(x)Zn1-xO epitaxial thin films

Abstract

In this article, we report on the Co atom incorporation, secondary phase formation and composition dependent magnetic and optical properties of mixed valence Co(II, III)(x)Zn1-xO epitaxial thin films grown by pulsed laser deposition. The intended total Co concentration is varied between similar to 6-60 at.% with relatively higher concentration of +3 over +2 charge state. Mixed valence Co(II, Ill) shows high solubility in ZnO (up to 38 at.%) and ferromagnetism is observed in samples with total Co incorporation of similar to 29 and 38 at.%. Electron diffraction pattern and high resolution transmission electron microscopy images reveal single crystalline nature of the thin films with wurtzite structure. Co oxide interlayer, with both rock salt and spinel structure, are observed to be formed between the substrate and wurtzite film for total Co concentration at similar to 17 at.% and above. Magnetization shows composition dependence with a saturation moment value of similar to 93 emu cm(-3) and a coercive field of similar to 285 Oe observed for similar to 38 at.% Co:ZnO films. Ferromagnetism was not observed for films with Co concentration 17 and 9 at.%. The Co oxide interlayer does not show any ferromagnetism. All the films are n-type with carrier concentration similar to 10(19) cm(-3). The observed magnetism is probably resulting from direct antiferromagnetic exchange interaction between Co2+ and Co3+ ions favored by heavy Co alloying giving rise to ferrimagnetism in the system. (C) 2013 Elsevier B.V. All rights reserved,