Nanometer scale mapping of cobalt in Al-doped ferromagnetic Zn _0.7Co_0.3O thin film

There has been a renewed interest in zinc oxide in the materials science community after it was shown to be ferromagnetic when doped with cobalt. However, it has remained difficult to tell whether the origin of the phenomenon was intrinsic or due to secondary phases. Here, we examine with analytical transmission electron microscopy the distribution of cobalt in a thin film of ferromagnetic Al-doped Zn_0.7Co_0.3O that we have grown by pulsed laser deposition on alumina. We show that precipitation of a secondary phase does occur, but that it concerns less than 10% of the cobalt atoms. The precipitates appear to be made of hexagonal metallic Co, and their average diameter is 4 nm. Their magnetism could be the reason for the low measured Curie temperature of the sample (50 K). On the other hand, the overall measured magnetization of 0.7 μ_B per cobalt atom suggests that the Co atoms in solution are at the origin of most of the signal.