  The prediction of ferromagnetism at room temperature in Co-ZnO thin films has
generated a large interest in the community due to the possible applications.
However, the results are controversial, going from ferromagnetism to
non-ferromagnetism, leading to a large debate about its origin (secondary
phase, Co clusters or not). By carefully studying the micro-structure of
various Co-ZnO films, we show that the Co2+ partly substitutes the ZnO wurtzite
matrix without forming Co clusters. Surprisingly, the ferromagnetism nature of
the films disappears as the Co content increases. In addition, our results
suggest that the observed ferromagnetism is likely associated to a large amount
of defects- close to the interface and strongly depending on the growth
temperature- which may explained the spreading of the results.
