Hidden ferromagnetic secondary phases in cobalt-doped ZnO epitaxial thin films

The origin of ferromagnetism is investigated in epitaxial Co:ZnO thin films which become weakly ferromagnetic after annealing in Zn vapor. Conventional characterization techniques indicate no change after treatment. However, x-ray photoelectron spectroscopy depth profiling clearly indicates the presence of Co(0) in the Zn-treated films; x-ray absorption fine structure is utilized to identify the secondary phase as ferromagnetic CoZn. This work demonstrates that the potential for ferromagnetic secondary phases must be thoroughly discounted, through painstaking materials characterization, before claims of intrinsic ferromagnetism can be made.