Magnetism of Co doped ZnO with Al codoping: Carrier-induced mechanisms versus extrinsic origins

Zn_1-xCoₓO epitaxial films codoped with Al were studied using a combination of synchrotron-based x-ray absorption spectroscopies and classical magnetometry. Phase purity was verified by comparing the x-ray linear dichroism with simulations and previously published reference spectra. The existence of weak ferromagnetism or inhomogeneous superparamagnetism is evidenced at low temperatures by classical magnetometry. A combination of x-ray absorption spectroscopies indicates that its origin is most likely of extrinsic character such as magnetic impurities or the onset of phase separation rather than weak, electron mediated ferromagnetism.