X-ray absorption fine structure and magnetization characterization of the metallic Co component in Co-doped ZnO thin films

X-ray absorption fine-structure (XAFS) measurements have been used to characterize a series of Co-doped ZnO films grown on sapphire substrates by pulsed laser deposition. The emphasis is on characterization of the fate of the Co dopant: metallic particles or substitutional Co2+. It is shown that analysis of both the near edge and extended fine structure can provide a measurement of the fraction of metallic Co. Any quantitative understanding of magnetism in this system needs to take into account both types of Co. Results are reported for two types of films from two different groups that show distinctly different behaviors. Films grown with high concentrations of Co show varying amounts of metallic Co that could be identified as a close-packed form of Co. Another set of films was annealed in Zn vapor to induce magnetism. These films also showed significant metallic Co, but of a different type similar to the CoZn intermetallic. The bulk forms of both metals are magnetic and should contribute to the magnetism. However, the measured room-temperature magnetic moments for some films are inconsistent with the expected moments based on the bulk magnetic values for either Co metal or CoZn. The magnetic properties of the small metal particles are likely changed by their surroundings. Low-temperature magnetic measurements for one of the samples confirmed this with an estimated blocking temperature of 50 K.