Structure and properties of Co-doped ZnO films prepared by thermal oxidization under a high magnetic field

The effect of a high magnetic field applied during oxidation on the structure, optical transmittance, resistivity, and magnetism of cobalt (Co)-doped zinc oxide (ZnO) thin films prepared by oxidizing evaporated Zn/Co bilayer thin films in open air was studied. The relationship between the structure and properties of films oxidized with and without an applied magnetic field was analyzed. The results show that the high magnetic field obviously changed the structure and properties of the Co-doped ZnO films. The Lorentz force of the high magnetic field suppressed the oxidation growth on nanowhiskers. As a result, ZnO nanowires were formed without a magnetic field, whereas polyhedral particles formed under a 6 T magnetic field. This morphology variation from dendrite to polyhedron caused the transmittance below 1,200 nm of the film oxidized under a magnetic field of 6 T to be much lower than that of the film oxidized without a magnetic field. X-ray photoemission spectroscopy indicated that the high magnetic field suppressed Co substitution in the ZnO lattice, increased the concentration of oxygen vacancies, and changed the chemical state of Co. The increased concentration of oxygen vacancies affected the temperature dependence of the resistivity of the film oxidized under a magnetic field of 6 T compared with that of the film oxidized without a magnetic field. The changes of oxygen vacancy concentration and Co state caused by the application of the high magnetic field also increase the ferromagnetism of the film at room temperature. All of these results indicate that a high magnetic field is an effective tool to modify the structure and properties of ZnO thin films.