||Doping of transition ions and introducing oxygen vacancies (VO) in several oxides can trigger spectacular room temperature ferromagnetic coupling and becomes diluted magnetic oxides. How the magnetic dopant and vacancies collaborate to induce ferromagnetism is still unclear. A general picture of the carrier mediated mechanism is widely accepted, such as: the free carriers mediate the long-range magnetic coupling via RKKY mechanism when the material is in metallic state while the localized carriers mediate magnetic coupling in a limited radius via BMP mechanism. For those semiconducting materials, the carrier is neither as free as metals nor localized as insulator, how electron induces magnetic coupling and who provides magnetic moments and what are the roles of doped transition ions and introduced oxygen vacancies are unclear and are needed to be answered before real applications. This study aims at developing a technique to reliably reproduce oxygen vacancies in Co doped ZnO films and growth a series samples with systematically varied oxygen vacancies to study these mentioned issues.|
In this study, five percent Co doped ZnO, Zn0.95Co0.05O (CZO), targets were prepared by solid state reaction method at 860oC. CZO films with various oxygen vacancies were grown by RF sputtering technique in a mixed H2/Ar gas with different percentages, denoted as H2%. It was found that the base vacuum of growth chamber is crucial for reliably reproduce exact vacancy concentration. With our effort, the base vacuum of our chamber is now lower than 1x10-7Torr and the VO reproducibility is reliable. These as grown films degraded very fast when were exposed to air. In order to know basic properties of the as grown films, the resistance and optical transmittance were measured right when films were taken out from the growth chamber. Films grown in high H2% atmosphere exhibit lower resistance, higher transmittance and wider optical bandgap which could be due to Burstein-Moss effect. The effective oxygen vacancies were measured by X-ray photon emission (XPS). Due to XPS is accessible in the core facility center which is usually few days or week after the film growth, only the data at the mid layer of films were taken. A slightly discrepancy between H2% and XPS data is found indicating a small uncontrollable in our film growth system. Reliable data has to be taken for only the as grown films. MCD data proves the doped Co ions substitutes successfully at Zn sites and shows magnetic coupling at around 3.4eV same as SQUID-VSM data indicated. These data strongly indicates that the doping of 5% Co alone cannot generate magnetic coupling, and only when coexist with oxygen vacancies the magnetic coupling appears.