Correlation between donor defects and ferromagnetism in insulating Sn _1-x Co_xO₂ films

Sn_1-xCo_xO₂ films have been fabricated to study the local structure of Co dopant and the mediation effects of donor defects (oxygen vacancies and Sn interstitials) on magnetic properties. Compared to as-grown film, the ferromagnetism is evidently enhanced after annealing in vacuum at 400 °C due to the increase in oxygen vacancies. While annealing at higher temperature, the ferromagnetism declines because of the domination of decrease in Sn interstitials over increase in oxygen vacancies in the films. The incorporation of Co dopant as well as the presence of oxygen vacancies and Sn interstitials is verified using x-ray absorption fine structure spectroscopy. The variations in the concentration of defects as a function of annealing temperature are obtained by positron annihilation spectroscopy technique. Additionally, the changes in structure and ferromagnetism after annealing in different atmospheres further demonstrate the crucial roles of oxygen vacancies and Sn interstitials in tuning ferromagnetism.