Effects of Annealing on Structural and Magnetic Properties of Cobalt Implanted TiO₂ Thin Films

Abstract

Since the observation of room-temperature ferromagnetism (RTFM) in Co-doped anatase TiO₂ [1], there have been many reports on the study of the magnetic properties of Co-doped TiO₂ prepared by various methods with diversified results. The origin of the RTFM in these systems is still a topic of controversy today. In this work, TiO₂ thin films were prepared by RF sputtering onto thermally grown oxide layers on Si substrates. Cobalt implantation was performed using a metal vapor vacuum arc (MEVVA) ion source to various doses ranging from 3×10¹⁵ cm^-2 to 4×10¹⁶ cm^-2. Post-implantation annealing was performed in a vacuum chamber at various temperatures ranging from 400°C to 700°C for 2 hours and 4 hours. Characterization of these films as-implanted and after thermal annealing under various conditions was performed using Rutherford backscattering spectrometry, energy filtered and high-resolution transmission electron microscopy, x-ray diffractometry, x-ray photoelectron spectroscopy, and vibrating sample magnetometry. The dependence of the magnetic properties on the implantation and annealing conditions were studied in detail. Clear RTFM properties were observed. The saturation magnetic moment per implanted Co atom (M_S) seems to increase with increasing dose within the implantation dose range in this study. At a fixed dose, the MS value also shows a generally increasing trend with increasing annealing temperature and annealing time. Quite a number of samples showed MS values exceeding the bulk Co value of 1.69 ì_B/Co significantly and the maximum MS value observed is about 3.16 µ_B/Co. Such high MS values indicate that the RTFM must not come from Co clusters alone. Possible origins of the RTFM properties will be discussed in conjunction with the structural properties.