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“Background Magnetic-ion-doped TiO2 with room-temperature ferromagnetism is one kind of promising diluted magnetic semiconductors (DMS). It has been widely studied due to its potential applications in spintronics [1–3]. Many efforts have been made to understand the mechanism of ferromagnetism (FM) in magnetic-ion-doped TiO2. The most important

point for industrial applications is if such room-temperature FM could originate 3-oxoacyl-(acyl-carrier-protein) reductase from the doped matrices and not from the dopant clusters. Some theory models, such as the Ruderman-Kittel-Kasuya-Yosida exchange [4], super exchange [5], double exchange [6], magnetic polarons [7], and F-center exchange mechanism [8], have been used to explain ferromagnetism in transition-metal-element-doped TiO2. However, many controversies still exist in the magnetic origin of DMS. Recently, room-temperature FM [9] and reversible FM [10] in undoped TiO2 films, and reversible FM in transition metal-doped TiO2 nanocrystals [11], have been reported. These reports suggest that the structural defects can induce FM order, which brings new challenges in elucidating the magnetic mechanism in this kind of DMS.