Activation and Enhancement of Room-Temperature Ferromagnetism in Cu-Doped Anatase TiO₂ Films by Bound Magnetic Polaron and Oxygen Defects

Abstract

Cu-doped anatase TiO2 films grown by magnetron sputtering at room temperature showed the unexpected observation of room-temperature ferromagnetism, which was enhanced or destroyed corresponding to low or high impurity concentration via vacuum annealing. On the basis of the analysis of composition and structure, the most important factor for activating ferromagnetism can be identified as the creation of grain boundary defects. In addition, oxygen defects can be the dominating factor for increasing the saturation moment of the 0.19 at. % Cu-doped TiO2 film from 0.564 to 26.41 emu/cm(3). These results help elucidate the origin of ferromagnetism and emphasize the role of oxygen defects for the application of ferromagnetic films.

Structural, optical, and magnetic properties of Cu‐doped TiO2 samples

“…The 2.37 eV emission correlated with singly ionized V O , generally found in TiO 2 nanoparticles. Singly occupied V O (V O with one electron) with a spin of 1/2 has been previously ascribed to the origin of RTFM in TiO 2 powders and nanoparticles , and TiO 2 :Cu thin films , nanorods , nanocrystals , and nanoparticles . Note that the PL intensity of the Cu‐doped TiO 2 samples was much higher than that of dopant‐free TiO 2 , demonstrating that Cu ions replaced the Ti site in TiO 2 , confirming that the obtained FM signals presented below were from Cu replacement but not from impurities and that Cu doping can enhance the reintegration of photo‐created electrons and holes.…”

“…have reported a similar observation in Cu‐doped rutile TiO 2 films and Zheng et al. in Cu‐doped anatase TiO 2 films and has been interpreted as due to the substitution of Cu ion into the Ti site. On the contrary, when the Cu concentration goes to 6% the unit cell volume reduce.…”

“…It can be seen that the dominant crystal phase was the rutile‐type, in addition to a very small amount of diffraction lines attributed to anatase‐type . Detailed XRD analysis also revealed a slight shifting of the diffraction peaks towards the lower angles for the Cu‐doped samples compared to the dopant‐free TiO 2 . Weak diffraction lines attributed to CuO could also be detected in the XRD patterns , whose concentration increases with Cu doping.…”

“…Detailed XRD analysis also revealed a slight shifting of the diffraction peaks towards the lower angles for the Cu‐doped samples compared to the dopant‐free TiO 2 . Weak diffraction lines attributed to CuO could also be detected in the XRD patterns , whose concentration increases with Cu doping. Note that increasing the Cu content also increased the CuO intensity peak.…”

Room temperature ferromagnetism in Cu2+ doped TiO2 nanocrystals: The impact of their size, shape and dopant concentration

“…Such increase of lattice parameters values could be an indication of substitution of Ti 4+ by Cu 2+ within crystal structure of doped TiO 2 nanoparticles [34]. It should be mentioned here that the ionic radius of Cu 2+ ions in octahedral coordination (0.87 Å) is not too much larger than ionic radius of Ti 4+ ions (0.745 Å) in the same coordination, to cause significant deformation of anatase crystal lattice [35].…”