Local environment surrounding magnetic impurity atoms in a structural phase transition of Co-doped TiO₂ nanocrystal ferromagnetic semiconductors

The local environment surrounding magnetic impurity atoms and the host crystal structure of codoped TiO₂ (TiO₂:Co) nanocrystal ferromagnetic semiconductors have been investigated using the x-ray absorption fine structure and powder diffraction techniques. It has been found that the magnetic Co impurity atoms substitute for the Ti sites in an anataselike local environment through a structural phase transition when the material changes from an amorphous phase to a mixture of anatase and rutile crystal structures and then to a rutile structure as a result of increasing the anneal temperature. This result reveals an interesting feature that the local structure around magnetic impurity atoms can remain practically unchanged while the material undergoes drastic structural variations and a loss of room-temperature ferromagnetism.