  We report a detailed magnetic and structural analysis of highly reduced Co
doped rutile TiO2 films displaying an anomalous Hall effect (AHE). The
temperature and field dependence of magnetization, and transmission electron
microscopy clearly establish the presence of nano-sized superparamagnetic
cobalt clusters of 8-10 nm size in the films at the interface. The
co-occurrence of superparamagnetism and AHE raises questions regarding the use
of the AHE as a test of the intrinsic nature of ferromagnetism in diluted
magnetic semiconductors.
