Reversible ferromagnetism in rutile TiO2 single crystals induced by nickel impurities

We report a Ni impurity induced reversible ferromagnetism and surface conduction in rutile TiO2 crystals subjected to specific thermal annealing. For annealing in vacuum at 800 degrees C, a growing ferromagnetic signal is seen with time while for a similar annealing in air, the magnetism vanishes. The magnetism is concomitant with a surface conductivity which at low temperatures shows tunneling characteristics. Here, we show that Ni magnetic impurity (in TiO2 crystals at <100 ppm) under vacuum annealing segregates to the surface over a 50 nm layer where the Ni concentration exceeds 10%-20% and drops with subsequent air annealing. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4756799]