Room-temperature ferromagnetism in highly-resistive Ni-doped TiO2

We investigated the magnetic and transport properties of Ni-doped TiO2 films grown by a sol-gel method with Ni concentrations from 1% to 8%. All the samples exhibited ferromagnetism at room temperature identified by optical magnetic circular dichroism along with magnetometer measurement. The microstructural and compositional analysis revealed Ni-rich regions with rather uniform background, indicating that at least two different sources contribute to the observed ferromagnetism. The Hall effect study showed that the carriers are electrons with density lower than 10(18) cm(-3) at room temperature, and no anomalous Hall effect has been observed due to the high resistivity of our samples. The observation of ferromagnetism in highly-resistive Ni-doped TiO2 films questions the carrier-mediated exchange interaction as an origin of ferromagnetism in this material.