Anisotropic super-paramagnetism in cobalt implanted rutile-TiO2 single crystals

The present study investigates the magnetic properties of single crystal rutile TiO₂ (110) implanted with cobalt ions for fluences between 5 × 10¹⁶ and 1 × 10¹⁷ ions/cm² with energy of 200 keV. The temperature and field dependent magnetization for magnetic fields along [001] and [11¯0] directions of the as implanted TiO₂ crystals show anisotropic super-paramagnetic behaviour due to formation of Co (hcp) nano-clusters unlike the complete ferromagnetic behaviour from previous reports. The ZFC and FC magnetization curves show a much higher blocking temperature (TB) along [11¯0] with values ranging from 30 to 150 K, while for field along [001] shows a lower TB ranging from 8 to 70 K. The magnetization isotherms above TB show a sharper rise and universal scaling behaviour, for field along [11¯0] direction. At 2 K, M - H curves show hysteresis behaviour similar of easy and hard axis of a ferromagnet. The magnetic anisotropy of Co nano clusters are coupled by the magneto crystalline anisotropy of secondary phases of cobalt with TiO₂, thus indicating the highly oriented nature of the Co clusters. Role of dipole interactions and inter cluster exchange interactions have also been discussed.