Anisotropic super-paramagnetism in cobalt implanted rutile-TiO^sub 2^ 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 [0 0 1] 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 [0 0 1] 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.