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

Abstract The present study investigates the magnetic properties of single crystal rutile TiO 2 (110) implanted with cobalt ions for fluences between 5 × 10 16 and 1 × 10 17 ions/cm 2 with energy of 200 keV. The temperature and field dependent magnetization for magnetic fields along [0 0 1] and [ 1 1 ¯ 0 ] directions of the as implanted TiO 2 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 ( T B ) along [ 1 1 ¯ 0 ] with values ranging from 30 to 150 K, while for field along [0 0 1] shows a lower T B ranging from 8 to 70 K. The magnetization isotherms above T B show a sharper rise and universal scaling behaviour, for field along [ 1 1 ¯ 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 TiO 2 , thus indicating the highly oriented nature of the Co clusters. Role of dipole interactions and inter cluster exchange interactions have also been discussed.