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

Highlights • This study investigates the magnetic properties of rutile TiO 2 implanted with cobalt ions. • SPM behavior here is highly anisotropic coupled with magneto-crystalline anisotropy of the Ti 1−x Co x O 2. • The Co nanoclusters possess an easy and hard axis of magnetization. • The M-T variation reveals the prominent role of dipole and inter-cluster exchange interaction. • Highly anisotropic nature of the SPM presents applications in magnetic switching devices. Abstract The present study investigates the magnetic properties of single crystal rutile TiO 2 (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 [ 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. [ABSTRACT FROM AUTHOR]