Perpendicular Anisotropy in Cobalt-nanodots on Rutile TiO2(110)*

We report the structural and magnetic properties of self-assembled Co-nanodots on rutile TiO$_{2}$(110) substrates. Co-dots with different coverage density were prepared inside a UHV chamber (base pressure $<$ 1x10$^{-10}$ Torr) by thermally evaporating Co-source on the well ordered rutile TiO$_{2}$(110) surface. The size of the Co-dots can be tuned by changing the coverage density and the dose rate of Co independently. The in-situ STM imaging indicates that the size of the Co-dots ranges from 0.5 nm to 1 nm. The magnetization of the systems was measured ex-situ using a Quantum Design SQUID magnetometer after they were capped with NaCl. The magnetization as a function of field measured at various temperatures for 2$<$T$<$250K reveals significant perpendicular anisotropy. The origin of the anisotropy is explained in terms of the competition between magnetocrystalline anisotropy and shape anisotropy of the nanodots.