Roughening of cobalt thin films on sapphire (110) upon annealing and superparamagnetic behavior of cobalt nanodots on sapphire (001)

Atomic force microscopy (AFM) was employed to investigate the surface roughness of 5.0 nm thick Co films as a function of the annealing temperature. Films were epitaxially grown on (110) sapphire via molecular beam epitaxy (MBE) and magnetron sputtering. Images were acquired in-situ at temperatures ranging between room temperature and 600°C. Surface roughening starts at 425°C and increases depending on the final annealing temperature and time. The films did not show a reversal of the roughening process upon sample cooling. Also a sub-monolayer film of Co was grown on Al2O 3 (0001) via MBE at room temperature. The film annealed at 400°C revealed the formation of dots with an average diameter of 17.6 nm and a height of 0.3 nm. Dots showed a superparamagnetic behavior with a blocking temperature of 130 K and an enhancement of the atomic magnetic moment over the bulk of approximately 9%.