Annealed Co thin films: Pit formation and magnetic anisotropy

We present a study of the morphological and magnetic transformation of hcp~0001! Co thin films, grown via sputtering on Al2O3(112̄0), which occurs when they are annealed in vacuum in the temperature range of 500 °C,TA,600 °C. In this temperature range, the films undergo a reentrant smoothening transition. For very thin films ~thickness t,20 nm!, this coincides with a fcc~111! to fcc~001! recrystallization, and with the appearance of rectangular pits. For thicker films, the recrystallization does not occur, even though the surface smoothens out leaving hexagonal pits behind. The number of pits per unit area decreases exponentially as the thickness of the film is increased, demonstrating that interactions at the film surface and film–substrate interface cause the smoothening and pit formation during the annealing process. Whereas the hcp~0001! and fcc~111! phases are magnetically isotropic in the film’s plane, the fcc~001! films exhibit a biaxial in-plane anisotropy. © 2000 American Institute of Physics. @S0021-8979~00!50808-1#