Magnetic properties and microstructure of ultrathin Co/Si(111) films

The magnetic properties and microstructure of ultrathin Co films grown on a Si(111)-7x7 surface were investigated. The experimental results observed by surface magneto-optic Kerr effect (SMOKE) and scanning tunneling microscopy show that the surface morphological evolution of x ML (monolayer) Co/Si(111) films is strongly related to their magnetic properties. Due to the formation of a CoSi2 layer, no magnetic signal could be detected by SMOKE for x=2.1. Both longitudinal and polar hysteresis loops appear for 4.2-8.5 ML Co/Si(111) films because of their rougher surfaces. When the Co thickness is increased to 11 ML, a magnetic hysteresis loop only occurs in the longitudinal configuration, which can be attributed to the contribution of volume anisotropy. After annealing an 11 ML Co/Si(111) film at 400 and 500 K, the surface becomes rougher, inducing magnetic anisotropy on the polar configuration. When the annealing temperature was increased to 600 K, however, the Co could react with Si to form a nonmagnetic cobalt silicide compound, and consequently, the hysteresis loops disappeared.