Effect of annealing on the magnetic properties of ultrathin Co/Ge(1 1 1) films

Effect of annealing on the magnetic properties of ultrathin Co/Ge(1 1 1) films has been investigated using the surface magneto-optic Kerr effect technique. As the annealing temperature increases from 200 to 450 K, the Kerr intensity decreases to zero for all the samples. The temperature, where ferromagnetism vanishes, increases from below 200 to 375 K as the coverage of the cobalt film increases from 5 to 28 monolayers (ML). Corresponding compositions obtained from the Auger electron spectroscopy measurements show an intermixing of the cobalt adlayer and the Ge(1 1 1) substrate. For a 20-ML-thick Co/Ge(1 1 1) film, the intensity ratio of Co to Ge Auger signals drops to roughly half of the original value as the annealing temperature increases to 325 K, at which the hysteresis disappears. This indicates the formation of a CoGe compound at the surface layers. Cobalt films possess an in-plane anisotropy as deposited at a substrate temperature of 200 K. The magnetic easy axis retains in the surface plane at an elevated temperature.