Magnetic studies of ultrathin cobalt layers on a Ge(1 0 0) surface

We present the magnetic properties of ultrathin Co films deposited at 120 and 300 K on a Ge(1 0 0) surface. As deposited at 120 K, coercive force increases monotonically from 30 to 70Oe as the coverage of the cobalt film increases from 8 to 28 monolayers (M L). No Kerr intensity has been observed for samples with Co thickness below 5 ML. However, this nonmagnetic Co layer increases to 12 ML as the substrate temperature increases to 300 K, and the coercive force is roughly 15±5Oe lower than the sample with the same thickness deposited at 120 K. This could be attributed to the thermal annealing effect. From the linear extrapolation of the Kerr intensity below 28 ML, the nonmagnetic behavior of the initial deposit (∼5 ML) is attributed to the formation of nonmagnetic Co-Ge compound at the interface.