Magnetic phases of Co/Ge(1 1 1) studied using surface magneto-optic Kerr effect

Co/Ge(111) films are nonferromagnetic up to 9.5 monolayers as deposited at 300 K. Upon cryogenic treatments, magnetic phases are experimentally resolved for Co/Ge(111) films thinner than 12 monolayers using the surface magneto-optic Kerr effect technique. Systematic investigations of the magnetic properties for Co/Ge(111) films revealed a transition of nonferromagnetism to ferromagnetism at low temperatures. The transition temperature increases from below 125 K to above 300 K as the cobalt coverage increases from 6.3 to 12 monolayers. The critical exponent β in the power law relationship of magnetization is about 0.23 that is close to the value expected by a two-dimensional XY model. Auger electron spectroscopy measurements suggest the co-existence of a pure cobalt and a Co-Ge compound layers in the nonferromagnetic Co/Ge(111) films.