Magnetic phases of the interfacial layers were comparatively investigated for Co/Ge(100) and Co/Oe(111) films thinner than 13 ML using surface magneto-optic Kerr effect technique. Co/Ge(100) films are nonferromagnetic up to 12 ML as deposited and measured at 300 K. A transition from nonferromagnetism to ferromagnetism occurs upon cryogenic treatments. Systematic investigations of the magnetic properties for Co/Ge(100) reveal a magnetic phase diagram. The boundary between nonferromagnetic and ferromagnetic phases was experimentally determined to be from below 150 K to above 300 K as the cobalt thickness increases from 9 to 13 ML. This behavior is consistent with the thickness-dependent scaling law of the Curie temperature for a thin film system. Because of the difference of the densities of surface atoms, the different thickness of the formed interfacial compounds could explain the shift of the phase boundary to higher Co thickness side as compared to the Co/Ge(111) system. In addition, the critical exponent beta in the power law relationship of magnetization for Co/Ge(100) films lies close to the value expected by the three-dimensional Heisenberg model, while the critical exponent of Co/Ge(111) is close to the value of the two-dimensional XY model. The different behaviors of the magnetic transition are attributed to the different interfacial structures. (c) 2005 American Vacuum Society.