The magnetic properties of ∼2500‐A˚‐thick Co films evaporated in various oxygen pressures have been studied. Our results show that as the saturation magnetization is reduced, the coercive force in the perpendicular direction (Hc⊥) increases to a maximum (1.6 kOe); this change is consistent with results of previous investigations. The sign of the effective perpendicular anisotropy field (Hk eff=2K/Ms−4&pgr;Ms), as determined by ferromagnetic resonance (FMR), is always negative (<−4 kOe), indicating that the anisotropy field (2K/Ms) does not overcome the demagnetizing field (4&pgr;Ms). Our FMR results also confirm that films having anHc⊥value larger than that in‐plane coercive force (Hc∥ ) do not always demonstrate a value ofHk eff≳0. ©1995 American Institute of Physics.