We report on Fe films grown on Au-buffered Si(001) showing perpendicular magnetization at ambient temperatures. The in-plane magnetization in ultra-thin Fe films grown at 150 K reorients irreversibly out-of-plane as a result of annealing close to room temperature. A Au cap layer which is expected to segregate to the top of the Fe film may be responsible for this behaviour. This hypothesis is based on the observation of an in-plane to out-of-plane reorientation of magnetization upon the growth of an ultra-thin Au cap layer on a slightly thicker Fe film. The origin of such a strong out-of-plane anisotropy of Fe induced by Au is discussed.