  We give experimental and theoretical evidence of the Rashba effect at the
magnetic rare-earth metal surface Gd(0001). The Rashba effect is substantially
enhanced and the Rashba parameter changes its sign when a metal-oxide surface
layer is formed. The experimental observations are quantitatively described by
ab initio calculations that give a detailed account of the near-surface charge
density gradients causing the Rashba effect. Since the sign of the Rashba
splitting depends on the magnetization direction, the findings open up new
opportunities for the study of surface and interface magnetism.
