Surface magnetism of Gd(0001) films: Evidence for an unexpected phase transition

By spin- and angle-resolved photoelectron spectroscopy we have investigated the surface magnetism of ultrathin Gd(0001) films on W(110). The binding energy shift of the Gd 4f surface emission allows us to study separately the surface and the subsurface in-plane magnetization. These two components show an unexpected and distinctly different behavior over an extended temperature range. The results indicate that the in-plane ordering temperature of the surface is by 80 K higher than that of the inner layers. In correspondence to the onset of the in-plane magnetic ordering inside the film the surface magnetization shows changes which could be related to an extraordinary phase transition.