Ferromagnetic order at Tb surfaces above the bulk Curie temperature

The magnetic order at surfaces of the 4f rare-earth metal terbium is investigated using electron capture spectroscopy (ECS), which probes the electron spin polarization (ESP) of the topmost surface layer. In ECS, the capture of spin-polarized electrons during grazing-angle ion-surface interaction is used to determine the ESP due to long- and short-ranged surface magnetic order. It is found that long-ranged ferromagnetic order exists far above the bulk Curie temperature, measured to be T/sub C//sub b/ = 220 K. At 140 K, the long-ranged ESP amounts to 24%. With increasing temperature, the ESP first decreases montonically up to the bulk Neel temperature T/sub N//sub b/ = 228 K, then exhibits a pronounced maximum at T = 238 K, and ultimately vanishes at the surface Curie temperature T/sub C//sub s/ = 248 K. These striking results on enhanced magnetic order at Tb surfaces suggest the presence of very large surface anisotropies