Ferromagnetic order at Tb surfaces above the bulk Curie temperature

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Abstract
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 TCb =220 K. At 140 K, the long‐ranged ESP amounts to 24%. With increasing temperature, the ESP first decreases montonically up to the bulk Néel temperature TNb=228 K, then exhibits a pronounced maximum at T=238 K, and ultimately vanishes at the surface Curie temperature TCs =248 K. These striking results on enhanced magnetic order at Tb surfaces suggest the presence of very large surface anisotropies.