It is observed that a reversible step period rearrangement on clean nickel single-crystal surfaces occurs in the immediate vicinity of the Curie temperature. Reversible carbon segregation is observed on the same crystal surfaces below the Curie point. The segregated carbon is carbidic, not graphitic, and indicates a change in the nickel surface electronic structure occurring at the ferromagnetic transition. Measured carbon coverages indicate a change greater than 0.2 eV per carbon atom in the heat of segregation at the Curie point.

• Received 13 January 1981

DOI:https://doi.org/10.1103/PhysRevLett.46.745