Double-exchange-driven spin pairing at the (001) surface of manganites

Study of the (001) surface of the La1-xCaxMnO3 system in various magnetic orderings points to a general occurrence: z(2) dangling bond charge-which is invisible in the formal valence picture-is promoted to the bulk gap and/or Fermi level region. This unexpected occurrence, obtained from first-principles calculations, drives a double-exchange-like process that serves to align the surface Mn spin with its subsurface neighbor, regardless of the bulk magnetic order. For heavy doping, the locally ferromagnetic coupling is very strong and the moment enhanced by as much as 30% over the bulk value.