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

The (001) surface of La_{1-x}Ca_xMnO_3 system in various magnetic orderings is studied by first principle calculations. A general occurrence is that z^2 dangling bond charge – which is invisible'' in the formal valence picture -- is promoted to the bulk gap/Fermi level region. This 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.