Abstract
Most previous investigations have shown that the surface of a ferromagnetic material may have antiferromagnetic
tendencies. However, experimentally, the opposite effect has been recently observed���
ferromagnetism appears in some nanosized manganites with a composition such that the antiferromagnetic
charge-ordered CE state is observed in the bulk. A possible origin is the development of ferromagnetic
correlations at the surface of these small systems. To clarify these puzzling experimental observations, we have
studied the two-orbital double-exchange model near half doping, n=0.5, using open boundary conditions to
simulate the surface of either bulk or nanosized manganites. Considering the enhancement of surface charge
density due to a possible AO termination A=trivalent/ divalent ion composite, O=oxygen, an unexpected
surface phase-separated state emerges when the model is studied using Monte Carlo techniques on small
clusters. This tendency suppresses the CE charge ordering and produces a weak ferromagnetic signal that could
explain the experimental observations.
