Ferromagnetic tendency at the surface of CE charge-ordered manganites

摘要

Most previous investigations have shown that the surface of a ferromagneticmaterial may have antiferromagnetic tendencies. However, experimentally theopposite effect has been recently observed: ferromagnetism appears in somenano-sized manganites with a composition such that the antiferromagneticcharge-ordered CE state is observed in the bulk. A possible origin is thedevelopment of ferromagnetic correlations at the surface of these smallsystems. To clarify these puzzling experimental observations, we have studiedthe two-orbital double-exchange model near half-doping n=0.5, using openboundary conditions to simulate the surface of either bulk or nano-sizedmanganites. Considering the enhancement of surface charge density due to apossible AO termination (A = trivalent/divalent ion composite, O = oxygen), anunexpected surface phase-separated state emerges when the model is studiedusing Monte Carlo techniques on small clusters. This tendency suppresses the CEcharge ordering and produces a weak ferromagnetic signal that could explain theexperimental observations.