Ferromagnetic tendency at the surface of CE charge-ordered manganites

TL;DR

This study uses a theoretical model to explain recent experimental findings of ferromagnetic tendencies at the surfaces of nano-sized CE charge-ordered manganites, revealing a surface phase separation that suppresses charge order and induces weak ferromagnetism.

Contribution

It demonstrates that surface charge density enhancement can lead to a phase-separated state with ferromagnetic tendencies, providing a theoretical explanation for experimental observations.

Findings

Surface phase separation suppresses CE charge order.

Weak ferromagnetic signals emerge at the surface.

Model reproduces experimental ferromagnetic tendencies.

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 nano-sized 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 nano-sized 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.