On the magnetic stability at the surface in strongly correlated electron systems

TL;DR

This paper investigates how strong electron correlations affect the magnetic stability at the surface of strongly correlated materials, revealing that correlations can destabilize surface ferromagnetism contrary to simple band theory predictions.

Contribution

It demonstrates that electron correlations beyond the Hartree-Fock level can destabilize surface ferromagnetism, contrasting with traditional expectations based on reduced surface coordination.

Findings

Surface magnetization decreases faster than bulk with temperature.

Reduced hopping integral at the surface destabilizes ferromagnetism.

Enhanced hopping integral stabilizes surface ferromagnetism.

Abstract

The stability of ferromagnetism at the surface at finite temperatures is investigated within the strongly correlated Hubbard model on a semi-infinite lattice. Due to the reduced surface coordination number the effective Coulomb correlation is enhanced at the surface compared to the bulk. Therefore, within the well-known Stoner-picture of band ferromagnetism one would expect the magnetic stability at the surface to be enhanced as well. However, by taking electron correlations into account well beyond the Hartree-Fock (Stoner) level we find the opposite behavior: As a function of temperature the magnetization of the surface layer decreases faster than in the bulk. By varying the hopping integral within the surface layer this behavior becomes even more pronounced. A reduced hopping integral at the surface tends to destabilize surface ferromagnetism whereas the magnetic stability gets enhanced by an increased hopping integral. This behavior represents a pure correlation effect and can be understood in terms of general arguments which are based on exact results in the limit of strong Coulomb interaction.