Persistent half-metallic ferromagnetism in a (111)-oriented manganite superlattice

TL;DR

This study demonstrates that a (111)-oriented LaMnO$_3$|SrMnO$_3$ superlattice maintains half-metallic ferromagnetism over larger thicknesses than the (001) orientation, due to charge transfer and octahedral connectivity effects.

Contribution

It reveals the enhanced stability of half-metallic ferromagnetism in (111)-oriented superlattices and links this behavior to charge transfer, symmetry effects, and Hund's metal characteristics.

Findings

Half-metallic ferromagnetism persists at larger thicknesses in (111) orientation.

Charge transfer and octahedral connectivity influence magnetic properties.

System exhibits features of a Hund's metal, with charge and spin oscillations.

Abstract

Heterostructures of mixed-valence manganites are still under intense scrutiny, due to the occurrence of exotic quantum phenomena linked to electronic correlation and interfacial composition. For instance, if two anti-ferromagnetic insulators as LaMnO$_3$ and SrMnO$_3$ are grown in a (001)-oriented superlattice, a half-metallic ferromagnet may form, provided that the thickness is sufficiently small to allow tunneling across interfaces. In this article, we employ electronic structure calculations to show that all the layers of a (111)-oriented LaMnO$_3$|SrMnO$_3$ superlattice retain a half-metallic ferromagnetic character for a much larger thickness than in its (001) counterpart. This behavior is shown to be linked to the charge transfer across the interface, favored by the octahedral connectivity between the layers. This also results in a symmetry-induced quenching of the Jahn-Teller distortions, which are replaced by breathing modes. The latter are coupled to charge and spin oscillations, whose components have a pure e g character. Most interestingly, the magnetization reaches its maximum value inside the LaMnO$_3$ region and not at the interface, which is fundamentally different from what observed for the (001) orientation. The analysis of the inter-atomic exchange coupling shows that the magnetic order arises from the double-exchange mechanism, despite competing interactions inside the SrMnO$_3$ region. Finally, the van Vleck distortions and the spin oscillations are found to be crucially affected by the variation of Hund's exchange and charge doping, which allows us to speculate that our system behaves as a Hund's metal, creating an interesting connection between manganites and nickelates.