  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.

Strongly Correlated Electrons;Mesoscale and Nanoscale Physics;Materials Science