Orientation-driven route to an intrinsic insulating ferromagnetic state in manganite superlattices

TL;DR

This paper predicts that (111)-oriented LaMnO3/SrTiO3 superlattices can exhibit an intrinsic insulating ferromagnetic state driven by structural and electronic factors, offering new pathways for spintronic device development.

Contribution

It demonstrates that orientation and structural order can induce insulating ferromagnetism in manganite superlattices, a phenomenon not reliant on extrinsic effects.

Findings

Insulating ferromagnetic state predicted in (111)-oriented superlattices.

Bandgap varies with composition, being direct or indirect.

Superlattices behave as Kugel-Khomskii materials.

Abstract

Increasing precision in the growth of superlattices sparks hope in applications that may arise from engineering layered structures. Heterostructuring and functionalization of magnetic oxides have been very popular due to their versatility and readiness for integration in modern electronics. In this study, we provide yet another example of this phenomenology by predicting that an insulating ferromagnetic state can be realized in superlattices of LaMnO$_3$ and SrTiO$_3$ oriented along the (111) direction. In strike contrast with respect to other orientations, these properties are not of extrinsic origin but arise from the interplay of structural order, strain and quantum confinement. The bandgap is shown to be either direct and indirect, depending on the precise composition, which can be explained in terms of the geometrical properties of (111)-oriented bilayers of LaMnO$_3$. The electronic structure shows narrow bands indicating localized $e_g$ states for all the investigated superlattices. These features and the analysis of the inter-atomic magnetic coupling suggest that the investigated superlattices behave as a Kugel-Khomskii material, at least for the explored compositions. Our results provide not only a new route to an insulating ferromagnet, but also novel insight into the intricate interplay between lattice symmetry, Hubbard physics and Hund's coupling to be exploited in next-generation spintronic applications.