Exchange interactions and finite-temperature magnetism in (111)-oriented (LaMnO$_3$)$_{2n}$|(SrMnO$_3$)$_n$ superlattices

Abstract

We present a first-principles investigation of magnetic exchange interactions and critical behavior in (111)-oriented (LaMnO$_3$)$_{2n}$|(SrMnO$_3$)$_n$ superlattices for $n=2,4,6$. For all superlattices under investigation, we find robust half-metallic ferromagnetism extending across all the layers of both component regions. Changing octahedral tilt patterns is found to have negligible effects on the magnetic properties, despite determining the presence or absence of small Jahn-Teller distortions. The analysis of the response of the magnetic coupling to a variation of the Coulomb interaction parameters demonstrates that ferromagnetism is driven by a double-exchange mechanism involving itinerant $e_g$ electrons, while its final strength is hampered by antiferromagnetic contributions due to the superexchange of localized $t_{2g}$ electrons. Multi-scale simulations based on atomistic spin dynamics show that the thinnest superlattices, $n=2,4$, possess an ordering temperature that is at least comparable to that of La$_{2/3}$Sr$_{1/3}$MnO$_3$. Conversely, as thickness increases, a two-phase behavior emerges, where the SrMnO$_3$ region loses long-range order faster than the LaMnO$_3$ region. While the global ordering temperature increases together with thickness, we argue that the high-temperature regime for the observed two-phase behavior is not representative of the real physical system, which will undergo a combined electronic, magnetic and structural phase transition as soon as the long-range order is lost inside the SrMnO$_3$ region. This study provides insights into the emergent magnetic phases and transition temperatures relevant to oxide heterostructures.