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

Shivalika Sharma, Julio do Nascimento, Imran Ahamed, Fabrizio Cossu, Heung-Sik Kim, Igor Di Marco

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.

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)|(SrMnO) superlattices for . 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 electrons, while its final strength is hampered by antiferromagnetic contributions due to the superexchange of localized electrons. Multi-scale simulations based on atomistic spin dynamics show that the thinnest superlattices, , possess an ordering temperature that is at least comparable to that of LaSrMnO. Conversely, as thickness increases, a two-phase behavior emerges, where the SrMnO region loses long-range order faster than the LaMnO 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 region. This study provides insights into the emergent magnetic phases and transition temperatures relevant to oxide heterostructures.
Paper Structure (6 sections, 2 equations, 6 figures, 1 table)

This paper contains 6 sections, 2 equations, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Methods and models
  3. Zero-temperature magnetic properties
  4. Nature of the exchange coupling
  5. Critical Temperature and Magnetic Ordering
  6. Conclusions

Figures (6)

  • Figure 1: Side view of the (111)-oriented (LMO)$_{4}$|(SMO)$_2$ superlattice, emphasizing the stacking sequence of the various layers.
  • Figure 2: Layer-resolved magnetic moments at the Mn sites in the (111)-oriented (LMO)$_{2n}$|(SMO)$_n$ superlattices in the $a^-a^-a^-$ tilting system for $n=2,4,6$ (a-c). Corresponding layer-resolved interatomic exchange interactions $J_{ij}$ (in meV) between the magnetic moments at the Mn sites for the first (straight segments) and fourth (semi-circular arcs) nearest neighbors along the (001) direction (d-f). The data for $n=6$ have already been reported in Ref. Cossu2022 and are shown here for an easier comparison.
  • Figure 3: Layer-resolved magnetic moments at the Mn sites in the (111)-oriented (LMO)$_{8}$|(SMO)$_4$ superlattice in the $a^-a^-c^+$ tilting system, separated in two distinct chains arising from the asymmetric tilt pattern (a). Corresponding layer-resolved interatomic exchange interactions $J_{ij}$ (in meV) between the magnetic moments at the Mn sites for the first (straight segments) and fourth (semi-circular arcs) nearest neighbors along the (001) direction (b-c).
  • Figure 4: Layer-resolved magnetic moments at the Mn sites in the (111)-oriented (LMO)$_{4}$|(SMO)$_2$ superlattice in the $a^-a^-a^-$ tilting system, for two different sets of Coulomb interaction parameters (a). Layer-integrated Mn-$3d$ and O-$2p$ PDOS as obtained for $U = 3.8$ eV (solid lines) and $U = 5.0$ eV (dashed lines), with a fixed $J = 1$ eV (b).
  • Figure 5: Orbital-resolved interatomic exchange interactions $J_{ij}$ as a function of the interatomic distance $d/a$ for Mn atoms located inside the LMO and SMO regions as well as at the IF (see main text). Top panels (a-c) correspond to calculations for $U = 3.8$ eV and $J = 1$ eV, while bottom panels (d-f) correspond to calculations for $U = 5.0$ eV and $J = 1$ eV.
  • ...and 1 more figures