By means of first-principles electronic structure calculations, we hereby investigate the structural transitions induced by epitaxial strain in (111)-oriented (LaMnO$_3$)$_{2n}|$(SrMnO$_3$)$_n$ superlattices, with $n=2,4,6$. All superlattices in the explored range of strain are shown to prefer a half-metallic ferromagnetic order where the local magnetic moments are coupled to volume-breathing distortions. More in detail, our results reveal that thickness plays a crucial role in the response to epitaxial strain, which is particularly evident in the resulting tilt pattern of the oxygen octahedra. The thinnest superlattice, for $n=2$, always adopts the $a^-a^-a^-$ tilt pattern and the competing $a^-a^-c^+$ tilt pattern can be stabilized as a metastable state only in presence of compressive strain. Instead, the superlattice with $n=4$ favours the $a^-a^-c^+$ tilt pattern at equilibrium conditions, but the in-phase rotations around the third pseudocubic axis are so fragile that the $a^-a^-a^-$ pattern is recovered under a tiny amount of either compressive or tensile strain. The superlattice with $n=6$ exhibits a more nuanced behaviour: compressive strain drives a transition from $a^-a^-c^+$ to $a^-a^-a^-$, whereas tensile strain preserves the $a^-a^-c^+$ tilt pattern and significantly accentuates the structural differences between the two inequivalent sublattices within this symmetry. In fact, the Jahn-Teller distortions are quenched in one of the sublattices, leading to enhanced volume-breathing distortions and corresponding enhanced charge and spin oscillations. This suggests that Hund's physics may be more relevant in this regime of tensile strain, maximizing the interplay between strong electronic correlations and structural effects.
The modulation of charge density and spin order in (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ ($n$=1-4) superlattices is studied via Monte Carlo simulations of the double-exchange model. G-type antiferromagnetic barriers in the SrMnO$_{3}$ regions with low charge density are found to separate ferromagnetic LaMnO$_{3}$ layers with high charge density. The recently experimentally observed metal-insulator transition with increasing $n$ is reproduced in our studies, and $n=3$ is found to be the critical value.
We have performed ab initio calculations within the LDA+U method in the multilayered system (LaMnO$_3$)$_{2n}$ / (SrMnO$_3$)$_n$. Our results suggest a charge-ordered state that alternates Mn$^{3+}$ and Mn$^{4+}$ cations in a checkerboard in-plane pattern is developed at the interfacial layer, leading to a gap opening. Such an interfacial charge-ordered situation would be the energetically favored reconstruction between LaMnO$_3$ and SrMnO$_3$. This helps understanding the insulating behavior observed experimentally in these multilayers at intermediate values of $n$, whose origin is known to b…
Polarized neutron reflectivity measurements of a ferromagnetic [(LaMnO$_3$)$_{11.8}$/(SrMnO$_3$)$_{4.4}$]$_6$ superlattice reveal a modulated magnetic structure with an enhanced magnetization at the interfaces where LaMnO$_3$ was deposited on SrMnO$_3$ (LMO/SMO). However, the opposite interfaces (SMO/LMO) are found to have a reduced ferromagnetic moment. The magnetic asymmetry arises from the difference in lateral structural roughness of the two interfaces observed via electron microscopy, with strong ferromagnetism present at the interfaces that are atomically smooth over tens of nanometers. …
Photoinduced magnetization dynamics is investigated in chemically ordered $(\mathrm{LaMnO}_3)_{2n}/(\mathrm{SrMnO}_3)_n$ superlattices using the time-resolved magneto-optic Kerr effect. A monotonic frequency-field dependence is observed for the $n=1$ superlattice, indicating a single spin population consistent with a homogeneous hole distribution. In contrast, for $n\geq2$ superlattices, a large precession frequency is observed at low fields indicating the presence of an exchange torque in the dynamic regime. We propose a model that ascribes the emergence of exchange torque to the coupling bet…
Using first-principles calculations based on the density functional theory, we show a strong strain dependence of magnetic order in (LaMnO$_{3}$)$_n$/(SrMnO$_{3}$)$_{2n}$ (001) superlattices with $n=1,2$. The epitaxial strain lifts the degeneracy of Mn $e_{g}$ orbitals, thus inducing an inherent orbital order, which in turn strongly affects the ferromagnetic double exchange of itinerant $e_{g}$ electrons, competing with the antiferromagnetic superexchange of localized $t_{2g}$ electrons. For the case of tensile strain induced by SrTiO$_3$ (001) substrate, we find that the ground state is A-typ…
We have studied the influence of epitaxial strain on magnetic orderings and the couplings between the spin and polar phonons in the 1:1 SrMnO$_3$/LaMnO$_3$ superlattice from first principles. Magnetic phase transitions of the superlattice induced by epitaxial strain are observed, consistent with previous reports. We find that oxygen octahedral rotations lower the ground state energy but do not destroy the magnetic phase transitions induced by strain. We compute zone center phonon frequencies and eigenvectors as functions of epitaxial strain and magnetic ordering. A substantial increase of the …
Electronic reconstruction at the polar interface LaMnO$_3$/SrMnO$_3$ (LMO/SMO) (100) resulting from the polar catastrophe is studied from a model Hamiltonian that includes the double and super exchange interactions, the Madelung potential, and the Jahn-Teller coupling terms relevant for the manganites. We show that the polar catastrophe, originating from the alternately charged LMO layers and neutral SMO layers, is quenched by the accumulation of an extra half electron per cell in the interface region as in the case of the LaAlO$_3$/SrTiO$_3$ interface. In addition, the Mn e$_g$ electrons leak…
We measure the optical conductivity of (SrMnO3)n/(LaMnO3)2n superlattices (SL) for n=1,3,5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator to metal transition (IMT) for n \leq 3, driven by the softening of a polaronic mid-infrared band. At n = 5 that softening is incomplete, while at the largest-period n=8 compound the MIR band is independent of T and the SL remains insulating. One can thus first observe the IMT in a manganite system in the absence of the disorder due to chemical doping. Unsuccessful reconstruction of the SL optical properties from those of the original bulk mater…
In this Letter, we present investigations of thermal transport in (LaMnO$_3$)$_n$/(SrMnO$_3$)$_n$ superlattices (SLs) with SL periods $Λ=2n$ between $2$ and $12$ unit cells. The analysis of the experimental data clearly reveals a minimum in the thermal conductivity $κ$ at $Λ=6$ u.c. Furthermore, we theoretically estimate the phonon mean free path to be about $3Λ$ in our SLs. These findings clearly show that, in a wide temperature range of $\pm 100\,$K around room temperature, thermal transport in our manganite SLs is coherent. In addition, we demonstrate that antiferromagnetic magnetic orderin…
We conducted lateral force microscopy measurements on seven [LaMnO$_{3}$]$_{m}$/[SrMnO$_{3}$]$_{n}$ superlattices with varied layer thicknesses. We observe that the friction forces and the friction coefficients initially increase with increasing LaMnO3 top layer thickness, followed by saturation when the top layer thickness exceeds a few nanometers. These observations clearly demonstrate that sliding friction is affected by sub-surface material properties to a depth of several nanometers and is not just determined by dynamics in the contact interface. We argue that the sub-surface dissipated e…
We study the magnetic structure of the (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices from density-functional calculations. In agreement with the experiments, we find that the magnetism changes with the layer thickness `n'. The reason for the different magnetic structures is shown to be the varying potential barrier across the interface, which controls the leakage of the Mn-e$_g$ electrons from the LMO side to the SMO side. This in turn affects the interfacial magnetism via the carrier-mediated Zener double exchange. For n=1 superlattice, the Mn-e$_g$ electrons are more or less spread over t…
Oxygen octahedral rotations have been measured in short-period (LaNiO$_3$)$_n$/(SrMnO$_3$)$_m$ superlattices using synchrotron diffraction. The in-plane and out-of-plane bond angles and lengths are found to systematically vary with superlattice composition. Rotations are suppressed in structures with $m>n$, producing a nearly cubic form of LaNiO$_3$. Large rotations are present in structures with $m<n$, leading to reduced bond angles in SrMnO$_3$. The metal-oxygen-metal bond lengths decrease as rotations are reduced, in contrast to behavior previously observed in strained, single layer films. …
We study how strain affects orbital ordering and magnetism at the interface between SrMnO$_3$ and LaMnO$_3$ from density-functional calculations and interpret the basic results in terms of a three-site Mn-O-Mn model. Magnetic interaction between the Mn atoms is governed by a competition between the antiferromagnetic superexchange of the Mn t$_{2g}$ core spins and the ferromagnetic double exchange of the itinerant e$_g$ electrons. While the core electrons are relatively unaffected by the strain, the orbital character of the itinerant electron is strongly affected, which in turn causes a large c…
The exchange coupling at the interfaces of magnetic superlattices consisting of ferromagnetic $SrRuO_3$ and antiferromagnetic $SrMnO_3$ grown on (001) oriented $SrTiO_3$ is studied with in-plane and out-of-plane orientations, with respect to the substrate plane, of the cooling magnetic field. The magnetization of the in-plane, field cooled hysteresis loop is lower than the corresponding in-plane zero-field-cooled hysteresis loop. The out-of-plane field cooled hysteresis loop is shifted, from the origin, along the graphical magnetization axis. We attribute this irreversible rotation of the mome…
A direct correlation is seen between the coercive field (HC) and the magnetic-field-dependent resistivity (MR) in SrMnO3/SrRuO3 superlattices of perpendicular magnetic anisotropy. The magnetoresistance shows a sharp jump at Hc for in-plane current and the out-of-plane magnetic field. Both HC and high-field MR also oscillate with the thickness of the SrMnO3 spacer layers separating the metallic ruthenate. Since the spacer in these superlattices has no mobile carriers to facilitate an oscillatory coupling, we attribute the observed behavior to the spin-polarized quantum tunneling of electrons be…
Using first-principles density functional theory calculations, we investigated the electronic structure and magnetic properties of (LaMnO$_3$)$_m$/(LaNiO$_3$)$_n$ superlattices stacked along (001)-direction. The electrons are transferred from Mn to Ni, and the magnetic moments are induced at Ni sites that are paramagnetic in bulk and other types of superlattices. The size of induced moment is linearly proportional to the amount of transferred electrons, but it is larger than the net charge transfer because the spin and orbital directions play important roles and complicate the transfer process…
The emergence of magnetic reconstructions at the interfaces of oxide heterostructures are often explained via subtle modifications in the electronic densities, exchange couplings, or strain. Here an additional possible route for induced magnetism is studied in the context of the (LaNiO$_3$)$_n$/(LaMnO$_3$)$_n$ superlattices using a hybrid tight-binding model. In the LaNiO$_3$ region, the induced magnetizations decouple from the intensity of charge leakage from Mn to Ni, but originate from the spin-filtered quantum confinement present in these nanostructures. In general, the induced magnetizati…
Motivated by recent experiments, we use the $+U$ extension of the generalized gradient approximation to density functional theory to study superlattices composed of alternating layers of LaNiO$_3$ and LaMnO$_3$. For comparison we also study a rocksalt ((111) double perovskite) structure and bulk LaNiO$_3$ and LaMnO$_3$. A Wannier function analysis indicates that band parameters are transferable from bulk to superlattice situations with the exception of the transition metal d-level energy, which has a contribution from the change in d-shell occupancy. The charge transfer from Mn to Ni is found …
Artificially fabricated 3$d$/5$d$ superlattices (SLs) involve both strong electron correlation and spin-orbit coupling in one material by means of interfacial 3$d$-5$d$ coupling, whose mechanism remains mostly unexplored. In this work we investigated the mechanism of interfacial coupling in LaMnO$_3$/SrIrO$_3$ SLs by several spectroscopic approaches. Hard x-ray absorption, magnetic circular dichroism and photoemission spectra evidence the systematic change of the Ir ferromagnetism and the electronic structure with the change of the SL repetition period. First-principles calculations further re…
Taking advantage of the large electron escape depth of soft x-ray angle resolved photoemission spectroscopy we report electronic structure measurements of (111)-oriented [LaNiO3/LaMnO3] superlattices and LaNiO3 epitaxial films. For thin films we observe a 3D Fermi surface with an electron pocket at the Brillouin zone center and hole pockets at the zone vertices. Superlattices with thick nickelate layers present a similar electronic structure. However, as the thickness of the LaNiO3 is reduced the superlattices become insulating. These heterostructures do not show a marked redistribution of spe…
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-metal…
Charge transfer in superlattices consisting of SrIrO$_3$ and SrMnO$_3$ is investigated using density functional theory. Despite the nearly identical work function and non-polar interfaces between SrIrO$_3$ and SrMnO$_3$, rather large charge transfer was experimentally reported at the interface between them. Here, we report a microscopic model that captures the mechanism behind this phenomenon, providing a qualitative understanding of the experimental observation. This leads to unique strain dependence of such charge transfer in iridate-manganite superlattices. The predicted behavior is consist…
The functional properties of transition metal perovskite oxides are known to result from a complex interplay of magnetism, polarization, strain, and stoichiometry. Here, we show that for materials with a cooperative Jahn-Teller distortion, such as LaMnO$_3$ (LMO), the orbital order can also couple to the defect chemistry and induce novel material properties. At low temperatures, LMO exhibits a strong Jahn-Teller distortion that splits the $e_g$ orbitals of the high-spin Mn$^{3+}$ ions and leads to alternating long, short, and intermediate Mn--O bonds. Our DFT+$U$ calculations show that, as a r…
Perovskite bilayers with (111)-orientation combine a honeycomb lattice as a key feature with the strongly correlated, multiorbital nature of electrons in transition metal oxides. In a systematic DFT+$U$ study of (111)-oriented (La$X$O$_3$)$_2$/(LaAlO$_3$)$_4$ superlattices, we establish trends in the evolution of ground states versus band filling in (111)-oriented (La$X$O$_3$)$_2$/(LaAlO$_3$)$_4$ superlattices, with $X$ spanning the entire $3d$ transition metal series. The competition between local quasi-cubic and global triangular symmetry triggers unanticipated broken symmetry phases, with m…
