Control of octahedral rotations in (LaNiO3)n/(SrMnO

Abstract

Oxygen octahedral rotations have been measured in short-period (LaNiO3)n/(SrMnO3)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 LaNiO3. Large rotations are present in structures with m < n, leading to reduced bond angles in SrMnO3. The metal-oxygen-metal bond lengths decrease as rotations are reduced, in contrast to behavior previously observed in strained, single layer films. This result demonstrates that superlattice structures can be used to stabilize non-equilibrium octahedral behavior in a manner distinct from epitaxial strain, providing a novel means to engineer the electronic and ferroic properties of oxide heterostructures.The ABO 3 perovskite oxides exhibit an array of physical properties making them attractive for applications in electronics and energy conversion. 1-3 Recent work has demonstrated that the functionality of these materials can be further expanded through the formation of superlattices, which can exhibit enhanced properties compared to bulk compounds. While the majority of work in this field has focused on the electronic and ferroic properties of oxide superlattices, 4-9 the impact of heterointerfaces on the local atomic structure has received less attention. 10-13 Of particular importance are the rotations and distortions of the BO 6 octahedra, which determine the B-O-B bond angles (θ) and B-O bond lengths (d). As both θ and d couple to the electronic bandwidth, a quantitative understanding of how octahedral rotations are modified in short-period superlattices is necessary in order to control novel electronic phenomena in oxide heterostructures.Motivated by the question of what octahedral behavior is stabilized at the interface between two structurally dissimilar perovskites, we have investigated the bond angles in a systematic series of (LaNiO 3 ) n /(SrMnO 3 ) 2 superlattices. In bulk, LaNiO 3 (LNO) exhibits robust octahedral rotations leading to θ = 165.2 • along all < 001 > directions, 14 with an a − a − a − rotation pattern. 15 In contrast, bulk SrMnO 3 (SMO) is a cubic perovskite (a 0 a 0 a 0 ) lacking octahedral rotations (θ = 180 • ). 16 Due to the short-period of the superlattices investigated and the geometric constraint requiring that the BO 6 octahedra maintain corner connectivity, the octahedral rotations remain coherent throughout the superlattices. We demonstrate that the in-plane and out-of-plane bond angles and lengths can be tuned by altering the superlattice composition [n/(n+m)], allowing for the stabilization of octahedral behavior that differs substantially from that found in bulk compounds.The (LNO) n /(SMO) m superlattices, herein referred to as LnSm, were deposited on SrTiO 3 substrates by ozoneassisted molecular beam epitaxy. 17 The sample thicknesses are between 200 and 215Å. Previous structural (00L) in three samples exhibiting distinct satellite peaks due to t...

Correlating interfacial octahedral rotations with magnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices

“…4 and Supplementary Note 1). Owing to the biaxial strain from substrates, these layers actually assume distorted rhombohedral (DR) structures with a À a À c À rotations 5,6 . For the N ¼ 16 superlattice and the LaMnO 3 þ d film, the Mn valence is close to þ 3, and correspondingly, a distorted orthorhombic (DO) phase emerges with in-phase rotation around one of the in-plane axis [15][16][17]31,32 , resulting in obvious two-half-integer peaks.…”

“…3 describe the average effect of all the involved octahedra, including all MnO 6 and the interfacial TiO 6 . Although half-order peaks in N ¼ 2 sample should be dominated by interfacial OOR, inhomogeneity of OOR could exist for superlattices with larger N and affect the data analysis.…”

“…Since the half-order Bragg peaks in the Nr6 superlattices are expected to arise from the a À a À c À OOR, the rotation angles could be determined by numerically fitting the intensity pattern according to the model in refs 5,6. In this model, the positions of 24 oxygen atoms in a 2 Â 2 Â 2 supercell were individually calculated based on the OOR, and their X-ray scattering amplitudes were added up coherently to yield the best fit to the experimental peak intensities (Supplementary Note 1).…”

“…Extensive studies have been focused on introducing novel physical properties at the interface 1,2 . Recently, oxygen octahedral rotation (OOR) manipulation in artificial superlattices was made possible through accurate OOR detection based on cutting-edge scanning transmission electron microscopy 3,4 and synchrotron X-ray diffraction (XRD) 5,6 . Firstprinciples calculations have demonstrated that interface OOR can be altered or entirely different from the bulk phase by proximity effects, electronic instabilities or OOR-strain coupling [7][8][9][10] .…”

“…We conducted a combined experimental and theoretical study to find the correlation between the OOR and the magnetic properties in (LaMnO 3 þ d ) N /(SrTiO 3 ) N (N ¼ 2, 3,4,6,8,16) superlattices. The maximized ferromagnetic (FM) moment is found in the N ¼ 6 superlattice accompanied by minimal octahedral rotations.…”