Control of octahedral rotations via octahedral connectivity in an epitaxially strained [1 u.c. // 4 u.c. ] LaNiO3/LaGaO3 superlattice

Abstract

ABO 3 provskites represent a board spectrum of intriguing functionalities such as metal‐insulator transitions, multiferroicity, colossal magnetoresistance and superconductivity owing to the strong correlation between charge, spin and orbital degrees of freedom [1]. In recent years, heterostructures and superlattices formed by two or more different ABO 3 perovskites have received intense research interest. Due to the electronic and structural reconstructions at the heterointerfaces, novel phenomena may emerge opening the way to novel functionalities that are not accessible in their bulk counterparts [2]. In ABO 3 perovskites, the magnetic and electronic states are strongly coupled to the B‐O‐B bond angles and B‐O bond lengths. Therefore, precise control of the BO 6 octahedral rotations and distortions via epitaxial strain and interfacial octahedral connectivity offers a promising route to tailoring the material properties in a controllable way [3]. In our previous study of a [(4 u.c.// 4 u.c.) ×8] LaNiO 3 /LaGaO 3 superlattice grown on (001) SrTiO 3 by using aberration‐corrected high‐resolution transmission electron microscopy (AC‐HRTEM), we have shown that the response of NiO 6 rotations to epitaxial strain in the LaNiO 3 /LaGaO 3 superlattice is significantly different from that in LaNiO 3 thin films [4]. In LaNiO 3 thin films, epitaxial strain effectively modifies the NiO 6 rotational magnitudes throughout the entire film therefore stabilizing new electronic and magnetic states [3]. However, in the [(4 u.c.// 4 u.c.) ×8] LaNiO 3 /LaGaO 3 superlattice, the [100] and [010] rotational magnitudes of NiO 6 and GaO 6 relax to the bulk values of LaNiO 3 and LaGaO 3 even though the superlattice is still coherently strained. Here, we investigated the local octahedral rotations in a [(1 u.c.// 4 u.c.) ×13] LaNiO 3 /LaGaO 3 superlattice grown on a (001) SrTiO 3 substrate. Figure 1 shows the atomic models of bulk LaNiO3 and LaGaO3 and the simulated [110] AC‐HRTEM images. Figure 2 represents an experimental AC‐HRTEM image near the surface of the superlattice. We found that, due to the octahedral connectivity at LaNiO3‐LaGaO3 interfaces (see Fig. 1e), the octahedral rotations of NiO 6 adopted the same [100] and [010] rotational magnitudes as the neighbouring GaO 6 till the surface of the superlattice. Our results indicate that in LaNiO 3 based superlattices, the octahedral rotations of NiO 6 can be precisely controlled via interfacial octahedral connectivity when the thickness of the LaNiO 3 layer is reduced to 1 unit cell.