Research Update: Interface-engineered oxygen octahedral tilts in perovskite oxide heterostructures

Interface engineering of structural distortions is a key for exploring the functional properties of oxide heterostructures and superlattices. In this paper, we report on our comprehensive investigations of oxygen octahedral distortions at the heteroin-terface between perovskite oxides SrRuO 3 and BaTiO 3 on GdScO 3 substrates and of the influences of the interfacially engineered distortions on the magneto-transport properties of the SrRuO 3 layer. Our state-of-the-art annular bright-field imaging in aberration-corrected scanning transmission electron microscopy revealed that the RuO 6 octahedral distortions in the SrRuO 3 layer have strong dependence on the stacking order of the SrRuO 3 and BaTiO 3 layers on the substrate. This can be attributed to the di ff erence in the interfacial octahedral connections. We also found that the stacking order of the oxide layers has a strong impact on the magneto-transport properties, allowing for control of the magnetic anisotropy of the SrRuO 3 layer through interface engineering. Our results demonstrate the significance of the interface engineering of the octahedral distortions on the structural and physical properties of perovskite oxides. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http: // dx.doi.org / 10.1063 / 1.4918965]

A broad spectrum of fascinating functional properties seen in heterointerfaces, which consist of dissimilar transition-metal oxides, has attracted much attentions. [1][2][3][4][5][6][7] Due to recent advances in atomic level synthesis technique, interface engineering [8][9][10][11][12][13] of oxides has emerged as a useful approach to explore and fine tune their functional properties. Because manipulating the interface is often associated with the added structural distortions, it is crucial to understand how the interfacial modification affects not only the structural but also the physical properties of entire constituent oxide layers. However, full understanding of the influence of interfacial distortions remains elusive because precise measurements of the positions of both cation and oxygen are required.
Cross-sectional imaging in aberration-corrected scanning transmission electron microscopy (STEM) has been shown to be useful for revealing oxygen octahedral distortions including deformations and tilts (or rotations) in oxide heterostructures. [14][15][16][17][18][19][20][21] The recent development of annular bright-field (ABF) imaging in STEM allows for simultaneous mapping of both light and heavy elements [22][23][24] and makes it possible to visualize oxygen coordination environments in the heterostructures. 18,25,26 Fast multiple-image acquisition and drift correction techniques using a cross correlation of the images 27 have been also demonstrated to minimize the image distortions due to drifts in both the specimen and incident probe in STEM, and consequently, the atomic positions for all constituent atoms-including oxygen-can be determined with sub-Ångström precision. 18,19,28 In this article, we present our investigations of oxygen octahedral distortions in heterostructures consisting of perovskite oxides SrRuO 3 (SRO) and BaTiO 3 (BTO) grown on GdScO 3 (GSO) substrates and our investigations of their influences on the magneto-transport properties of the SRO layer. Our high-resolution STEM observations reveal that octahedral distortion in the SRO layer a dkan@scl.kyoto-u.ac.jp is strongly dependent on the stacking order of the SRO and BTO layers. When a SRO layer is deposited on a BTO layer, a tetragonal SRO layer with a negligibly small RuO 6 tilt is stabilized. In contrast, for a SRO layer with a BTO layer on top of it, the gradually changing octahedral tilts occur in the SRO layer, but a monoclinic structure is maintained. We also found that the magneto-transport properties of the SRO layer are closely correlated with the interfacially engineered RuO 6 octahedral distortions. Based on the experimental results, we discuss the structural and physical properties of the interfacially engineered SRO layer. SRO (10 nm)/BTO (1.2 nm)/GSO and BTO (5 nm)/SRO (11 nm)/GSO heterostructures were fabricated by successively depositing SRO and BTO layers on (110) ortho GSO substrates using pulsed laser deposition. (The subscript ortho denotes the orthorhombic perovskite notation.) The thickness of each layer is indicated in parentheses. All depositions were carried out at a constant substrate temperature of 700 • C. The SRO and BTO layers were grown under oxygen pressures of 100 mTorr and 25 mTorr, respectively. The details of the growth conditions were given in our previous report. 20 For the cross-sectional observations, the fabricated heterostructures were thinned down to electron transparency by mechanical polishing and Ar-ion milling. The cross-sectional images were obtained using a spherical aberration-corrected STEM (JEM-9980TKP1; accelerating voltage = 200 kV, C s = −0.025 nm, and C 5 = 15 mm) equipped with a cold field emission gun. The details of the image acquisition and analysis processes have been provided in our previous paper. 18 Figures 1 and 2 show typical cross-sectional high-angle annular dark-field (HAADF-) and  Fig. 1(a)), respectively. The orange lines denote the positions of the BTO/GSO and SRO/BTO interfaces. (c) ABF image taken from the same region as the HAADF image ( Fig. 1(a)). In the ABF image, the oxygen atoms (dotted in red) can be clearly seen. were determined from the oxygen atom positions that were extracted with Bragg filtering and cubic interpolation techniques in the "Find Peaks" option of the HREM Research Peak Pairs Analysis software package. 30,31 The results for the SRO (10 nm)/BTO (1.2 nm)/GSO and BTO (5 nm)/SRO (11 nm)/GSO heterostructures are shown in Figures 3(a) and 4, respectively. The definitions of the θ tilt and ∆x are also shown in Fig. 3. We note that θ tilt used in this study corresponds to the tilt angle projected on the (001) ortho plane. For comparison, the tilt angle and in-plane oxygen displacement in the SRO (15 nm)/GSO heterostructure, of which the SRO layer has a monoclinic structure, 18,20,32 are included in Fig. 3(b). For the SRO/BTO/GSO heterostructure ( Fig. 3(a)), the octahedral tilt is strongly suppressed across the BTO/GSO interface. The θ tilt changes from 156 • in the substrate region to 180 • in the three unit-cell-thick BTO layer. Consequently, the θ tilt at the interface between the SRO and BTO layers is 180 • , and the RuO 6 octahedra in the SRO layer have negligibly small tilts, indicating that the SRO layer in the SRO/BTO/GSO heterostructure has a tetragonal structure. 32 The observed behavior is in contrast to that of the SRO/GSO heterostructure shown in Fig. 3(b). The SRO/GSO heterostructure has only a slight decrease in the tilt angle across the interface, and the SRO layer maintains the octahedral tilts comparable to those in the bulk counterpart, indicating that the SRO layer has the monoclinic structure. The results indicate that the BTO layer inserted between SRO and GSO results in the structural change in the SRO layer from the monoclinic phase to the tetragonal one.
The observed structural changes due to the insertion of the BTO layer can be attributed to changes in the octahedral connection at the interface. For the SRO/BTO/GSO heterostructure, the in-plane displacement ∆x of oxygen atoms, which is directly related to the octahedral tilt, is significantly reduced at the BTO/GSO interface and decreases to 0 pm within the three unit-cell-thick BTO layer, as shown in Fig. 3(a). The results indicate that a RuO 6 -TiO 6 octahedral connection with negligibly small tilts is formed at the SRO/BTO interface and that the propagation of the octahedral tilt from the GSO substrate into the SRO layer is blocked by the three unit-cell-thick BTO layer, stabilizing the tetragonal SRO structure. However a ∆x as large as 26 pm is maintained at the SRO/GSO interface (Fig. 3(b)), and the tilted RuO 6 and ScO 6 are connected, facilitating the propagation of the octahedral tilts and consequently stabilizing the monoclinic SRO structure. These results indicate that the BTO layer inserted underneath the SRO layer modifies the interfacial octahedral connections associated with the change in the oxygen displacement, thereby determining the structural phase of the SRO layer.
For the BTO/SRO/GSO heterostructure (Fig. 4), the RuO 6 octahedral tilt, which remains comparable to the bulk counterpart near the SRO/GSO interface, undergoes a gradual decrease toward the BTO layer and is completely suppressed at the upper BTO/SRO interface. Although the SRO layer in the BTO/SRO/GSO heterostructure has a structure very close to the monoclinic structure seen in the SRO/GSO heterostructure, the observed behavior in the octahedral tilt is in stark contrast to those for the SRO/GSO heterostructures ( Fig. 3(b)), in which the SRO layer has the almost constant octahedral tilt. Interestingly, the inversion symmetry of the SRO layer of the BTO/SRO/GSO heterostructure is broken by the gradual change in the tilt angle, while the inversion symmetry of the SRO layer with the constant tilt-seen either in bulk or in the SRO/GSO heterostructure-is maintained. The results indicate that the introduction of the spatially inhomogeneous structural distortions through the interface engineering is a good way to stabilize oxides with structural symmetry not seen in bulk.
The oxygen displacement in Fig. 4 also shows the concomitant changes with the gradual decrease in the tilts. A large displacement of ∼29 pm at the SRO/GSO interface shows that the interfacial octahedral connection consists of the tilted RuO 6 and ScO 6 octahedra. However, the displacement at the BTO/SRO interface is largely suppressed to ∼2 pm, indicating that the RuO 6 octahedra with negligibly small tilts are connected to non-tilted TiO 6 . We ascribed the gradually changing RuO 6 tilts to the difference in the octahedral connection angle between the SRO/GSO and BTO/SRO interfaces. Given that the oxygen displacement at the interface determines the degree of the tilt propagation, 19 the top BTO layer, which suppresses the RuO 6 tilts induced by the tilt propagation from the substrate, influences the octahedral tilts in the underneath SRO layer through the interfacial octahedral connection, thus stabilizing the gradually changing RuO 6 tilt in the BTO/SRO/GSO heterostructure. It is interesting to compare the behavior of octahedral tilts between the BTO/SRO/GSO and BTO/GSO 19 heterostructures. While the BTO layers in both heterostructures stabilize the interfacial octahedral connections with the suppressed oxygen displacement (the TiO 6 -RuO 6 connection for BTO/SRO/GSO and the TiO 6 -ScO 6 one for BTO/GSO), the gradual change in the octahedral tilt is seen only in the BTO/SRO/GSO heterostructure. This observation implies that while the octahedral connection angle at the interface is controlled by the A-site cation size, 19 the number of the octahedral layer whose octahedral tilt can be modified through the interfacial octahedral connection strongly depends on the relative size mismatch between the A-and B-site cations in the constituent layer (Sr and Ru for SRO and Gd and Sc for GSO), which is in principle characterized by the Goldschmidt tolerance factor. [33][34][35][36] Given that the tolerance factor of SRO (0.994) is close to unity, the results suggest that perovskite oxides whose A-and B-site cations size mismatch is relatively small, preferentially accommodates spatially inhomogeneous structural distortions.
To evaluate the influences of the interfacially engineered RuO 6 octahedral tilts on the functional properties, we investigated the magneto-transport properties of the SRO/BTO/GSO and BTO/SRO/GSO heterostructures. The obtained data are summarized in Fig. 5. Regardless of the BTO layer, all heterostructures exhibit metallic conductions down to 10 K and undergo ferromagnetic transitions at 110-140 K, which can be seen as humps in the ρ xx -T curves (Fig. 5(a)). We note no clear relationships between the transition temperature (T c ) and interface structure. The magnetic field dependence of the Hall resistivity ρ xy in Fig. 5(b) reveals a strong influence of the stacking order of the heterostructure on the magnetic anisotropy of the SRO layer. The anomalous part in ρ xy for the SRO/BTO/GSO heterostructure shows no hysteresis, which is characteristic of the in-plane magnetic anisotropy. In contrast, a clear hysteresis due to the reversal of the magnetic moment is seen for the anomalous part in ρ xy of the SRO/GSO and BTO/SRO/GSO heterostructures, indicating that the magnetic moment has a component along the out-of-plane ([110] GSO ) direction.
To further determine the direction of the magnetic easy axis E a of the SRO layer, we measured the dependence of ρ xy on the magnetic field angle θ H . The results are shown in Fig. 5(c). Clear jumps with the hysteresis in the clockwise and counter-clockwise field rotations due to the fieldinduced magnetization reversal are seen. A jump in ρ xy is observed at every 180 • in θ H , revealing the uniaxial magnetic anisotropy of the SRO layer in all heterostructures. From the center angle of the hysteresis, the E a angle α is determined to be ∼90 • for SRO/BTO/GSO, ∼45 • for BTO/SRO/GSO, and ∼45 • for SRO/GSO. Figure 5(d) shows α as a function of the thickness of the SRO layer in the BTO/SRO/GSO heterostructure. While the SRO layer has various degrees of the RuO 6 tilt, α remains ∼45 • independent of the SRO layer thickness, indicating little influence of the gradually changing tilt angle on the magnetic anisotropy. We also note that α is independent of both temperature and the thickness of the BTO layer for all heterostructures. Our observation indicates that α is determined by the SRO layer's structure, highlighting the significant role of the magnetocrystalline effect on the magnetic anisotropy.
In summary, we demonstrated that the interface engineering is useful in controlling oxygen octahedral distortions in oxide heterostructures. We showed that the RuO 6 octahedral distortions in the SRO epitaxial layer on the GSO substrate can be engineered by adding a thin (a few unit cell thick) BTO layer either underneath or above the SRO layer. The BTO layer inserted underneath the SRO layer induces the monoclinic-to-tetragonal structural phase transition associated with the large suppression in the RuO 6 octahedral tilt. The BTO layer on the SRO layer, on the other hand, stabilizes the gradually changing RuO 6 tilts across the SRO layer, leading to the broken inversion symmetry. We ascribed the observed distortions to the interfacial octahedral connections modified by the added BTO layer. We also investigated the influence of interface engineering on the magneto-transport properties of the SRO layer, revealing that the magnetic anisotropy is controlled by the interfacially engineered RuO 6 octahedral distortions through the magnetocrystalline effect. The results imply that the interface engineering enables accommodating various types of octahedral distortions, specifically, oxygen coordination environments that are strongly correlated with functional properties such as dielectric and magnetic properties and metal-insulator transitions. Designing and stabilizing oxygen coordination environments not seen in bulk materials would allow for exploring novel phenomena emergent in oxide heterostructures.