Strong Dependence of Oxygen Octahedral Distortions in SrRuO3

Nov 11, 2014 - The findings highlight the important role of the type of the epitaxial strain ... This work investigates the effects of types of epitax...
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Strong Dependence of Oxygen Octahedral Distortions in SrRuO3 Films on Types of Substrate-Induced Epitaxial Strain Ryotaro Aso,†,‡ Daisuke Kan,*,† Yoshifumi Fujiyoshi,† Yuichi Shimakawa,†,§ and Hiroki Kurata*,†,§ †

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan § Japan Science and Technology Agency, CREST, Uji, Kyoto 611-0011, Japan ‡

ABSTRACT: This work investigates the effects of types of epitaxial strain on the structural and magneto-transport properties of SrRuO3 (SRO) thin films grown on (110)ortho NdGaO3 (NGO) and (110)ortho SmScO3 (SSO) substrates that result in a −1.66% compressive strain and a +1.63% tensile strain, respectively. Although the epitaxial strains induced by the NGO and SSO substrates are almost equal in magnitude, the film properties were found to be strongly dependent on the type of strain. Highresolution scanning transmission electron microscopy revealed that the compressively strained SRO films possess a tetragonal structure with no octahedral tilts, while the tensilely strained SRO films undergo a thickness-dependent transition from a monoclinic structure with octahedral tilts to a tetragonal structure with small tilts. These findings indicate that octahedral tilt propagation from the substrate into the film is preferentially induced under tensile rather than compressive strain. We further found that the magneto-transport properties of SRO films exhibit a significant dependence on the type of the epitaxial strain, demonstrating the close correlation between strain-induced octahedral distortions and magnetic anisotropy. These results highlight the important role of the type of the epitaxial strain on the structural and physical properties of epitaxial thin films.

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INTRODUCTION

previous studies revealed that the displacement of the oxygen atom shared by the two different octahedra at the heterointerface is closely correlated with the octahedral tilts in the films and consequently plays a crucial role in determining their physical properties.16,17 The implication of these results is that the oxygen atom positions in films under a given epitaxial strain possess some degrees of freedom. Given that the epitaxial thin films have an in-plane lattice spacing imposed by the substrate and are subject to various strains depending on the lattice mismatch between the film and the substrate, it is important to investigate how the oxygen atom positions in the heterostructures, which characterize the octahedral tilts, are influenced under various epitaxial strains. In this study, we investigated the effects of the strain type on the structural and magneto-transport properties of SrRuO3 (SRO) epitaxial thin films grown on (110)ortho NdGaO3 (NGO) and (110)ortho SmScO3 (SSO) substrates. The NGO

Transition-metal oxides under epitaxial strain have been the subject of intensive investigation, since they exhibit physical properties not seen in their bulk counterparts.1−8 Because the strain-induced distortions of oxygen octahedra modify the couplings between lattices (or orbitals), electrons, and spins, it is understood that the physical properties of these materials are closely linked to these distortions.9−14 Thus, it is critical to investigate correlations between the strain-induced octahedral distortions and the physical properties of epitaxial thin films. Since such distortions often involve slight displacements of oxygen atoms, a complete understanding of the distortions is an experimentally challenging task. Recently, we demonstrated that determination of the positions of both cations and oxygen atoms with sub-angstrom precision is achievable using complementary high-angle annular dark-field (HAADF) and annular bright-field (ABF) imaging in aberration-corrected scanning transmission electron microscopy (STEM). This technique represents useful means of characterizing oxygen octahedral distortions including deformations and tilts (or rotations) in the strained films.15 Our © 2014 American Chemical Society

Received: September 5, 2014 Revised: October 28, 2014 Published: November 11, 2014 6478

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Figure 1. (a) X-ray 2θ−θ patterns of SRO films on NGO (orange, red) and SSO substrates (green, blue). The dashed line in the figure indicates the position of the (220)o reflection of the bulk SRO. The inset shows the typical 2 μm × 2 μm AFM surface morphology of an SRO film on an SSO substrate. X-ray reciprocal space mappings of (b) a 5 nm thick SRO film on an NGO substrate and (c) 8 nm and (d) 11.7 nm thick SRO films on SSO substrates. The reflections were acquired around the (620)o, (260)o, and (444)o Bragg reflections of the substrate. For the mappings, Q⊥ (on the vertical axis) and Q// (on the horizontal axis) correspond to the inverse of the distance along the out-of-plane and in-plane directions of the film, respectively. In (b), the reflections indicated by arrows originate from a minority of the crystallographic domain in the substrate. The subscript o denotes orthorhombic perovskite notation.

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and SSO substrates, both of which have a Pbnm orthorhombic perovskite structure with an a−a−c+ octahedral tilt pattern,18 provide compressive and tensile strains of −1.66 and +1.63%, respectively, to the SRO films. X-ray diffraction-based structural characterizations show that the compressively strained SRO films (grown on the NGO substrate) have a tetragonal structure, while the tensilely strained films (grown on the SSO substrate) undergo a structural transition from a monoclinic structure at thicknesses less than 11 nm to a tetragonal structure above that thickness. Our cross-sectional HAADF- and ABF-STEM observations further revealed that the behavior of the octahedral tilts in the films is strongly dependent on the type of substrate-induced epitaxial strain (compressive or tensile). In a compressively strained tetragonal film, the octahedral tilts introduced by the substrate are greatly suppressed at the heterointerface. In contrast, a monoclinic film under a tensile strain is stabilized by the octahedral tilt propagation from the substrate and the octahedra in the film exhibit slight tilts even after the change to the tetragonal structure. These results indicate that octahedral tilts are more easily introduced under a tensile strain. We have also characterized the magneto-transport properties of the SRO films under compressive and tensile strains and demonstrated that the magnetic anisotropy of the SRO films depends on the octahedral distortions. On the basis of these experimental results, the influence of the strain-induced octahedral distortions on the physical properties of the strained films is also discussed.

EXPERIMENTAL DETAILS

SRO thin films ranging from 5 to 20 nm in thickness were grown epitaxially on (110)ortho NGO and (110)ortho SSO substrates using pulsed laser deposition. We note that SRO, NGO, and SSO have the Pbnm orthorhombic perovskite structure with the octahedral tilt pattern described as a−a−c+ in the Glazer notation.19 The lattice constants of the substrates were: aortho = 5.43 Å, bortho = 5.50 Å, and cortho = 7.71 Å for NGO and aortho = 5.53 Å, bortho = 5.76 Å, and cortho = 7.97 Å for SSO. The subscript ortho denotes orthorhombic perovskite notation. The corresponding average substrate-induced strains along the in-plane direction of the SRO (aortho = 5.57 Å, bortho = 5.53 Å, and cortho = 7.85 Å in bulk), (asub − aSRO)/aSRO, are −1.66% for the NGO substrate and +1.63% for the SSO substrate, where asub and aSRO are the pseudocubic lattice parameters of the substrate and SRO, respectively. SRO deposition was performed at a substrate temperature of 700 °C under an oxygen partial pressure of 100 mTorr. During the deposition, a Ru-rich SrRu1.1O3 target was ablated using a KrF excimer laser (λ = 248 nm, Coherent COMPex-Pro 205 F) with a laser spot density of 1.0 J/cm and a spot area of 5 mm × 2 mm. The fabricated films have an atomically smooth surface, as confirmed by tappingmode atomic force microscopy (AFM) at room temperature. For structural characterizations of the films, we obtained X-ray diffraction patterns with a conventional 4-circle diffractometer (X’pert MR, PANalytical) and also performed cross-sectional HAADF- and ABF-STEM observations. Prior to these cross-sectional observations, the fabricated specimens were thinned down to electron transparency by mechanical polishing and Ar-ion milling. The images were obtained using a spherical aberration-corrected STEM (JEM-9980TKP1; accelerating voltage = 200 kV, Cs = −0.025 nm, C5 = 15 mm) equipped with a cold field emission gun. Details of the image acquisition and analysis processes have been provided in our previous paper.15 The octahedral tilt angles and the in-plane oxygen displacements were obtained from the oxygen atom positions in ABF-STEM images determined with Bragg filtering and cubic 6479

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interpolation techniques in the “Find Peaks” option of the HREM Research Peak Pairs Analysis software package.20,21 The precision for the determination of the tilt angles and oxygen displacements was also evaluated by analyzing simulated ABF-STEM images based on hypothetical SRO/NGO and SRO/SSO heterostructures using the multislice simulation software (WinHREM, HREM Research).22 The results were reflected in the error bars of the corresponding experimental data. To characterize the magneto-transport properties of the films, we patterned a 50 μm wide Hall bar by photolithography and Ar-ion milling. The longitudinal and transverse Hall resistivities were measured in a conventional four-terminal configuration with a Physical Property Measurement System (PPMS, Quantum Design) equipped with a sample rotator.

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substrates is summarized in Figure 2a. These lattice spacings were obtained from the d620_ortho, d260_ortho, and d110_ortho values.

RESULTS AND DISCUSSION

Figure 1a presents the X-ray 2θ−θ profiles of the SRO thin films grown on the NGO and SSO substrates. Both films exhibit (220)ortho reflections and no peaks from a secondary phase are observed, thus confirming the epitaxial growth of the SRO films. In addition, interference fringes (so-called Laue oscillations) with a period corresponding to the film thickness are observed. AFM observations of surface morphologies also indicate that each fabricated film exhibits a step-and-terrace surface structure with a pseudocubic unit cell height of ∼4 Å, as shown in the inset of the figure, indicating the high quality of the fabricated SRO thin films. It can be seen that the (220)ortho reflections of the SRO films on the NGO and SSO substrates are situated at lower and higher 2θ values than that of the bulk SRO, respectively. This result provides evidence that SRO films grown on NGO and SSO substrates are indeed subject to compressive and tensile strains. Figure 1b−d shows X-ray reciprocal space mappings for the SRO films grown on the NGO and SSO substrates, acquired around the (620) ortho, (260)ortho, and (444)ortho Bragg reflections of the substrate. It is clearly evident that, in the case of each film, there is no multiple reflections from the SRO film and that all SRO reflections appear at the same position along the in-plane direction (the horizontal axis) as those of the substrate. This indicates that, regardless of the type of epitaxial strain, the SRO film has the single crystallographic domain without any formations of twins and that each of the in-plane lattices of the films is fixed by the substrate lattice. In the compressively strained films on the NGO substrate (Figure 1b), the (620)ortho and (260)ortho SRO reflections are observed in the same position along the out-of-plane direction (the vertical axis), demonstrating that the film structure is tetragonal. These results are consistent with our previously reported results.23 We also note that this tetragonal structure is maintained when the film thickness is reduced to 5 nm (Figure 1b). In contrast, the structures of the tensilely strained films on the SSO substrate are dependent on the thickness, as shown in Figure 1c,d. The (620)ortho, (260)ortho, and (444)ortho SRO reflections of the 8 nm thick film appear at different positions along the out-of-plane direction, while all the reflections of the 11.7 nm thick film are observed at the same position. Thus, although the in-plane lattice of the SRO film remains fixed by the SSO substrate, the film structure changes from a monoclinic to a tetragonal phase with increasing film thickness. We also note that a similar structural transition has been reported in the case of a +1.0% tensilely strained SRO film grown on a (110)ortho GdScO3 (GSO) substrate.24 The thickness dependence of the d100_ortho and d010_ortho lattice spacings of the SRO thin films on the NGO and SSO

Figure 2. (a) Thickness dependence of the d100_ortho and d010_ortho lattice spacings of SRO films on NGO and SSO substrates. (b) The out-of-plane lattice stain, εZZ, plotted against the in-plane lattice stain, εXX, of the SRO films. Data for SRO films on STO and GSO substrates are included to demonstrate the overall trend of the substrate-induced strain effect. In the GSO and SSO data, the open and filled squares correspond to εZZ values for the monoclinic and tetragonal films, respectively. All films were grown coherently, and the in-plane lattice of the SRO film was fixed by the substrate lattice.

The d100_ortho and d010_ortho values of the compressively strained tetragonal films on the NGO substrate are identical at ∼5.57 Å and independent of the film thickness values employed in this study (above 5 nm). In the tensilely strained films on the SSO substrate, a monoclinic structure in which d100_ortho = 5.59 Å and d010_ortho = 5.55 Å undergoes a transition to a tetragonal structure with d100_ortho = d010_ortho = 5.57 Å at a thickness of 11 nm. Given that both the compressively and tensilely strained films are subjected to strains of very similar magnitudes, the observed differences in the thickness dependence of each film structure imply that the substrate-induced lattice distortions are strongly dependent on the type of epitaxial strain (either compressive or tensile). To systematically investigate the manner in which the type of epitaxial strain affects lattice distortions, we subsequently plotted the out-of-plane lattice strain of SRO films, εZZ, against the in-plane lattice strain, εXX, as shown in Figure 2b. Both εZZ and εXX were calculated based on the out-of-plane (d110_ortho) and average in-plane lattice spacings ((d1−10_ortho + d001_ortho)/ 2), respectively. The lattice strains of SRO films on SrTiO3 (STO)25 and GSO24 substrates are also included in this plot to establish the strain effect on octahedral distortions. Under a compressive strain, εZZ values exhibit linear relationships with εXX, suggesting that the film lattices on the NGO and STO substrates deform in an almost elastic manner. In contrast, under a tensile strain, εZZ values show little variation with changes in εXX, indicating that the film lattices on the GSO and SSO substrates undergo anomalous expansion along the out-ofplane direction. Similar lattice expansion has been observed with ATiO3 films (A = Sr, Sr0.7Ca0.3, and Sr0.5Ca0.5) grown on a GSO substrate,26 in which the propagation of the octahedral tilts from the substrate played a crucial role in the lattice distortions. The observed lattice expansion in the SRO films implies that the dependence of the lattice distortions on the type of strain is related to the substrate-induced octahedral tilts. 6480

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Figure 3. High-resolution HAADF-STEM images of (a) 12 nm thick tetragonal SRO/NGO, (b) 8 nm thick monoclinic SRO/SSO, and (c) 11.7 nm thick tetragonal SRO/SSO heterostructures taken along the [001]ortho direction. (d−f) ABF-STEM images acquired from almost the same region as the corresponding HAADF images. Red squares indicate projected oxygen octahedra.

Figure 4. (a) Definition of the octahedral tilt angles θ1 (blue) and θ2 (red). Both θ1 and θ2 correspond to the angle between two oxygen octahedra projected on the (001)ortho plane. (b−d) Variations of octahedral tilt angles in (b) 12 nm thick tetragonal SRO/NGO, (c) 8 nm thick monoclinic SRO/SSO, and (d) 11.7 nm thick tetragonal SRO/SSO heterostructures. The error bars correspond to the standard deviation of each determined octahedral tilt angle. The orange dashed lines represent the SRO/NGO and SRO/SSO heterointerfaces. The green, purple, and pink dotted lines indicate the octahedral tilt angles (θ1 and θ2) projected on the (001)ortho plane of SRO (168° or 192°), NGO (162° or 198°), and SSO (156° or 204°) in bulk, respectively. These tilt angles do not correspond to the B−O−B bond angles in the three-dimensional lattice, which are 162°, 154°, and 144° for bulk SRO, NGO, and SSO, respectively.

We acquired cross-sectional high-resolution STEM images of the SRO/NGO and SRO/SSO heterostructures so as to

analyze the RuO6 octahedral distortions under compressive and tensile strains. Figure 3a−c shows HAADF-STEM images of 6481

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the tetragonal SRO film on the NGO substrate and the monoclinic and tetragonal SRO films on the SSO substrates, respectively. These images were acquired along the [001]ortho direction of both the film and the substrate. This figure demonstrates that none of the films exhibit any misfit dislocations at the heterointerfaces, confirming the coherent growth of the films. On the basis of the observed Z-contrast in the HAADF images, the termination layers of the substrates in the SRO/NGO and SRO/SSO heterostructures are identified as GaO2 and ScO2 layers, respectively, indicating that B-site termination is preserved in both heterointerfaces. In the substrate area, the image contrasts of Nd and Sm columns are slightly distorted, confirming the A-site cation displacements in the Pbnm orthorhombic structure.15 ABF-STEM images of the tetragonal SRO/NGO and the monoclinic and tetragonal SRO/SSO are provided in Figure 3d−f, respectively. In these images, not only the constituent cations but also the oxygen atoms are clearly visible in dark contrast. Extracting the oxygen atom positions from the ABF images allows us to reveal the octahedral distortions in the SRO films as well as the octahedral connections between the distorted octahedra across the heterointerface. Figure 4 represents the variations of the oxygen octahedral tilt angles across the SRO/NGO and SRO/SSO heterointerfaces, extracted from the corresponding ABF images (Figure 3d−f). Figure 4a illustrates the manner in which the octahedral tilt angle was defined. For each heterostructure, the measured tilt angles of the substrate perfectly match the bulk counterparts. For the compressively strained film (Figure 4b), the octahedral tilts seen in the substrate region undergo a significant decrease beginning at the interface and disappear completely by five RuO6 octahedral layers from the interface. This is consistent with the tetragonal structure of the compressively strained film. In contrast, as seen in Figure 4c,d, the behavior of the octahedral tilts in the tensilely strained SRO films is different from that in the compressively strained film. For the monoclinic SRO/SSO (Figure 4c), tilted octahedra are observed in the film, while the RuO6 octahedral tilts are seen to gradually decrease toward the film surface. This indicates that the octahedral tilts propagate from the substrate into the film, stabilizing the monoclinic structure of the SRO film. In the tetragonal SRO/SSO (Figure 4d), the RuO6 octahedra exhibit slight tilts even at the film surface, although the octahedral tilts are suppressed at the interface. Given that the magnitudes of the epitaxial strains induced by the NGO and SSO substrates are nearly equal, it appears that tilt propagation into the SRO film preferentially occurs under a tensile strain. A similar structural transition associated with a change in the octahedral tilts was observed in SRO films on a GSO substrate inducing a +1.0% tensile strain.16 It is also of interest that the tetragonal film subjected to solely tensile strain (on the GSO and SSO substrates) shows slightly tilted RuO6 octahedra even though the magnitude of the octahedral tilts is quite small (177° or 183°). These results strongly suggest that the octahedral tilts result in the anomalous expansion of the tensilely strained SRO lattice, which hinders its elastic deformation under epitaxial strain (Figure 2b). Since the oxygen octahedra are corner-connected by sharing of oxygen atoms, the octahedral tilt propagation from the substrate to the film region is correlated with the octahedral connection at the film/substrate interface. Figure 5 compares the variations of the in-plane displacements of the oxygen atoms across the heterointerfaces. As shown in the inset of the

Figure 5. Variations of in-plane oxygen displacement, Δx, of 12 nm thick tetragonal SRO/NGO (red), 8 nm thick monoclinic SRO/SSO (green), and 11.7 nm thick tetragonal SRO/SSO (blue) heterostructures. Δx is defined as the distance from the middle position between A-site cations along the in-plane direction, as shown in the inset. The error bars represent the standard deviation of each determined oxygen displacement. The orange dashed line represents the SRO/NGO and SRO/SSO heterointerfaces. The green, purple, and pink dotted lines indicate the oxygen displacements of the SRO, NGO, and SSO in bulk, respectively.

figure, the in-plane oxygen displacement, Δx, is related to the connection angle between the octahedra. It can be seen that the Δx value of the substrate is suddenly and significantly reduced at the interface of the tetragonal films on both NGO and SSO substrates, leading to the formation of GaO6−RuO6 and ScO6− RuO6 octahedral connections with small tilts. This finding is consistent with the fact that the propagation of the octahedral tilts is suppressed at the interface between the tetragonal film and the substrate. Conversely, at the interface between the monoclinic film and the substrate, the value of Δx is larger than that in the bulk SRO, facilitating the tilt propagation. We also note that, despite the tilt propagation, Δx in the monoclinic film undergoes a gradual decrease over the span of nine RuO6 octahedral layers from the interface. The observed behavior is in stark contrast to the constant Δx induced in a monoclinic SRO film on a GSO substrate15,16 that provides a tensile strain lower than that generated by the SSO substrate, as shown in Figure 2b. These results suggest that, under a large tensile strain, deformed octahedra with negligibly small tilts are further stabilized, hindering octahedral tilt propagation from the substrate into the film region. This also explains why the film thickness at which the transition from the monoclinic to tetragonal structure occurs becomes smaller with increasing tensile strain, such that a thickness of 16 nm is required at a +1.0% tensile strain in the GSO study24 but only 11 nm is required at +1.63% in the case of the SSO substrates. To evaluate how the strain-induced RuO6 octahedral distortions in the SRO films influence their physical properties, we carried out magneto-transport characterizations. Figure 6 summarizes the temperature dependence of the longitudinal electrical resistivity, ρxx, and the magnetic field dependence of the transverse Hall resistivity, ρxy, of the SRO films grown on the NGO and SSO substrates. The tetragonal SRO film on the NGO substrate exhibits metallic conduction down to low temperatures and undergoes a ferromagnetic transition at ∼135 K, indicated by a hump in the ρxx−T curve in Figure 6a. Similar behavior was also observed in our previous work.23 The 6482

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Figure 6. Temperature dependence of the longitudinal electrical resistivity, ρxx, of (a) tetragonal SRO/NGO (red), (b) monoclinic SRO/SSO (green), and (c) tetragonal SRO/SSO (blue) heterostructures. Magnetic field dependence of the transverse Hall resistivity, ρxy, of (d) the tetragonal SRO/NGO (red), (e) monoclinic SRO/SSO (green), and (f) tetragonal SRO/SSO (blue) heterostructures. The data were acquired at 10 K, and currents were applied along the [001]o direction.

ferromagnetic ordering in the compressively strained tetragonal film is further confirmed by the observation that the anomalous part of ρxy, associated with the magnetization, M, displays a square-shaped hysteresis against the magnetic field resulting from the magnetization reversal (Figure 6d). Similar results were also obtained for the tensilely strained SRO films, as shown in Figure 6b,c. Here, the hump in the temperature dependence of ρxx due to the ferromagnetic transition of the monoclinic and tetragonal films on the SSO substrate is seen at 125 and 128 K, respectively. The observed difference in the ferromagnetic transition temperature between the compressively and tensilely strained films might be related to straininduced modification of the crystal field splitting.23,27 The anomalous part of ρxy of both tensilely strained films (Figure 6e,f) also displays hysteresis resulting from the field-induced magnetization reversal, indicating that the magnetic moment has a component along the [110]ortho (the out-of-plane) direction. To determine the direction of the magnetic easy axis (EA) in the SRO films, we measured the magnetic field angle dependence of the ρxy values of the fabricated films. Since SRO exhibits strong magnetic anisotropy,28 the field-induced magnetization reversal takes place only when the angle between the EA and applied field exceeds 90°.24,29 This can be seen as jumps in ρxy. Figure 7 shows the field angle dependence of ρxy obtained under a magnetic field rotated in the (001)ortho plane with a current applied along the [001]ortho direction. The ρxy values of all films show a clear jump with hysteresis in response to the clockwise and counterclockwise field rotations, due to the field-induced magnetization reversal, and the jumps are observed at each 180° increment of the field angle. This observation indicates uniaxial magnetic anisotropy in the SRO films on both NGO and SSO substrates. The center of the hysteresis of the tetragonal SRO film on the NGO substrate lies at ±90°, implying that the EA points in the out-of-plane direction. This is in good agreement with the data acquired in

Figure 7. Magnetic field angle, θH, dependence of Hall resistivity, ρxy, of (a) tetragonal SRO/NGO (red), (b) monoclinic SRO/SSO (green), and (c) tetragonal SRO/SSO (blue) heterostructures. All data were collected at 10 K. The magnetic fields were rotated in the (001)o plane, and the currents were applied along the [001]o direction. (d) Thickness dependence of the magnetic easy axis angle, α, of SRO films on NGO and SSO substrates. The definitions of the magnetic field angle, θH, and the magnetic easy axis angle, α, are also shown in (d).

our previous study.23 In contrast, the EA directions of the monoclinic and tetragonal SRO films on the SSO substrate were determined to be 26° and 36° with respect to the [110]ortho direction (the out-of-plane direction), respectively. These EA directions for the strained films are not seen in the bulk whose EA is along either the [100]ortho axis or the 6483

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[010]ortho one.30−32 This indicates the significance of the octahedral distortions on the magnetic anisotropy of SRO. Figure 7d summarizes the thickness dependence of the EA angle, α, of our SRO films. Due to the strong magnetocrystalline effect of the SRO films, the EA angle remains constant with the film thickness but changes in response to the film structural transition that, in turn, is associated with the RuO6 octahedral distortions. It is of interest that, in contrast to an SRO film on a GSO substrate,16 in which the EA angle changes by 45° in response to a monoclinic-to-tetragonal structural transition, the difference in the EA angles of the monoclinic and tetragonal SRO films on the SSO substrate is as low as 10°. This likely occurs because the octahedral tilts of the monoclinic structure gradually decrease toward the film surface, such that the octahedra in the vicinity of the surface have distortions similar to those in the tetragonal film. The observed correlation between the EA angles of the films and the strain-induced octahedral distortions, including the tilts and deformations, indicates that the type of epitaxial strain is an important factor influencing the magneto-transport properties of strained oxide films.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.K.). *E-mail: [email protected] (H.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research (Grant No. 24760009) and a Joint Project of Chemical Synthesis Core Research Institutions grant from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The work was also supported by the Japan Science and Technology Agency, CREST.

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REFERENCES

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CONCLUSIONS We investigated the effects of strain type on the structural and magneto-transport properties of SRO thin films grown on (110)ortho NGO and (110)ortho SSO substrates that result in a −1.66% compressive strain and a +1.63% tensile strain, respectively. We found that, even though the magnitudes of the epitaxial strains induced by the NGO and SSO substrates are almost equal, the film properties are strongly dependent on the type of strain. X-ray diffraction characterizations revealed that the compressively strained SRO films (on the NGO substrate) have a tetragonal structure, while the tensilely strained films (on the SSO substrate) undergo a thicknessdependent transition from a monoclinic structure below a thickness of 11 nm to a tetragonal structure above the thickness. High-resolution HAADF- and ABF-STEM observations further showed that the RuO6 octahedral distortions in the films are strongly dependent on the type of substrateinduced epitaxial strain. In the compressively strained tetragonal films, no octahedral tilts are introduced in the film and the RuO6 octahedra are almost elastically deformed in response to the epitaxial strain. In contrast, in the tensilely strained films in which the lattices are anomalously expanded along the out-of-plane direction, octahedral tilt propagation plays an important role in the structural distortions of the films. The monoclinic structure under a tensile strain is stabilized by octahedral tilt propagation from the substrate, facilitated by the large displacement of the oxygen atom shared by RuO6 and ScO6 octahedra at the interface. The structural transition to the tetragonal phase is associated with significant suppression of the octahedral tilt propagation, although the octahedra do exhibit slight tilts even at the film surface. These results indicate that octahedral tilt propagation preferentially occurs under a tensile strain as opposed to a compressive strain. We also showed that the magneto-transport properties of the SRO films are strongly dependent on the type of epitaxial strain, revealing the close correlation between the magnetic easy axis direction of the films and the strain-induced RuO6 octahedral distortions. These findings highlight the importance of the type of epitaxial strain on the structural and physical properties of oxide thin films. 6484

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