| 1. |
Ryotaro Aso, Takuya Katsumata, Takashi Nakamura, Yusuke Watase, Koji Amezawa, Yasukazu Murakami, Structural and electronic characterization of fluorine-doped La0.5Sr0.5CoO3−δ using electron energy-loss spectroscopy, Microscopy, 2023.05. |
| 2. |
Ryotaro Aso, Hajime Hojo, Yoshio Takahashi, Tetsuya Akashi, Yoshihiro Midoh, Fumiaki Ichihashi, Hiroshi Nakajima, Takehiro Tamaoka, Kunio Yubuta, Hiroshi Nakanishi, Hisahiro Einaga, Toshiaki Tanigaki, Hiroyuki Shinada, Yasukazu Murakami, Direct identification of the charge state in a single platinum nanoparticle on titanium oxide, Science, 10.1126/science.abq5868, 378, 202-206, 2022.10, A goal in the characterization of supported metal catalysts is to achieve particle-by-particle analysis of the charge state strongly correlated with the catalytic activity. Here, we demonstrate the direct identification of the charge state of individual platinum nanoparticles (NPs) supported on titanium dioxide using ultrahigh sensitivity and precision electron holography. Sophisticated phase-shift analysis for the part of the NPs protruding into the vacuum visualized slight potential changes around individual platinum NPs. The analysis revealed the number (only one to six electrons) and sense (positive or negative) of the charge per platinum NP. The underlying mechanism of platinum charging is explained by the work function differences between platinum and titanium dioxide (depending on the orientation relationship and lattice distortion) and by first-principles calculations in terms of the charge transfer processes.. |
| 3. |
Masatoshi Kimura, Xinyi He, Takayoshi Katase, Terumasa Tadano, Jan M. Tomczak, Makoto Minohara, Ryotaro Aso, Hideto Yoshida, Keisuke Ide, Shigenori Ueda, Hidenori Hiramatsu, Hiroshi Kumigashira, Hideo Hosono, Toshio Kamiya, Large phonon drag thermopower boosted by massive electrons and phonon leaking in LaAlO3/LaNiO3/LaAlO3 heterostructure, Nano Letters, 10.1021/acs.nanolett.1c03143, 21, 21, 9240-9246, 2021.10. |
| 4. |
T. Tamaoka, R. Aso, Y. Murakami, Magnetism of antiphase boundaries in ordered alloys studied using electron holography, J. Magn. Magn. Mater., 2021.08. |
| 5. |
Takashi Nakamura, Kento Ohta, Xueyan Hou, Yuta Kimura, Kazuki Tsuruta, Yusuke Tamenori, Ryotaro Aso, Hideto Yoshida, Koji Amezawa, Oxygen defect engineering for the Li-rich cathode material Li1.2Ni0.13Co0.13Mn0.54O2−δ, Journal of Materials Chemistry A, 9, 3657-3667, 2020.12. |
| 6. |
Takashi Nakamura, Kento Ohta, Yuta Kimura, Kazuki Tsuruta, Yusuke Tamenori, Ryotaro Aso, Hideto Yoshida, Koji Amezawa, Impact of Oxygen Defects on Electrochemical Processes and Charge Compensation of Li-Rich Cathode Material Li1.2Mn0.6Ni0.2O2−δ, ACS Applied Energy Materials, 3, 9703-9713, 2020.09. |
| 7. |
Yutaka Ohno, Hideto Yoshida, Naoto Kamiuchi, Ryotaro Aso, Seiji Takeda, Yasuo Shimizu, Yasuyoshi Nagai, Jianbo Liang, Naoteru Shigekawa, Impact of focused ion beam on structural and compositional analysis of interfaces fabricated by surface activated bonding, Japanese Journal of Applied Physics, 10.7567/1347-4065/ab4b15, 59, SB, 2020.02, We have shown that the structural and compositional properties of semiconductor interfaces fabricated by surface activated bonding (SAB) would be modified during focused ion beam (FIB) processes operated at room temperature (RT), especially for wide band-gap materials, and such a modification can be suppressed by FIB processes operated at lower temperatures. During FIB processes operated at RT, SAB-fabricated Si/Si and GaAs/GaAs interfaces are amorphized along the interfaces, even at the internal locations deeper than the penetration depth of the FIB, and the impurity distribution across the interfaces is modified. This phenomenon is presumably due to the atomic diffusion assisted by the point defects that are introduced by FIB irradiation. By using FIB processes operated at -150 °C, the FIB-induced atomic diffusion would be ignored for Si/Si interfaces. Meanwhile, the diffusion would be still effective for GaAs/GaAs interfaces, presumably due to the effects of recombination-enhanced defect motion under FIB irradiation.. |
| 8. |
Ryotaro Aso, Yohei Ogawa, Takehiro Tamaoka, Hideto Yoshida, Seiji Takeda, Visualizing Progressive Atomic Change in the Metal Surface Structure Made by Ultrafast Electronic Interactions in an Ambient Environment, Angewandte Chemie - International Edition, 10.1002/anie.201907679, 58, 45, 16028-16032, 2019.11, Understanding the atomic and molecular phenomena occurring in working catalysts and nanodevices requires the elucidation of atomic migration originating from electronic excitations. The progressive atomic dynamics on metal surface under controlled electronic stimulus in real time, space, and gas environments are visualized for the first time. By in situ environmental transmission electron microscopy, the gas molecules introduced into the biased metal nanogap could be activated by electron tunneling and caused the unpredicted atomic dynamics. The typically inactive gold was oxidized locally on the positive tip and field-evaporated to the negative tip, resulting in the atomic reconstruction on the negative tip surface. This finding of a tunneling-electron-attached-gas process will bring new insights into the design of nanostructures such as nanoparticle catalysts and quantum nanodots and will stimulate syntheses of novel nanomaterials not seen in the ambient environment.. |
| 9. |
Takehiro Tamaoka, Ryotaro Aso, Hideto Yoshida, Seiji Takeda, Reversible gas-solid reaction in an electronically-stimulated palladium nanogap, Nanoscale, 10.1039/c9nr00806c, 11, 18, 8715-8717, 2019.05, We investigated a nanogap between a pair of palladium electrode tips with gas (nitrogen, hydrogen, and oxygen) and a biasing voltage using in situ atomic resolution environmental transmission electron microscopy (ETEM). We found an unexpected gas-solid (nitrogen-palladium) reaction that occurs on the surface of the positive electrode tip. A palladium nitride compound was synthesized with gaseous nitrogen at low pressure at room temperature. The nitridation of palladium was previously reported and predicted to occur only under high pressure and at high temperature. The reaction in ETEM apparatus was reversible with the change in the magnitude of an electric field in the nanogap. Additionally, the asymmetrical surface dynamics on the pair of electrode tips in gas (nitrogen, hydrogen, and oxygen) were revealed by ETEM observation. It is likely that the electrons in the gap induce the reversible reaction. This study has opened a new route toward creating nanoscale materials because the creation, stabilization, and annihilation of the material in a nanogap can be controlled electrically and electronically on demand for various applications.. |
| 10. |
Direct Observation of Oxygen Displacement in Oxide Heterointerface by Atomic-Resolution STEM. |
| 11. |
Naoto Kamiuchi, Keju Sun, Ryotaro Aso, Masakazu Tane, Takehiro Tamaoka, Hideto Yoshida, Seiji Takeda, Self-Activated surface dynamics in gold catalysts under reaction environments, Nature communications, 10.1038/s41467-018-04412-4, 9, 1, 2018.12, Nanoporous gold (NPG) with sponge-like structures has been studied by atomic-scale and microsecond-resolution environmental transmission electron microscopy (ETEM) combined with ab initio energy calculations. Peculiar surface dynamics were found in the reaction environment for the oxidation of CO at room temperature, involving residual silver in the NPG leaves as well as gold and oxygen atoms, especially on {110} facets. The NPG is thus classified as a novel self-Activating catalyst. The essential structure unit for catalytic activity was identified as Au-AgO surface clusters, implying that the NPG is regarded as a nano-structured silver oxide catalyst supported on the matrix of NPG, or an inverse catalyst of a supported gold nanoparticulate (AuNP) catalyst. Hence, the catalytically active structure in the gold catalysts (supported AuNP and NPG catalysts) can now be experimentally unified in low-Temperature CO oxidation, a step forward towards elucidating the fascinating catalysis mechanism of gold.. |
| 12. |
Kentaro Soma, Stan Konings, Ryotaro Aso, Naoto Kamiuchi, Genki Kobayashi, Hideto Yoshida, Seiji Takeda, Detecting dynamic responses of materials and devices under an alternating electric potential by phase-locked transmission electron microscopy, Ultramicroscopy, 10.1016/j.ultramic.2017.04.018, 181, 27-41, 2017.10, An apparatus is developed for transmission electron microscopy (TEM) to acquire image and spectral data, such as TEM images, electron holograms, and electron energy loss spectra, synchronized with the measurement of the dynamic response of a specimen under an applied alternating current (AC) electric potential (voltage, denoted VAC). From a VAC of frequency f, a shutter pulse signal is generated to open and close a pre-specimen shutter in a base TEM apparatus. A pulse is generated per VAC cycle from the targeted phase Φ to Φ + ∆Φ with phase width ∆Φ (∆Φ AC ranges from 2 mV to 1 V depending on observation conditions. The quality of phase-locked strobe images can be improved markedly using a phase-locked strobe electron beam. Under specific conditions, the spatial resolution in images is better than 0.12 nm, even though the spatial resolution generally depends on VAC, f, the base TEM, and the conductivity of the specimen. For the model specimens, it is shown that electrochemical impedance spectroscopy and cyclic voltammetry can be performed in a TEM apparatus, and could potentially be synchronized with phase-locked (strobe) imaging and spectroscopy. Severe electron irradiation damage is detected during phase-locked (strobe) electron holography of the model LIB.. |
| 13. |
Kei Hirai, Ryotaro Aso, Yusuke Ozaki, Daisuke Kan, Mitsutaka Haruta, Noriya Ichikawa, Hiroki Kurata, Yuichi Shimakawa, Melting of Oxygen Vacancy Order at Oxide-Heterostructure Interface, ACS Applied Materials and Interfaces, 10.1021/acsami.7b08134, 9, 35, 30143-30148, 2017.09, Modifications in oxygen coordination environments in heterostructures consisting of dissimilar oxides often emerge and lead to unusual properties of the constituent materials. Although lots of attention has been paid to slight modifications in the rigid oxygen octahedra of perovskite-based heterointerfaces, revealing the modification behaviors of the oxygen coordination environments in the heterostructures containing oxides with oxygen vacancies have been challenging. Here, we show that a significant modification in the oxygen coordination environments - melting of oxygen vacancy order - is induced at the heterointerface between SrFeO2.5 (SFO) and DyScO3 (DSO). When an oxygen-deficient perovskite (brownmillerite structure) SrFeO2.5 film grows epitaxially on a perovskite DyScO3 substrate, both FeO6 octahedra and FeO4 tetrahedra in the (101)-oriented SrFeO2.5 thin film connect to ScO6 octahedra in DyScO3. As a consequence of accommodating a structural mismatch, the alternately ordered arrangement of oxygen vacancies is significantly disturbed and reconstructed in the 2 nm thick heterointerface region. The stabilized heterointerface structure consists of Fe3+ octahedra with an oxygen vacancy disorder. The melting of the oxygen vacancy order, which in bulk SrFeO2.5 occurs at 1103 K, is induced at the present heterointerface at ambient temperatures.. |
| 14. |
Daisuke Kan, Ryotaro Aso, Riko Sato, Mitsutaka Haruta, Hiroki Kurata, Yuichi Shimakawa, Tuning magnetic anisotropy by interfacially engineering the oxygen coordination environment in a transition metal oxide, Nature Materials, 10.1038/nmat4580, 15, 4, 432-437, 2016.04, Strong correlations between electrons, spins and lattices - stemming from strong hybridization between transition metal d and oxygen p orbitals - are responsible for the functional properties of transition metal oxides. Artificial oxide heterostructures with chemically abrupt interfaces provide a platform for engineering bonding geometries that lead to emergent phenomena. Here we demonstrate the control of the oxygen coordination environment of the perovskite, SrRuO3, by heterostructuring it with Ca0.5Sr0.5TiO3 (0-4 monolayers thick) grown on a GdScO3 substrate. We found that a Ru-O-Ti bond angle of the SrRuO3/Ca0.5Sr0.5TiO3 interface can be engineered by layer-by-layer control of the Ca0.5Sr0.5TiO3 layer thickness, and that the engineered Ru-O-Ti bond angle not only stabilizes a Ru-O-Ru bond angle never seen in bulk SrRuO3, but also tunes the magnetic anisotropy in the entire SrRuO3 layer. The results demonstrate that interface engineering of the oxygen coordination environment allows one to control additional degrees of freedom in functional oxide heterostructures.. |
| 15. |
Daisuke Kan, Ryotaro Aso, Hiroki Kurata, Yuichi Shimakawa, Phase control of a perovskite transition-metal oxide through oxygen displacement at the heterointerface, Dalton Transactions, 10.1039/c4dt03749a, 44, 23, 10594-10607, 2015.06, Controlling structural distortions that are closely related to functional properties in transition-metal oxides is a key not only to exploring novel phenomena but also to developing novel oxide-based electronic devices. In this review article, we overview investigations revealing that oxygen displacement at the heterointerface is a key parameter characterizing structure-property relationships of heterostructures. We further demonstrate that the interface engineering of the oxygen displacement is useful to control structural and electronic properties of strained oxides.. |
| 16. |
Daisuke Kan, Ryotaro Aso, Hiroki Kurata, Yuichi Shimakawa, Research Update
Interface-engineered oxygen octahedral tilts in perovskite oxide heterostructures, APL Materials, 10.1063/1.4918965, 3, 6, 2015.06, 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 heterointerface between perovskite oxides SrRuO3 and BaTiO3 on GdScO3 substrates and of the influences of the interfacially engineered distortions on the magneto-transport properties of the SrRuO3 layer. Our state-of-the-art annular bright-field imaging in aberration-corrected scanning transmission electron microscopy revealed that the RuO6 octahedral distortions in the SrRuO3 layer have strong dependence on the stacking order of the SrRuO3 and BaTiO3 layers on the substrate. This can be attributed to the difference 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 SrRuO3 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.. |
| 17. |
Ryotaro Aso, Daisuke Kan, Yoshifumi Fujiyoshi, Yuichi Shimakawa, Hiroki Kurata, Strong dependence of oxygen octahedral distortions in SrRuO3 films on types of substrate-induced epitaxial strain, Crystal Growth and Design, 10.1021/cg501340e, 14, 12, 6478-6485, 2014.12, 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. High-resolution 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.. |
| 18. |
Ryotaro Aso, Daisuke Kan, Yuichi Shimakawa, Hiroki Kurata, Control of structural distortions in transition-metal oxide films through oxygen displacement at the heterointerface, Advanced Functional Materials, 10.1002/adfm.201303521, 24, 33, 5177-5184, 2014.09, Structural distortions in the oxygen octahedral network in transition-metal oxides play crucial roles in yielding a broad spectrum of functional properties, and precise control of such distortions is a key for developing future oxide-based electronics. Here, it is shown that the displacement of apical oxygen atom shared between the octahedra at the heterointerface is a determining parameter for these distortions and consequently for control of structural and electronic phases of a strained oxide film. The present analysis by complementary annular dark- and bright-field imaging in aberration-corrected scanning transmission electron microscopy reveals that structural phase differences in strained monoclinic and tetragonal SrRuO3 films grown on GdScO3 substrates result from relaxation of the octahedral tilt, associated with changes in the in-plane displacement of the apical oxygen atom at the heterointerface. It is further demonstrated that octahedral distortions and magnetrotransport properties of the SrRuO3 films can be controlled by interface engineering of the oxygen displacement. This provides a further degree of freedom for manipulating structural and electronic properties in strained films, allowing the design of novel oxide-based heterostructures.. |
| 19. |
Ryotaro Aso, Daisuke Kan, Yuichi Shimakawa, Hiroki Kurata, Octahedral tilt propagation controlled by a-site cation size at perovskite oxide heterointerfaces, Crystal Growth and Design, 10.1021/cg500285m, 14, 5, 2128-2132, 2014.05, A clear correlation between the A-site cation size and the octahedral tilt propagation from the substrates into the ATiO3 (A = Ba2+, Sr2+, Sr2+0.7Ca2+0.3, and Sr2+0.5Ca2+0.5) epitaxial thin films was found from the observations of ATiO3/GdScO3 heterostructures using high-resolution annular bright-field scanning transmission electron microscopy. The in-plane oxygen displacements at the interface increase with decreasing the A-site cation size and facilitate the TiO6 octahedral tilt propagation across the interface. The results highlight the significance of the A-site cation size as a controlling factor for structural distortions at oxide-based heterointerfaces.. |
| 20. |
Daisuke Kan, Ryotaro Aso, Hiroki Kurata, Yuichi Shimakawa, Unit-cell thick BaTiO3 blocks octahedral tilt propagation across oxide heterointerface, Journal of Applied Physics, 10.1063/1.4875839, 115, 18, 2014.05, We fabricated SrRuO3/BaTiO3/GdScO3 heterostructures in which the BaTiO3 layer is one unit cell thick by pulsed laser deposition and elucidated how the BaTiO3 layer influences structural and magneto-transport properties of the SrRuO3 layer through octahedral connections across the heterointerface. Our X-ray-diffraction-based structural characterizations show that while an epitaxial SrRuO3 layer grown directly on a GdScO3 substrate is in the monoclinic phase with RuO6 octahedral tilts, a one-unit-cell-thick BaTiO3 layer inserted between SrRuO3 and GdScO3 stabilizes the tetragonal SrRuO3 layer with largely reduced RuO6 tilts. Our high-angle annular dark-field and annular bright-field scanning transmission electron microscopy observations provide an atomic-level view of the octahedral connections across the heterostructure and reveal that the BaTiO3 layer only one unit cell thick is thick enough to stabilize the RuO6-TiO6 octahedral connections with negligible in-plane oxygen atomic displacements. This results in no octahedral tilts propagating into the SrRuO3 layer and leads to the formation of a tetragonal SrRuO3 layer. The magneto-transport property characterizations also reveal a strong impact of the octahedral connections modified by the inserted BaTiO3 layer on the spin-orbit interaction of the SrRuO3 layer. The SrRuO3 layer on BaTiO3/ GdScO3 has in-plane magnetic anisotropy. This is in contrast to the magnetic anisotropy of the monoclinic SrRuO 3 films on the GdScO3 substrate, in which the easy axis is ∼45° to the film surface normal. Our results demonstrate that the one-unit-cell-thick layer of BaTiO3 can control and manipulate the interfacial octahedral connection closely linked to the structure-property relationship of heterostructures.. |
| 21. |
Daisuke Kan, Takuya Shimizu, Yasuhiro Yamada, Ryotaro Aso, Hiroki Kurata, Yoshihiko Kanemitsu, Yuichi Shimakawa, Band-to-band photoluminescence as a probe of electron carriers in Nb-doped SrTiO3 epitaxial thin films, Applied Physics Express, 10.7567/APEX.7.015503, 7, 1, 2014.01, In this paper, we describe the use of band-to-band photoluminescence (PL) as a tool for evaluating the quality of Nb-doped STO (Nb: 0.1 at.%) epitaxial thin films. We found that the films with the bulk-equivalent lattice parameters show a large variation in their band-to-band PL properties. In combination with the transport property characterizations, we ascribe the variation to the change in the carrier density owing to the carrier compensation by a small amount of point defects, which cannot be detected in structural characterizations. We also show that the band gap of the film is 10meV smaller than that of the single crystal. Our results imply that even a small amount of defects has strong influences on the physical properties of the Nb-doped STO thin films and that the band-to-band PL is useful for elucidating these influences.. |
| 22. |
Ryotaro Aso, Hiroki Kurata, Takeshi Namikoshi, Tamotsu Hashimoto, Shiao Wei Kuo, Feng Chih Chang, Hirokazu Hasegawa, Masahiko Tsujimoto, Mikio Takano, Seiji Isoda, Quantitative imaging of T g in block copolymers by low-angle annular dark-field scanning transmission electron microscopy, Macromolecules, 10.1021/ma4014934, 46, 21, 8589-8595, 2013.11, It is often difficult to observe nanoscale structures of polymeric materials using conventional transmission electron microscopy (TEM) because of their weak scattering contrast. To produce quantitative image contrast without any staining, low-angle annular dark-field scanning transmission electron microscopy (LAADF-STEM) was studied for its applicability for observing fine structures in block copolymers. The LAADF-STEM images displayed microphase-separated morphologies of block copolymers with high S/N contrast depending on the intrinsic density difference because of nonstaining. We found that the temperature dependence of the image contrast showed a kink around the glass-transition of the constituent phase, from which one can estimate glass transition temperatures and thermal expansion coefficients at nanoscale. This indicates that the LAADF-STEM imaging is an effective tool to quantitatively image nanoscale phases of polymers..  |
| 23. |
Kei Hirai, Daisuke Kan, Ryotaro Aso, Noriya Ichikawa, Hiroki Kurata, Yuichi Shimakawa, Anisotropic in-plane lattice strain relaxation in brownmillerite SrFeO 2.5 epitaxial thin films, Journal of Applied Physics, 10.1063/1.4817505, 114, 5, 2013.08, Anisotropic in-plane lattice relaxation behavior of brownmillerite SrFeO2.5 epitaxial thin films grown on (110) DyScO3 substrates was investigated. The in-plane lattices in the films less than 50 nm thick are fixed by the substrate lattice, whereas partial in-plane lattice relaxation along the [010] direction occurs in a 50 nm thick film. When the thickness reaches 98 nm, the film eventually exhibits lattice relaxation in both the [010] and the [10-1] in-plane directions. In the bottom region of the partially relaxed film, a dislocation, at which additional Fe atoms are seen, leads to formations of the stacking faults. In the surface region of the film, the complicated lattice defects propagated from the bottom result in the partial in-plane lattice relaxation associated with the disordered arrangements of the FeO4 tetrahedra and the FeO6 octahedra in the surface region. The preferential generation of the dislocations in the (10-1) plane can be explained by taking into account the anisotropic thermal expansion of SrFeO2.5, which results in the increase in the lattice mismatch between the film and the substrate only along the [010] direction in the cooling process after the film deposition.. |
| 24. |
Ryotaro Aso, Daisuke Kan, Yuichi Shimakawa, Hiroki Kurata, Atomic level observation of octahedral distortions at the perovskite oxide heterointerface, Scientific reports, 10.1038/srep02214, 3, 2013.07, For perovskite oxides, ABO3, slight octahedral distortions have close links to functional properties. While perovskite oxide heterostructures offer a good platform for controlling functionalities, atomistic understanding of octahedral distortion at the interface has been a challenge as it requires precise measurements of the oxygen atomic positions. Here we demonstrate an approach to clarify distortions at an atomic level using annular bright-field imaging in aberration-corrected scanning transmission electron microscopy, which provides precise mappings of cation and oxygen atomic positions from distortion-minimized images. This technique revealed significant distortions of RuO6 and ScO6 octahedra at the heterointerface between a SrRuO3 film and a GdScO3 substrate. We also found that structural mismatch was relieved within only four unit cells near the interface by shifting the oxygen atomic positions to accommodate octahedral tilt angle mismatch. The present results underscore the critical role of the oxygen atom in the octahedral connectivity at the perovskite oxide heterointerface.. |
| 25. |
Daisuke Kan, Ryotaro Aso, Hiroki Kurata, Yuichi Shimakawa, Epitaxial strain effect in tetragonal SrRuO3 thin films, Journal of Applied Physics, 10.1063/1.4803869, 113, 17, 2013.05, We report on our characterization of the structural, electrical, and magnetic properties of tetragonal SrRuO3 (SRO) thin films stabilized under both compressive and tensile strain. These tetragonal films consisting of the deformed RuO6 octahedra without rotations were coherently grown on (110)ortho NdGaO3 and (110)ortho GdScO 3 substrates, which provide compressive (-1.7) and tensile (1.0) strains, respectively. The ferromagnetic transition temperature TC for the compressively strained film is found to be as high as 155 K, while TC of the film under tensile strain is only 100 K. The longitudinal resistivity ρxx of the compressively strained films is lower than that of the films under the tensile strain. This is attributed to the enhanced mobility for the compressive-strain case. The magnetic anisotropy also exhibits strong dependence on the substrate-induced epitaxial strain. The film under the compressive strain has the uniaxial magnetic easy axis along the out-of-plane direction, while the easy axis of the film under the tensile strain lies along the in-plane direction parallel to the [1-10]GSO one. The results demonstrate that the electrical and magnetic properties of the tetragonal SRO thin films are closely related to the RuO6 octahedral deformations due to the substrate-induced strain.. |
| 26. |
Daisuke Kan, Ryotaro Aso, Hiroki Kurata, Yuichi Shimakawa, Thickness-dependent structure-property relationships in strained (110) SrRuO3 Thin Films, Advanced Functional Materials, 10.1002/adfm.201202402, 23, 9, 1129-1136, 2013.03, Thickness-dependent structure-property relationships in strained SrRuO 3 thin films on GdScO3 (GSO) substrates are reported. The film is found to have epitaxially stabilized crystal structures that vary with the film thickness. Below 16 nm, the √2apc × √2apc × 2apc monoclinic structure is stabilized while above 16 nm the film has the apc × 2a pc × apc tetragonal structure. The thickness-dependent structural changes are ascribed to the substrate-induced modification in the RuO6 octahedral rotation pattern, which highlights the significance of the octahedral rotations for the epitaxial strain accommodation in the coherently-grown films. Close relationships between the structural and physical properties of the films are also found. The monoclinic film has the uniaxial magnetic easy axis 45° away from the [110] GSO direction while the tetragonal film has the one that lies along the in-plane [1-10]GSO direction. The results demonstrate that the octahedral rotations in the strained perovskite oxide thin films are a key factor for determining their structure phases and physical properties. Thickness-dependent structure-property relationships in strained SrRuO 3 thin films are reported. The thin film changes from the monoclinic structure below 16 nm to the tetragonal structure above the thickness. The thickness-dependent structure is ascribed to the substrate-induced modification in the RuO6 octahedral rotation pattern. Physical properties such as magnetic anisotropy are closely related to the thin-film structure.. |
