Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

Abstract

The charge state of supported metal catalysts is the key to understanding the elementary processes involved in catalytic reactions. However, high-precision charge analysis of the metal catalysts at the atomic level is experimentally challenging. To address this critical challenge, high-sensitivity electron holography has recently been successfully applied for precisely measuring the elementary charges on individual platinum nanoparticles supported on a titanium dioxide surface. In this review, we introduce the latest advancements of high-precision charge analysis and discuss mechanisms of charge transfer at the metal–support interface. The development of charge measurements is entering a new era, and charge analyses under conditions closer to practical working environments, such as real-time, real-space, and reactive gas environments, are expected to be realized in near future.

DOI： 10.1093/jmicro/dfae018

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Evaluating ion-conductive solid electrolytes (SEs) accurately is crucial for advancing solid-state battery technology. Scanning/transmission electron microscopy (S/TEM) is a valuable technique for examining the local characteristics of battery materials. However, the tolerance of SEs to high-energy electron beams is notably lower than that of electrode active materials, because SEs generally have low electron conductivity. Here, we propose a specialized coating technique for TEM samples, termed “nano-shield,” which enables stable and accurate observation of their micro-/nano-structures. Nano-shield consists of a dual-layer coating combining an amorphous insulating layer of aluminum oxide (AlOx) with a conductive carbon (C) film. The AlOx layer blocks ion diffusion in the TEM samples, and the C layer minimizes electron charging during electron-beam irradiation. By applying a nano-shield, we successfully visualized the atomic structures of a lithium-ion-conductive SE, Li1.3Al0.3T1.7(PO4)3 (LATP), by using STEM at room temperature. Additionally, we performed a precise elemental analysis of a sodium-ion-conductive SE, Na3Zr2Si2PO12 (NZSP), via STEM equipped with energy-dispersive X-ray spectroscopy (STEM-EDS). Our findings demonstrate that nano-shield enhances the reliability of S/TEM observations of SEs and sheds light on its underlying protective mechanisms.

DOI： 10.1007/s10008-024-05869-8

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Other Link： https://link.springer.com/article/10.1007/s10008-024-05869-8/fulltext.html

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

Abstract

Perovskite oxides, ABO3, are potential catalysts for the oxygen evolution reaction, which is important in the production of hydrogen as a sustainable energy resource. Optimizing the chemical composition of such oxides by substitution or doping with additional elements is an effective approach to improving the activity of such catalysts. Here, we characterized the crystal and electronic structures of fluorine-doped La0.5Sr0.5CoO3−δ particles using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). High-resolution STEM imaging demonstrated the formation of a disordered surface phase caused by fluorine doping. In addition, spatially resolved EELS data showed that fluorine anions were introduced into the interiors of the particles and that Co ions near the surfaces were slightly reduced by fluorine doping in conjunction with the loss of oxygen ions. Peak fitting of energy-loss near-edge structure data demonstrated an unexpected nanostructure in the vicinity of the surface. An EELS characterization comprising elemental mapping together with an energy-loss near-edge structure analysis indicated that this nanostructure could not be assigned to Co-based materials but rather to the solid electrolyte BaF2. Complementary structural and electronic characterizations using STEM and EELS as demonstrated herein evidently have the potential to play an increasingly important role in elucidating the nanostructures of functional materials.

DOI： 10.1093/jmicro/dfad031

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Institute of Metals and Materials  

DOI： 10.2320/materia.63.95

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Abstract

Instead of conventional cation doping strategy, anion doping is a promising new strategy for advances of energy conversion and storage technologies such as batteries, catalysts, electrolysis, and fuel cells. To synthesize mixed‐anion compounds, novel synthesis techniques such as topochemical reaction, high‐pressure reaction, solvothermal reaction have been developed. Despite these excellent synthesis techniques, synthesizable mixed‐anion compounds are still limited. For further expansion of the material exploration of mixed‐anion compounds, herein, an electrochemical anion doping technique is developed, which can flexibly control a species of anion, the doping rate and the degree of anion doping. The concept of the new synthesis technique is verified by F doping to the perovskite oxide La_0.5Sr_0.5CoO₃₋_δ. Quantitative control of F in the perovskite host material is succeeded by using an electrochemical reactor composed of La_0.5Sr_0.5CoO₃₋_δ‐BaF₂|BaF₂|PbF₂‐Pb, and phase‐pure F‐doped La_0.5Sr_0.5CoO₃₋_δ powder is obtained. Moreover, nano‐size crystalline domains with amorphous phase are formed on the particle surface under the high‐rate F doping, suggesting that tuning the anion doping rate enables the control of the formation of metastable phase. As demonstrated, the electrochemical anion doping technique opens up new possibilities for advances of energy materials by utilizing function of anionic species.

DOI： 10.1002/adfm.202307116

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

We grew epitaxial thin films of hematite (α-Fe2O3) on (0 0 0 1) Al2O3 substrates by pulsed laser deposition and investigated their magnetic properties. α-Fe2O3 films grown at lower temperatures are found to undergo the Morin transition at higher temperatures, implying that lowering the growth temperature and managing lattice defects associated with strain relaxation are key to realizing Morin transition. We also characterized films' magnetic properties by spin Hall magnetoresistance (SMR). We show that tri-axial magnetic anisotropy can be detected as SMR features having a 60° period with respect to in-plane magnetic field rotations. Furthermore, a large change in SMR ratio associated with Néel vector re-orientation due to Morin transition is seen. Details of SMR properties for α-Fe2O3 films and their analysis results, including the influence of the fabrication process on SMR behaviors, are discussed.

DOI： 10.1063/5.0087643

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.nanolett.1c03143

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.7567/1347-4065/ab4b15

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1039/c9nr00806c

Language：Japanese   Publishing type：Research paper (scientific journal)  

Direct Observation of Oxygen Displacement in Oxide Heterointerface by Atomic-Resolution STEM

DOI： 10.11410/kenbikyo.54.1_14

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1038/s41467-018-04412-4

Language：English   Publishing type：Research paper (scientific journal)  

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 V_AC). From a V_AC 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 V_AC cycle from the targeted phase Φ to Φ + ∆Φ with phase width ∆Φ (∆Φ < 2π). ∆Φ corresponds to the temporal pulse width τ (τ < 1/f) of an electron beam; i.e., ∆Φ = 2πfτ. Because of the high sensitivity of the TEM camera used in this study, the images and spectra that are acquired at the same target phase are integrated by means of stroboscopic illumination to obtain the final phase-locked images and spectra with sufficiently small S/N ratio. Phase-locked (strobe) images and/or spectra are obtained for model specimens of polycrystalline aluminum and an all-solid-state lithium ion battery (LIB). In the phase-locked TEM conditions, f ranges from 1 Hz to about 40 kHz and ∆Φ from 2π/80 to π. V_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 V_AC, 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.

DOI： 10.1016/j.ultramic.2017.04.018

Language：English   Publishing type：Research paper (scientific journal)  

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 SrFeO_2.5 (SFO) and DyScO₃ (DSO). When an oxygen-deficient perovskite (brownmillerite structure) SrFeO_2.5 film grows epitaxially on a perovskite DyScO₃ substrate, both FeO₆ octahedra and FeO₄ tetrahedra in the (101)-oriented SrFeO_2.5 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 SrFeO_2.5 occurs at 1103 K, is induced at the present heterointerface at ambient temperatures.

DOI： 10.1021/acsami.7b08134

Language：English   Publishing type：Research paper (scientific journal)  

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 Ca_0.5Sr_0.5TiO₃ (0-4 monolayers thick) grown on a GdScO₃ substrate. We found that a Ru-O-Ti bond angle of the SrRuO₃/Ca_0.5Sr_0.5TiO₃ interface can be engineered by layer-by-layer control of the Ca_0.5Sr_0.5TiO₃ 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.

DOI： 10.1038/nmat4580

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1039/c4dt03749a

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1063/1.4918965

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1021/cg501340e

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1021/cg500285m

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1063/1.4875839

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.7567/APEX.7.015503

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1021/ma4014934

Repository Public URL： https://hdl.handle.net/2324/7172123

Language：English   Publishing type：Research paper (scientific journal)  

Anisotropic in-plane lattice relaxation behavior of brownmillerite SrFeO_2.5 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 SrFeO_2.5, 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.

DOI： 10.1063/1.4817505

Language：English   Publishing type：Research paper (scientific journal)  

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 T_C for the compressively strained film is found to be as high as 155 K, while T_C of the film under tensile strain is only 100 K. The longitudinal resistivity ρ_xx 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.

DOI： 10.1063/1.4803869

Language：English   Publishing type：Research paper (scientific journal)  

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 √2a_pc × √2a_pc × 2a_pc monoclinic structure is stabilized while above 16 nm the film has the a_pc × 2a _pc × a_pc 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.

DOI： 10.1002/adfm.201202402