Daisuke Kan

Atomic level observation of octahedral distortions at the perovskite oxide heterointerface

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.

Blue luminescence from electron-doped SrTiO3

Blue-light emissions from electron-doped SrTiO3 single crystals at room temperature were observed. Substituting La3+ for Sr2+ and Nb5+ for Ti4+ in SrTiO3 provides electron carriers in Ti 3d conduction bands; these carriers are responsible for the room-temperature blue-light emission. This blue-light emission is essentially the same as that observed in Ar+-irradiated oxygen-deficient SrTiO3. This blue luminescence is independent of the defect type. The chemical substitution of La3+ for Sr2+ changes the temperature of the structural phase transition from cubic to tetragonal symmetry. The relation between the photoluminescence properties and the structural phase transition is also discussed.

Atomic-scale evolution of modulated phases at the ferroelectric–antiferroelectric morphotropic phase boundary controlled by flexoelectric interaction

Physical and structural origins of morphotropic phase boundaries (MPBs) in ferroics remain elusive despite decades of study. The leading competing theories employ either low-symmetry bridging phases or adaptive phases with nanoscale textures to describe different subsets of the macroscopic data, while the decisive atomic-scale information has so far been missing. Here we report direct atomically resolved mapping of polarization and structure order parameter fields in a Sm-doped BiFeO(3) system and their evolution as the system approaches a MPB. We further show that both the experimental phase diagram and the observed phase evolution can be explained by taking into account the flexoelectric interaction, which renders the effective domain wall energy negative, thus stabilizing modulated phases in the vicinity of the MPB. Our study highlights the importance of local order-parameter mapping at the atomic scale and establishes a hitherto unobserved physical origin of spatially modulated phases existing in the vicinity of the MPB.

Multiferroic thin film of Bi2NiMnO6 with ordered double-perovskite structure

Epitaxial thin films of Bi2NiMnO6 were synthesized on SrTiO3 substrates by pulsed laser deposition. The resulting film had the rock-salt-type arrangement of Ni2+ and Mn4+ ions in a double-perovskite unit cell. The films clearly showed multiferroic properties, both ferromagnetic behavior with a Curie temperature of about 100K and ferroelectric behavior with a saturated polarization of about 5μC∕cm2.

Epitaxial growth of ferromagnetic La2NiMnO6 with ordered double-perovskite structure

Epitaxial thin films of ordered double-perovskite La2NiMnO6 were deposited on SrTiO3, (LaAlO3)0.3–(Sr2AlTaO6)0.7, and LaAlO3 substrates by a pulsed-laser deposition method. A rock-salt-type ordering for Ni2+ and Mn4+ ions was confirmed through structural and magnetic measurements. Despite the difference in heteroepitaxial constraints on the crystal structure, the magnetic properties of the films were quite similar to each other and also to those of bulk La2NiMnO6.

Phase coexistence near a morphotropic phase boundary in Sm-doped BiFeO3 films

We have investigated heteroepitaxial films of Sm-doped BiFeO3 with a Sm-concentration near a morphotropic phase boundary. Our high-resolution synchrotron x-ray diffraction, carried out in a temperature range of 25 to 700 °C, reveals substantial phase coexistence as one changes temperature to crossover from a low-temperature PbZrO3-like phase to a high-temperature orthorhombic phase. We also examine changes due to strain for films exhibiting anisotropic misfit between film and substrate. Additionally, thicker films exhibit a substantial volume collapse associated with the structural transition that is suppressed in thinner films.

Microstructure-electromechanical property correlations in rare-earth-substituted BiFeO3 epitaxial thin films at morphotropic phase boundaries

Structure-electromechanical property correlations in rare-earth (RE)-substituted (001) BiFeO3 (BFO) epitaxial thin films have been investigated. Quantitative piezoelectric coefficient (d33) and dielectric constant (e33) measurements, in conjunction with selected area electron diffraction, reveal that the enhancement in d33 and e33 at the morphotropic phase boundary (MPB) of the RE-substituted films (RE=Dy3+, Gd3+, and Sm3+) is correlated with the presence of a competing intermediate antipolar phase with the rhombohedral ferroelectric and nonpolar orthorhombic phase. This leads to a complex nanoscale phase coexistence at the MPB. Extending the studies to RE=La3+ case, we find the nanoscale phase coexistence to be less pronounced. This explains the lack of increase in d33 for the La3+-substituted BFO films, in contrast to the Dy3+, Gd3+, and Sm3+-substituted films.

Tuning magnetic anisotropy by interfacially engineering the oxygen coordination environment in a transition metal oxide

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.

Anomalous ferromagnetism in TbMnO3 thin films

Magnetometry, x-ray, and neutron scattering have been used to study the structural and magnetic properties of a TbMnO3 thin film grown on a [001] SrTiO3 substrate by pulsed laser deposition. Although bulk TbMnO3 is a low temperature antiferromagnet, magnetometry measurements indicate the presence of low temperature ferromagnetism. Depth profiling by x-ray and polarized neutron reflectometry reveals a net sample magnetization that is commensurate with the film thickness, indicating that the observed ferromagnetism is not due to an altered surface phase (such as Mn3O4), or external impurities that might give rise to an artificial magnetic signal. Instead, these results show that the ferromagnetism is an intrinsic property of the TbMnO3 film.

Chemical Substitution‐Induced Ferroelectric Polarization Rotation in BiFeO3

and multiferroic [ 2 ] materials. For instance, ferroelectric materials at morphotropic phase boundaries (MPB), where multiple structural phases with ferroelectric polarizations pointing in different crystallographic directions coexist, often display large piezoelectric coeffi cients. [ 3– 5 ] It is the ferroelectric distortions, which accompany the polarization rotation that leads to enhancements in the piezoelectric coeffi cient. In multiferroic BiFeO 3 (BFO), it has been shown [ 2 ] that the coupled antiferromagnetic order can be altered by switching the ferroelectric polarization vector. In fact, the ability of a material to display polarization rotation is recognized as an important precursor to occurrence of an MPB. [ 5 , 6 ]