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Compositional tuning of correlations between electrons, spins, and lattices in solids is a key strategy for exploring quantum phenomena. In contrast to uniform chemical substitutions that are widely used for the compositional tunings, continuous variations in compositions along a given direction of materials (composition gradients) are expected to modulate translational crystal symmetries and electronic band structures, leading to unique properties not seen in homogeneously substituted materials. Preparing compositionally graded materials, however, is still challenging as it requires elaborate fabrication processes, and how composition gradients affect materials' properties remains elusive. Here, we show that composition gradients can be made by simply depositing films under continuously varying oxygen pressures by pulsed laser deposition. Applying this technique to the inverse-spinel-structured ferrimagnet NiCo2O4, we found that composition gradients in NiCo2O4 epitaxial films tune the magnetic anisotropy and stabilize the magnetizations modulated along the gradient, which are not seen in compositionally uniform films. The spatially modulated magnetization can also be reversed in controlled manners through distinct antiferromagnetically coupled magnetization configurations in the single material. Our results demonstrate the potential of composition gradients to tune materials' properties and explore a new class of materials with added functionalities. [ABSTRACT FROM AUTHOR]

We investigated the influence of oxygen vacancies on the magnetic and transport properties of ferrimagnetic NiCo2O4 (NCO) epitaxial films. Oxygen vacancies were introduced by annealing under reducing atmospheres NCO films whose cation composition was close to the stoichiometric one. We find that annealing NCO films under the vacuum reduces their magnetizations and increases their electrical resistivities. The perpendicular magnetic anisotropy, on the other hand, is almost unaffected by annealing treatments. X-ray absorption spectroscopy shows that oxygen vacancies introduced in NCO films preferentially lower the Ni valence state while leaving the Co valence state unchanged. The lowering in the Ni valence state explains the reduced magnetizations and the increased resistivities for the films annealed under the vacuum. On the other hand, the Co valence state, which dominantly determines the orbital magnetic moments responsible for magnetic anisotropy, is insensitive to oxygen vacancies. Therefore, the perpendicular magnetic anisotropy is maintained even when the oxygen vacancies are introduced. [ABSTRACT FROM AUTHOR]

We grew SnO2 thin films on (001) YSZ substrates by a mist chemical vapor deposition (mist CVD) method and investigated their structural properties by measuring synchrotron x-ray diffraction. We found that the structural phases of the films depend on the growth temperature. SnO2 deposited at 400 °C was found to be epitaxially grown on the substrate and to have the high-pressure-stabilized structure [i.e., columbite structure with the (100)-orientation]. On the other hand, films deposited at 700 °C have a mixture of the epitaxially grown columbite structure and a polycrystalline rutile structure. We also found that while films deposited at both temperatures are grown in island-growth manners, the size of the islands in the initial stage of growth depends on the growth temperature. The islands of the films grown at 400 °C have diameters of 59 nm ± 10 nm, while the islands of the films grown at 700 °C have diameters 2 times larger (110 nm ± 16 nm). These observations indicate that the coalescence of the deposited mist particles resulting from the higher temperature growth, which makes the island larger, leads to partial relaxation of the substrate-induced strain. This explains why the films deposited at 700 °C consist of a mixture of the columbite and rutile structures. Our results highlight the significance of the growth-temperature-dependent coalescence and its induced strain relaxation as the factor determining the structural phase of mist-CVD-grown films. [ABSTRACT FROM AUTHOR]

We carried out the synchrotron X-ray diffraction measurements and investigated the oxygen octahedral distortions in compressively strained SrRuO3 epitaxial thin films on NdGaO3 substrates. We observed half-order Bragg reflections originating from the RuO6 octahedral distortions accommodated in the compressively strained films. Our structural analysis shows that the RuO6 octahedra are elongated along the out-of-plane direction and are rotated in the out-of-phase manner only about the out-of-plane direction. The RuO6 rotation angle γrot is 10.9° ± 1.2°, which is larger than that in the bulk SrRuO3. The results indicate that the substrate-induced compressive strain enhances the out-of-plane rotations while strongly suppressing the in-plane rotations. In fact, the half-order Bragg reflections arising from the octahedral rotations about the in-plane direction are found to be weak and broad, implying that the in-plane rotations exist only in the interface region, not in the entire film. This indicates that while the in-plane octahedral rotation propagates through the interfacial octahedral connections into the film, its propagation decays within the interface region because of the compressive strain. Our results indicate that both the substrate-induced strain and the interfacial octahedral connection play important roles in determining the octahedral distortions accommodated in the strained SrRuO3 films. [ABSTRACT FROM AUTHOR]

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 RuO3 octahedral tilts, a one-unit-cell-thick BaTiO3 layer inserted between SrRuO3 and GdScO3 stabilizes the tetragonal SrRuO3 layer with largely reduced RuO3 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 RuO3-TiO3 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 SrRuO3 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Detailed structural investigations on the substitution-induced structural phase transition from the rhombohedral phase to an orthorhombic phase in (Bi,RE)FeO3 epitaxial thin films (RE = La, Sm, and Dy) grown on (100) SrTiO3 substrates are presented. X ray diffraction reveals that the unit cell dimensions of the orthorhombic phase are strongly dependent on the type of the RE dopant. For RE = La3+ ion, which has an ionic size comparable to the Bi3+ ion, the unit cell is found to be a0 × a0 × 2a0, where a0 is the pseudo-cubic lattice parameter. This is in contrast with the [Square_Root]2a0 × [Square_Root]2a0 × 2a0 unit cell for the case of smaller ionic radius RE (= Sm and Dy) elements. While clear double-hysteresis loops in the polarization versus electric-field curves due to field-induced transitions are observed for smaller ionic radius RE (= Sm and Dy), no signature of the double hysteresis loops is observed for the RE = La case across the structural boundary. We have also performed systematic tracking of the structural phases as functions of the RE composition and temperature, based on which we propose a phase diagram. This work reveals that the ionic size of the RE element plays a critical role in the evolution of the structural and functional properties of RE-substituted BiFeO3 thin film materials systems. [ABSTRACT FROM AUTHOR]