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Epitaxial multiferroic BaTiO3-BiFeO3 composite thin films exhibit a correlation between the magnetoelectric (ME) voltage coefficient αME and the oxygen partial pressure during growth. The ME coefficient αME reaches high values up to 43 V/(cm·Oe) at 300 K and at 0.25 mbar oxygen growth pressure. The temperature dependence of αME of the composite films is opposite that of recently-reported BaTiO3-BiFeO3 superlattices, indicating that strain-mediated ME coupling alone cannot explain its origin. Probably, charge-mediated ME coupling may play a role in the composite films. Furthermore, the chemically-homogeneous composite films show an oxygen vacancy superstructure, which arises from vacancy ordering on the {111} planes of the pseudocubic BaTiO3-type structure. This work contributes to the understanding of magnetoelectric coupling as a complex and sensitive interplay of chemical, structural and geometrical issues of the BaTiO3-BiFeO3 composite system and, thus, paves the way to practical exploitation of magnetoelectric composites.

Epitaxial multiferroic BaTiO3-BiFeO3 composite thin films exhibit a correlation between the magnetoelectric (ME) voltage coefficient αME and the oxygen partial pressure during growth. The ME coefficient αME reaches high values up to 43 V/(cm·Oe) at 300 K and at 0.25 mbar oxygen growth pressure. The temperature dependence of αME of the composite films is opposite that of recently-reported BaTiO3-BiFeO3 superlattices, indicating that strain-mediated ME coupling alone cannot explain its origin. Probably, charge-mediated ME coupling may play a role in the composite films. Furthermore, the chemically-homogeneous composite films show an oxygen vacancy superstructure, which arises from vacancy ordering on the {111} planes of the pseudocubic BaTiO3-type structure. This work contributes to the understanding of magnetoelectric coupling as a complex and sensitive interplay of chemical, structural and geometrical issues of the BaTiO3-BiFeO3 composite system and, thus, paves the way to practical exploitation of magnetoelectric composites. [ABSTRACT FROM AUTHOR]

Multiferroic composites with ferromagnetic and ferroelectric properties can be used in nanobiomedicine and multifunctional drug load owing to their strong magnetoelectric performance at room temperature. In this study, cobalt ferrite (CFO) and barium titanate (BTO) nanocomposites are synthesized by chemical co-precipitation method, the fraction of CFO with respect to the BTO is 20, 40, and 60 wt% (hereafter abbreviation as C20, C40, and C60). In this way, CFO as the growth site of BTO grains, forms a structure that BTO grains coated CFO grains. With the addition of CFO, the crystalline size of BTO in nanocomposites become smaller than pure BTO, particles size of nanocomposites is between 48 and 60 nm, lower than pure BTO. Smaller particle size of nanocomposites lead to higher dielectric constant in nanocomposites. The magnetization (M) versus field (H) loops reveal the ferromagnetic nature of CFO–BTO nanocomposites. All nanocomposite samples present outstanding ME effect with high αME value of exceeding 41 mV Oe−1 all samples, reaching 44.13 mV Oe−1 of C20. [ABSTRACT FROM AUTHOR]

We demonstrate that a combination of electronic structure modification and oxygen vacancy ordering can stabilize a long-range ferromagnetic ground state in epitaxial LaCoO3 thin films. Highest saturation magnetization values are found in the thin films in tension on SrTiO3 and (La,Sr)(Al,Ta)O3and the lowest values are found in thin films in compression on LaAIO3. Electron microscopy reveals oxygen vacancy ordering to varying degrees in all samples, although samples with the highest magnetization are the most defective. Element-specific x-ray absorption techniques reveal the presence of high spin Co2+ and Co3+ as well as low spin Co3+ in different proportions depending on the strain state. The interactions among the high spin Co ions and the oxygen vacancy superstructure are correlated with the stabilization of the long-range ferromagnetic order. [ABSTRACT FROM AUTHOR]

Herein, the magnetoelectric (ME) performance of epitaxial multilayer composite films built from nanometer‐thick layers of multiferroic BiFeO3 and ferroelectric BaTiO3 is reviewed. A successful implementation of shadow‐mask pulsed laser deposition considerably reduces the interface and surface roughness of the multilayers. In dependence of double‐layer thickness and the degree of structural perfection, the multilayers show high ME voltage coefficients up to 480 V cm−1 Oe−1 at 300 K and 0 T bias magnetic field. With decreasing double‐layer thickness, an interface‐driven effect critically enhances the ME coupling in this strain and charge–comediated system. Interestingly, the characteristics of temperature and DC magnetic field dependencies of magnetoelectric voltage coefficients change with the transition from the 2D to 3D character of the single layers, i.e., for BiFeO3 layers thicker than 5 nm within the multilayers. These changes are attributed to variations of the contributing ME coupling mechanisms. Furthermore, scanning transmission electron microscopy (STEM) with energy‐dispersive X‐ray (EDX) spectroscopy mapping‐based nanoanalysis indicates that chemical effects at the interfaces play an important role for the ME performance of the BiFeO3–BaTiO3 multilayer thin films. [ABSTRACT FROM AUTHOR]

The properties of the TbBa(Co[sub 2-x]Fe[sub x-])O[sub 5+δ](00.1, at which a concentration-dependent orthorhombictetragonal transition takes place. The orthorhombic composition with x=0.1 exhibits a transition from an antiferromagnetic to weak ferromagnetic state at a temperature T[sub i]=200 K on warming and at 160 K on cooling, and a transition from a weak ferromagnetic to paramagnetic state at T[sub N] =304 K. A metal-insulator transition occurs at T[sub M]=348 K on warming. Transformation into a tetragonal phase (x ≥ 0.12) leads to the disappearance of both the weak ferromagnetic state and the metal-insulator phase transition. Also, the Néel point drops. The orthorhombic and tetragonal phases coexist in the concentrational range (0.1

The ultrafast motion of oxygen vacancies in solids is crucial for various future applications, such as oxide electrolytes. Visualization and quantification can offer unforeseen opportunities to probe the collective dynamics of defects in crystalline solids, but little research has been conducted on oxygen vacancy electromigration using these approaches. Here, we visualize electric-field-induced creation and propagation of oxygen-vacancy-rich and -poor competing phases and their interface with optical contrast in Ca-substituted BiFeO3 that contains a high density of mobile oxygen vacancies. We quantitatively determined the drift velocity of collective migration to be on the order of 100 μm s−1 with an activation barrier of 0.79 eV, indicating a significantly large ionic mobility of 2 × 10−6 cm2 s−1 V−1 at a remarkably low temperature of 390 °C. In addition, visualization enables direct observation of fluidic behavior, such as the enhancement of conduction at channel edges, which results in U-shaped viscous propagation of the phase boundary and turbulence under a reverse electric field. All of these results provide new insights into the collective motion of defects. The collective propagation of defects in an oxide material has been visualized by researchers in South Korea and the USA. Most crystalline materials are imperfect. One type of imperfection, known as a vacancy, is the absence of an atom from the crystal lattice. These vacancies move through the material as surrounding atoms shift to fill the empty spot. A better understanding of this motion could aid the development of novel memories, solid oxide fuel cells and batteries. Chan-Ho Yang from the Korea Advanced Institute of Science and Technology, Daejeon, and colleagues recorded the movement of oxygen vacancies in calcium-substituted BiFeO3 because of the color difference between regions of high and low vacancy concentration. This ability to visualize oxygen-vacancies in real-time offers an opportunity to measure their movement and better understand defect-induced material changes. We visualize an electric-field-induced collective propagation of oxygen vacancies spontaneously contained in Ca-substituted BiFeO3 films, using the fact that the oxygen-rich and poor regions have different color contrasts and thus they are optically distinguishable forming a sharp boundary. We quantitatively determine the drift velocity to be of the order of 100 μm s−1 with an activation barrier of 0.79 eV indicating a significantly large ionic mobility 2 × 10−6 cm2 s−1 V−1 at a remarkably low temperature of 390 °C. Furthermore, U-shaped propagation and turbulence under backward electric field provide insights into fluidic defects in crystalline solids. [ABSTRACT FROM AUTHOR]

Multiferroic superlattices consisting of 15 double layers BaTiO3-BiFeO3 show a very high magnetoelectric voltage coefficient αME of up to 49 V cm−1 Oe−1 at 300 K, measured at 1 kHz with zero direct current (DC) bias magnetic field. However, the microscopic origins of such high αME values, and the temperature and DC magnetic field dependencies of αME are not understood up to now. Therefore, in this study two superlattices grown at high/low oxygen partial pressures having high/low αME values, respectively, are compared. While at high growth pressure the strain contrast in high-resolution transmission electron microscopy images is limited to few layers at the substrate interface, the low-pressure sample shows much more pronounced microstrain. This is additionally visualized in Fourier transformed images, and the out-of-plane lattice parameter of the single BaTiO3 layer is increased near the interface. In addition to a low density of oxygen vacancies, it seems to be important to avoid micromechanical clamping of the 2D single-phase nanolayers of the multiferroic BaTiO3-BiFeO3 superlattices to achieve high magnetoelectric coupling. [ABSTRACT FROM AUTHOR]

The energetics at oxide semiconductor/La1-xSrxCoO3 heterojunctions, including the respective alignment of the valence and conduction bands, govern charge transfer and have to be determined for the design of future La1-xSrxCoO3-based devices. In this letter, the electronic and atomic structures of epitaxial La1-xSrxCoO3 on Nb-doped strontium titanate are revealed by scanning transmission electron microscopy, electron energy loss spectroscopy, and in situ x-ray and ultra violet photoelectron spectroscopies. For LaCoO3, a valence band (VB) offset of 2.8±0.1 eV is deduced. The large offset is attributed to the orbital contributions of the Co 3d states to the VB maximum of the LaCoO3 thin films, with no evidence of interface dipole contributions. The sensitivity of the valence band orbital character to spin state ordering and oxygen vacancies is assessed using density functional theory. [ABSTRACT FROM AUTHOR]

This work presents results of HRTEM and XRD studies on microstructure of ceria-based mixed oxides subjected to high-temperature heat treatment in air. Nano-sized (4–5 nm) crystallites of Ce0.5Ln0.5O1.75 oxides (Ln = Yb, Lu) with narrow size distribution were prepared by a microemulsion method (W/O). Presence of an intermediate cubic phase, with oxygen vacancy ordering, was found for the Ce0.5Ln0.5O1.75 mixed oxides. A simple model of the intermediate phase has been proposed and tested. [ABSTRACT FROM AUTHOR]

The influence of oxygen content on the magnetization and electrical resistivity of Ln[sub 0.5]A[sub 0.5]MnO[sub 3] (Ln=La, Pr, Nd; A=Ca, Ba) manganites with the perovskite structure is investigated. It is shown that the La[sub 0.5]Ca[sub 0.5]MnO[sub 3-γ] compound undergoes a sequence of transitions from an antiferromagnetic (γ=0) to a spin-glass (γ=0.17) state and then to an inhomogeneous ferromagnetic (γ=0.3) state. A transition from an antiferromagnetic charge-ordered state to a ferromagnetic charge-disordered state in Nd[sub 0.5]Ca[sub 0.5]MnO[sub 3-γ] is observed as the oxygen content is reduced to where γ=0.07. The Nd[sub 0.5]Ba[sub 0.5]MnO[sub 3-γ] compound shows an increase of the Curie point from 110 K(γ=0) to 310 K(γ=0.3). In addition, a large magnetoresistance is revealed which develops below their Curie temperature despite the absence of Mn[sup 3+]-Mn[sup 4+] pairs. A Zener double-exchange interaction is usually used in literature to explain the magnetic and electrical properties of hole-doped perovskite manganites. The data obtained support the mechanism of superexchange interactions between magnetic moments of the manganese ions via oxygen. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The magnetization and magnetoresistance are studied in La[sub 0.5]Ca[sub 0.5]MnO[sub 3-γ] as a function of oxygen content. It is found that as the oxygen content is decreased, this compound undergoes a sequence of transitions from an antiferromagnetic to a ferromagnetic state (γ>=0.04), from the ferromagnetic to a spin-glass state (γ>=0.14), and from the spin glass to an inhomogeneous ferromagnetic state (γ>=0.25). Strongly reduced samples (γ>=0.25) show a large magnetoresistance despite the absence of Mn[sup 3+]–Mn[sup 4+] pairs. It is suggested that the oxygen vacancies in the strongly reduced samples (γ>=0.25) are long-range ordered. © 1999 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Proton conduction is an important property for fuel cell electrolytes. The search for molecular details on proton transport is an ongoing quest. Here, we show that in hydrated yttrium doped barium zirconate using X-ray and neutron diffraction that protons tend to localize near the dopant yttrium as a conjugated superstructure. The proton jump time measured using quasi-elastic neutron scattering follows the Holstein-Samgin polaron model, revealing that proton hopping is weakly coupled to the high-frequency O-H stretching motion, but strongly coupled to low-frequency lattice phonons. The ratio of the proton polaron effective mass, m*, and the proton mass is m*/m = 2, when coupled to the Zr-O stretching mode, giving experimental evidence of proton pairing in perovskites, as a result of proton-phonon coupling. Possible pathways of a proton pair are provided through Nudge Elastic Band calculations. The pairing of protons, when jumping, is discussed in context of a cooperative protonic charge transport process. Proton conduction through ceramic electrolyte membranes involves the ionic transport of H+ ions through a crystalline lattice. Here, the authors investigate the underlying molecular transport mechanism and demonstrate that proton conduction in yttrium-doped barium zirconate occurs through cooperative proton motion. [ABSTRACT FROM AUTHOR]

In this paper, the magnetoelectric (ME) effect is investigated in two kinds of A-line shape Terfenol-D/PZT-5A structures by changing the position of the NdFeB permanent magnet. The experimental results show that both ME composite structures had multiple resonance peaks. For the ME structure with acrylonitrile-butadiene-styrene (ABS) trestles, the resonance peak was different for different places of the NdFeB permanent magnet. Besides, the maximum of the ME coefficient was 4.142 V/A at 32.2 kHz when the NdFeB permanent magnet was on top of the Terfenol-D layer. Compared with the ME coefficient with a DC magnetic field, the ME coefficient with NdFeB magnets still maintained high values in the frequency domain of 65~87 kHz in the ME structure with mica trestles. Through Fourier transform analysis of the transient signal, it is found that the phenomenon of multiple frequencies appeared at low field frequency but not at high field frequency. Moreover, the output ME voltages under different AC magnetic fields are shown. Changing the amplitude of AC magnetic field, the magnitude of the output voltage changed, but the resonant frequency did not change. Finally, a finite element analysis was performed to evaluate the resonant frequency and the magnetic flux distribution characteristics of the ME structure. The simulation results show that the magnetic field distribution on the surface of Terfenol-D is non-uniform due to the uneven distribution of the magnetic field around NdFeB. The resonant frequencies of ME structures can be changed by changing the location of the external permanent magnet. This study may provide a useful basis for the improvement of the ME coefficient and for the optimal design of ME devices. [ABSTRACT FROM AUTHOR]

Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addition to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-volume ratio. Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelectrics, piezoelectrics, power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge. The research related to the oxide electronic field is at an impressionable stage, and this has motivated us to contribute with a roadmap on ‘oxide electronic materials and oxide interfaces’. This roadmap envisages the potential applications of oxide materials in cutting edge technologies and focuses on the necessary advances required to implement these materials, including both conventional and novel techniques for the synthesis, characterization, processing and fabrication of nanostructured oxides and oxide-based devices. The contents of this roadmap will highlight the functional and correlated properties of oxides in bulk, nano, thin film, multilayer and heterostructure forms, as well as the theoretical considerations behind both present and future applications in many technologically important areas as pointed out by Venkatesan. The contributions in this roadmap span several thematic groups which are represented by the following authors: novel field effect transistors and bipolar devices by Fortunato, Grundmann, Boschker, Rao, and Rogers; energy conversion and saving by Zaban, Weidenkaff, and Murakami; new opportunities of photonics by Fompeyrine, and Zuniga-Perez; multiferroic materials including novel phenomena by Ramesh, Spaldin, Mertig, Lorenz, Srinivasan, and Prellier; and concepts for topological oxide electronics by Kawasaki, Pentcheva, and Gegenwart. Finally, Miletto Granozio presents the European action ‘towards oxide-based electronics’ which develops an oxide electronics roadmap with emphasis on future nonvolatile memories and the required technologies. In summary, we do hope that this oxide roadmap appears as an interesting up-to-date snapshot on one of the most exciting and active areas of solid state physics, materials science, and chemistry, which even after many years of very successful development shows in short intervals novel insights and achievements. Guest editors: M S Ramachandra Rao and Michael Lorenz [ABSTRACT FROM AUTHOR]