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Eu1−xBaxTi1−yMyO3 (M = Co or Ni) was sintered at 1400 °C under a reduction atmosphere. X-ray photoelectron spectroscopy revealed the mixed valences of Eu2+/Eu3+ and Ti4+/Ti3+ in EuTiO3 and Eu0.7Ba0.3TiO3, as well as some oxygen vacancies required to keep the charge neutrality. The co-doping of Co2+/Ni2+ in Eu0.7Ba0.3TiO3 resulted in the disappearance of oxygen vacancies, as a result of a reduction in Ti3+ numbers and an increase in Eu3+ numbers. On the other hand, Ba2+ doping led to an increased lattice parameter due to its larger ionic size than Eu2+, whereas the Co2+/Ni2+ co-doping resulted in smaller lattice parameters because of the combined effects of ionic size and variation in the oxygen-vacancy numbers. Eu0.7Ba0.3TiO3 exhibited a clear ferroelectricity, which persisted in the Co2+/Ni2+ co-doped samples until the doping levels of y = 0.05 and 0.10, respectively. Eu0.7Ba0.3TiO3 remained to be antiferromagnetic with a reduced transition temperature of 3.1 K, but co-doping of Co2+/Ni2+ turned the samples from antiferromagnetic to ferromagnetic with transition temperatures of 2.98 K and 2.72 K, respectively. The cause for such a transition could not be explained by the larger lattice volume, oxygen vacancies and mixed valences of Eu2+/Eu3+, which were proposed in previous works. Instead, it was more likely to arise from a large asymmetric distortion of the Eu–O polyhedron introduced by the aliovalent doping, which promotes the admixture of Eu 5d and 4f states. [ABSTRACT FROM AUTHOR]

Abstract BiFeO 3 (BFO) samples with nearly perfect ferroelectric hysteresis loops were synthesized from chemical solution via hydrothermal route at 200 °C. However, for many applications, ceramic samples of reasonable bulk density (>80%) have to be sintered at temperature over 700 °C, which in this case results in a significant reduction in resistivity due to increased amounts of Fe2+. Interestingly, doping of a few percent Sb minimized such a problem and the sintered Sb:BFO ceramics retained a similarly high resistivity as samples cold-pressed from the chemical-solution synthesized powders. However, for cold-pressed samples, Sb:BFO actually had higher conductivity than undoped BFO. Temperature-dependent conductivity showed that cold-pressed samples of both undoped and Sb doped BFO had the similar activation energy of 1.0 eV, typical for electrons trapped in oxygen vacancies. After sintering, the activation energy of Sb:BFO remained almost unchanged, but the activation energy of undoped BFO changed to 0.4 eV, which is associated to electron hopping between Fe2+/Fe3+. X-ray photoelectron spectroscopy (XPS) showed a significant increase in Fe2+/Fe3+ ratio from 6.6/93.4 to 25.7/74.3 in undoped BFO after sintering, while for 1% Sb doped BFO the increase was much milder from 10.9/89.1 to 14.1/85.9. XPS also showed that Sb had single +3 oxidation state before sintering, but after sintering a fairly large portion of Sb5+ occurred. So, charge compensation for oxygen vacancies in undoped BFO was achieved dominantly by reduction of Fe3+ to Fe2+, while in Sb:BFO it was achieved more by cation vacancies. Highlights • Pure and Sb-doped BiFeO 3 were synthesized by hydrothermal method at 200 °C. • As-prepared samples showed typical hysteresis loops of ideal ferroelectrics. • Sintering the hydrothermal powders resulted in a great decrease in resistivity. • Sb doping helped to retain a similarly high resistivity in the sintered samples. • Sb doping kept oxygen vacancies rather than Fe2+ ions as the main defects. [ABSTRACT FROM AUTHOR]

Ni 0.5 Zn 0.5 Fe 2 O 4 (NZFO) + BiFeO 3 (BFO) composite films were co-sputtered on (001) SrTiO 3 (STO) substrates, which were confirmed to be epitaxial by both 1D and 2D X-ray diffractions. Conductive LaNiO 3 (LNO) was sputtered on STO as bottom electrode for magnetoelectric (ME) measurement across film. The LNO layer was epitaxial and composed of nanocolumns perpendicular to substrate, which are helpful in reducing “substrate clamp” effect because of the flexibility of nanocolumns. NZFO + BFO films co-sputtered on LNO/STO were also epitaxial but both out-of-plane and in-plane textures of BFO were degraded with full width at half maximum of 1.974° and 2.55°, respectively, compared to 1.163° and 1.71° in films grown on bare STO. There were apparent tetragonal distortions in NZFO and BFO due to compressive in-plane strains imposed by epitaxial growth of larger NZFO and BFO lattices on smaller LNO lattice. Transmission electron microscopy (TEM) showed that the composite films were compact with clear boundaries between the NZFO and BFO phases, which were uniformly distributed with the ratio of about 35% NZFO and 65% BFO. Cross-sectional TEM revealed that crystallographic planes were very well aligned between BFO and LNO, but NZFO planes tilted 7.5° due to large lattice misfit. When measured vertically from LNO to top surface of composite film, large ME voltage coefficient at zero bias field was observed, which was 911 mVcm −1 Oe −1 at the frequency of 8 kHz. This was ascribed to the large heteroepitaxial strains in composite films. [ABSTRACT FROM AUTHOR]

Tetragonal tungsten-bronze (TTB) structured niobates were doped with transition metal (TM) ions in order to search for multiferroic materials. Single crystals of Ba6FeNb9O30, Ba6CoNb9O30, Ba6Mn0.67Nb9.33O30, K6Fe3.0Nb9.6O30, etc. were grown by spontaneous nucleation method and their structures were determined by X-ray single crystal diffraction, which revealed that TM substituted with Nb instead of entering any of the three interstitial sites. Polarisation vs. electrical field measurements showed hysteresis loops for Ba6FeNb9O30, suggesting that it is likely to be ferroelectric just as many other TTB niobates. Magnetic measurements indicated some ordered magnetic structure at low temperature. [ABSTRACT FROM PUBLISHER]

Abstract: Multiferroic BiFeO3 films have been deposited on a number of substrates by RF magnetron sputtering. Two routes were adopted in order to obtain the films of high phase purity and accurate stoichiometry. The first was to sputter films at room temperature and then the BiFeO3 phase was formed after sintering at various temperatures under controlled ambient atmosphere. The second was to grow BiFeO3 in-situ at high temperature during sputtering. Although the sintered films grown on SrTiO3 substrates were epitaxial and showed better texture than the in-situ films, they had much poorer ferroelectric properties, due to the residual traces of intermediate phases formed during heating. Under right growth parameters, the in-situ films grown on the LaNiO3−x buffered SrTiO3 showed fully saturated ferroelectric hysteresis loops with large remanent polarisation of 64μC/cm2. Piezoresponse force microscopy (PFM) was used to examine the ferroelectric domain structures. When scanned without DC bias, fine spontaneous domains were observed. Under ±10V DC bias, PFM confirmed that the domains could be poled and switched. [Copyright &y& Elsevier]

Abstract: Multiferroic BiFeO3 films of smooth surface and fully-saturated ferroelectric hysteresis loops have been grown by RF magnetron sputtering. The (001)-oriented epitaxial films showed a large remanent polarisation of 61 µC/cm2. A strategy to grow BiFeO 3 films of good ferroelectric property was demonstrated, that was using fast growth rate to achieve accurate stoichiometry for the BiFeO3 phase and at the same time to avoid the formation of impurity phases associated with the fast growth by accurate control of thermodynamic parameters such as oxygen partial pressure and temperature, as well as proper selection of substrates. Piezoresponse force microscopy revealed fine spontaneous domains for highly resistive epitaxial films, which were switchable under DC biases. For the polycrystalline films of increased density of free carriers, single-domain grains of about 200 nm in diameter were observed due to effective compensation of depolarisation field by free carriers and therefore allowing larger domains. [Copyright &y& Elsevier]