Direct Epitaxial Growth of Polar Hf0.5Zr0.5O2 Films on Corundum.

Single-phase epitaxial Hf0.5Zr0.5O2 films with non-centrosymmetric orthorhombic structure have been grown directly on electrode-free corundum (α-Al2O3) substrates by pulsed laser deposition. A combination of high-resolution X-ray diffraction and X-ray absorption spectroscopy confirms the epitaxial growth of high-quality films belonging to the Pca21 space group, with [111] out-of-plane orientation. The surface of a 7-nm-thick sample exhibits an atomic step-terrace structure with a corrugation of the order of one atomic layer, as proved by atomic force microscopy. Scanning transmission electron microscopy reveals that it consists of grains with around 10 nm lateral size. The polar nature of this film has been corroborated by pyroelectric measurements. These results shed light on the mechanisms of the epitaxial stabilization of the ferroelectric phase of hafnia.

Multiferroic Aurivillius Bi4Ti2-xMnxFe0.5Nb0.5O12 (n = 3) compounds with tailored magnetic interactions.

Aurivillius compounds with the general formula (Bi2O2)(An-1BnO3n+1) are a highly topical family of functional layered oxides currently under investigation for room-temperature multiferroism. A chemical design strategy is the incorporation of magnetically active BiMO3 units (M: Fe3+, Mn3+, Co3+ …) into the pseudo-perovskite layer of known ferroelectrics like Bi4Ti3O12, introducing additional oxygen octahedra. Alternatively, one can try to directly substitute magnetic species for Ti4+ in the perovskite slab. Previous reports explored the introduction of the M3+ species, which required the simultaneous incorporation of a 5+ cation, as for the Bi4Ti3-2xNbxFexO12 system. A larger magnetic fraction might be attained if Ti4+ is substituted with Mn4+, though it has been argued that the small ionic radius prevents its incorporation into the pseudo-perovskite layer. We report here the mechanosynthesis of Aurivillius Bi4Ti2-xMnxNb0.5Fe0.5O12 (n = 3) compounds with increasing Mn4+ content up to x = 0.5, which corresponds to a magnetic fraction of 1/3 at the B-site surpassing the threshold for percolation, and equal amounts of Mn4+ and Fe3+. The appearance of ferromagnetic superexchange interactions and magnetic ordering was anticipated and is shown for phases with x ≥ 0.3. Ceramic processing was accomplished by spark plasma sintering, which enabled electrical measurements that demonstrated ferroelectricity for all Mn4+-containing Aurivillius compounds. This is a new family of layered oxides and a promising alternative single-phase approach for multiferroism.

A Glance at Processing-Microstructure-Property Relationships for Magnetoelectric Particulate PZT-CFO Composites.

In this work, we investigated the processing-microstructure-property relationships for magnetoelectric (ME) particulate composites consisting of hard ferromagnetic CoFe2O4 (CFO) particles dispersed in a Nb-doped PbZrxTi1-xO3 (PZT) soft ferroelectric matrix. Several preparation steps, namely PZT powder calcination, PZT-CFO mixture milling and composite sintering were tailored and a range of microstructures was obtained. These included open and closed porosities up to full densification, PZT matrices with decreasing grain size across the submicron range down to the nanoscale and well dispersed CFO particles with bimodal size distributions consisting of submicron and micron sized components with varying weights. All samples could be poled under a fixed DC electric field of 4 kV/mm and the dielectric, piezoelectric and elastic coefficients were obtained and are discussed in relation to the microstructure. Remarkably, materials with nanostructured PZT matrices and open porosity showed piezoelectric charge coefficients comparable with fully dense composites with coarsened microstructure and larger voltage coefficients. Besides, the piezoelectric response of dense materials increased with the size of the CFO particles. This suggests a role of the conductive magnetic inclusions in promoting poling. Magnetoelectric coefficients were obtained and are discussed in relation to densification, piezoelectric matrix microstructure and particle size of the magnetic component. The largest magnetoelectric coefficient α33 of 1.37 mV cm-1 Oe-1 was obtained for submicron sized CFO particles, when closed porosity was reached, even if PZT grain size remained in the nanoscale.

Selection and Optimization of a K0.5Na0.5NbO3-Based Material for Environmentally-Friendly Magnetoelectric Composites.

Li- and Ta-modified K 0.5 Na 0.5 NbO 3 compounds are among the most promising lead-free ferroelectrics for high-sensitivity piezoelectric ceramic materials, and are potentially capable of replacing Pb(Zr,Ti)O 3 . They are also being investigated as piezoelectric components in environmentally friendly magnetoelectric composites. However, most suitable modifications for this application have not been identified. We report here a simulation study of how the magnetoelectric voltage responses of layered composite structures based on Li x (K 0.5 Na 0.5 ) 1 - x Nb 1 - y Ta y O 3 varies with the chemical composition of the piezoelectric. Instead of relying on material coefficients from the literature, which would have required using different sources, an ad hoc set of materials was prepared. This demanded tailoring preparation by conventional means to obtain dense ceramics while controlling alkali volatilization, perovskite phase and microstructure, as well as characterizing their dielectric, elastic and electromechanical properties. This provided the set of relevant material coefficients as a function of composition, which was used to obtain the magnetoelectric responses of model layered structures including a reference magnetostrictive spinel oxide by simulation. The piezoelectric material leading to the highest magnetoelectric coefficient was identified, and shown to be different to that showing the highest piezoelectric coefficient. This reflects the dependence of the magnetoelectric response on all material coefficients, along with the complex interplay between composition, processing and properties in K 0.5 Na 0.5 NbO 3 -based ceramics.

Mechanosynthesis of the Whole Y1-xBixMn1-xFexO3 Perovskite System: Structural Characterization and Study of Phase Transitions.

Perovskite BiFeO3 and YMnO3 are both multiferroic materials with distinctive magnetoelectric coupling phenomena. Owing to this, the Y1-xBix Mn1-xFexO3 solid solution seems to be a promising system, though poorly studied. This is due to the metastable nature of the orthorhombic perovskite phase of YMnO3 at ambient pressure, and to the complexity of obtaining pure rhombohedral phases for BiFeO3-rich compositions. In this work, nanocrystalline powders across the whole perovskite system were prepared for the first time by mechanosynthesis in a high-energy planetary mill, avoiding high pressure and temperature routes. Thermal decomposition temperatures were determined, and structural characterization was carried out by X-ray powder diffraction and Raman spectroscopy on thermally treated samples of enhanced crystallinity. Two polymorphic phases with orthorhombic Pnma and rhombohedral R3c h symmetries, and their coexistence over a wide compositional range were found. A gradual evolution of the lattice parameters with the composition was revealed for both phases, which suggests the existence of two continuous solid solutions. Following bibliographic data for BiFeO3, first order ferroic phase transitions were located by differential thermal analysis in compositions with x ≥ 0.9. Furthermore, an orthorhombic-rhombohedral structural evolution across the ferroelectric transition was characterized with temperature-dependent X-ray diffraction.

Multilayer Ceramic Magnetoelectric Composites with Tailored Interfaces for Enhanced Response.

Composite materials consisting of two dissimilar ferroic phases are an excellent alternative to single-phase multiferroics for a wide range of magnetoelectric technologies. In composites with strain-mediated magnetoelectric coupling the response is strongly dependent on the characteristics of the interface between the two mechanically coupled phases. Among the different material approaches considered, cofired ceramic composites offer improved reliability in applications and are more adequate for free-forming and miniaturization. However, their magnetoelectric response often suffers from poor reproducibility, which has been reiteratively associated with the quality of the interfaces with little experimental support. Here, we report an in-depth study of the local material properties across the interfaces of 0.36BiScO3-0.64PbTiO3/NiFe2O4 multilayer ceramic composites, processed by spark plasma sintering of nanocrystalline powders. Tailored microstructures and low residual stress levels were obtained by adjusting the sintering mismatch between the two ferroic phases, which also resulted in fully functional interfaces and enhanced magnetoelectric responses.

A novel perovskite oxide chemically designed to show multiferroic phase boundary with room-temperature magnetoelectricity.

There is a growing activity in the search of novel single-phase multiferroics that could finally provide distinctive magnetoelectric responses at room temperature, for they would enable a range of potentially disruptive technologies, making use of the ability of controlling polarization with a magnetic field or magnetism with an electric one (for example, voltage-tunable spintronic devices, uncooled magnetic sensors and the long-searched magnetoelectric memory). A very promising novel material concept could be to make use of phase-change phenomena at structural instabilities of a multiferroic state. Indeed, large phase-change magnetoelectric response has been anticipated by a first-principles investigation of the perovskite BiFeO3-BiCoO3 solid solution, specifically at its morphotropic phase boundary between multiferroic polymorphs of rhombohedral and tetragonal symmetries. Here, we report a novel perovskite oxide that belongs to the BiFeO3-BiMnO3-PbTiO3 ternary system, chemically designed to present such multiferroic phase boundary with enhanced ferroelectricity and canted ferromagnetism, which shows distinctive room-temperature magnetoelectric responses.

High-sensitivity piezoelectric perovskites for magnetoelectric composites.

A highly topical set of perovskite oxides are high-sensitivity piezoelectric ones, among which Pb(Zr,Ti)O3 at the morphotropic phase boundary (MPB) between ferroelectric rhombohedral and tetragonal polymorphic phases is reckoned a case study. Piezoelectric ceramics are used in a wide range of mature, electromechanical transduction technologies like piezoelectric sensors, actuators and ultrasound generation, to name only a few examples, and more recently for demonstrating novel applications like magnetoelectric composites. In this case, piezoelectric perovskites are combined with magnetostrictive materials to provide magnetoelectricity as a product property of the piezoelectricity and piezomagnetism of the component phases. Interfaces play a key issue, for they control the mechanical coupling between the piezoresponsive phases. We present here main results of our investigation on the suitability of the high sensitivity MPB piezoelectric perovskite BiScO3-PbTiO3 in combination with ferrimagnetic spinel oxides for magnetoelectric composites. Emphasis has been put on the processing at low temperature to control reactions and interdiffusion between the two oxides. The role of the grain size effects is extensively addressed.

Thin film multiferroic nanocomposites by ion implantation.

Thin film multiferroic nanocomposites might enable a range of potentially disruptive integrated magnetoelectric devices for information storage, spintronics, microwave telecommunications, and magnetic sensing. With this aim, we have investigated ion implantation of magnetic species into ferroelectric single crystal targets as a radically novel approach to prepare film nanoparticulate magnetic-metal ferroelectric-oxide composites. These materials are an alternative to multiferroic oxide epitaxial columnar nanostructures that are under intensive research, but whose magnetoelectric response is far from expectations. Here, we unambiguously demonstrate the preparation of such a thin film multiferroic nanocomposite of Co and BaTiO3 by ion implantation of a high dose of the magnetic species, followed by rapid thermal processing under tailored conditions. Results thus constitute a proof of concept for the feasibility of obtaining the materials by this alternative approach. Ion implantation is a standard technique for the microelectronic industry in combination with well-established patterning procedures.

High-temperature behavior and polymorphism in novel members of the perovskite family Pb2LnSbO6 (Ln=Ho, Er, Yb, Lu).

The synthesis, crystal structure, and dielectric properties of four novel members of the family of double perovskites Pb(2)LnSbO(6) are described. The room-temperature crystal structures were refined from neutron powder diffraction (NPD) data in the monoclinic C2/c (No. 15) space group. They contain a completely ordered array of alternating LnO(6) and SbO(6) octahedra sharing corners, tilted in antiphase along the three pseudocubic axes, with a a(-)b(-)b(-) tilting scheme, which is very unusual in the crystallochemistry of perovskites. The lead atoms occupy highly asymmetric voids with 8-fold coordination due to the stereoactivity of the Pb(2+) electron lone-pair. Several trends are observed for the entire family of compounds upon heating. The Ln = Lu, Yb, and Er oxides display three successive phase transitions in a narrow temperature range, as shown by differential scanning calorimetry (DSC) data, while the Ln = Ho shows only two transitions. Different crystal structure evolutions have been found from temperature-dependent NPD and DSC, following the space-group sequence C2/c → P2(1)/n → R ̅3 → Fm ̅3m for Ln = Lu and Yb, the sequence C2/c → unknown → P2(1)/n → Fm ̅3m for Ln = Er, and C2/c → P2(1)/n → Fm ̅3m for Ln = Ho. The Ln/Sb long-range ordering is preserved across the consecutive phase transitions. Dielectric permittivity measurements indicate the presence of a paraelectric/antiferroelectric transition (associated with the last structural transition), as suggested by the negative Curie temperature from the Curie-Weiss fit of the reciprocal permittivity.

An original polymorph sequence in the high-temperature evolution of the perovskite Pb2TmSbO6.

The synthesis, crystal structure, and dielectric properties of the novel double perovskite Pb(2)TmSbO(6) are described. The room-temperature crystal structure was determined by ab initio procedures from neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (SXRPD) data in the monoclinic C2/c (No. 15) space group. This double perovskite contains a completely ordered array of alternating TmO(6) and SbO(6) octahedra sharing corners, tilted in antiphase along the three pseudocubic axes, with an a(-)b(-)b(-) tilting scheme, which is very unusual in the crystallochemistry of perovskites. The lead atoms occupy a highly asymmetric void with 8-fold coordination due to the stereoactivity of the Pb(2+) lone electron pair. This compound presents three successive phase transitions in a narrow temperature range (at T1 = 385 K, T2 = 444 K, and T3 = 460 K in the heating run) as shown by differential scanning calorimetry (DSC) data. The crystal structure and temperature-dependent NPD follow the space-group sequence C2/c → P2(1)/n → R3 → Fm3m. This is a novel polymorph succession in the high-temperature evolution of perovskite-type oxides. The Tm/Sb long-range ordering is preserved across the consecutive phase transitions. Dielectric permittivity measurements indicate the presence of a paraelectric/antiferroelectric transition (associated with the last structural transition), as suggested by the negative Curie temperature obtained from the Curie-Weiss fit of the reciprocal permittivity.

The role of the Pb2+ 6s lone pair in the structure of the double perovskite Pb2ScSbO6.

The new double perovskite Pb2ScSbO6 was synthesized by standard ceramic procedures; the Rietveld refinement of room temperature neutron powder diffraction data shows that the crystal structure is well defined in the space group Fm3[combining macron]m. It contains a completely ordered array of alternating ScO6 and SbO6 octahedra sharing corners; the PbO12 polyhedra present an off-center displacement of the lead atoms along the [111] direction, due to the electrostatic repulsion between the Pb2+ 6s lone pair and the Pb-O bonds of the cuboctahedron. Dielectric permittivity measurements show a peak near 343 K, with a Curie-Weiss response above this temperature, which suggests an antiferroelectric behavior. Finally we present a DFT study of the electronic structure of Pb2ScSbO6, showing a great difference between the electronic density within SbO6 and ScO6 octahedra.

Relaxor behavior, polarization buildup, and switching in nanostructured 0.92 PbZn1/3Nb2/3O3-0.08 PbTiO3 ceramics.

The relaxor-type behavior, electrical polarization buildup, and switching in 0.92Pb(Zn(1/3)Nb(2/3))O(3)-0.08PbTiO(3) nanostructured ceramics with a grain size of approximately 20 nm is reported for the first time. This composition presents the highest-known piezoelectric coefficients, yet phase stability is an issue. Ceramics can only be obtained by the combination of mechanosynthesis and spark-plasma sintering. The results raise the possibility of using nanoscale, perovskite-relaxor-based morphotropic-phase-boundary materials for sensing and actuation in nanoelectromechanical systems.