J.M. Vieira

Effect of diamagnetic Ca, Sr, Pb, and Ba substitution on the crystal structure and multiferroic properties of the BiFeO3 perovskite

In this work, we studied the effect of heterovalent Ca, Sr, Pb, and Ba substitution on the crystal structure, dielectric, local ferroelectric, and magnetic properties of the BiFeO3 multiferroic perovskite. Ceramic solid solutions with the general formula Bi0.7A0.3FeO3 (A is a doping element) were prepared and characterized by x-ray diffraction, dielectric, piezoresponse force microscopy (PFM), and magnetic measurements. It is shown that the crystal structure of the compounds is described within the space group R3c, permitting the spontaneous polarization, whose existence was confirmed by the PFM data. Magnetic properties of the solid solutions are determined by the ionic radius of the substituting element. Experimental results suggest that the increase in the radius of the A-site ion leads to the effective suppression of the spiral spin structure of BiFeO3, resulting in the appearance of net magnetization.

Synthesis and multiferroic properties of Bi0.8A0.2FeO3 (A=Ca,Sr,Pb) ceramics

Bi1−xAxFeO3 ceramics (A=Ca,Sr,Pb) were sintered by conventional mixed oxide route. The crystallographic structure of all samples is characterized by the rhombohedral symmetry (space group R3c). The existence of switchable ferroelectric polarization is verified by piezoresponse force microscopy that is proven to be a useful technique in semi-insulating ferroelectrics. Magnetic properties of Ca and Sr-doped ceramics are found to reproduce the antiferromagnetic behavior of undoped BiFeO3 without any enhancement of the magnetization. On the contrary, Pb-doped compound demonstrates appearance of a weak ferromagnetism. It is thus shown that Pb doping of BiFeO3 is a promising way for preparing multiferroic materials.

Crystal structure and multiferroic properties of Gd-substituted BiFeO3

Room-temperature crystal structure, local ferroelectric, and magnetic properties of the Bi1−xGdxFeO3 (x=0.1,0.2,0.3) polycrystalline samples have been investigated by x-ray diffraction, piezoresponse force microscopy, and magnetometry techniques. Performed measurements have revealed a sequence of the composition-driven structural phase transitions R3c→Pn21a (occurs at x∼0.1) and Pn21a→Pnma (takes place within the concentrational range of 0.2<x<0.3). The latter structural transformation is attributed to the substitution-induced suppression of the polar displacements. Gd substitution has been shown to effectively induce the appearance of the spontaneous magnetization, thus indicating a promising way for improving multiferroic properties of antiferromagnetic BiFeO3.

Intrinsic nature of the magnetization enhancement in heterovalently doped Bi1−xAxFeO3 (A = Ca, Sr, Pb, Ba) multiferroics

The mechanism of the formation of heterovalent-substitution-induced defects as well as their influence on the magnetic properties of BiFeO3-based multiferroics has been studied. It has been shown that heterovalent A2+ substitution results in the formation of oxygen vacancies in the host lattices of both antiferromagnetic and weak ferromagnetic Bi1−xAxFeO3 (A = Ca, Sr, Pb, Ba; x = 0.2, 0.3) compounds, thus indicating the intrinsic (i.e. not related to defects themselves) mechanism of doping-induced enhancement of magnetization. A correlation between the ionic radius of the substituting element and the value of the spontaneous magnetization of the corresponding solid solution has been found. The experimental results suggest that A-site substitution with the biggest ionic radius ions effectively suppresses the spiral spin configuration of antiferromagnetic BiFeO3.

Crystal structure and magnetic properties of Bi0.8(Gd1?xBax)0.2FeO3 (x = 0, 0.5, 1) multiferroics

Investigations of crystal structure and magnetic properties of Bi0.8(Gd1−xBax)0.2FeO3 (x = 0, 0.5, 1) samples have been performed. The Bi0.8Gd0.2FeO3 and Bi0.8Ba0.2FeO3 compounds have been shown to crystallize in the polar space groups Pn21a and R3c, respectively. It has been found that no continuous series of solid solutions is formed in the Bi0.8(Gd1−xBax)0.2FeO3 system: the crystal structure of the Bi0.8Gd0.1Ba0.1FeO3 sample is characterized by a coexistence of Pnma and R3c structural phases which differ in their chemical compositions. All of the Bi0.8(Gd1−xBax)0.2FeO3 (x = 0, 0.5, 1) compounds have been found to possess a spontaneous magnetization at room temperature. For Gd-containing samples, a significant enhancement of the magnetization takes place with decreasing temperature.

Microstructure, toughness and flexural strength of self-reinforced silicon nitride ceramics doped with yttrium oxide and ytterbium oxide.

Self‐reinforced silicon nitride ceramics with additions of either yttrium oxide or ytterbium oxide have been investigated at room temperature after various processing heat treatments. Devitrification of the intergranular phase in these materials is very sensitive to the heat treatment used during processing and does not necessarily improve their strength and toughness. Hot‐pressed ceramics without a subsequent devitrification heat treatment were the strongest. The ytterbium oxide‐doped silicon nitride ceramics were consistently tougher, but less strong, than the yttrium oxide‐doped silicon nitride ceramics. In all the ceramics examined, the fracture toughness showed evidence for R‐curve behaviour. This was most significant in pressureless sintered ytterbium oxide‐doped silicon nitride ceramics. A number of toughening mechanisms, including crack deflection, bridging, and fibre‐like grain pull‐out, were observed during microstructural analysis of the ceramics. In common with other silicon nitride‐based ceramics, thin amorphous films were found at the grain boundaries in each of the ceramics examined. Arrays of dislocations left in the elongated silicon nitride grains after processing were found to belong to the {101¯0}<0001> primary slip system.

Relationship between flexural strength and surface roughness for hot-pressed Si3N4 self-reinforced ceramics

Abstract The hot-pressed Si3N4 self-reinforced ceramics with Y2O3 and Al2O3 addition were first ground using the diamond wheel with the diamond grit of 64 μm size, and then polished using the diamond abrasives of 15, 6 and 1 μm size. The surface morphologies were examined by scanning electron microscopy (SEM) and atomic force microscope (AFM). The flexural strength at room temperature showed a net relationship with surface roughnesses (Ra, Rz, R3z, Rt, Rq) the increase of which followed the surface finished and defect disappearance. The flexural strength and the negative square root of surface defect size, maximum peak-to-valley height of surface, fit the following equation: σ f = A R t − B R t +C , where surface compressive residual stress and defect size interact competitively on strength. Sharp peaks and deep grooves were undoubtedly the fracture origins after the first grinding by the diamond wheel. The obvious surface defects had been eliminated by the following polishing, where fracture may be initiated from subsurface defects; however, there was still some local damage observed by AFM due to diamond abrasion. The mirror finished surfaces should be stress free.

Phase transformation kinetics during thermal annealing of LFZ Bi–Sr–Ca–Cu–O superconducting fibers in the range 800–870°C

Abstract Phase development of laser floating zone (LFZ) fibers of Bi 2 SrO 2 Ca 2 Cu 4 O 11 (2224) nominal composition during isothermal annealing at 800–870°C for 1.5–24 h is investigated. The as-grown fibers have axially aligned Sr 0.3 Ca 0.7 CuO 2 (“1/1”) dendrites of constant average thickness δ =7.8 μm, an interdendritic matrix with the Bi 4 Sr 4 CaCu 3 O 14 (“4413”) intergrowth, the Bi 2 Sr 2 CaCu 2 O 8 (“2212”) phase and CuO cubic grains. Primary “1/1” cuprate and “4413” constituent are metastable and soon react at T ≥800°C transforming into equilibrium phases, “2212”, Sr 0.3 Ca 1.7 CuO 3 (“2/1”) and Sr 7 Ca 7 Cu 24 O 41 (“14/24”). The growth kinetics of “2212”, “2/1” and “14/24” fit a parabolic dependence on time, revealing a diffusion controlled mechanism of reaction in the solid state with activation energies in the 170–400 kJ mol −1 range. For T ≥835°C, the Bi 2 Sr 2 Ca 2 Cu 3 O 10 (“2223”) phase develops. The reaction rates of all phases are dependent on the volume fraction of “1/1” in the as-grown fibers. Although the growth mechanism of “2223” resembles that of “2212”, by enrichment of the “2212” plates in Ca and Cu from “2/1” and CuO phases, the overall reaction is a four-step reaction with an activation energy close to 1.5 MJ mol −1 . Present results confirm the “2212”–“2223”–“2/1”–“14/24” as the set of compatibility phases for the 2224 composition at 835–870°C when the “2223” phase is thermodynamically stable in air.

Anisotropic electrical transport in epitaxial La2/3Ca1/3MnO3 thin films

Epitaxial thin films of La0.62±0.05Ca0.33±0.02MnO3−δ were grown by laser ablation on SrTiO3. On (100) substrates the films grow with the larger c axis perpendicular to the plane. The films deposited on (110) SrTiO3 grow with both the c (long) axis and a (or b, short) axis in the plane of the film. The electrical resistivity (ρ) and the magnetoresistance (Δρ/ρ) show crystalline anisotropy. The resistivity ratio between the a and c axis is constant (0.8) from 10 K up to 120 K and decreases to 0.77 between 125 and 225 K, shows a small peak anomaly at Tc (257 K), and is almost constant in the paramagnetic phase. This temperature dependence is associated with anisotropic local lattice distortions. The magnetoresistance anisotropy (Δρ/ρ∥−Δρ/ρ⊥) with the applied field in the plane of the film, is small at low temperatures, peaks close to Tc, and is slightly larger for measurements along the a axis. The contributions of domain rotation and magnetocrystalline anisotropy to the anisotropic magnetoresistance associa...

Electrical field freezing effect on laser floating zone (LFZ)-grown Bi2Sr2Ca2Cu4O11 superconducting fibres

The electrical assisted laser floating zone (EALFZ) solidification process makes the tailoring of fibre microstructures possible. The application of a dc electrical current of during the solidification process of Bi2Sr2Ca2Cu4O11 nominal composition fibres strongly modified phase development, crystal shapes and effective distribution coefficients. Growth conditions with the solidification interface positively polarized deviate the system from metastability, leading to the development of the equilibrium cuprate (SrxCa1?x)14Cu24O41?(14/24) as primary phase dendrites. Compared to the morphology of the SrxCa1?xCuO2?(1/1) primary crystals of the conventional LFZ process, the 14/24 crystals are aligned higher along the fibre axis, with half the thickness and twice the extension. One of the major effects of EALFZ is the control of the effective distribution coefficients, k. At equal values of the fibre pulling rate, R, the copper partition between the liquid and the solid is the most affected, the kCu increasing from 1 to 1.22 due to the ionic drift from the zone melt to the negative polarized feed rod. Bismuth and calcium effective distribution coefficients present the lowest values (kBi = 0.69 and kCa = 1.13) in these conditions, according to the field-modified BPS theory. When the reverse current is applied, the dendritic morphology disappears and a globular structure of completely new phases develops.