D. A. Kiselev

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.

Real space mapping of polarization dynamics and hysteresis loop formation in relaxor-ferroelectric PbMg1/3Nb2/3O3–PbTiO3 solid solutions

Polarization switching in ergodic relaxor and ferroelectric phases in the PbMg1/3Nb2/3O3–PbTiO3 (PMN-PT) system is studied using piezoresponse force microscopy, single point electromechanical relaxation measurements, and voltage spectroscopy mapping. The dependence of relaxation behavior on voltage pulse amplitude and time is found to follow a universal logarithmic behavior with a nearly constant slope. This behavior is indicative of the progressive population of slow relaxation states, as opposed to a linear relaxation in the presence of a broad relaxation time distribution. The role of relaxation behavior, ferroelectric nonlinearity, and the spatial inhomogeneity of the tip field on hysteresis loop behavior is analyzed in detail. The hysteresis loops for ergodic PMN-10%PT are shown to be kinetically limited, while in PMN with larger PT content, true ferroelectric hysteresis loops with low nucleation biases are observed.

Nanoscale polarization patterning of ferroelectric Langmuir–Blodgett P(VDF-TrFE) films

This paper reports nanoscale piezoelectric measurements on ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) films prepared by the Langmuir–Blodgett (LB) technique. Polarization patterning, piezoelectric hysteresis and relaxation after poling are studied in this work by piezoresponse force microscopy. High quality P(VDF-TrFE) films with a copolymer content of 30% were fabricated using a Schaefer monolayer transfer setup permitting precise control of the film microstructure. The thickness of the films deposited with 100 transfers was ~64 nm. Local switching resulted in written polarization lines with the lateral size in the range 70–300 nm depending on polarization time. Local hysteresis loop (at a fixed tip position) demonstrated clear ferroelectric switching with the coercive voltage ≈8–10 V that corresponds to a macroscopic switching field (~1.5 MV cm−1) at a ~10 nm depth below the tip. Relatively slow ageing after poling was observed with the characteristic relaxation time of about 1500–2000 s depending on the polarization direction. The obtained results demonstrate that the stable polarization patterns can be created in LB P(VDF-TrFE) films and attest them as suitable candidates for memory and nanotemplate applications.

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.

Quasi-one-dimensional domain walls in ferroelectric ceramics: Evidence from domain dynamics and wall roughness measurements

Direct measurements of the motion of domain walls in disordered ferroelectrics provide important information on their interaction with defects, which is crucial for the functioning of various ferroelectric devices such as high-K capacitors and ferroelectric-based memories. In this work, we report on the results of the nanoscale measurements of domain-wall velocity and static domain-wall roughness in disordered (Pb,La)(Zr,Ti)O3 (PLZT) ceramics prepared by hot press sintering. Relaxation of domain walls to their equilibrium configurations at short length scales yields the apparent roughness exponent ζ ≈ 0.67 in PLZT 9.5/65/35. At the same time, the dynamical exponent μ was found to be about 0.25 from the independent measurements of domain-wall creep under local application of electric field. An analysis of these two values points to the effective dimensionality deff = 1 for the domain walls in PLZT with high La content. This result is confirmed by the layer-by-layer polishing followed by the imaging of an a...

Coexistence of spontaneous ferroelectricity and weak ferromagnetism in Bi0.8Pb0.2FeO2.9 perovskite

Polycrystalline samples with the nominal composition Bi0.8Pb0.2FeO3 have been studied via x-ray diffraction, M?ssbauer spectroscopy, dielectric, magnetic, and local ferroelectric measurements. It has been found that the heterovalent Pb2+ substitution in Bi0.8Pb0.2FeOy is realized through the formation of oxygen vacancies. The crystal structure of the compound has been shown to be described by the non-centrosymmetric space group R3c. Investigations of local ferroelectric and magnetic properties have confirmed that spontaneous polarization and weak ferromagnetism coexist in this material at room temperature. The nature of the weak ferromagnetic moment in this compound is discussed in terms of a doping-induced change in the magnetic anisotropy.

Effect of annealing on the structure and phase composition of thin electro-optical lithium niobate films

We have studied the formation of thin textured LiNbO3 films in originally amorphous samples produced by rf magnetron sputtering of a single-crystal target on silicon substrates containing a native oxide layer. The results demonstrate that postgrowth annealing leads to the formation of two phases, LiNbO3 and LiNb3O8, and that the percentage of the nonferroelectric phase LiNb3O8 is minimal after annealing at a temperature of 700°C. Annealing at 700°C is optimal because it ensures the lowest surface roughness of the film, the highest degree of structuring of the ferroelectric phase, and the maximum contrasts corresponding to the vertical and lateral components of the ferroelectric polarization in piezoresponse force microscopy.

The influence of Mn substitution on the local structure of Na0.5Bi0.5TiO3 crystals: Increased ferroelectric ordering and coexisting octahedral tilts

The ferroelectric domain structure of pure Na1/2Bi1/2TiO3 (NBT) and 1 at.% Mn-doped NBT (Mn-NBT) crystals was investigated by piezoresponse force microscopy. The correlation length of the polar regions was found to increase upon Mn substitution. High resolution transmission electron microscopy revealed that the coherency of the lattice across the domain boundaries between polar regions was also enhanced. Selected area electron diffraction showed that Mn favored coexisting 1/2 (ooo) and 1/2 (ooe) oxygen octahedral tiltings, over only 1/2 (ooo) for pure NBT.