Change in periodicity of the incommensurate magnetic order towards commensurate order in bismuth ferrite lead titanate

xBiFeO₃-(1−x)PbTiO₃ has been synthesised in bulk polycrystalline ceramic form and then self-disintegrated to form powder (P4mm) (x=0.7) or crushed (R3c) (x=0.75, 0.9) to relieve stress. High-resolution neutron powder diffraction has been employed to observe incommensurate antiferromagnetic ordering within the materials, and its dependence on phase at 7.5 K. It is shown that in the rhombohedral phase the period of the magnetic structure depends on PbTiO₃ addition, and increases from 790 Å (x=0.9) to 840 Å (x=0.75). The tetragonal phase restores the collinear antiferromagnetic order.     

Magnetic phases of bismuth ferrite

We have recently shown that BiFeO₃ has at least four different magnetic phases, contrary to the conventional wisdom. Below room temperature it undergoes spin reorientation transitions at T₂=200 K and T₁=140 K analogous to those in orthoferrites; and above room temperature it undergoes a structural transition near 185°C first reported by Polomska et al. This may help explain the apparent linear magnetoelectric effect at 20°C reported by D. Lebeugle et al. [Phys. Rev. Lett. 100, 227602 (2008)] which is nominally forbidden due to the long wavelength cycloidal spin structure assumed. We also find evidence of an unusual acentric spin glass below ca. 200 K, related not to T_N but to T₁ and T₂.     

Magnetic,ferroelectric and magnetoelectric properties of Ba-doped BiFeO3

Bi_1–xBa_xFeO₃ (0.0≤x≤0.25) ceramics are prepared by chemical synthesis route. At room temperature, antiferromagnetic BiFeO₃ is converted to ferromagnetic on doping Ba. A large change in the magnetization is observed around 370 °C which is close to the Neel temperature (T_N) of parent compound. Another magnetic transition is also observed near 600 °C. Spin canting or impurity phase could be a probable reason for the origin of ferromagnetism in both cases. Ferroelectric and magnetic transitions of the compounds shift towards higher temperature with Ba-doping concentration. Anomaly in the dielectric constant is also observed near the T_N of BiFeO₃. The composition x=0.15 shows the maximum magnetic moment at room temperature while better fatigue resistance and maximum magnetoelectric coupling are observed for x=0.20 composition.     

Dielectric,ferroelectric and field-induced strain response of lead-free BaZrO3-modified Bi0.5Na0.5TiO3 ceramics

Lead-free piezoelectric ceramics (1 − x)Bi_0.5Na_0.5TiO₃–xBaZrO₃ (BNT–BZ100x, with x = 0–0.10) were prepared using a conventional solid-state reaction method. The crystal structure, microstructure, dielectric, ferroelectric, and piezoelectric properties of BNT–BZ100x ceramics were studied as functions of different BZ content. X-ray diffraction patterns revealed that the BZ completely diffused in the BNT lattice in the studied composition range. An appropriate amount of BZ addition improved the dielectric, ferroelectric, and piezoelectric properties of BNT ceramics. The remanent polarization (P_r) and piezoelectric constant (d₃₃) increased from 22 μC/cm² and 60 pC/N for pure BNT to 30 μC/cm² and 112 pC/N for x = 0.040, respectively. In addition, electric field-induced strain was enhanced to its maximum value (S_max = 0.40%) with normalized strain (d*₃₃ = S_max/E_max = 500 pm/V) at an applied electric field of 8 kV/mm for x = 0.055. The enhanced strain can be attributed to the coexistence of ferroelectric and relaxor ferroelectric phases.     

Synthesis of bismuth titanate with urea as fuel by solution combustion route and its dielectric and ferroelectric properties

Preparation of ferroelectric bismuth titanate (Bi₄Ti₃O₁₂) is carried out by solution combustion route with urea as fuel at much lower calcinations temperatures. The single phase bismuth titanate was obtained after calcinations at 800 °C. SEM micrographs of the calcined powders show agglomerated, flaky and foamy morphology, which is typical of combustion synthesis and that of sintered ceramics shows the grain formation. Behavior of dielectric constant and dielectric loss as a function of temperature of as-prepared sample are reported in this communication. Ferroelectric to paraelectric phase transition occurs at the temperature T_c ∼ 660 °C. Its remnant polarization (2P_r) is very less of the order of 0.012 μC/cm².     

Effects of excess Bi2O3 on grain orientation and electrical properties of CaBi4Ti4O15 ceramics

A CaBi₄Ti₄O₁₅ (CBTO) ceramic in which the Bi₂O₃ concentration was controlled from 0 to 10 wt% was fabricated using a solid-state reaction method. Structural analysis by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) indicated differences in the preferred grain orientation and size of the plate-like grains according to the Bi₂O₃ concentration. The orientation of plate-like grains was also found to vary with the Bi₂O₃ concentration. There was no noticeable change trend of dielectric properties with different Bi₂O₃ concentrations. Relatively low dielectric constants (about 135) were exhibited by the CBTO ceramic with 1 wt% Bi₂O₃ and CBTO ceramic with 10 wt% Bi₂O₃ only, and similar values (about 150) were exhibited by the other ceramics. The dielectric loss exhibited a low value in the range of 0.01–0.09 for all samples (frequency range of 1–100 kHz). Regarding the ratio changes of the piezoelectric coefficient (d₃₃) and the ratio of a-axis orientation of plate-like grains, the trends of these two values were shown to be similar. These results suggest that the addition of Bi₂O₃ greatly influences the microstructure of CBTO ceramics, including the grain size and orientation of plate-like grains. In particular, the change in the preferred grain orientation is closely related to the change in the piezoelectric properties.     

Surface morphology, chemical contrast, and ferroelectric domains in TGS bulk single crystals differentiated with UHV non-contact force microscopy

Ferroelectric bulk single crystals of tri-glycine sulphate (TGS) have been investigated in ultra-high vacuum (UHV) using dynamic force microscopy (DFM) in the non-contact (nc) mode. Both chemical contrast from different sub-unit cell cleavage steps, and ferroelectric domains were differentiated by recording the variation in interaction force affecting the excitation amplitude A_exc applied to the piezo shaker in constant amplitude DFM. No chemical difference was found for steps measuring half the unit cell height b, in contrast to b/4—steps where sulphate ions change the local short-range chemical forces. By varying the bias voltage applied to the TGS counter electrode, the sign of bound surface charge within each ferroelectric domain was determined. Domain walls separating regions with antiparallel polarisation vectors are resolved down to a 9 nm domain wall width. Furthermore, we achieved atomic resolution with nc-DFM on cleaved TGS samples indicating the monoclinic unit cell at the ferroelectric sample surface with a=1.0±0.05 nm, c=0.55±0.05 nm, and β=107±3°.     

Effect of film thickness on interface and electric properties of BiFeO3 thin films

Bismuth ferrite (BFO) thin films were fabricated by RF-magnetron sputtering deposition method on Pt/Ti/SiO₂/Si(1 0 0) substrate. The effect of the thickness of BFO films varying from 85 to 280 nm on electrical properties was investigated. Saturated coercive fields were found to increase with the BFO film thickness. The dielectric constant of BFO thin films measured at 1 kHz decreased with decreasing thickness from 98 to 86, while tangent losses increased from 0.013 to 0.021. The presence of bismuth oxide at the interface between BFO films and Pt bottom electrodes was responsible for the high leakage currents in thin BFO thin films as was demonstrated by X-ray diffraction, grazing-incident X-ray diffraction, and secondary ion mass spectroscopy analysis.     

Superdomain structure and high conductivity at the vertices in the (111)-oriented epitaxial tetragonal Pb(Zr,Ti)O3 thin film

Recently, in ferroelectric materials, there have been many experimental efforts to find out more intriguing topological objects and their functionalities, such as conduction property. Here we investigated ferroelectric domain structures and related topological defects in the (111)-oriented epitaxial tetragonal PbZr_0.35Ti_0.65O₃ thin film. Systematic piezoresponse force microscopy measurements revealed that the field-induced polarization switching can form thermodynamically stable superdomain structures composed of nano-sized stripe subdomains. Within such superdomain structures, we observed the exotic equilateral triangular in-plane flux-closure domains composed of three stripe domain bundles with 120/120/120 degrees of separation. The conductive-atomic force microscopy measurements under vacuum showed that some vertices have significantly higher conductivity compared to other surrounding regions. This work highlights electric field-driven polarization switching and unique crystallographic symmetry (here, three-fold rotational symmetry) can generate exotic ferroelectric domain structures and functional topological defects, such as conductive vertices.     

Structure and electrical properties of tungsten oxide nanorods epitaxially organized on a mica substrate

Our objective was to fabricate nanosized tungsten oxide rods and to test their sensing properties. In the present report, we focus on the crystallographical structure and the electrical properties of tungsten nanorods. The tungsten oxide nanorods were grown by vapor transport from a WO₃ layer onto a substrate (Mica). The nanorods growth was controlled by the temperature gradient between the WO₃ layer and the substrate. Their morphology was investigated by AFM and their structure by TED and TEM. We have investigated the conductivity of the WO₃ nanorods with a technique derived from atomic force microscopy operating in contact mode with a conductive tip (C-AFM). Its provides at the same time a classical topographic image of the sample surface and an image representative of the local electrical resistance between the tip and a metallic contact on the substrate. We also investigated the electrical properties of the WO₃ nanorods by the current-voltage responses in a bias range of 0 ± 1 V. We have performed experiments in an environmental chamber and characterized the role of water vapor on the electrical conductivity of WO₃ nanorods.     

Ferroelectric domain structure of the BiFeO3 film grown on different substrates

Domain structure of BiFeO₃ (BFO) films grown on different substrates, with a conductive La_0.7Sr_0.3MnO₃ underlayer, has been experimentally studied. Two oppositely orientated polarizations, along the long body diagonal to the perovskite unit cell of BFO, are detected in the BFO films on the (0 0 1)-oriented NdGaO₃. Electric pulses applied in the [0 0 1] direction produce a polarization switching, resulting in the domain structure characterized by the 109° domain walls. Contrary to the BFO films on NdGaO₃, the BFO films on SrTiO₃ (0 0 1) exhibit a much complex domain structure. Both 71° and 109° domain walls are possible with a uniform polarization component pointing to the bottom electrode.     

Modification of electrical properties of tungsten oxide nanorods using conductive atomic force microscopy

Electrical conduction of tungsten oxide nanorods has been characterized by conductive atomic force microscopy (C-AFM). The conduction measurements were carried out in air using a conductive P⁺-type diamond-coated tip. This technique allows either the simultaneous measuring of the topography and the special current distribution or the recording of the current voltage distribution in a single point mode. We have proposed an equivalent electrical circuit which allows us to understand the I(V) curves. During C-AFM observations we have observed significant changes in image contrast and hysteresis in the I(V) characteristics which depend on the applied bias voltage. The bias dependence effect is interpreted as being due to a local oxidation-reduction phenomenon induced by the tip in the presence of water.     

Effects of yttrium substitution on structural and electrical properties of barium zirconate titanate ferroelectric ceramics

In the present study, structural, dielectric and ferroelectric properties of Ba_1?3x/2Y_x Zr_0.025Ti_0.975O₃ ferroelectric ceramics have been investigated. The compound was synthesized using solid state reaction technique. The effect of yttrium substitution on structural, dielectric and ferroelectric properties was studied using X-ray diffractometer, scanning electron microscopy (SEM), LCR meter and P–E loops. Phase analysis shows the formation of secondary phase YTi₂O₆ for Y ≥ 2.5 mol% substitution. The microstructural investigation shows that Y substitution significantly reduces the grain size. An increase in Y content up to 2.5 mol% increases the Curie temperature (T_c) initially but decreases subsequently. The maximum dielectric constant at T_c has been observed for 2 mol% Y substitution and with further increase in Y content the dielectric constant decreases considerably. The solubility limit is found to be 2.5 mol% of Y and after that some of the yttrium atoms enter B-sites and leading to the formation of the secondary phase. The P–E loop studies show that there is an increase in the coercive field with increasing Y content.     

Atomic force microscopic studies of thin oriented films of ferroelectric liquid crystalline elastomers with varying network architecture

Cross-linked ferroelectric liquid crystalline (FLC) polymers were studied by atomic force microscopy (AFM). Polysiloxane copolymers were synthesized with mesogenic and photo-cross-linkable side groups, the latter connected either directly to the backbone via a short spacer or as a terminal group on a part of the mesogens. These elastomers were prepared as thin, freely suspended films in homeotropic orientation. Topographic measurements depict plateaus separated by steps of characteristic height corresponding to the surfaces and edges of smectic layers. From the temperature behavior, as well as from the reaction to mechanical deformation, a model for the network architectures is proposed. In the first case (“intralayer cross-linking”), a predominantly two-dimensional network is formed within the backbone layers separating the smectic layers; in the second case (“interlayer cross-linking”), a three-dimensional network is established that is dependent on the mesophase present during cross-linking.     

Effect of sintering temperature on the structural, dielectric and ferroelectric properties of tungsten substituted SBT ceramics

Structural, dielectric and ferroelectric properties of tungsten (W) substituted SrBi₂(Ta_1−xW_x)₂O₉ (SBTW) [x=0.0, 0.025, 0.05, 0.075, 0.1 and 0.2] have been studied as a function of sintering temperature (1100-1250 °C). X-ray diffraction patterns confirm the single-phase layered perovskite structure formation up to x=0.05 at all sintering temperatures. The present study reveals an optimum sintering temperature of 1200 °C for the best properties of SBTW samples. Maximum T_c of ∼390 °C is observed for x=0.20 sample sintered at 1200 °C. Peak-dielectric constant (ε_r) increases from ∼270 to ∼700 on increasing x from 0.0 to 0.20 at 1200 °C sintering temperature. DC conductivity of the SBTW samples is nearly two to three orders lower than that of the pristine sample. Remnant polarization (P_r) increases with the W content up to x≤0.075. A maximum 2P_r (∼25 μC/cm²) is obtained with x=0.075 sample sintered at 1200 °C. The observed behavior is explained in terms of improved microstructural features, contribution from the oxygen and cationic vacancies in SBTW. Such tungsten substituted samples sintered at 1200 °C exhibiting enhanced dielectric and ferroelectric properties should be useful for memory applications.     

Effect of zinc substitution on magnetic and electrical properties of nanocrystalline nickel ferrite synthesized by refluxing method

Nanocrystalline Nickel ferrite (NiFe₂O₄) and Zn substituted nickel ferrite (NiZnFe₂O₄) have been synthesized by the refluxing method. These ferrites were characterized by XRD, TEM, Mossbauer spectroscopy and VSM in order to study the effect of zinc substitution in nickel ferrite. XRD diffraction results confirm the spinel structure for the prepared nanocrystalline ferrites with an average crystallite size of 14-16 nm. Lattice parameter was found to increase with the substitution of Zn²⁺ ions from 8.40 Å to 8.42 Å. TEM images confirmed average particle size of about 20 nm and indicates nanocrystalline nature of the compounds. A shift in isomeric deviation with the doublet was observed due to the influence of Zn substitution in the nickel ferrite. The Zn content has a significant influence on the magnetic behavior and electrical conductivity of NiFe₂O₄. Saturation magnetization drastically increased whereas room temperature electrical conductivity decreased due to the addition of Zn content in NiFe₂O₄, indicating super magnetic material with lesser coercivity.     

Nanoembossing and piezoelectricity of ferroelectric Pb(Zr0.3,Ti0.7)O3 nanowire arrays

Arrays of ferroelectric PZT nanowires with lateral size down to 200 nm were fabricated by nanoembossing technology. Structural characterization of the embossed PZT film was studied by Raman spectroscopy. Multidomain configurations of a single nanowire have been explored by vertical mode piezoresponse force microscopy (VPFM). The local electric polarization of the individual ferroelectric nanowire has also been investigated. Excellent ferroelectric and piezoelectric characteristics observed in the embossed PZT nanowires suggest nanoembossing technique proposed in this work is promising to become a useful method for ferroelectric nanowires fabrication.     

Vertical electron transport study in GaN/AlN/GaN heterostructures

In this work, we investigate the electronic structure and vertical electron transport through GaN/AlN/GaN single-barrier structures with different AlN thickness, grown by plasma-assisted molecular beam epitaxy. Conductive and capacitive characterization has been performed, and the experimental results are interpreted by comparison with 1D self-consistent simulations. Capacitive measurements reveal a complete depletion of the top GaN layer, and the formation of a two-dimensional electron gas at the bottom interface of the AlN barrier, even for barrier thicknesses of 0.5 nm (2 monolayers of AlN). Conductive atomic force microscopy reveals discrete leakage current locations with a density of 10⁷ cm⁻², more than one order of magnitude lower than the dislocation density in these samples. These results are promising for the fabrication of resonant tunnelling diodes using the GaN/AlN material system.     

Pre- and post-breakdown electrical studies in ultrathin Al2O3 films by conductive atomic force microscopy

The loss of local dielectric integrity in ultrathin Al₂O₃ films grown by atomic layer deposition is investigated using conducting atomic force microscopy. I–V spectra acquired at different regions of the samples by constant and ramping voltage stress are analyzed for their pre- and post-breakdown signatures. Based on these observations, the thickness dependent dielectric reliability and failure mechanism are discussed. Our results show that remarkable enhancement in breakdown electric field as high as 130 MV/cm is observed for ultrathin films of thickness less than 1 nm.