Ferroelectric precursor behavior in PbSc<inf>0.5</inf>Ta <inf>0.5</inf>O<inf>3</inf> detected by field-induced resonant piezoelectric spectroscopy

A novel experimental technique, resonant piezoelectric spectroscopy (RPS), has been applied to investigate polar precursor effects in highly (65%) B-site ordered PbSc0.5Ta0.5O3 (PST), which undergoes a ferroelectric phase transition near 300 K. The cubic-rhombohedral transition is weakly first order, with a coexistence interval of ∼4 K, and is accompanied by a significant elastic anomaly over a wide temperature interval. Precursor polarity in the cubic phase was detected as elastic vibrations generated by local piezoelectric excitations in the frequency range 250–710 kHz. The RPS resonance frequencies follow exactly the frequencies of elastic resonances generated by conventional resonant ultrasound spectroscopy (RUS) but RPS signals disappear on heating beyond an onset temperature, Tonset, of 425 K. Differences between the RPS and RUS responses can be understood if the PST structure in the precursor regime between Tonset and the transition point, Ttrans=300 K, has locally polar symmetry even while it remains macroscopically cubic. It is proposed that this precursor behavior could involve the development of a tweed microstructure arising by coupling between strain and multiple order parameters, which can be understood from the perspective of Landau theory. As a function of temperature the transition is driven by the polar displacement P and the order parameter for cation ordering on the crystallographic B site Qod. Results in the literature show that, as a function of pressure, there is a separate instability driven by octahedral tilting for which the assigned order parameter is Q. The two mainly displacive order parameters, P and Q, are unfavorably coupled via a biquadratic term Q2P2, and complex tweedlike fluctuations in the precursor regime would be expected to combine aspects of all the order parameters. This would be different from the development of polar nanoregions, which are more usually evoked to explain relaxor ferroelectric behavior, such as occurs in PST with a lower degree of B-site order.RUS facilities in Cambridge were established through support from the NERC NE/B505738/1) to MAC. EKHS thanks the Leverhulme foundation (RG66640) and EPSRC (RG66344) for financial support.This is the accepted version of an original article published in Physical Review B and available online at http://link.aps.org/doi/10.1103/PhysRevB.88.174112

Ultra high resolution of PZT 30/70 domains as imaged by PFM

iezoforce microscopy (PFM) has been used to determine the domain structure of lead zirconate titanate (PZT) (30/70) on an indium tin oxide (ITO)/glass substrate with a TiO2 boundary layer. The PZT nucleates into the perovskite form in a random crystallographic manner, which leads to a random domain structure in the final film. Using PFM it has been possible to visualize the domain structure of the PZT and determine that the domain structure has features as fine as 8 nm herringbone patterns. The possible impact of these structures for future devices utilizing nanoscale features of PZT and especially FeRAM developments is highlighted

Quasi-indirect measurement of electrocaloric temperature change in PbSc0.5Ta0.5O3 via comparison of adiabatic and isothermal electrical polarization data

Electrically driven adiabatic changes of temperature are identified in the archetypal electrocaloric material PbSc0.5Ta0.5O3 by comparing isothermal changes of electrical polarization due to the slow variation of electric field and adiabatic changes of electrical polarization due to the fast variation of electric field. By obtaining isothermal (adiabatic) electrical polarization data at measurement (starting) temperatures separated by <0.4 K, we identify a maximum temperature change of ∼2 K due to a maximum field change of 26 kV cm−1 for starting temperatures in the range of 300 K–315 K. These quasi-indirect measurements combine with their direct, indirect, and quasi-direct counterparts to complete the set and could find routine use in the future

Ferroelectric Behavior in Exfoliated 2D Aurivillius Oxide Flakes of Sub‐Unit Cell Thickness

Ferroelectricity in ultrasonically exfoliated flakes of the layered Aurivillius oxide Bi5Ti3Fe0.5Co0.5O15 with a range of thicknesses is studied. These flakes have relatively large areas (linear dimensions many times the film thickness), thus classifying them as 2D materials. It is shown that ferroelectricity can exist in flakes with thicknesses of only 2.4 nm, which equals one‐half of the normal crystal unit cell. Piezoresponse force microscopy (PFM) demonstrates that these very thin flakes exhibit both piezoelectric effects and that the ferroelectric polarization can be reversibly switched. A new model is presented that permits the accurate modeling of the field‐on and field‐off PFM time domain and hysteresis loop responses from a ferroelectric during switching in the presence of charge injection, storage, and decay through a Schottky barrier at the electrode–oxide interface. The extracted values of spontaneous polarization, 0.04(±0.02) C m−2 and electrostrictive coefficient, 2(±0.1) × 10−2 m4 C−2 are in good agreement with other ferroelectric Aurivillius oxides. Coercive field scales with thickness, closely following the semi‐empirical scaling law expected for ferroelectric materials. This constitutes the first evidence for ferroelectricity in a 2D oxide material, and it offers the prospect of new devices that might use the useful properties associated with the switchable ferroelectric spontaneous polarization in a 2D materials format

Direct visualization of magnetic-field-induced magnetoelectric switching in multiferroic aurivillius phase thin films

Multiferroic materials displaying coupled ferroelectric and ferromagnetic order parameters could provide a means for data storage whereby bits could be written electrically and read magnetically, or vice versa. Thin films of Aurivillius phase Bi6Ti2.8Fe1.52Mn0.68O18, previously prepared by a chemical solution deposition (CSD) technique, are multiferroics demonstrating magnetoelectric coupling at room temperature. Here, we demonstrate the growth of a similar composition, Bi6Ti2.99Fe1.46Mn0.55O18, via the liquid injection chemical vapor deposition technique. High-resolution magnetic measurements reveal a considerably higher in-plane ferromagnetic signature than CSD grown films (MS=24.25 emu/g (215 emu/cm3), MR=9.916 emu/g (81.5 emu/cm3), HC=170 Oe). A statistical analysis of the results from a thorough microstructural examination of the samples, allows us to conclude that the ferromagnetic signature can be attributed to the Aurivillius phase, with a confidence level of 99.95%. In addition, we report the direct piezoresponse force microscopy visualization of ferroelectric switching while going through a full in-plane magnetic field cycle, where increased volumes (8.6% to 14% compared with 4% to 7% for the CSD-grown films) of the film engage in magnetoelectric coupling and demonstrate both irreversible and reversible magnetoelectric domain switching

Active layers of high-performance lead zirconate titanate at temperatures compatible with silicon nano- and microelecronic devices

Applications of ferroelectric materials in modern microelectronics will be greatly encouraged if the thermal incompatibility between inorganic ferroelectrics and semiconductor devices is overcome. Here, solution-processable layers of the most commercial ferroelectric compound ─ morphotrophic phase boundary lead zirconate titanate, namely Pb(Zr0.52Ti0.48)O3 (PZT) ─ are grown on silicon substrates at temperatures well below the standard CMOS process of semiconductor technology. The method, potentially transferable to a broader range of Zr:Ti ratios, is based on the addition of crystalline nanoseeds to photosensitive solutions of PZT resulting in perovskite crystallization from only 350 °C after the enhanced decomposition of metal precursors in the films by UV irradiation. A remanent polarization of 10.0 μC cm−2 is obtained for these films that is in the order of the switching charge densities demanded for FeRAM devices. Also, a dielectric constant of ~90 is measured at zero voltage which exceeds that of current single-oxide candidates for capacitance applications. The multifunctionality of the films is additionally demonstrated by their pyroelectric and piezoelectric performance. The potential integration of PZT layers at such low fabrication temperatures may redefine the concept design of classical microelectronic devices, besides allowing inorganic ferroelectrics to enter the scene of the emerging large-area, flexible electronics

Steps towards ceramic process scale-up: analysis of a laboratory process

In this paper the first steps towards a scaling up of a ceramic process are described, using as case study a lab-scale process, in which PZT ceramics are produced via an aqueous tape casting route. The process, the result of a feasibility study, was firstly analysed by drawing flowcharts; potential problems and flow bottlenecks were then identified and their solutions were pursued using a "soft" problem solving approach