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Ferroelectric materials exhibit spontaneous polarization that can be switched by electric field. Beyond traditional applications as nonvolatile capacitive elements, the interplay between polarization and electronic transport in ferroelectric thin films has enabled a path to neuromorphic device applications involving resistive switching. A fundamental challenge, however, is that finite electronic conductivity may introduce considerable power dissipation and perhaps destabilize ferroelectricity itself. Here, tunable microwave frequency electronic response of domain walls injected into ferroelectric lead zirconate titanate (PbZr0.2Ti0.8O3) on the level of a single nanodomain is revealed. Tunable microwave response is detected through first-order reversal curve spectroscopy combined with scanning microwave impedance microscopy measurements taken near 3 GHz. Contributions of film interfaces to the measured AC conduction through subtractive milling, where the film exhibited improved conduction properties after removal of surface layers, are investigated. Using statistical analysis and finite element modeling, we inferred that the mechanism of tunable microwave conductance is the variable area of the domain wall in the switching volume. These observations open the possibilities for ferroelectric memristors or volatile resistive switches, localized to several tens of nanometers and operating according to well-defined dynamics under an applied field.

Domain switching pathways fundamentally control performance in ferroelectric thin film devices. In epitaxial bismuth ferrite (BiFeO3) films, the domain morphology is known to influence the multiferroic orders. While both striped and mosaic domains have been observed, the origins of the latter have remained unclear. Here, it is shown that domain morphology is defined by the strain profile across the film–substrate interface. In samples with mosaic domains, X-ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using scanning transmission electron microscopy finds that within 5 nm of the film–substrate interface, the out-of-plane strain shows an anomalous dip while the in-plane strain is constant. Conversely, if uniform strain is maintained across the interface with zero strain gradient, striped domains are formed. Critically, an ex situ thermal treatment, which eliminates the interfacial strain gradient, converts the domains from mosaic to striped. The antiferromagnetic state of the BiFeO3 is also influenced by the domain structure, whereby the mosaic domains disrupt the long-range spin cycloid. This work demonstrates that atomic scale tuning of interfacial strain gradients is a powerful route to manipulate the global multiferroic orders in epitaxial films.

© 2019, The Author(s). Understanding and manipulating complex spin texture in multiferroics can offer new perspectives for electric field-controlled spin manipulation. In BiFeO 3 , a well-known room temperature multiferroic, the competition between various exchange interactions manifests itself as non-collinear spin order, i.e., an incommensurate spin cycloid with period 64 nm. We report on the stability and systematic expansion of the length of the spin cycloid in (110)-oriented epitaxial Co-doped BiFeO 3 thin films. Neutron diffraction shows (i) this cycloid, despite its partly out-of-plane canted propagation vector, can be stabilized in thinnest films; (ii) the cycloid length expands significantly with decreasing film thickness; (iii) theory confirms a unique [11 2 ¯] cycloid propagation direction; and (iv) in the temperature dependence the cycloid length expands significantly close to T N . These observations are supported by Monte Carlo simulations based on a first-principles effective Hamiltonian method. Our results therefore offer new opportunities for nanoscale magnonic devices based on complex spin textures.

BiFeO3 thin films have attracted considerable attention by virtue of their potential application in low-energy spintronic and magnonic devices. BiFeO3 possesses an intricate magnetic structure, characterized by a spin cycloid with period ∼62 nm that governs the functional magnonic response, and which can be modulated or even destroyed by strain, magnetic and electric fields, or chemical doping. The literature on (110)-oriented BiFeO3 films is not explicit in defining the conditions under which this cycloid persists, as its presence depends on synthesis method and thin-film boundary conditions, especially in the sub-100 nm thickness regime. This report aims to end "trial and error" approaches in determining the conditions under which this cycloid and its associated functional magnonic response exist. We show that in specific crystallographic orientations of epitaxial BiFeO3, an unexplored strain parameter - the distortion in the ab plane of the monoclinic unit cell - significantly influences the spin structure. Combining Mössbauer spectroscopy and low-energy Raman spectroscopy with first-principles-based effective Hamiltonian calculations, we show that both average strain and this distortion destroy the cycloid. For films grown on (110)-oriented SrTiO3 substrates, if the BiFeO3 lattice parameters a and b differ by more than about 1.2%, the cycloid is destabilized, resulting in a pseudocollinear magnetic order ground state. We are thereby able to construct a phase diagram of the spin structure for nanoscale epitaxial BiFeO3 films, which aims to resolve long-standing literature inconsistencies and provide powerful guidelines for the design of future magnonic and spintronic devices.

The influence of film orientation, strain relaxation, and flexoelectric fields on the stability of the spin cycloid in (110)-oriented BiFeO3 epitaxial films grown on LaAlO3 substrates is investigated. By means of advanced x-ray-diffraction techniques, we show that thinner films have very large strain gradients which give rise to high flexoelectric fields. Using low-energy Raman spectroscopy and conversion electron Mössbauer spectroscopy (CEMS) we show that films up to 53 nm thick possess collinear antiferromagnetic order, with no cycloidal modulation. This suppression of the cycloid is proposed to be from strain and strain-gradient-induced flexoelectric fields. On the other hand, films thicker than 90 nm show a complex spin texture consistent with two separate cycloids, likely with different propagation directions. Interestingly, CEMS analysis suggests that the two cycloids have the same spin rotation plane. The multiple cycloids are suggested to arise from different ferroelastic domains (in turn influenced by twinning in the substrate) with different strain relaxation behaviors. These results offer insight into the factors that influence cycloid stability in the less common (110) film orientation and have implications for future magnonic devices.

Domain walls in ferroelectric materials have tantalizing potential in disruptive memory and reconfigurable nanoelectronics technologies. Here, a ferroelectric domain wall switch with three distinct addressable resistance states is demonstrated. The device operation hinges on fully controllable and reversible conformational changes of the domain wall. As validated by atomistic simulations consistent with the experiments, using electric field, the shape—and hence the charge state—of the domain wall and ultimately its conduction are altered. Sequential nanoscale transitions of the walls are visualized directly using stroboscopic-piezoresponse force microscopy and Kelvin probe microscopy. Anisotropic head-to-head domain wall injection, stabilized by the majority carrier type of the ferroelectric, BiFeO 3 , is identified as the key factor that enables conformational control.

The stability of the “T-like” (T$) phase in BiFeO3 films grown on LaAlO3(001) is investigated. We show that the T$ phase can be stabilized for thicknesses >70nm under ultralow incident flux conditions in pulsed laser ablation growth. This low flux results in a low growth rate; thus, the sample is held at high temperatures (>600°C) for much longer than is typical. Transmission electron microscopy and X-ray diffraction analysis suggest that such growth conditions favor the formation of nanoscale “defect pockets”, which apply a local compressive strain of >1.8%. We propose that the cumulative effect of local stresses induced by such “designer defects” maintains macroscale strain coherence mechanical boundary conditions, which then preserves the T$ phase to thicknesses beyond conventional wisdom. Finally, by intentionally introducing an amorphous phase at the film-substrate interface, it is shown that the mixed-phase proportion can be tuned for a given thickness.

A detailed structural and optical band gap characterization study for more than 40 epitaxial bismuth ferrite (BiFeO3—BFO) thin films, measured by X-ray diffraction, atomic force microscopy, and optical transmission spectroscopy, is reported. The films are grown in different deposition systems to varying thicknesses (10–140 nm), on several substrates, and under different growth and cooling conditions. Using the results and literature data, first it is shown that the band gap measured by transmission is systematically lower than the gap found by ellipsometry, suggesting that sufficient caution must be exercised when comparing optical properties of BFO thin films. Then, statistical analysis is used to look for correlations between the band gap and structural parameters. While earlier works show the band gap to be sensitive to epitaxial (homogeneous) strain, it is found that it appears not to exhibit a dependence on inhomogeneous strain, out-of-plane lattice constant, or substrate/film interface roughness. Rather, it is found that surface roughness as well as film thickness generally tends to enhance the gap. Overall, the insensitivity of the band gap to structural parameters—aside from homogeneous strain—makes BiFeO3 largely immune to deviations in processing parameters, which should be an asset for photonic devices based on this material.

Epitaxial mixed-phase bismuth ferrite (BiFeO3, BFO) thin films were successfully synthesized on (001) lanthanum aluminate (LaAlO3, LAO) substrates by a chemical solution deposition (CSD) technique. X-ray diffraction measurements confirm the co-existence of a completely relaxed rhombohedral-like (R′) phase and a strained tetragonal-like (T′) phase. Atomic resolution scanning transmission electron microscopy (STEM) measurements reveal that the T′ and R′ phases in our CSD derived BFO/LAO films are mixed homogeneously at the nanoscale. This is in stark contrast to the typical physical vapor deposition derived mixed-phase BFO thin films, which show R′ phase striations embedded in a T′ phase matrix. This phenomenon is attributed to the specific deposition-nucleation-crystallization-relaxation pathway characteristic of the CSD route. This homogenously mixed-phase still demonstrates the well-known morphotropic phase boundary effect, i.e. superior electromechanical properties compared to either the pure T′ phase or R′ phase constituents themselves. Moreover, the maximum piezoelectric coefficient measured by using nanoscale top electrodes shows surprising insensitivity to the clamping effect from the substrate, thereby offering considerable promise in thin film applications.

© 2017 Author(s). We investigate photo-induced ferroelectric domain switching in a series of Pb(Zr0.2Ti0.8)O3/La0.7Sr0.3MnO3 (PZT/LSMO) bilayer thin films with varying surface morphologies by piezoresponse force microscopy under light illumination. We demonstrate that reverse poled ferroelectric regions can be almost fully recovered under laser irradiation of the PZT layer and that the recovery process is dependent on the surface morphology on the nanometer scale. The recovery process is well described by the Kolmogorov-Avrami-Ishibashi model, and the evolution speed is controlled by light intensity, sample thickness, and initial write voltage. Our findings shed light on optical control of the domain structure in ferroelectric thin films with different surface morphologies.

BACKGROUND: Several recent studies have attempted to measure the prevalence of disrespect and abuse (D&A) of women during childbirth in health facilities. Variations in reported prevalence may be associated with differences in study instruments and data collection methods. This systematic review and comparative analysis of methods aims to aggregate and present lessons learned from published studies that quantified the prevalence of Disrespect and Abuse (D&A) during childbirth. METHODS: We conducted a systematic review of the literature in accordance with PRISMA (Preferred Reporting Items for Systematic Review and Meta-Analysis) guidelines. Five papers met criteria and were included for analysis. We developed an analytical framework depicting the basic elements of epidemiological methodology in prevalence studies and a table of common types of systematic error associated with each of them. We performed a head-to-head comparison of study methods for all five papers. Using these tools, an independent reviewer provided an analysis of the potential for systematic error in the reported prevalence estimates. RESULTS: Sampling techniques, eligibility criteria, categories of D&A selected for study, operational definitions of D&A, summary measures of D&A, and the mode, timing, and setting of data collection all varied in the five studies included in the review. These variations present opportunities for the introduction of biases - in particular selection, courtesy, and recall bias - and challenge the ability to draw comparisons across the studies' results. CONCLUSION: Our review underscores the need for caution in interpreting or comparing previously reported prevalence estimates of D&A during facility-based childbirth. The lack of standardized definitions, instruments, and study methods used to date in studies designed to quantify D&A in childbirth facilities introduced the potential for systematic error in reported prevalence estimates, and affected their generalizability

The Tanzania HIV Care and Treatment Plan was launched in October 2004 aiming at providing 440,000 AIDS patients with antiretroviral drugs (ARVs) and track disease progression in 1.2 million HIV+ persons by the end of the 2008. This paper is intended to provide information to stake holders of the achievements and challenges of the HIV Care and Treatment Plan since its inception in 2004. Facility patient reports are aggregated at district and then regional level before being sent to the national level where they are aggregated to form a national report. By December 2007, 210 health facilities were offering HIV care and treatment services in Tanzania. About 123,147 (5 %) of the 2,636,785 estimated people living with HIV and AIDS were enrolled, and 71,439 (13.6 %) of the estimated 527,357 AIDS cases commenced ART. More females than males started ART, F: M ratio being 3: 2. Most (49 %) patients were started ART due to low CD4 counts (<200). About 6,618 patients had their initial ARV regimen changed due to starting anti-TB treatment 679 (10 %), peripheral neuropathy 812 (12%), skin rash 378 (6 %), and stock out 247 (4 %) or other reasons (18 %), while 2,653 (42 %) had no reason recorded. The proportion of patients still alive and on ART at 6, 12 and 24 months after initiation of treatment was 60 %, 60 % and 50 %, respectively, while those collecting ARVs on schedule was 34 %, 25 % and 10 % respectively. About 3,084 patients developed TB after starting ART, of whom 1,557 (~50%) patients during the first three months of treatment. During the three years (2004-2007) of HIV care and treatment services in Tanzania, there has been an increase in the number of CTC facilities, geographical coverage of services, the number of enrolled patients and those on ART. However, the set target for ART services has not been achieved and there are significant geographical variations in these achievements, which do not correspond with either population density or disease burden. Efforts sh