arXiv subjects

J. F. Scott

Publications and source records attributed to J. F. Scott.

At least 19 recordsLinked to original sources

arXiv2019

Origin of Ferroelectricity in Orthorhombic LuFeO$_3$

We demonstrate that small but finite ferroelectric polarization ($\sim$0.01 $μ$C/cm$^2$) emerges in orthorhombic LuFeO$_3$ ($Pnma$) at $T_N$ ($\sim$600 K) because of commensurate (k = 0) and collinear magnetic structure. The synchrotron x-ray and neutron diffraction data suggest that the polarization could originate from enhanced bond covalency together with subtle contribution from lattice. The theoretical calculations indicate enhancement of bond covalency as well as the possibility of structural transition to the polar $Pna2_1$ phase below $T_N$. The $Pna2_1$ phase, in fact, is found to be energetically favorable below $T_N$ in orthorhombic LuFeO$_3$ ($albeit$ with very small energy difference) than in isostructural and nonferroelectric LaFeO$_3$ or NdFeO$_3$. Application of electric field induces finite piezostriction in LuFeO$_3$ via electrostriction resulting in clear domain contrast images in piezoresponse force microscopy.

cond-mat.str-el↗

arXiv2019

Tin Titanate: the hunt for a new ferroelectric perovskite

We review all the published literature and show that there is no experimental evidence for homogeneous tin titanate SnTiO3 in bulk or thin-film form. Instead a combination of unrelated artefacts are easily misinterpreted. The X-ray Bragg data are contaminated by double scattering from the Si substrate, giving a strong line at the 2-theta angle exactly where perovskite SnTiO3 should appear. The strong dielectric divergence near 560K is irreversible and arises from oxygen site detrapping, accompanied by Warburg/Randles interfacial anomalies. The small (4 uC/cm2) apparent ferroelectric hysteresis remains in samples shown in pure (Sn,Ti)O2 rutile/cassiterite, in which ferroelectricity is forbidden. Only very recent German work reveals real bulk SnTiO3, but this is completely inhomogeneous, consisting of an elaborate array of stacking faults, not suitable for ferroelectric devices. Unpublished TEM data reveal an inhomogeneous SnO layered structured thin films, related to shell-core structures. The harsh conclusion is that there is a combination of unrelated artefacts masquerading as ferroelectricity in powders and ALD films; and only a trace of a second phase in Cambridge PLD data suggests any perovskite content at all. The fact that X-ray, dielectric, and hysteresis data all lead to the wrong conclusion is instructive and reminds us of earlier work on copper calcium titanate (a well-known boundary-layer capacitor).

cond-mat.mtrl-sci↗

arXiv2019

Electrical Studies of Barkhausen Switching Noise in Ferroelectric lead zirconate titanate (PZT) and BaTiO3: Critical Exponents and Temperature-dependence

Previous studies of Barkhausen noise in PZT have been limited to the energy spectrum (slew rate response voltages versus time), showing agreement with avalanche models; in barium titanate other exponents have been measured acoustically, but only at ambient temperatures. In the present study we report the Omori exponent (-0.95$\pm$0.03) for aftershocks in PZT and extend the barium titanate studies to a wider range of temperature.

cond-mat.mtrl-sci↗

arXiv2018

Studies of Barkhausen Pulses in Ferroelectrics

Systems that produce crackling noises such as Barkhausen pulses are statistically similar and can be compared with one another. In this project, the Barkhausen noise of three ferroelectric lead zirconate titanate (PZT) samples were demonstrated to be compatible with avalanche statistics. The peaks of the slew-rate (time derivative of current $dI/dt$) squared, defined as jerks, were statistically analysed and shown to obey power-laws. The critical exponents obtained for three PZT samples (B, F and S) were 1.73, 1.64 and 1.61, respectively, with a standard deviation of 0.04. This power-law behaviour is in excellent agreement with recent theoretical predictions of 1.65 in avalanche theory. If these critical exponents do resemble energy exponents, they were above the energy exponent 1.33 derived from mean-field theory. Based on the power-law distribution of the jerks, we demonstrate that domain switching display self-organised criticality and that Barkhausen jumps measured as electrical noise follows avalanche theory.

cond-mat.mtrl-sci↗

arXiv2018

Quantum Critical Points in Ferroelectric Relaxors: Stuffed Tungsten Bronze K3Li2Ta5O15 and Lead Pyrochlore (Pb2Nb2O7)

We have synthesised ceramic specimens of the tetragonal tungsten bronze K3Li2Ta5O15 (KLT) and characterized its phase transition via X-ray, dielectric permittivity, ultrasonic spectroscopy and heat capacity measurements. The space group of KLT is reported as both P4/mbm or Cmmm with the orthorhombic distortion occurring when there are higher partial pressures of volatile K and Li used within the closed crucibles for the solid state synthesis. The data show strong relaxor behaviour, with the temperature at which the two dielectric relative permittivity peaks decreasing with 104 K $\geqslant$ Tm1 $\geqslant$ 69 K and 69 K $\geqslant$ Tm2 $\geqslant$ 46 K as probe frequency f is reduced from 1 MHz to 316 Hz. The data satisfy a Vogel-Fulcher model with an extrapolated freezing temperature for ε' and ε" of Tf1 = + 15.8 and - 11.8 K and Tf2 = - 5.0 and - 15.0 K for f $\rightarrow$ 0 (tending to dc). Therefore by tuning frequency, the transition could be shifted to absolute zero suggesting KLT has a relaxor-type quantum critical point. In addition, we have reanalysed the conflicting literature for Pb2Nb2O7 pyrochlore which suggests that this is also a relaxor-type quantum critical point as the freezing temperature from Vogel-Fulcher fitting is below absolute zero. Since the transition temperature evidenced in the dielectric data at ca. 100 kHz shifts below zero Kelvin for very low frequencies, heat capacity data collected in the zero-frequency (dc) limit, should not indicate a transition. Both of these materials show promise as possible new relaxor-type quantum critical points within non-perovskite based structures as multiple compounds are reported with low-temperature transitions.

cond-mat.mtrl-sci↗

arXiv2017

Prospects and Applications Near Ferroelectric Quantum Phase Transitions

The emergence of complex and fascinating states of quantum matter in the neighborhood of zero temperature phase transitions suggests that such quantum phenomena should be studied in a variety of settings. Advanced technologies of the future may be fabricated from materials where the cooperative behavior of charge, spin and current can be manipulated at cryogenic temperatures. The propagating lattice dynamics of displacive ferroelectrics make them appealing for the study of quantum critical phenomena that is characterized by both space- and time-dependent quantities. In this Key Issues article we aim to provide a self-contained overview of ferroelectrics near quantum phase transitions. Unlike most magnetic cases, the ferroelectric quantum critical point can be tuned experimentally to reside at, above or below its upper critical dimension; this feature allows for detailed interplay between experiment and theory using both scaling and self-consistent field models. Empirically, the sensitivity of the ferroelectrics $T_c$s to external to chemical pressure gives practical access to a broad range of temperature behavior over several hundreds of Kelvin. Additional degrees of freedom like charge and spin can be added and characterized systematically. Satellite memories, electrocaloric cooling and low-loss phased-array radar are among possible applications of low-temperature ferroelectrics. We end with open questions for future research that include textured polarization states and unusual forms of superconductivity that remain to be understood theoretically.

cond-mat.str-el↗

arXiv2017

Quantum percolation phase transition and magneto-electric dipole glass in hexagonal ferrites

Hexagonal ferrites do not only have enormous commercial impact (£2 billion/year in sales) due to applications that include ultra-high density memories, credit card stripes, magnetic bar codes, small motors and low-loss microwave devices, they also have fascinating magnetic and ferroelectric quantum properties at low temperatures. Here we report the results of tuning the magnetic ordering temperature in PbFe$_{12-x}$Ga$_x$O$_{19}$ to zero by chemical substitution $x$. The phase transition boundary is found to vary as $T_N \sim (1-x/x_c)^{2/3}$ with $x_c$ very close to the calculated spin percolation threshold which we determine by Monte Carlo simulations, indicating that the zero-temperature phase transition is geometrically driven. We find that this produces a form of compositionally-tuned, insulating, ferrimagnetic quantum criticality. Close to the zero temperature phase transition we observe the emergence of an electric-dipole glass induced by magneto-electric coupling. The strong frequency behaviour of the glass freezing temperature $T_m$ has a Vogel-Fulcher dependence with $T_m$ finite, or suppressed below zero in the zero frequency limit, depending on composition $x$. These quantum-mechanical properties, along with the multiplicity of low-lying modes near to the zero-temperature phase transition, are likely to greatly extend applications of hexaferrites into the realm of quantum and cryogenic technologies.

cond-mat.str-el↗

arXiv2017

Optically Controlled Polarization in Highly Oriented Ferroelectric Thin Films

The out-of-plane and in-plane polarization of (Pb0.6Li0.2Bi0.2)(Zr0.2Ti0.8)O3(PLBZT) thin film has studied in the dark and under illumination of a weak light source of a comparable bandgap. A highly oriented PLBZT thin film was grown on LaNiO3 (LNO)/LaAlO3(LAO) substrate by pulsed laser deposition system which illustrates well-saturated polarization and its significant enhancement under illumination of light. We have employed two configurations for polarization characterization; first deals with out of plane polarization with single capacitor under investigation, whereas second demonstrates the two capacitors connected in series via the bottom electrode. Two different configurations were illuminated using different energy sources and their effects were studied. The latter configuration shows a significant change in polarization under illumination of light that may provide an extra degree of freedom for device miniaturization. The polarization was also tested using positive-up & negative-down (PUND) measurements which confirm robust polarization and their switching under illumination.

cond-mat.mtrl-sci↗

arXiv2017

Giant Enhancement in Ferroelectric Polarization under Illumination

We report optical enhancement in polarization and dielectric constant near room temperature in Pb0.6Li0.2Bi0.2Zr0.2Ti0.8O3 (PLBZT) electro-ceramics; these are doubly substituted members of the most important commercial ferroelectric PbZr0.2Ti0.8O3 (PZT:20/80). Partial (40%) substitution of equal amounts of Li+1 and Bi+3 in PZT: 20/80 retains the PZT tetragonal structure with space group P4mm. Under illumination of white light and weak 405-nm near-ultraviolet laser light (30 mW), an unexpectedly large (200-300%) change in polarization and displacement current was observed. Light also changes the dc conduction current density by one to two orders of magnitude with a large switchable open circuit voltage (Voc ~ 2 V) and short circuit current (Jsc ~ 5x10-8 A). The samples show a photo-current ON/OFF ratio of order 6:1 under illumination of weak light.

cond-mat.mtrl-sci↗

arXiv2016

Giant Magnetoelectric coupling in Single Phase Pb(Zr0.20Ti0.80)0.70Pd0.30O3-δ Multiferroics

During the last fifteen years, multiferroic (MF) research communities have been searching for an alternative room temperature MF material with large magnetoelectric (ME) coupling for possible applications in high density electronic components, low heat dissipation memory and logic devices. We have studied Pb(Zr0.20Ti0.80)0.70Pd0.30O3-δ (PZTP30) system with an unusually large (30%) palladium occupancy in B site of PZT. This material exhibited a giant ME coupling coefficient ~0.36 mV/cm.Oe. Interestingly, this is the first time any room temperature single phase compound that showed ME trends, and magnitude similar to those in the well established mechanical strain-mediated ferroelectric and ferromagnetic composites; the latter ones are already in the commercial stage as nT/pT magnetic field sensors due to their large ME values. The presence of Pd in PZTP30 has been confirmed by XPS and XRF studies and assigned with related binding energies of Pd+2 and Pd+4 ions as 336.37 eV, 342.9 eV, and 337.53 eV, 343.43 eV, respectively, which may be the origin of room temperature magnetism in Pd substituted PZT ceramics. A sharp first order ferroelectric phase transition was observed at ~569 K (+/-5 K) that is confirmed from dielectric, Raman, and thermal analysis. Both ferromagnetic and ferroelectric orderings with large ME coupling were found above room temperature, a significant step forward in the development of single phase ME material with enhanced functionalities.

cond-mat.mtrl-sci↗

arXiv2016

Novel optically active lead-free relaxor ferroelectric (Ba0.6Bi0.2Li0.2)TiO3

We discovered a near room temperature lead-free relaxor-ferroelectric (Ba0.6Bi0.2Li0.2)TiO3 (BBLT) having A-site compositional disordered ABO3 perovskite structure. Microstructure-property relations revealed that the chemical inhomogeneities and development of local polar nano regions (PNRs) are responsible for dielectric dispersion as a function of probe frequencies and temperatures. Rietveld analysis indicates mixed crystal structure with 80% tetragonal structure (space group P4mm) and 20% orthorhombic structure (space group Amm2) which is confirmed by the high resolution transmission electron diffraction pattern. Dielectric constant and tangent loss dispersion with and without illumination of light obey nonlinear Vogel-Fulture relation. It shows slim polarization-hysteresis (P-E) loops and excellent displacement coefficients (d33 ~ 233 pm/V) near room temperature, which gradually diminish near the maximum dielectric dispersion temperature (Tm). The underlying physics for light-sensitive dielectric dispersion was probed by X-ray photon spectroscopy (XPS) which strongly suggests that mixed valence of bismuth ions, especially Bi5+ ions, are responsible for most of the optically active centers. Ultraviolet photoemission measurements showed most of the Ti ions are in 4+ states and sit at the centers of the TiO6 octahedra, which along with asymmetric hybridization between O 2p and Bi 6s orbitals appears to be the main driving force for net polarization. This BBLT material may open a new path for environmental friendly lead-free relaxor-ferroelectric research.

cond-mat.mtrl-sci↗

arXiv2015

Folding Catastrophes due to Viscosity in Multiferroic Domains: Implications for Room-Temperature Multiferroic Switching

Unusual domains with curved walls and failure to satisfy the Landau-Lifshitz-Kittel Law are modeled as folding catastrophes (saddle-node bifurcations). This description of ballistic motion in a viscous medium is based upon early work by Dawber et al., Appl. Phys. Lett. 82, 436 (2003). It suggests that ferroelectric films can exhibit folds or vortex patterns but not both.

cond-mat.mes-hall↗

arXiv2015

Anomalous change in leakage and displacement currents after electrical poling on lead-free ferroelectric ceramics

We report the polarization, displacement current and leakage current behavior of a trivalent nonpolar cation Al cation substituted lead free ferroelectric NBT-BT electroceramics with tetragonal phase and P4mm space group symmetry. Nearly three orders of magnitude decrease in leakage current were observed under electrical poling, which significantly improves microstructure, polarization, and displacement current. Effective poling neutralizes the domain pinning, traps charges at grain boundaries and fills oxygen vacancies with free charge carriers in matrix, thus saturated macroscopic polarization in contrast to that in upoled samples. E-poling changes bananas type polarization loops to real ferroelectric loops.

cond-mat.mtrl-sci↗

arXiv2015

Quantum Critical Point study in Multiferroic Hexaferrites: BaFe12O19, SrFe12O19, and PbFe3Ga9O19 -- Verification of the Khmelnitskii Theory

BaFe12O19 is a popular M-type hexaferrite with T(Neel) = 720 K of enormous commercial value (3 billion dollars/year). It exhibits an incipient ferroelectric phase transition (in violation of the Spaldin-Hill rule) extrapolated to lie at 6.0 K Kelvin but suppressed due to quantum fluctuations (as in SrTiO3). The QCP theory of Khmelnitskii for such uniaxial ferroelectrics predicts that the inverse isothermal electric susceptibility varies as T cubed, in contrast to that for pseudo-cubic materials such as SrTiO3 or KTaO3, a hypothesis we verify.

cond-mat.mtrl-sci↗

arXiv2015

Ferroelectric Capped Magnetization in Multiferroic PZT/LSMO Tunnel Junctions

Self-poled ultra-thin ferroelectric PbZr0.52Ti0.48O3 (PZT) (5 and 7 nm) films have been grown by pulsed laser deposition technique on ferromagnetic La0.67Sr0.33MnO3 (LSMO) (30 nm) to check the effect of polar capping on magnetization for ferroelectric tunnel junction (FTJ) devices. PZT/LSMO heterostructures with thick polar PZT (7 nm) capping show nearly 100% enhancement in magnetization compared with thin polar PZT (5 nm) films, probably due to excess hole transfer from the ferroelectric to the ferromagnetic layers. Core-level X-ray photoelectron spectroscopy studies revealed the presence of larger Mn 3s exchange splitting and higher Mn3+/Mn4+ ion ratio in the LMSO with 7 nm polar capping.

cond-mat.mtrl-sci↗

arXiv2014

Quantum criticality in a uniaxial organic ferroelectric

Tris-sarcosine calcium chloride (TSCC) is a highly uniaxial ferroelectric with a Curie temperature of approximately 130K. By suppressing ferroelectricity with bromine substitution on the chlorine sites, pure single crystals were tuned through a ferroelectric quantum phase transition. The resulting quantum critical regime was investigated in detail - the first time for a uniaxial ferroelectric and for an organic ferroelectric - and was found to persist up to temperatures of at least 30K to 40K. The nature of long-range dipole interactions in uniaxial materials, which lead to non-analytical terms in the free-energy expansion in the polarization, predict a dielectric susceptibility varying as $1/T^3$ close to the quantum critical point. Rather than this, we find that the dielectric susceptibility varies as $1/T^2$ as expected and observed in better known multi-axial systems. We explain this result by identifying the ultra-weak nature of the dipoles in the TSCC family of crystals. Interestingly we observe a shallow minimum in the inverse dielectric function at low temperatures close to the quantum critical point in paraelectric samples that may be attributed to the coupling of quantum polarization and strain fields. Finally we present results of the heat capacity and electro-caloric effect and explain how the time dependence of the polarization in ferroelectrics and paraelectrics should be considered when making quantitative estimates of temperature changes induced by applied electric fields.

cond-mat.mtrl-sci↗

arXiv2014

Faceting Oscillations in Nano-Ferroelectrics

We observe periodic faceting of 8-nm diameter ferroelectric disks on a 10 s time-scale when thin Pb(Zr0.52Ti0.48)O3 (PZT) film is exposed to constant high-resolution transmission electron microscopy (HRTEM) beams. The oscillation is between circular disk geometry and sharply faceted hexagons. The behavior is analogous to that of spin structure and magnetic domain wall velocity oscillations in permalloy [A. Bisig et al., Nature Commun. 4, 2328 (2013)], involving overshoot and de-pinning from defects [C. P. Amann, et al., J. Rheol. 57, 149-175 (2013)].

cond-mat.mtrl-sci↗

arXiv2014

Multifunctional Magnetoelectric Materials for Device Applications

Mutiferroics are a novel class of next generation multifunctional materials, which display simultaneous magnetic spin, electric dipole, and ferroelastic ordering, and have drawn increasing interest due to their multi-functionality for a variety of device applications. Since single-phase materials exist rarely in nature with such cross-coupling properties, an intensive research activity is being pursued towards the discovery of new single-phase multiferroic materials and the design of new engineered materials with strong magneto-electric (ME) coupling. This review article summarizes the development of different kinds of multiferroic material: single-phase and composite ceramic, laminated composite, and nanostructured thin films. Thin-film nanostructures have higher magnitude direct ME coupling values and clear evidence of indirect ME coupling compared with bulk materials. Promising ME coupling coefficients have been reported in laminated composite materials in which signal to noise ratio is good for device fabrication. We describe the possible applications of these materials.

cond-mat.mtrl-sci↗