arXiv subjects

J. F. Scott

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

At least 37 records · Page 2Linked to original sources

arXiv2014

Ferroelectric and Photovoltaic Properties of Transition Metal doped Pb(Zr0.14Ti0.56Ni0.30)O3-delta Thin Films

We report nearly single phase Pb(Zr0.14Ti0.56Ni0.30)O3-delta(PZTNi30) ferroelectric having large remanent polarization (15-30 μC/cm2), 0.3-0.4 V open circuit voltage (VOC), reduced band gap (direct 3.4 eV, and indirect 2.9 eV), large ON and OFF photo current ratio, and the fast decay time. Reasonably good photo current density (1-5 μA/cm2) was obtained without gate bias voltage which significantly increased with large bias field. Ferroelectric polarization dictates the polarity of VOC and direction of short circuit current (ISC), a step forward towards the realization of noncentrosymmetric ferroelectric material sensitive to visible light.

cond-mat.mtrl-sci↗

arXiv2013

Manipulating Ferroelectric Domains in Nanostructures Under Electron Beams

Freestanding BaTiO3 nanodots exhibit domain structures characterized by distinct quadrants of ferroelastic 90° domains in transmission electron microscopy (TEM) observations. These differ significantly from flux-closure domain patterns in the same systems imaged by piezoresponse force microscopy. Based upon a series of phase field simulations of BaTiO3 nanodots, we suggest that the TEM patterns result from a radial electric field arising from electron beam charging of the nanodot. For sufficiently large charging, this converts flux-closure domain patterns to quadrant patterns with radial net polarizations. Not only does this explain the puzzling patterns that have been observed in TEM studies of ferroelectric nanodots, but also suggests how to manipulate ferroelectric domain patterns via electron beams.

cond-mat.mtrl-sci↗

arXiv2013

High-Symmetry Polarization Domains in Low-Symmetry Ferroelectrics

We present experimental evidence for hexagonal domain faceting in the ferroelectric polymer PVDF-TrFE films having the lower orthorhombic crystallographic symmetry. This effect can arise from purely electrostatic depolarizing forces. We show that in contrast to magnetic bubble shape domains where such type of deformation instability has a predominantly elliptical character, the emergence of more symmetrical circular harmonics is favored in ferroelectrics with high dielectric constant.

cond-mat.mtrl-sci↗

arXiv2012

Reduced graphene oxide as ultra fast temperature sensor

We demonstrate the excellent temperature sensing property of a chemically synthesized reduced graphene oxide (rGO). It is found that with increase in temperature from 80 to 375K, the resistivity of reduced graphene oxide monotonically decreases. The ultra-fast temperature sensing property is demonstrated by keeping and removing a block of ice under the rGO sensor, which shows the resistance of rGO increases by 15% in 592 miliseconds and recovers in 8.92 seconds. The temperature sensing of rGO is compared with a standard platinum thermo sensor (Pt 111) and found the sensitivity is much better in rGO.

cond-mat.mes-hall↗

arXiv2012

In-plane Dielectric and Magnetoelectric Studies of BiFeO3

In-plane temperature dependent dielectric behavior of BiFeO3 (BFO) as-grown thin films show diffuse but prominent phase transitions near 450 (+/-10) K and 550 K with dielectric loss temperature dependences that suggest skin layer effects. The 450 K anomalies are near the "transition" first reported by Polomska et al. [Phys. Stat. Sol. 23, 567 (1974)]. The 550 K anomalies coincide with the surface phase transition recently reported [Xavi et al. PRL 106, 236101 (2011)]. In addition, anomalies are found at low temperatures: After several experimental cycles the dielectric loss shows a clear relaxor-like phase transition near what was previously suggested to be a spin reorientation transition (SRT) temperature (~ 201 K) for frequencies 1 kHz < f < 1MHz which follow a nonlinear Vogel-Fulcher (V-F) relation; an additional sharp anomaly is observed near ~180 K at frequencies below 1 kHz. As emphasized recently by Cowley et al. [Adv. Phys. 60, 229 (2011)], skin effects are expected for all relaxor ferroelectrics. Using the interdigital electrodes, experimental data and a theoretical model for in-plane longitudinal and transverse direct magnetoelectric (ME) coefficient are presented.

cond-mat.mtrl-sci↗

arXiv2012

Surface phase transitions in BiFeO3 below room temperature

We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, EPR, X-ray lattice constant measurements, conductivity and dielectric response, specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO3 nanotubes were also studied. The transition at T=140.3K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201K anomaly appears to signal the onset of glassy behaviour.

cond-mat.mtrl-sci↗

arXiv2012

High-temperature spin-wave propagation in BiFeO3: relation to the Polomska transition

In bismuth ferrite thin films the cycloidal spiral spin structure is suppressed, and as a result the spin-wave magnon branches of long wavelength are reduced from a dozen to one, at ω= 19.2 cm-1 (T=4K). This spin wave has not been measured previously above room temperature, but in the present work we show via Raman spectroscopy that it is an underdamped propagating wave until 455 K. This has important room-temperature device implications. The data show that ω(T) follows an S=5/2 Brillouin function and hence its Fe+3 ions are in the high-spin 5/2 state and not the low-spin S=1/2 state. The spin wave cannot be measured as a propagating wave above 455 K. We also suggest that since this temperature is coincident with the mysterious "Polomska transition" (M. Polomska et al., Phys. Stat. Sol. A 23, 567, (1974)) at 458+/-5 K, that this may be due to overdamping.

cond-mat.mtrl-sci↗

arXiv2011

Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane X-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

cond-mat.mtrl-sci↗

arXiv2010

Magnetic Effects on Dielectric and Polarization Behavior of Multiferroic Hetrostructures

PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3(PZT/LSMO) bilayer with surface roughness ~ 1.8 nm thin films have been grown by pulsed laser deposition on LaAlO3(LAO) substrates. High remnant polarization (30-54 micro C/cm2), dielectric constant(400-1700), and well saturated magnetization were observed depending upon the deposition temperature of the ferromagnetic layer and applied frequencies. Giant frequency-dependent change in dielectric constant and loss were observed above the ferromagnetic-paramagnetic temperature. The frequency dependent dielectric anomalies are attributed to the change in metallic and magnetic nature of LSMO and also the interfacial effect across the bilayer; an enhanced magnetoelectric interaction may be due to the Parish-Littlewood mechanism of inhomogeneity near the metal-dielectric interface.

cond-mat.mtrl-sci↗

arXiv2010

Processing and Characterization of Multiferroic Bi-relaxors

We compare chemical solution deposition (CSD), and pulsed-laser-deposition (PLD), specimens of the new room-temperature, single-phase, multiferroic magnetoelectric, [PbFe2/3W1/3O3]x[PbZr0.53Ti0.47O3]1-x (PZTFWx ~ 0.40<x<0.20) with polarization, loss (<1%), and resistivity (typically 108 ohm.cm) equal to or superior to BiFeO3. Single phase polycrystalline multiferroics PZTFWx thin films were fabricated on platinized silicon substrate by CSD and as epitaxial single-crystal films on MgO substrate by PLD. High dielectric constants (1200- 3000), high polarization (30 - 60 micro C/cm2), weak saturation magnetization (0.48 - 4.53 emu/cm3), a broad dielectric temperature peak, high-frequency dispersion, low dielectric loss and low leakage current were observed in these materials, suggesting the family as candidates for room-temperature multiferroic devices. The ferroelectric switching in these materials can be suppressed or quenched with applied magnetic field.

cond-mat.mtrl-sci↗

arXiv2010

Phonon Spectroscopy Near Phase Transition Temperatures in Multferroic BiFeO3 Epitaxial Thin Films

We report a Raman scattering investigation of multiferroic bismuth ferrite BiFeO3 epitaxial (c-axis oriented) thin films from -192 to 1000C. Phonon anomalies have been observed in three temperature regions: in the gamma-phase from 930C to 950C; at ~370C, Neel temperature (TN), and at ~123C, due to a phase transition of unknown type (magnetic or structural). An attempt has been made to understand the origin of the weak phonon-magnon coupling and the dynamics of the phase sequence. The disappearance of several Raman modes at ~820C (Tc) is compatible with the known structural phase transition and the Pbnm orthoferrite space group assigned by Arnold {\it et al.} \cite{arnold:09}. The spectra also revealed a {\it non-cubic} $β$-phase from 820-930\dc and the same {\it non-cubic} phase extends through the $γ$-phase between 930-950\dc, in agreement with Arnold {\it et al.} \cite{arnold2:09}, and an evidence of a cubic $δ$-phase around 1000\dc in thin films that is not stable in powder and bulk. Such a cubic phase has been theoretically predicted in \cite{vasquez:prb09}. Micro-Raman scattering and X-ray diffraction showed no structural decomposition in thin films during the thermal cycling from 22-1000\dc.

cond-mat.mtrl-sci↗

arXiv2009

Observation of Spin-glass-like Behavior in SrRuO3 Epitaxial Thin Films

We report the observation of spin-glass-like behavior and strong magnetic anisotropy in extremely smooth (~1-3 Å) roughness) epitaxial (110) and (010) SrRuO3 thin films. The easy axis of magnetization is always perpendicular to the plane of the film (unidirectional) irrespective of crystallographic orientation. An attempt has been made to understand the nature and origin of spin-glass behavior, which fits well with Heisenberg model.

cond-mat.mtrl-sci↗

arXiv2009

Topology of the polarization field in ferroelectric nanowires from first principles

The behaviour of the cross-sectional polarization field is explored for thin nanowires of barium titanate from first-principles calculations. Topological defects of different winding numbers have been obtained, beyond the known textures in ferroelectric nanostructures. They result from the inward accommodation of the polarization patterns imposed at the surface of the wire by surface and edge effects. Close to a topological defect the polarization field orients out of the basal plane in some cases, maintaining a close to constant magnitude, whereas it virtually vanishes in other cases.

cond-mat.mtrl-sci↗

arXiv2009

Effect of Chemical Pressure on the Magnetic Transition of Multiferroic Ca-BiFeO3

Multiferroic BiFeO3 ceramics have been doped with Ca. The smaller ionic size of Ca compared with Bi means that doping acts as a proxy for hydrostatic pressure, at a rate of 1%Ca=0.3GPa. It is also found that the magnetic Neel temperature (TNeel) increases as Ca concentration increases, at a rate of 0.66K per 1%Ca (molar). Based on the effect of chemical pressure on TNeel, we argue that applying hydrostatic pressure to pure BiFeO3 can be expected to increase its magnetic transition temperature at a rate around ~2.2K/GPa. The results also suggest that pressure (chemical or hydrostatic) could be used to bring the ferroelectric critical temperature, Tc, and the magnetic TNeel closer together, thereby enhancing magnetoelectric coupling, provided that electrical conductivity can be kept sufficiently low.

cond-mat.mtrl-sci↗

arXiv2009

Quantum criticality in ferroelectrics

Materials tuned to the neighbourhood of a zero temperature phase transition often show the emergence of novel quantum phenomena. Much of the effort to study these new effects, like the breakdown of the conventional Fermi-liquid theory of metals has been focused in narrow band electronic systems. Ferroelectric crystals provide a very different type of quantum criticality that arises purely from the crystalline lattice. In many cases the ferroelectric phase can be tuned to absolute zero using hydrostatic pressure or chemical or isotopic substitution. Close to such a zero temperature phase transition, the dielectric constant and other quantities change into radically unconventional forms due to the quantum fluctuations of the electrical polarization. The simplest ferroelectrics may form a text-book paradigm of quantum criticality in the solid-state as the difficulties found in metals due to a high density of gapless excitations on the Fermi surface are avoided. We present low temperature high precision data demonstrating these effects in pure single crystals of SrTiO3 and KTaO3. We outline a model for describing the physics of ferroelectrics close to quantum criticality and highlight the expected 1/T2 dependence of the dielectric constant measured over a wide temperature range at low temperatures. In the neighbourhood of the quantum critical point we report the emergence of a small frequency independent peak in the dielectric constant at approximately 2K in SrTiO3 and 3K in KTaO3 believed to arise from coupling to acoustic phonons. Looking ahead, we suggest that in ferroelectric materials supporting mobile charge carriers, quantum paraelectric fluctuations may mediate new effective electron-electron interactions giving rise to a number of possible states such as superconductivity.

cond-mat.str-el↗

arXiv2009

Origin of Ferroelastic Domains in Free-Standing Single Crystal Ferroelectric Films

The origin of the unusual 90^o ferroelectric / ferroelastic domains, consistently observed in recent studies on meso and nanoscale free-standing single crystals of BaTiO3 [Schilling et al., Physical Review B, 74, 024115 (2006); Schilling et al., Nano Letters, 7, 3787 (2007)], has been considered. A model has been developed which postulates that the domains form as a response to elastic stress induced by a surface layer which does not undergo the paraelectric-ferroelectric, cubic-tetragonal phase transition. This model was found to accurately account for the changes in domain periodicity as a function of size that had been observed experimentally. The physical origin of the surface layer might readily be associated with patterning damage, seen in experiment; however, when all evidence of physical damage is removed from the BaTiO3 surfaces by thermal annealing, the domain configuration remains practically unchanged. This suggests a more intrinsic origin, such as the increased importance of surface tension at small dimensions. The effect of surface tension is also shown to be proportional to the difference in hardness between the surface and the interior of the ferroelectric. The present model for surface tension induced twinning should also be relevant for finely grained or core-shell structured ceramics.

cond-mat.mtrl-sci↗

arXiv2009

Structural Origin of the Metal-Insulator Transition of Multiferroic BiFeO3

We report X-ray structural studies of the metal-insulator phase transition in bismuth ferrite, BiFeO3, both as a function of temperature and of pressure (931 oC at atmospheric pressure and ca. 45 GPa at ambient temperature). Based on the experimental results, we argue that the metallic gamma-phase is not rhombohedral but is instead the same cubic Pm3m structure whether obtained via high temperature or high pressure, that the MI transition is second order or very nearly so, that this is a band-type transition due to semi-metal band overlap in the cubic phase and not a Mott transition, and that it is primarily structural and not an S=5/2 to S=1/2 high-spin/low-spin electronic transition. Our data are compatible with the orthorhombic Pbnm structure for the beta-phase determined definitively by the neutron scattering study of Arnold et al .[Phys. Rev. Lett. 2009]; the details of this beta-phase had also been controversial, with a remarkable collection of five crystal classes (cubic, tetragonal, orthorhombic, monoclinic, and rhombohedral!) all claimed in recent publications.

cond-mat.mtrl-sci↗

arXiv2008

Almeida-Thouless Line in BiFeO3: Is Bismuth Ferrite a mean field spin glass?

Low-temperature magnetic properties of epitaxial BiFeO3 thin films grown on (111)-SrTiO3 substrates have been studied. ZFC and FC magnetization curves show a large discrepency beginning at a characteristic temperature Tf that is dependent on the magnetic-field strengh. Tf(H) varies according to the well known de Aleimda-Thouless line (varies in Hexp(2/3)) suggesting an acentric long rnage spin-glass behavior and mean field system.

cond-mat.mtrl-sci↗