Application of conducting ferroelectric domain walls (DW) as functional elements may facilitate d... more Application of conducting ferroelectric domain walls (DW) as functional elements may facilitate development of conceptually new resistive switching devices. In a conventional approach, several orders of magnitude change in resistance can be achieved by controlling the DWs density using super-coercive voltage. However, a deleterious characteristic of this approach is high-energy cost of polarization reversal due to high leakage current. Here, we demonstrate a new approach based on tuning the conductivity of DWs themselves rather than on domain rearrangement. Using LiNbO3 capacitors with graphene, we show that resistance of a device set to a polydomain state can be continuously tuned by application of sub-coercive voltage. The tuning mechanism is based on the reversible transition between the conducting and insulating states of DWs. The developed approach allows an energy-efficient control of resistance without the need for domain structure modification. The developed memristive devices are promising for multi-level memories and neuromorphic computing applications.
Near-field second-harmonic generation has been investigated from unpoled polycrystalline ferroele... more Near-field second-harmonic generation has been investigated from unpoled polycrystalline ferroelectric thin films. Images of the second-harmonic response obtained with both p and s polarizations of the fundamental light display ferroelectric domain contrast due to spontaneous macroscopic electric polarization. An approach towards an absolute determination of the direction of ferroelectric polarization using analysis of the images obtained with different polarizations of the fundamental light and contrast mechanisms available for various fundamental light polarizations are discussed on a basis of a simple theoretical model.
Laser-controlled reduction of individual graphene oxide films provide unprecedented work function... more Laser-controlled reduction of individual graphene oxide films provide unprecedented work function tuning with millivolt precision.
The localized crystallography of conducting and non-conducting phase boundaries in mixed-phase Bi... more The localized crystallography of conducting and non-conducting phase boundaries in mixed-phase BiFeO3 is directly compared using scanning transmission electron microscopy techniques.
The discovery of single-phase multiferroic materials and the understanding of intriguing physics ... more The discovery of single-phase multiferroic materials and the understanding of intriguing physics of the coupling mechanisms between their spin and polarization is important for the next generation of multifunctional devices. In this work, we report dielectric, ferroelectric, and magnetization results of Pdsubstituted room-temperature magnetoelectric multiferroic Pb(Zr 0.20 Ti 0.80) 0.7 Pd 0.3 O 3 (PZTPd) thin films. Highly c-axis oriented PZTPd thin films were grown on {(LaAlO 3) 0.3 (Sr 2 AlTaO 6) 0.7 } (LSAT) substrates using pulsed laser deposition technique. These films were phase pure and stabilized in tetragonal phase. Atomic force microscopic studies indicated a homogeneous distribution of grains on the films' surface with a surface roughness (R Q) of ~5.4 nm. A large dielectric constant of ~1780 and a low loss tangent value of ~0.52 were observed at 1 kHz measured at room temperature. Temperature dependent dielectric studies on Pt/PZTPd/La 0.7 Sr 0.3 MnO 3 (LSMO) metal-insulator-metal heterostructure capacitors between 80-660 K indicated a diffused ferroelectric to paraelectric phase transition around 515 K. The polarization hysteresis loops observed at room temperature were attributed to its ferroelectric behavior. Saturated magnetization hysteresis loop with remanent magnetization of 1.54 emu/cm 3 was obtained at room temperature. Ferromagnetic ordering in thin films was found to sustain in the entire temperature range from 5-395 K, as revealed from the constant behavior of dM/dT curve with temperature. The mixed oxidation states of palladium (Pd 2+ /Pd 4+) dispersed in the polar Pb(Zr0.20Ti0.80)O 3 matrix were revealed from our high resolution x-rays photoelectron spectroscopic studies, and is ascribed to the origin of ferromagnetic ordering in the film. These findings suggest that PZTPd thin films are multiferroic (ferroelectricferromagnetic) at room temperature.
Very recently, the discovery of ultra-tetragonal PbTiO3 thin films was reported [L. Zhang et al.,... more Very recently, the discovery of ultra-tetragonal PbTiO3 thin films was reported [L. Zhang et al., Science, 361, 494 (2018)], in which the switchable out-of-plane polarization was seen to be almost twice that of any previously known ferroelectric. To understand more about this system
Kelvin probe force microscopy (KPFM) has been used to directly and quantitatively measure Hall vo... more Kelvin probe force microscopy (KPFM) has been used to directly and quantitatively measure Hall voltages, developed at conducting tail-to-tail domain walls in ErMnO single crystals, when current is driven in the presence of an approximately perpendicular magnetic field. Measurements across a number of walls, taken using two different atomic force microscope platforms, consistently suggest that the active p-type carriers have unusually large room temperature mobilities of the order of hundreds of square centimeters per volt second. Associated carrier densities were estimated to be of the order of 10 cm. Such mobilities, at room temperature, are high in comparison with both bulk oxide conductors and LaAlO-SrTiO sheet conductors. High carrier mobilities are encouraging for the future of domain-wall nanoelectronics and, significantly, also suggest the feasibility of meaningful investigations into dimensional confinement effects in these novel domain-wall systems.
We present a study of the crystallography and transport properties of NdNiO 3 thin films, grown b... more We present a study of the crystallography and transport properties of NdNiO 3 thin films, grown by pulsedlaser deposition, on a variety of substrates and with a range of thicknesses. Results highlight the importance of epitaxy, and show that NdNiO 3 , with a sharp metal-insulator phase transition, can be fabricated without the need for high-pressure processing. The conductivity of the nickelate films was found to be well described by a linear sum of activated transport and Mott's variable range hopping in the entire measured temperature range of the semiconducting state, and this description was also found to provide an accurate fit for previously published transport properties of bulk ceramics. The transition was subsequently modeled using a percolative approach. It was found that the temperature of the metal-insulator phase transition, in both our films and in bulk, corresponded to a critical percolation threshold where the volume fraction of the semiconducting phase (V s) was 2 3 , as expected for a three-dimensional cubic lattice. For the thinnest films grown on NdGaO 3 , a possible crossover to two-dimensional percolation was indicated by V s ϭ 1 2 .
Advanced materials (Deerfield Beach, Fla.), Jan 9, 2015
The coupling between magnetization and polarization in a room temperature multiferroic (Pb(Zr,Ti)... more The coupling between magnetization and polarization in a room temperature multiferroic (Pb(Zr,Ti)O3 -Pb(Fe,Ta)O3 ) is explored by monitoring changes in capacitance that occur when a magnetic field is applied in each of three orthogonal directions. Magnetocapacitance effects, consistent with P(2) M(2) coupling, are strongest when fields are applied in the plane of the single crystal sheet investigated.
Application of conducting ferroelectric domain walls (DW) as functional elements may facilitate d... more Application of conducting ferroelectric domain walls (DW) as functional elements may facilitate development of conceptually new resistive switching devices. In a conventional approach, several orders of magnitude change in resistance can be achieved by controlling the DWs density using super-coercive voltage. However, a deleterious characteristic of this approach is high-energy cost of polarization reversal due to high leakage current. Here, we demonstrate a new approach based on tuning the conductivity of DWs themselves rather than on domain rearrangement. Using LiNbO3 capacitors with graphene, we show that resistance of a device set to a polydomain state can be continuously tuned by application of sub-coercive voltage. The tuning mechanism is based on the reversible transition between the conducting and insulating states of DWs. The developed approach allows an energy-efficient control of resistance without the need for domain structure modification. The developed memristive devices are promising for multi-level memories and neuromorphic computing applications.
Near-field second-harmonic generation has been investigated from unpoled polycrystalline ferroele... more Near-field second-harmonic generation has been investigated from unpoled polycrystalline ferroelectric thin films. Images of the second-harmonic response obtained with both p and s polarizations of the fundamental light display ferroelectric domain contrast due to spontaneous macroscopic electric polarization. An approach towards an absolute determination of the direction of ferroelectric polarization using analysis of the images obtained with different polarizations of the fundamental light and contrast mechanisms available for various fundamental light polarizations are discussed on a basis of a simple theoretical model.
Laser-controlled reduction of individual graphene oxide films provide unprecedented work function... more Laser-controlled reduction of individual graphene oxide films provide unprecedented work function tuning with millivolt precision.
The localized crystallography of conducting and non-conducting phase boundaries in mixed-phase Bi... more The localized crystallography of conducting and non-conducting phase boundaries in mixed-phase BiFeO3 is directly compared using scanning transmission electron microscopy techniques.
The discovery of single-phase multiferroic materials and the understanding of intriguing physics ... more The discovery of single-phase multiferroic materials and the understanding of intriguing physics of the coupling mechanisms between their spin and polarization is important for the next generation of multifunctional devices. In this work, we report dielectric, ferroelectric, and magnetization results of Pdsubstituted room-temperature magnetoelectric multiferroic Pb(Zr 0.20 Ti 0.80) 0.7 Pd 0.3 O 3 (PZTPd) thin films. Highly c-axis oriented PZTPd thin films were grown on {(LaAlO 3) 0.3 (Sr 2 AlTaO 6) 0.7 } (LSAT) substrates using pulsed laser deposition technique. These films were phase pure and stabilized in tetragonal phase. Atomic force microscopic studies indicated a homogeneous distribution of grains on the films' surface with a surface roughness (R Q) of ~5.4 nm. A large dielectric constant of ~1780 and a low loss tangent value of ~0.52 were observed at 1 kHz measured at room temperature. Temperature dependent dielectric studies on Pt/PZTPd/La 0.7 Sr 0.3 MnO 3 (LSMO) metal-insulator-metal heterostructure capacitors between 80-660 K indicated a diffused ferroelectric to paraelectric phase transition around 515 K. The polarization hysteresis loops observed at room temperature were attributed to its ferroelectric behavior. Saturated magnetization hysteresis loop with remanent magnetization of 1.54 emu/cm 3 was obtained at room temperature. Ferromagnetic ordering in thin films was found to sustain in the entire temperature range from 5-395 K, as revealed from the constant behavior of dM/dT curve with temperature. The mixed oxidation states of palladium (Pd 2+ /Pd 4+) dispersed in the polar Pb(Zr0.20Ti0.80)O 3 matrix were revealed from our high resolution x-rays photoelectron spectroscopic studies, and is ascribed to the origin of ferromagnetic ordering in the film. These findings suggest that PZTPd thin films are multiferroic (ferroelectricferromagnetic) at room temperature.
Very recently, the discovery of ultra-tetragonal PbTiO3 thin films was reported [L. Zhang et al.,... more Very recently, the discovery of ultra-tetragonal PbTiO3 thin films was reported [L. Zhang et al., Science, 361, 494 (2018)], in which the switchable out-of-plane polarization was seen to be almost twice that of any previously known ferroelectric. To understand more about this system
Kelvin probe force microscopy (KPFM) has been used to directly and quantitatively measure Hall vo... more Kelvin probe force microscopy (KPFM) has been used to directly and quantitatively measure Hall voltages, developed at conducting tail-to-tail domain walls in ErMnO single crystals, when current is driven in the presence of an approximately perpendicular magnetic field. Measurements across a number of walls, taken using two different atomic force microscope platforms, consistently suggest that the active p-type carriers have unusually large room temperature mobilities of the order of hundreds of square centimeters per volt second. Associated carrier densities were estimated to be of the order of 10 cm. Such mobilities, at room temperature, are high in comparison with both bulk oxide conductors and LaAlO-SrTiO sheet conductors. High carrier mobilities are encouraging for the future of domain-wall nanoelectronics and, significantly, also suggest the feasibility of meaningful investigations into dimensional confinement effects in these novel domain-wall systems.
We present a study of the crystallography and transport properties of NdNiO 3 thin films, grown b... more We present a study of the crystallography and transport properties of NdNiO 3 thin films, grown by pulsedlaser deposition, on a variety of substrates and with a range of thicknesses. Results highlight the importance of epitaxy, and show that NdNiO 3 , with a sharp metal-insulator phase transition, can be fabricated without the need for high-pressure processing. The conductivity of the nickelate films was found to be well described by a linear sum of activated transport and Mott's variable range hopping in the entire measured temperature range of the semiconducting state, and this description was also found to provide an accurate fit for previously published transport properties of bulk ceramics. The transition was subsequently modeled using a percolative approach. It was found that the temperature of the metal-insulator phase transition, in both our films and in bulk, corresponded to a critical percolation threshold where the volume fraction of the semiconducting phase (V s) was 2 3 , as expected for a three-dimensional cubic lattice. For the thinnest films grown on NdGaO 3 , a possible crossover to two-dimensional percolation was indicated by V s ϭ 1 2 .
Advanced materials (Deerfield Beach, Fla.), Jan 9, 2015
The coupling between magnetization and polarization in a room temperature multiferroic (Pb(Zr,Ti)... more The coupling between magnetization and polarization in a room temperature multiferroic (Pb(Zr,Ti)O3 -Pb(Fe,Ta)O3 ) is explored by monitoring changes in capacitance that occur when a magnetic field is applied in each of three orthogonal directions. Magnetocapacitance effects, consistent with P(2) M(2) coupling, are strongest when fields are applied in the plane of the single crystal sheet investigated.
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Papers by Marty Gregg