Saturated Ferroelectric Hysteresis Loops Research Articles

Large Electric Polarization and Exchange Bias in Multiferroic BiFeO3

Low-leakage BiFeO3 films have been grown with saturated ferroelectric hysteresis loops (see figure) and a very large remanent polarization. The antiferromagnetic nature of BiFeO3 films is confirmed by the appearance of an exchange bias in BiFeO3-based spin-valve multilayers. The results imply that there is potential for room-temperature applications of BiFeO3 that combine ferroelectric and antiferromagnetic functionality.

Microstructural, ferroelectric, and dielectric properties of Bi3.15Nd0.85Ti3O12 ceramics

Dense Bi3.15Nd0.85Ti3O12 (BNdT) ceramics of a layered perovskite structure were sintered at 1100 °C by solid state reaction. Ferroelectric domains were probably observed in plate-shaped grains. The BNdT ceramics exhibit saturated ferroelectric hysteresis loops with a remanent polarization (2Pr) of 45μC∕cm2 and a coercive field of 67.6kV∕cm. The dielectric constant and dissipation factor of the ceramics at 100 kHz are 221 and 0.0064, respectively. A broad dielectric peak was observed around 408 °C, and it might come from the oxygen-vacancy-related dielectric relaxation. The leakage current density of the ceramics is less than 7.5×10−7A∕cm2 under an applied field below 239kV∕cm. The BNdT ceramics show Schottky emission behavior under low electric field below 75kV∕cm.

Modeling saturated and unsaturated ferroelectric hysteresis loops: An analytical approach

In ferroelectric materials, hysteresis behavior is very difficult to model due to its nonlinear and history-dependent characteristics. Among approaches that are able to describe unsaturated loops, many of them are either very complicated (numerical procedures must be employed) or the resulting loops contain some undesirable or defective features. In this work, a simple hysteresis model based on a special construction of the Preisach function is proposed. Explicit expressions for the polarization-field (P-E) responses under increasing and decreasing applied fields have been derived. The saturated and unsaturated P-E loops can be conveniently calculated by piecing together such responses. The technique is widely applicable to the modeling of ferroelectric hysteresis behavior of ceramics and polymers. As examples we study the applied field dependence of dielectric permittivity of a ferroelectric film and the remanent polarization of ferroelectric composites after ac poling. We find that the model predictions agree well with the experimental results.

Ferroelectric and dielectric properties of Ca0.28Ba0.72Nb2O6 single crystals of tungsten bronzes structure

Large Ca0.28Ba0.72Nb2O6 (CBN-28) single crystals exhibit saturated ferroelectric hysteresis loops under an electric field of 115kV∕cm along the [001] direction. Their spontaneous polarization, remanent polarization and coercive field are 35.3μC∕cm2, 32.2μC∕cm2, and 38.1kV∕cm, respectively. The dielectric constant and dielectric loss tanδ of the crystals are 195 and 0.32 at 10 kHz, respectively. A diffused dielectric anomaly with relaxor characteristic was observed in the range of 325-500 °C at low frequencies, while the CBN-28 crystals experienced a first-order normal-relaxor ferroelectric phase transition around 252 °C on heating. A broad dielectric loss peak appears around 120 °C and it is interpreted in terms of the migration of oxygen vacancies. The conductance activation energy was determined to be 1.33 eV in the high temperature regime (500–560 °C).

Ferroelectric and dielectric properties of bismuth neodymium titanate ceramics prepared using sol–gel derived fine powders

Dense Bi 3.15Nd 0.85Ti 3O 12 (BNdT) ceramics of a layered perovskite structure were prepared using sol–gel derived fine powders. Grains in the ceramics are plate-shaped. The BNdT ceramics exhibit saturated ferroelectric hysteresis loops with the remanent polarization ( 2 P r ) of 43 μC/cm 2 and the coercive field ( E c ) of 67 kV/cm. The dielectric constant and dissipation factor of the ceramics at 100 kHz are 138 and 0.0039, respectively. A broad dielectric peak was observed at the phase transition temperature of around 390 °C, and the broading of the peak might come from the oxygen-vacancy-related dielectric relaxation.

Magnetic and electrical properties of single-phase multiferroic BiFeO3

We have reported the structural, thermal, microscopic, magnetization, polarization, and dielectric properties of BiFeO3 ceramics synthesized by a rapid liquid-phase sintering technique. Optimum conditions for the synthesis of single-phase BiFeO3 ceramics were obtained. Temperature-dependent magnetization and hysteresis loops indicate antiferromagnetic behavior in BiFeO3 at room temperature. Although saturated ferroelectric hysteresis loops were observed in single-phase BiFeO3 ceramic synthesized at 880 °C, the reduced polarization is found to be due to the high loss and low dielectric permittivity of the ceramic, which is caused by higher leakage current.

Prominent ferroelectricity of BiFeO3 thin films prepared by pulsed-laser deposition

Ferroelectric BiFeO3 thin films have been prepared on Pt/TiO2/SiO2/Si substrates in various oxygen pressures of 0.001–0.1 Torr at a temperature as low as 450 °C by pulsed-laser deposition. The crystallinity of the films was studied by x-ray diffraction. X-ray photoelectron spectroscopy showed that the films have a single phase of perovskite BiFeO3. The BiFeO3 thin films deposited at 0.01–0.1 Torr show good current-density–applied-voltage characteristics. It is obtained from polarization–electric-field characterization that 2Pr is about 71.3 μC/cm2 and 2Ec is 125 kV/cm. Stable current density and saturated ferroelectric hysteresis loop have been observed in BiFeO3 thin films.

Structural and electrical characterization of epitaxial, large area ferroelectric films of Ba2Bi4Ti5O18 grown by pulsed excimer laser ablation

Excimer laser ablation was used to deposit epitaxial thin films of Ba2Bi4Ti5O18 having mostly c-axis oriented grains. The ferroelectric layer was deposited on an epitaxial conducting oxide electrode layer of LaNiO3, the electrode layer in turn being grown on an epitaxial buffer layer stack on single crystalline (100)-oriented silicon wafers of 3 in. diameter. X-ray diffraction measurements indicate a strong c-axis orientation of the ferroelectric layer. Scanning electron microscopy and atomic force microscopy were used to observe the microstructure of the films and these revealed the homogenous quality of the film with good uniformity over the wafers. Well-defined, saturated ferroelectric hysteresis loops were observed in the compound with values of 1.3 and 0.6 μC/cm2 as saturation and remnant polarization values, respectively, and a coercive field of 47 kV/cm. The results of I–V measurements and complex impedance spectroscopy measurements are presented and a defect formula is proposed for the compound.

Orientation dependence of ferroelectricity in pulsed-laser-deposited epitaxial bismuth-layered perovskite thin films

Thin films of bismuth-layered perovskites such as SrBi2Ta2O9, Bi4Ti3O12 ,a ndBaBi4Ti4O15 with preferred orientations were grown by pulsed laser deposition on epitax- ial conducting LaNiO3 electrodes on single-crystalline (100) SrTiO3 or on top of epitaxial buffer layers on (100) sil- icon. A morphology and structure investigation by X-ray diffraction analysis, scanning probe microscopy, and scan- ning and transmission electron microscopy showed that the films consisted of both c-axis-oriented regions and mixed (110)-, (100)-, and (001)-oriented regions. The regions with mixed orientation featured rectangular as well as equiaxed crystalline grains protruding out of a smooth c-oriented back- ground. A closer examination revealed that the regions with mixed orientation actually consisted of a c-axis-oriented sub- layer growing directly on the epitaxial LaNiO3 electrode, on top of which the growth of either (110)-, (100)-, or (001)- oriented grains took place. Macroscopic as well as micro- scopic measurements of the ferroelectric properties of regions with pure c-orientation and of regions with mixed orienta- tions showed a clear relationship between their ferroelectric properties and their morphology and crystallographic orien- tation. In the regions with mixed orientation, the films exhib- ited saturated ferroelectric hysteresis loops with well-defined remnant polarisation Pr and coercive field Ec. The regions having c-axis orientation with a smooth surface morphology in contrast exhibited a linear P E curve with no hysteretic behaviour for SrBi2Ta2O9 and BaBi4Ti4O15 and a weak fer- roelectric behaviour for Bi4Ti3O12. This clearly showed that the ferroelectric properties of bismuth-layered ferroelectric oxides depended on the crystalline orientation of the film and that the observed ferroelectric hysteresis loops in SrBi2Ta2O9 and BaBi4Ti4O15 films were solely due to the (100)- and (110)-oriented grains. The size of the (110)- and (100)- oriented grains being of the order of 100 nm and spontaneous polarisation having been observed and switched in a con- trolled manner is a demonstration that ferroelectricity can exist in structures of submicrometer size. These results might have a technological impact due to the relevance of bismuth- layered ferroelectric oxides for the fabrication of non-volatile FeRAM memories.

Epitaxial bismuth-layer-structured perovskite ferroelectric thin films grown by pulsed laser deposition

Thin films of bismuth-layer-structured perovskites such as SrBi2Ta2O9 (SBT) and BaBi4Ti4O15 (BBiT) with preferred orientations have been grown by pulsed laser deposition (PLD) on epitaxial conducting LaNiO3 (LNO) electrodes on single crystalline (100) SrTiO3 (STO) or on top of epitaxial buffer layers on (100) silicon. X-ray diffraction (XRD) analysis and cross-section electron microscopy reveal that the films consist of c-axis oriented regions and mixed a-axis and c-axis oriented regions. The regions with mixed a-axis and c-axis orientation show high surface roughness due to the rectangular crystallites protruding out of the surface, whereas the c-axis oriented regions show a smooth surface morphology. In the mixed a-axis and c-axis oriented regions, the SBT and BBiT films exhibit saturated ferroelectric hysteresis loops with a remnant polarization Pr of about 2 μC/cm2 and coercive fields Ec of about 20 kV/cm for SBT and 60 kV/cm for BBiT. No polarization fatigue was observed up to 108 cycles neither for SBT nor for BBiT films. The regions having c-axis orientation with a smooth surface morphology in contrast exhibit a linear P-E curve. The results show that the ferroelectric properties of a planar capacitor consisting of ferroelectric Bi-layer-structured perovskites depend on the crystalline orientation of the film.

BaBi 4 Ti 4 O 15 ferroelectric thin films grown by pulsed laser deposition

BaBi 4 Ti 4 O 15 (BBiT) is an n=4 member of the Bi-layer-structured ferroelectric oxide family (Aurivillius phases). BBiT thin films with preferred orientations have been grown on epitaxial conducting LaNiO3 electrodes on (001) SrTiO3 by pulsed laser deposition. Cross-section electron microscopy analysis reveals that the films consist of ct-axis oriented regions and mixed at- and ct-axis oriented regions. The mixed at- and ct-axis oriented regions show high surface roughness due to the rectangular crystallites protruding out of the surface, whereas the ct-axis oriented regions show a smooth surface morphology. In the mixed at- and ct-axis oriented regions, the BBiT films exhibit saturated ferroelectric hysteresis loops with remnant polarization Pr of 2 μC/cm2 and coercive field Ec of 60 kV/cm and no polarization fatigue up to 108 cycles. The regions having ct-axis orientation with a smooth surface morphology exhibit a linear P–E curve. The results show that the ferroelectric properties of a planar capacitor consisting of BBiT depend on the crystalline orientation of the film.

Structure - Property Relationships of Thin Films of Epitaxial Ferroelectric Bismuth-Layered Perovskites with Even and Odd Aurivillius' Parameters

AbstractThin films of bismuth-layered perovskites with the generic formula (Bi2O2)2+(An- 1BnO3n+1)2- with an even Aurivillius' parameter n like SrBi2Ta2O9 (n = 2) and BaBi4Ti4O15 (n= 4) and with an odd Aurivillius' parameter like Bi4Ti3O12 (n= 3) and Ba2Bi4Ti5O18 (n=5) have been grown by pulsed laser deposition on epitaxial conducting LaNiO3 electrodes on single crystalline (100) SrTiO3 or on top of epitaxial buffer layers on (100) silicon. The films are shown to grow epitaxially, however, they do not have a single crystallographic orientation but several preferred orientations. The films consist of both c-axis-oriented regions and mixed (110)-, (100)- and (001)-oriented regions. The regions with mixed orientation feature rectangular as well as equiaxed crystalline grains embedded in a c-oriented matrix, protruding out of its smooth surface. Macroscopic as well as microscopic measurements of the ferroelectric properties of regions with pure c-orientation and of regions with mixed orientations showed a clear relationship between their ferroelectric properties and their morphology and crystallographic orientation. In the regions with mixed orientation, the films exhibited saturated ferroelectric hysteresis loops with well-defined remanent polarization Pr and coercive field Ec. The regions having c-axis orientation with a smooth surface morphology, in contrast, exhibited a linear P-E curve with no hysteretic behavior for SrBi2Ta2O9 and BaBi4Ti4O15, and weak ferroelectric behavior for Bi4Ti3O12. These results might have a technological impact due to the relevance of bismuth-layered ferroelectric oxides for the fabrication of non-volatile FeRAM memories.

Microstructures and Electrical Properties of (Pb, La)TiO3 Thin Films Grown on the Pt Electrodes with a Percolating Network Structure

We have investigated the microstructures and electrical properties of Pb0.95La0.05TiO3 (PLT) thin films deposited on the (100)-oriented Pt/(100)MgO substrates by rf magnetron sputtering and have fabricated a thin film infrared sensor. The (100)-oriented Pt film is formed via a coalescence of Pt islands and the Pt layer goes through a transition from nonconducting islands to a conductive percolating network as the Pt deposition time increases. The highly c-axis oriented PLT thin film has been successfully grown on the Pt bottom electrode with a network structure. The PLT thin film on the interconnected percolating Pt network exhibits a well saturated ferroelectric hysteresis loop with the remanent polarization of 1.6 µ C/cm2 and the coercive field of 70 kV/cm. The responsivity and detectivity of the thin film infrared sensor are 700 V/W and 6×108 cm√ Hz/W at 10 Hz, respectively, without any poling treatment.