Electrical Coercivity Research Articles

Improvement in the dielectric, magnetic, ferroelectric, and magnetoelectric coupling attributes of BaTiO3/CoNb0.02Fe1.98O4 composite systems

The search for new materials and compositions, in which the phenomena of ferrimagnetism and ferroelectricity are closely coupled, is of great technological interest. In the present study, composite systems of (BaTiO3)(100-x)%/(CoNb0·02Fe1·98O4)x% where x = 0, 2, 5, 10, 20, and 100% were produced. Initially, BaTiO3 (BTO) was made by sol-gel combustion approach and CoFe1·98Nb0·02O4 (CoFeNbO) was made by hydrothermal method. Both compounds were then combined via solid-state reaction route to form the composite systems. According to X-ray diffraction, the pristine BTO and CoNbFeO display tetragonal and cubic structures, respectively, and all composite systems consisted of the cubic phase for BTO and spinel phase for CoNbFeO. Scanning electron microscopy showed a decrease in grain size (from 0.552 μm to 0.194 μm) and porosity (from 18.65% to 4.57%) as the CoNbFeO content increases from x = 0 to x = 20%, respectively. The influence of magnetic CoNbFeO content on the magnetic, ferroelectric, magnetoelectric coefficient, electric, and dielectric properties of composite systems has been investigated. The ferromagnetic and ferroelectric natures of diverse composite systems are reflected via the registered M-H and P-E loops at ambient temperature. A steady increase in magnetic parameters such as saturation magnetization and remanent magnetization is observed as the spinel ferrite CoNbFeO content increases. Similarly, the values of the electric coercivity increased from 1.65 kV/cm for x = 0% to 3.99 kV/cm for x = 20%. The frequency-dependent dielectric and electrical measurements were also investigated at various temperatures. By raising the measurement temperature, the dielectric polarization was found to increase for composite systems with x > 2%. Interestingly, the tangent loss was reduced for different composite systems in comparison to pristine BTO and CoNbFeO materials. The lowest tangent loss values were noticed for x = 5% composite system. All composite systems revealed good magnetoelectric coupling with coupling coefficient magnitudes up to a maximum value of ∼22.5 mV/cm.Oe, indicating that they can be potential candidates for use in advanced multifunctional electric devices.

Ferroelectric perovskite‐type films with robust in‐plane polarization toward efficient room-temperature chemiresistive sensing

Ferroelectric materials have become key components for versatile device applications, and their thin films are highly desirable for integrating the miniaturized devices. Despite substantial endeavors, it is still challenging to achieve effective chemiresistive sensing in the ferroelectric films. Here, for the first time, we have exploited ferroelectric thin films of 2D hybrid perovskite BA2EA2Pb3I10 (1), to fabricate the high-performance chemiresistor gas sensors. The spin-coated films of 1 exhibit high orientation and good crystallinity, thus preserving robust in-plane spontaneous polarization (Ps ∼2.0 μC/cm2) and low electric coercivity. Notably, such ferroelectric film-based sensors after electric poling enable the dramatic room-temperature sensing responses to NO2 gas, including high sensitivity (0.05 ppm−1), extremely low detection limit (1 ppm) and fast responding rate (∼6 s). Besides, the chemiresistive responses are remarkably enhanced by threefold (up to 320%) through electric poling. It is proposed that this behavior closely involves with strong in-plane ferroelectric polarization of 1 that generates a built-in electric field inhibiting the recombination of charge carriers. As far as we know, this ferroelectric-based film chemiresisor is one of the best room-temperature sensors for NO2 gas among all the existing candidate materials. These findings highlight great potential of ferroelectrics toward effective chemiresistive performances, and also establish a bright direction to explore their future device applications.

Investigation of multiferroic behaviour at room temperature in Bi-induced orthoferrite: combined experimental and first principles studies

Perovskite oxides are an important and exciting class of materials owing to the structural diversity with remarkable magnetic, optical and electronic behaviours. Here, a single-phase multiferroic orthoferrite, Y0.74Bi0.25FeO2.97 (YBFO), has been synthesized by a conventional solid-state method. The crystal structure was refined using the Rietveld refinement method from powder X-ray diffraction data, and confirms an orthorhombic structure with the Pnma space group. The Curie-Weiss fitting to the magnetic susceptibility shows that the θ-value is –926 K and μeff = 5.59 µB, exhibiting canted antiferromagnetism. Room temperature magnetic and ferroelectric hysteresis loops verify the presence of magnetic as well as ferroelectric ordering in the material with a remnant polarization (Pr) of 0.27 μC cm–2 and electric coercivity (Ec) of 15 kV cm–1. Temperature-dependent P–E loops and pyrocurrent response confirm the inherent multiferroic nature of the material. The UV–visible study indicates a semiconductor nature with the bandgap of 1.86 eV. Density-functional theory calculations support the observed experimental behaviour and it signified the role of Bi-substitution. Even a very less concentration of Bi-doping can form an exciting room temperature multiferroic material.

Structural Analysis and Magnetoelectric Sensing in Cobalt Ferrite–BaTiO3 Composites

The fabrication of ferromagnetic–ferroelectric composites is of pronounced importance in science and technology. The efficiency with which magnetostrictive and piezoelectric strain gets transferred among each other decides the extent of coupling between polarization and magnetization. In this study, the solid-state reaction method was used to synthesize CFO-BT composites. X- ray diffraction patterns of CFO-BT composites revealed phase purity of constituent phases, i.e., spinel and tetragonal structures for CFO and BT, respectively. The enhancement in electrical polarization was observed in the CFO-BT composite compared with CFO. The electrical coercivity improved radically from 9 to 5 kV/cm, confirming the larger grain size and its correlation with domains switching. The magnetoelectric coupling was achieved in CFO-40%BT composite by studying the change in polarization with magnetic field which revealed a remarkable change in the maximum polarization from 3.5 to 0.5 μC/cm2 when the applied field changed from 0 to 5000 Gauss.

Anomalous dielectric behavior and correlation of barrier hopping mechanism with ferroelectricity in solvent assisted phase pure bismuth iron oxide nanoparticles

Phase pure bismuth iron oxide (BiFeO3) nanoparticles (NPs) are synthesized using sol-gel route; acetic acid (AA) and ethylene glycol (EG) are used as solvents. Nanoparticles synthesized using AA as solvent result in amorphous behavior under as-synthesized condition. Phase pure BiFeO3 is obtained after calcination at 200 °C. Whereas, mixed BiFeO3 and Bi2Fe4O9 phases are observed after calcination at 300 °C. Nanoparticles synthesized using EG as solvent result in phase pure BiFeO3 even under as-synthesized conditions. Strengthening of phase pure BiFeO3 is observed after calcination at 300 °C. TEM result shows spherical morphology of nanoparticles with average size ˜12 nm after calcination at 300 °C using EG as solvent. Higher value of dielectric constant (i.e. ˜1050) and low tangent loss (i.e. ˜0.001), at log f = 7.3, are observed for EG based nanoparticles. Variation in conductivity with change in temperature is studied in detail using Jonscher’s power law. AC electrical conduction is associated with correlated barrier-hopping mechanism. Bismuth iron oxide nanoparticles exhibit room temperature ferroelectricity for both of the solvents used i.e. acetic acid and ethylene glycol. Low electrical coercivity and maximum polarization (Pmax˜12.22 μC/cm2) are observed for EG based nanoparticles. Correlation of barrier hopping mechanism and ferroelectricity has been observed in these nanoparticles especially prepared with EG as solvent because of the phase purity.

Illumination effects on the ferroelectric properties of zinc oxide films grown by DC-unbalanced magnetron sputtering

We study the illumination effect on the ferroelectric properties of zinc oxide (ZnO) film grown by DC-unbalanced magnetron sputtering. We focus on the P–E hysteresis response of the as-grown ZnO (ag-ZnO) and annealed-ZnO (ann-ZnO) films under dark and light conditions. The measurement of ferroelectric properties is performed by driving a positive voltage on the top-side of the films. Under the dark condition, a strong P–E response is observable on the ann-ZnO film due to the structural enhancement. The value of electrical coercivity for ferroelectric polarization is strongly related to the light illumination. The illumination treatment changed the P–E hysteresis of the ZnO films from symmetric to asymmetric. We found that higher energy illumination promotes a higher electric coercivity. These results confirmed that ferroelectric properties could be effectively tailored by tuning the energy of the light source. This interrelated electrical and optical properties is an important phenomenon to design a new light-induced non-volatile device application.

Influence of annealing treatment on electric polarization behaviour of zinc oxide films grown by low-power dc- unbalanced magnetron sputtering

We study the annealing effect of highly oriented zinc oxide (ZnO) films grown by low-power dc-unbalanced magnetron sputtering (DC-UBMS). In this study, we compare the structural and electrical properties of thermal-annealed ZnO films (ann-ZnO) and as-growth ZnO films (ag-ZnO) by using x-ray diffraction (XRD), scanning electron microscopy (SEM) and RT66A standardized ferroelectric test system. We found that the ag-ZnO films and the ann-ZnO films show a high orientation in (101) plane. SEM images indicate that annealing treatment at 600°C in nitrogen ambient promote the surface atomics arrangement and convert a non-uniform ag-ZnO surface to relatively flat ann-ZnO film surface. Also confirm that the ag- ZnO and the ann-ZnO films have strong ferroelectric characteristics, while the values of remnant polarization and polarization saturation are almost similar. The electric coercivity (Ec) of the ann-ZnO film is larger than ag-ZnO films as an indication of structural defects elimination. Our results are beneficial for high energy electric-based storage devices with less depolarized structural systems.

Simultaneous enhancements of polarization and magnetization in epitaxialPb(Zr0.52Ti0.48)O3/La0.7Sr0.3MnO3multiferroic heterostructures enabled by ultrathinCoFe2O4sandwich layers

Layered thin films of $\mathrm{Pb}(\mathrm{Z}{{\mathrm{r}}_{1}}_{\ensuremath{-}x}\mathrm{T}{\mathrm{i}}_{x}){\mathrm{O}}_{3}$ (PZT) and $\mathrm{L}{\mathrm{a}}_{x}\mathrm{S}{{\mathrm{r}}_{1}}_{\ensuremath{-}x}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) are well-known multiferroic systems that show promise for numerous applications including data storage devices and spintronics. In this work, structure-property relationships are explored in novel $\mathrm{PZT}/\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ (CFO)/LSMO heterostructures with optimized ferroic properties. High quality, epitaxial PZT/CFO/LSMO heterostructures with the thickness of the CFO layer varying from 0 to 50 nm were grown on $\mathrm{SrTi}{\mathrm{O}}_{3}$ (100) substrates using an optimized pulsed laser deposition technique. An ultrathin (10--20 nm) CFO layer was found to simultaneously improve the ferromagnetic and ferroelectric (FE) characteristics of the system through distinct mechanisms. The increase in magnetization and magnetic coercivity in the CFO-containing samples was associated with a tetragonal distortion of the CFO lattice under epitaxial strain, while perpendicular anisotropy generated by the distortion stabilized an out-of-plane orientation of the easy axis of magnetization in the thinnest CFO layers. Trapped charge at the CFO/PZT interface in PZT/CFO/LSMO induced an internal built-in field in the heterostructures, resulting in the accumulation of higher switched charges during voltage cycling and enhanced polarization in the samples over PZT/LSMO. An increase in electric coercivity was also observed in the CFO-containing heterostructures, and is discussed in terms of a dielectric/FE layered capacitor model. Above a critical thickness, \ensuremath{\sim}50 nm, the presence of a CFO layer has a negative effect on both magnetization and polarization in PZT/CFO/LSMO as compared to PZT/LSMO.

Induced magneto-electric coupling at ferroelectric/ferromagnetic interface

We have demonstrated that at room temperature the electric polarization of a ferroelectric polymer, polyvinylidene fluoride (PVDF), in an artificial multiferroic structure can be controlled by applying an external magnetic field. We found that as the applied magnetic field is changed, the switching of electric polarization for the PVDF depends on the external magnetic field. We also noticed that both the electric coercivity and polarization of the PVDF polymer display hysteretic features as the applied magnetic field is changed. We found that the thickness of the PVDF layer has an effect on the magnetoelectric coupling strength in our samples. The coupling sensitivity increases as the PVDF layer thickness increases. It is possible to control the ferroelectric properties of a PVDF film by tuning the external magnetic field.

Piezoelectric control of the magnetic anisotropy via interface strain coupling in a composite multiferroic structure

We investigate theoretically the magnetic dynamics in a ferroelectric/ferromagnetic heterostructure mediated by a strain-induced magnetoelectric interaction. The epitaxial strain involves a strong piezoelectricity and interfacial magnetoelastic coupling, the interplay of both gives rise to an electrically tunable uniaxial magnetic anisotropy. We predict an electric-field–induced magnetic switching in the plane perpendicular to the magneto-crystalline easy axis while the total magnetization remains stable. We trace this effect back to the ferroelectric control of the magnetic anisotropy. We also inspect the magnetic remanence and the electric coercivity and demonstrate a pronounced dynamic magnetic response to an electric field in a thin ferromagnetic layer.

Effect of Strain on Ferroelectric and Magnetic Behavior in Pb(Zr0.52Ti0.48)O3-Based Magnetoelectric Heterostructures

In this paper, the "sandwich" structured magnetoelectric composite films of Pb(Zr0.52Ti0.48)O3/ NiFe2O4/Pb(Zr0.52Ti0.48)O3 and Pb(Zr0.52Ti0.48)O3/CoFe2O4/Pb(Zr0.52Ti0.48)O3 are epitaxially grown on SrRuO3/SrTiO3 substrates by pulsed-laser deposition. The crystalline quality and microstructures of these heterostructures are investigated by X-ray diffraction technique. The effects of strain on the ferroelectric, magnetic and magnetoelectric coupling properties of these thin films are systematically studied. The results show that the strain effect induced by lattice mismatch between the ferroelectric/ferromagnetic layers plays an important role in the ferroelectric and magnetic properties of these composite films. Compared to the strained Pb(Zr0.52Ti0.48)O3/ CoFe2O4/Pb(Zr0.52Ti0.48)O3 heterostructure, improved ferroelectric properties with an out-of-plane polarization (2P(r)) of 34.2 microC/cm2 and electric coercivity field of 158 kV/cm are obtained in the strain-free Pb(Zr0.52Ti0.48)O3/NiFe2O4/Pb(Zr0.52Ti0.48)O3 heterostructure. The ME measurement results not only show that the strain induced by lattice mismatch has great influence on the ME behavior, but also provide an understanding of the multilayers with full control over the interface structure at the atomic-scale.

Dielectric and Ferroelectric Properties of (Cr,Nb)-Doped Lead Zirconate Titanate Ceramics

The 3 mol% Nb2O5 doped Pb(Zr0.52Ti0.48)O3 with addition of 1-3 mol% Cr2O3 prepared via the solid-state reaction technique have been investigated. The XRD shows that a sample is primarily in both tetragonal and rhombohedral phases coexist and the amount of rhombohedral phase decreases that is due to the donor (Nb) dopant reduces the number of oxygen vacancies and leads to a lower amount of rhombohedral phase. The maximum dielectric constant tend to decrease with increasing Cr doping concentration from 0.1 to 1 mol%. Further increase in Cr concentration, the maximum dielectric constant increase dramatically. The hysteresis loop measurements, the electrical coercivity and the remnant polarization do not show a systematic trend at low concentration of Cr doping. It is possibly due to the partial solubility of Cr doping. The observations clearly indicate the hardening behavior at higher concentration because of more solubility of Cr.

ELECTRICAL PROPERTIES OF Nb-DOPED Pb(Zr0.52Ti0.48)O3 CERAMICS

The Pb ( Zr 0.52 Ti 0.48) O 3 ceramics with 0.75, 1.0, 1.25 and 1.50%wt of Nb 2 O 5 addition were prepared by a conventional mixed oxide technique. It was found that the average grain size of the ceramics decreased from 6 to 1 μm with increasing doping concentration to 1.5%wt. The room temperature dielectric constant and d33 reached maximum values of 1050 and 285 pC/N, respectively, at 1%wt Nb -doping. The electrical coercivity (Ec) of Nb -doped PZT decreased, while the polarization values increased, with increasing doping concentration. Most importantly, this study shows that Nb 2 O 5-doped PZT ceramics exhibits soft piezoelectric and enhanced ferroelectric behaviors, and the optimum properties can be obtained using suitable doping concentration.

Effect of strain on the ferroelectric properties in epitaxial perovskite titanate thin films grown on ferromagnetic CoFe 2O 4 layers

A BaTiO3/CoFe2O4 heterostructure was fabricated by laser molecular beam epitaxy. The effects of strain on the microstructures, domain morphologies and ferroelectric properties of BaTiO3 thin films were studied systematically. Ferroelectric properties in a 40-nm-thick BaTiO3 film were suppressed owing to a reduced tetragonality induced by the strain between BaTiO3 and the underlying CoFe2O4. For a 100-nm-thick film, BaTiO3 exhibits improved the ferroelectric properties with an out-of-plane polarization (2Pr) of 36.4 μC cm−2 and an electric coercivity field of 104 kV cm−1. The results show that the thickness of BaTiO3 can significantly affect the interfacial strain and hence the ferroelectric properties of the BaTiO3/CoFe2O4 heterostructure.

Effect of Sintering Temperature on Phase Formation, Dielectric, Piezoelectric, and Ferroelectric Properties of Nb-Doped Pb(Zr0.52Ti0.48)O3 Ceramics

The effects of sintering temperature on physical and electrical properties of the Pb(Zr 0.52 Ti 0.48 )O 3 ceramics with addition of 1wt% Nb 2 O 5 prepared by conventional mixed oxide technique were investigated. The samples were sintered over temperature rang 1200–1300 o C. At lower sintering temperature, the dominant phase was rhombohedral while the tetragonal phase became dominant at higher temperature. The dielectric constant increased with increasing sintering temperature from 1200 to 1250°C. Further increase in temperature resulted in a drop of the dielectric constant. Similar trend was also observed for the d 33 -constant. For the ferroelectric properties, the electrical coercivity and remnant polarization decreased with increasing sintering temperature.

マイクロ湿度センサへの応用を目的としたＦｅ‐Ｂ‐Ｏ系スパッタ薄膜の作製と感湿特性

Our previous study showed that the spontaneous electric polarization and electric coercivity of Fe-B-O sputtered films were subject to drastic change by ambient humidity, suggesting that these films are attractive as constiuent materials for humidity sensors. In the studies, however, the film was maintained almost irrespective of the film composition of the sputtering target with Fe2O3 and B2O3 mixture powders, and hence, correlation between the film composition and the humidity sensing characteristics could not be studied. In this study, using the composite target with B chips that were placed uniformly on an Fe2O3 target to form the Fe-B-O films, the humidity sensing characteristics were investigated. The results strongly indicate that there exists an optimum composition of film for micro-humidity sensors using these films.