Enhanced Magnetodielectric Effect in Graded CoFe2O4/Pb(Zr0.52Ti0.48)O3 Particulate Composite Films

Graded particulate composite films in which ferromagnetic CoFe₂O₄ nanoparticles are gradedly distributed in the ferroelectric Pb(Zr_0.52Ti_0.48)O₃ matrix along the thickness direction have been prepared on Pt/Ti/SiO₂/Si wafers via sol–gel spin‐coating method and rapid annealing process. Compared with the homogenous films, the graded ones not only exhibited magnetic anisotropy, but also showed evident increase in both ferroelectric polarization and dielectric tunability. Moreover, great enhancement of magnetodielectric effect was observed in such graded films. We elucidated the origin of enhanced magnetodielectric coupling and attributed it to the combined influence of two factors, that is, the enhanced ferroelectric polarization caused by CoFe₂O₄ distribution gradient and flexoelectric polarization induced by strain gradient under external magnetic field. This work presents a feasible way to modulate the magnetoelectric coupling in ferromagnetic–ferroelectric composite films for developing high‐performance multiferroic materials at nanoscale.     

Effect of Substrate on Structure and Multiferrocity of (La,Mn) CoSubstituted BiFeO3 Thin Films

In this article, we report the substrate effect on ferroelectric and magnetic properties of epitaxial BiFeO₃‐based thin films at room temperature. (La, Mn) cosubstituted BiFeO₃ (BFOLM) thin films were deposited on differently lattice mismatched single‐crystal substrates to manipulate the strain states in the as‐deposited films. All the films with 30‐nm thick CaRuO₃ bottom electrodes exhibited highly epitaxial growth behavior with a slightly monoclinic distorted lattice structure while their strain states are drastically different as confirmed by X‐ray reciprocal space mapping. These films possessed significantly different macroscopic ferroelectric properties with giant remanent polarization of 101 ± 2, 65 ± 2, and 48 ± 2 μC/cm² for the films grown on SrTiO₃, (La, Sr)(Al, Ta)O₃, and LaAlO₃, respectively. It is found that the room‐temperature magnetic properties are also in accordance with their strain state, having a reciprocal relationship with polarization. For example, the enhanced magnetization is associated with the suppressed polarization and vice versa. The stain tunability of multiferroic properties in BFOLM thin films are presumably ascribed to the polarization rotation and oxygen octahedral tilts.     

Electrophoretic deposition of multiferroic BiFeO3 sub-micrometric particles from stabilized suspensions

The electrophoretic deposition (EPD) was applied to BiFeO₃ (BFO) powders, one of the most interesting multiferroic compounds characterized by simultaneous magnetic and ferroelectric activity, to form homogeneous films. The preparation and characterization of stable BFO colloidal suspensions in aqueous, organic and mixed solvents were investigated by zeta potential measurements at room temperature in the presence of surfactants. BFO thin films were then deposited on steel substrates from stabilized BFO suspensions, by adjusting the preparative parameters to optimize the film quality. The compositional, morphological and electrical characteristics of the obtained BFO films, together with thickness measurements, were studied using SEM, XRD, AFM, EIS and optical surface profilometer. EPD method applied to BFO stable suspensions produced homogeneous thickness BFO films, free from pinholes and cracks, that were successively sintered and characterized also in terms of photocatalytic response.     

Solution-processed BiFeO3 thin films with low leakage current

Bismuth ferrite (BiFeO₃) is an attractive multiferroic material that shows strong ferroelectric and antiferromagnetic properties. Nevertheless, producing high-quality oriented BiFeO₃ on technology-important platinized silicon substrates by low-cost solution deposition methods is still challenging. In this work, polycrystalline Mn and Ti co-doped BiFeO₃ (BFO) thin films were fabricated on platinized silicon substrates by a solution deposition method. PbTiO₃ nanocrystals were used as a seed layer between the electrode and the BFO thin films to induce a preferential (100) pseudocubic orientation. We show that the introduction of a PbTiO₃ seed layer strongly reduces the leakage current. The films show excellent room-temperature ferroelectric properties at low frequencies (300 Hz), with epitaxial-like remanent polarization as high as 51 μC/cm² and coercive field of 500 kV/cm.     

Magnetic and electrical properties of single-phase multiferroic (1-x)Pb(Zr0.52Ti0.48)O3–xPb(Fe0.5Nb0.5)O3 thin films prepared by sol-gel route

Single-phase multiferroic (1-x)Pb(Zr_0.52Ti_0.48)O₃-xPb(Fe_0.5Nb_0.5)O₃ (0≤x≤0.5) thin films were synthesized by sol-gel route and characterized to understand their structural, electrical, and magnetic properties. The films were thermally treated by conventional furnace (CFA) and rapid thermal annealing (RTA). A pyrochlore-free perovskite phase is stabilized only by RTA in samples with high Fe³⁺/Nb⁵⁺ content. The films displayed excellent dielectric and ferroelectric properties in the whole concentration range, with saturated hysteresis loops and remanent polarization values of ～15μC/cm². Films with x>0.3 showed ferromagnetic behavior at room temperature. Consequently, the multiferroic behavior in the films occurs in a different concentration range than that observed in bulk ceramics. The origin of the weak ferromagnetism is discussed.     

Epitaxial Bi5Ti3FeO15 thin films on Nb-doped SrTiO3 substrates

We investigated the ferroelectric switching dynamics as well as the multiferroic and piezoelectric properties of highly a-oriented epitaxial Bi₅Ti₃FeO₁₅ (BTFO) thin films on Nb-doped SrTiO₃ single crystal substrates. The BTFO thin films favored highly a-oriented crystallinity because c-oriented crystallinity decreased under deposition conditions in which substrate temperature and thin film deposition rate were simultaneously lowered. The highly a-oriented epitaxial BTFO thin films showed the best ferroelectric properties, whereas the highly c-oriented epitaxial BTFO thin films showed the best ferromagnetic properties. In particular, the BTFO thin films in which a- and c-oriented crystallinity were properly mixed showed the best piezoelectric properties.     

Polarization behavior of sol‐gel‐derived relaxor Ba(Zr,Ti)O3 films

Films of the relaxor ferroelectric BaZr_0.25Ti_0.75O₃ (0.25‐BZT) were synthesized via a sol‐gel route to investigate the effect of film thickness on the dielectric properties and for comparison with normal ferroelectric BaTiO₃ (BT). The as‐prepared films on Nb‐doped SrTiO₃ (Nb–ST) displayed a (100) orientation; thinner films had stronger (100) orientations. Microwave dielectric measurements up to a few GHz quantified the polarizations, that is, the dipole contribution, ε_dipole, the combination of the ionic and electronic polarizations, ε_ionic+el., and the total contribution, ε_total. The ε_dipole in the relaxors at a film thickness of t=630 nm was 360, which was double that for the normal ferroelectric BT (ε_dipole=180) at t=735 nm. The larger apparent permittivity of the BZT therefore originated from the larger ε_dipole of the polar nanoregions (PNRs), while the nanograins of BT with few domain walls led to a comparably smaller ε_dipole. The volume ratio of the surface and film‐substrate interface lacking the dipole interactions increased with the reduction in the film thickness, leading to the significant depression in the permittivity for both specimens. The difference in the thickness dependence of the dielectric properties of the sol‐gel derived relaxor BZT and the normal ferroelectric BT films was attributed to the different origins of their dipole contribution, that is, the PNRs and ferroelectric domains, respectively.     

Enhanced switchable ferroelectric photovoltaic in BiFeO3 based films through chemical-strain-tuned polarization

Ferroelectric photovoltaic (FE-PV) materials have generated widespread attention due to their unique switchable photovoltaic behavior, but suffering from low photocurrent and remanent polarization. Herein, enhanced ferroelectric polarization and switchable photovoltaic in BiFeO₃ based thin films were achieved by the optimization of Bi content. The compact and uniform films with few defects were obtained by the control of chemical composition. The remanent polarization increased from 3.4 to 73.9 μC cm^?2 showing a qualitative leap. Intriguingly, the control range of photovoltaic signal between two polarization directions of the short-circuit current density (J_SC) and open circuit (V_OC) in present films exhibited an increase of 99.2% and 278.9%, respectively. It is suggested that the ferroelectric polarization was the main driving force for enhancing switchable ferroelectric photovoltaic. Therefore, the present work outstands a simple idea to enhance switchable ferroelectric photovoltaic based on the chemical engineering, providing a promising pathway for the development of photovoltaic devices.     

Thickness Dependence of Dielectric,Leakage, and Ferroelectric Properties of Bi6Fe2Ti3O18 Thin Films Derived by Chemical Solution Deposition

We prepared Bi₆Fe₂Ti₃O₁₈ thin films on Pt/Ti/SiO₂/Si substrates with thickness ranging from ～300 to ～900 nm by using a chemical solution deposition route and investigated the thickness effects on the microstructure, dielectric, leakage, and ferroelectric properties of Bi₆Fe₂Ti₃O₁₈ thin films. Increasing thickness improves the surface morphology, dielectric, and leakage properties of Bi₆Fe₂Ti₃O₁₈ thin films and a well‐defined ferroelectric hysteresis loops can form for the thin films with the thickness above 400 nm. Moreover, the thickness dependence of saturation polarization is insignificant, whereas the remnant polarization decreases slightly with increasing thickness and it possesses a maximal value of ～20 μC/cm² for the 500 nm‐thick thin films. The mechanisms of the thickness dependence of microstructure, dielectric, and ferroelectric properties are discussed in detail. The results will provide a guidance to optimize the ferroelectric properties in Bi₆Fe₂Ti₃O₁₈ thin films by chemical solution deposition, which is important to further explore single‐phase multiferroics in the n = 5 Aurivillius thin films.     

Synthesis and ferroelectric investigations of poly(vinylidene fluoride‐co‐hexafluoropropylene)‐Mg(NO3)2 films

The free‐standing, flexible, and ferroelectric films of poly(vinylidenefluoride‐co‐hexafluoropropylene) [P(VDF‐HFP)] were prepared by spin coating method. The ferroelectric phase of the films was enhanced by adding magnesium nitrate Mg(NO₃)₂ in different wt % as the additive during the film fabrication. The effects on the structural, compositional, morphological, ferroelectric, dielectric, and leakage current behaviors of the films due to the addition of salt were analyzed. Based on the X‐ray diffraction (XRD) patterns and Fourier Transform Infrared (FTIR) spectra, it is confirmed that the addition of Mg(NO₃)₂ promotes the electroactive β phase that induces the ferroelectric property. The fiber‐like topography of the films exhibits a nodule‐like structure, and the roughness of the films increases by the addition of Mg(NO₃)₂. The ferroelectric studies show the higher polarization values for the composite films than that of the plain P(VDF‐HFP) film. The Piezo‐response force microscope images also confirm the domain switching behavior of the samples. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44008.     

Structural and electrical properties of the sol-gel derived multiferroic BiFeO3 monolayer and NiTiO3-BiFeO3 bilayer thin films

In this paper, the multiferroic BeFiO₃ monolayer and NiTiO₃–BiFeO₃ bilayer thin films were fabricated by spin-coating method on the SrRuO₃/n⁺-Si substrate. The structural and ferroelectric properties of multiferroic BeFiO₃ monolayer and NiTiO₃–BiFeO₃ bilayer thin films were investigated. Both multiferroic films showed the typical XRD patterns of the perovskite structure without presence of the second phase. The electrical properties, such as leakage current and remnant polarization, of the NiTiO₃–BiFeO₃ bilayer film were superior to those of BeFiO₃ monolayer film, which those values were 1.94 × 10⁻⁴ A/cm² at electric field of 0.75 MV/cm and 14.05 μC/cm², respectively. This outcome is due to the NiTiO₃–BiFeO₃ bilayer film with a high Schottky barrier height as well as a top NiTiO₃ layer on the BiFeO₃ film inducing the strain-induced polarization rotation and forming the strong domain-wall pinning.     

Enhanced electrical and photocurrent characteristics of sol-gel derived Ni-doped PbTiO3 thin films

Partial substitution of group 10 metal for titanium is predicted theoretically to be one of the most effective ways to decrease the band gap of PbTiO₃-based ferroelectric photovoltaic materials. It is therefore of interest to experimentally investigate their ferroelectric and photovoltaic properties. In this work, we focus on the electrical and photocurrent properties of Ni-doped PbTiO₃ thin films prepared via a sol-gel route. The nickel incorporation does not modify the crystalline structure of PbTiO₃ thin film, but it can increase the dielectric constant, ferroelectric polarization and photocurrent, and simultaneously decrease the band gap. The maximum remnant polarization (P_r) of 58.1 μC/cm² is observed in PbTi_0.8Ni_0.2O₃ thin film, and its photocurrent density is improved to be approximately one order larger than that of PbTiO₃ thin film and simultaneously exhibits the polarization-dependent switching characteristic, which may be a promising choice for ferroelectric photovoltaic applications.     

Ferroelectric polarization controlled surface potential and charge transport across ITO-Nd-doped BiFeO3 polycrystalline ceramic bulk for photoelectrochemical solar water splitting

The spontaneous polarization in ferroelectrics provides an effective way to engineer interfacial charge distribution at the ferroelectric heterojunctions, thereby depletion width in p-n junction and energy band structures. Here, we integrated p-type multiferroic 7% neodymium-doped bismuth ferrite Bi_0.93Nd_0.07FeO₃ (BFO7Nd) polycrystalline ceramic bulk with n-type transparent conductive indium tin oxide (ITO) to demonstrate the influence of ferroelectric polarization on surface potential and its effect on photoelectrochemical (PEC) water reduction. Kelvin probe force microscopy (KPFM) results reveal the surface potential change during ferroelectric polarization switching, suggesting that the polarization bound charges may have an effect on space charge region between ITO-BFO7Nd, thus dominating surface potential at the electrode/electrolyte interface. This implies that the interface barrier between ITO and BFO7Nd can be easily modulated by polarization bound charges. The electron lifetime of upward-polarization (P-up) ITO-BFO7Nd is estimated 29.30 ms, about 15 times longer than unpoled ITO-BFO7Nd. The photocurrent density of P-up ITO-BFO7Nd is ～1.2 × 10^?6 A/cm² (at 0 V vs RHE), which is one-order higher compared to that of the unpoled and P-down states under illumination with an intensity of 100 mWcm^?2. This work shows that ferroelectric polarization combined with photogenerated carriers is an effective approach for effective charge transfer in multiferroic ceramic to boost PEC solar water splitting.     

Hexagonal YMnO3 films as promising ultraviolet photodetectors

Hexagonal rare-earth manganites (h-RMnO₃, R = Sc, Y, Ho-Lu) exhibit intricate multiferroic properties and may be utilized for various technological applications ranging from non-volatile memories to solar cells and photo-detectors. Here, we report the epitaxial growth of hexagonal YMnO₃ (h-YMO) films with different thicknesses deposited on yttria-stabilized zirconia (YSZ) substrates and their photoresponses to ultraviolet (UV) light. The structural characterizations illustrate that all the films present high quality epitaxy with out-of-plane h-YMO(0001)//YSZ(111) and in-plane h-YMO[1230]//YSZ[110] relationship. Meanwhile, UV light driven photoresponses of these as-prepared films based on the planar photodetector structure with transparent Al-doped ZnO (AZO) as top electrodes are explored. It is revealed that the photocurrent will first increase and then decrease with increasing film thickness. The highest photoconductivity gain is ~387.4 observed at +4?V bias for the device with 160?nm YMO film. These results provide a guideline for hexagonal rare-earth manganites to work for UV photodetector application.     

Rearrangement of Multiferroic BiFeO3 Nanodots Smaller than 15 nm Using Atomic Force Microscopy

In this study, we demonstrate an atomic force microscopy process for manipulating multiferroic BiFeO₃ nanodots smaller than 15 nm to desired positions on a Nb‐doped SrTiO₃ substrate. For formation of the BiFeO₃ nanodot array, nanocrystal movement was achieved using a +1.2 V biased conducting atomic force microscopy (CAFM) followed by nanocrystal attachment to the tip. Using this method, high‐density BiFeO₃ nanodot arrays with a density greater than 0.5 Tb/in.² can be achieved. Perfectly flipped ferroelectric polarization with an external electric field was observed for each BiFeO₃ nanodot, whose ferroelectric properties were confirmed using piezoelectric force microscopy.     

Influence of the Strain on Dielectric and Ferroelectric Properties of 0.5BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3

0.5BaZr_0.2Ti_0.8O₃‐0.5Ba_0.7Ca_0.3TiO₃ ceramic and its epitaxial films on (0 0 1) SrTiO₃ substrate were prepared to compare their dielectric and ferroelectric properties. The ceramic has a high dielectric permittivity, a weak dielectric relaxation, a low ferroelectric Curie temperature (T_C) of 60°C and a fast polarization relaxation. The films show much lower dielectric permittivities and mild dielectric relaxations. Furthermore, the T_C of film with 40, 100, and 200 nm thickness is 155°C, 110°C, and 60°C, respectively, because the epitaxial strain decreases with the film thickness increasing. The higher the T_C is, the more stable the room‐temperature polarization is.     

Magnetoelectric coupling effect of Ga0.8Fe1.2O3/Bi0.5(Na0.85K0.15)0.5TiO3 multiferroic composite films

Lead-free Ga_0.8Fe_1.2O₃/Bi_0.5(K_0.15Na_0.85)_0.5TiO₃ (GFO/BKNT) bilayer multiferroic composite films were fabricated on Pt(100)/Ti/SiO₂/Si substrates via sol-gel methods. The microstructure, domain structure, ferroelectric, piezoelectric, magnetic properties as well as magnetoelectric coupling effect were investigated for the composite films at room temperature. Well-defined interfaces between GFO and BKNT layers and clear electric domain structures are observed. A strong magentoelectric effect is obtained with magnetoelectric voltage coefficient of α_E=30.89 mV/cm Oe, which is attributed to excellent ferroelectric, piezoelectric, and magnetic properties, as well as the coupling interaction between ferromagnetic GFO and ferroelectric BKNT phases for lead-free bilayer composite films. Besides, GFO and BKNT demonstrate the similar perovskite structure with well lattice matching, which endows the outstanding coupling and fascinating magnetoelectric properties. The present work opens up the opportunity of lead-free magnetoelectric composite films for both further fundamental studies and practical device applications such as sensors, transducers and multistate memories.     

Enhanced photovoltaic properties of PbTiO3-based ferroelectric thin films prepared by a sol-gel process

PbTiO₃ (PTO), Pb(Mn_0.1Ti_0.9)O₃ (PMTO), Pb(Sr_0.1Ti_0.9)O₃ (PSTO), and Pb(Zr_0.1Ti_0.9)O₃ (PZTO) were prepared on an indium tin oxide (ITO)/glass substrate by a sol-gel method. PTO, PMTO, PSTO, and PZTO films exhibited energy band gaps of 3.55 eV, 3.63 eV, 3.59 eV, and 3.66 eV, respectively. All these films generated high photocurrents due to high shift currents, because carrier migration channels were successfully introduced by a lattice mismatch between the films and ITO substrates. The PMTO thin film exhibited the best ferroelectric and photovoltaic properties, with a photovoltage of 0.74 V, a photocurrent density of 70 μA/cm², and a fill factor of 43.34%, which confirms that shift current and ferroelectric polarization are two main factors that affect the ferroelectric photovoltaic properties. The PSTO, PZTO, and PTO thin films displayed space-charge-limited current (SCLC) when the electric field strength was below 10 kV/cm, and these three films broke down when the electric field strength was above 10 kV/cm. Analysis of the shift current mechanism confirmed that the breakdown of the PZTO and PSTO thin films resulted from Pool Frenkel emission current. The PMTO thin film displayed SCLC in the test range, which indicates that doping with Mn could inhibit defect formation in ferroelectric thin films.     

In situ study of electric-field-induced ferroelectric and antiferromagnetic domain switching in polycrystalline BiFeO3

Antiferromagnetic domain switching induced by ferroelectric polarization switching has previously been observed in situ in both multiferroic BiFeO₃ single crystals and thin films. Despite a number of reports on macroscopic magnetoelectric measurements on polycrystalline BiFeO₃, direct in situ observation of electric-field-induced antiferromagnetic domain switching in this material has not been addressed due to the lack of high-quality samples capable of electrical poling. Here, the electric field control of antiferromagnetic domain texture is identified in polycrystalline BiFeO₃ using in situ neutron diffraction, showing the resultant magnetic domain reorientation induced by an electric field. An antiferromagnetic domain reorientation to a value of 2.2-2.5 multiples of a random distribution (MRD) is found to be induced by an electric field that provides a non-180° ferroelectric-ferroelastic domain texture of 2.2-2.5 MRD along the field direction. The current results show well-controlled coupling of multiferroic domain texturing in single-phase polycrystalline BiFeO₃.     

Thickness‐dependent domain wall reorientation in 70/30 lead magnesium niobate‐ lead titanate thin films

Continued reduction in length scales associated with many ferroelectric film‐based technologies is contingent on retaining the functional properties as the film thickness is reduced. Epitaxial and polycrystalline lead magnesium niobate‐lead titanate (70PMN‐30PT) thin films were studied over the thickness range of 100‐350 nm for the relative contributions to property thickness dependence from interfacial and grain‐boundary low permittivity layers. Epitaxial PMN‐PT films were grown on SrRuO₃/(001)SrTiO₃, while polycrystalline films with {001}‐Lotgering factors >0.96 were grown on Pt/TiO₂/SiO₂/Si substrates via chemical solution deposition. Both film types exhibited similar relative permittivities of ~300 at high fields at all measured thicknesses with highly crystalline electrode/dielectric interfaces. These results, with the DC‐biased and temperature‐dependent dielectric characterization, suggest irreversible domain wall mobility is the major contributor to the overall dielectric response and its thickness dependence. In epitaxial films, the irreversible Rayleigh coefficients reduced 85% upon decreasing thickness from 350 to 100 nm. The temperature at which a peak in the relative permittivity is observed was the only measured small signal quantity which was more thickness‐dependent in polycrystalline than epitaxial films. This is attributed to the relaxor nature present in the films, potentially stabilized by defect concentrations, and/or chemical inhomogeneity. Finally, the effective interfacial layers are found to contribute to the measured thickness dependence in the longitudinal piezoelectric coefficient.