Enhanced ferroelectric properties of (Zn,Ti) equivalent co-doped BiFeO3 films prepared via the sol-gel method

High-quality BiFe_1-2xZn_xTi_xO₃ (BFZTO with x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) films were successfully prepared on fluorine-doped tin oxide (FTO)/glass substrates via the sol-gel method. The influence of (Zn, Ti) equivalent co-doping on the structure, surface morphology, and ferroelectric properties of BFZTO films was investigated systematically. X-ray diffraction (XRD) and Raman spectra analysis indicate that co-doping results in structural transformations. Scanning electron microscope (SEM) images show that BFZTO films with x = 0.02 exhibit uniform fine grains and higher density, which is instrumental for the development of ferroelectric properties. X-ray photoelectron spectroscopy (XPS) analysis reveals that BiFe_0.96Zn_0.02Ti_0.02O₃ film can inhibit the conversion of Fe³⁺ into Fe²⁺, thereby greatly reducing oxygen vacancy concentration. Therefore, under the electric field strength of 150 kV/cm, BiFe_0.96Zn_0.02Ti_0.02O₃ film was found to have the lowest leakage current density, J = 1.13 × 10^?6 A/cm², which is five orders of magnitude lower than that of pure BiFeO₃ (BFO) film. Furthermore, this film exhibits the largest remnant polarization at room temperature, P_r = 131.9 μC/cm², which is more than twice as large as that of pure BFO (P_r = 52.6 μC/cm²). Additionally, by comparing P-E hysteresis loops of different regions on the surface of BiFe_0.96Zn_0.02Ti_0.02O₃ film, it was found that the film has high uniformity and stable overall performance. Dielectric and magnetic properties were also enhanced via (Zn, Ti) co-doping.     

Structural,ferroelectric, and optical properties of Pr-NBT-xCTO relaxor ferroelectric thin films

Sol-gel method was used to prepare the Pr³⁺ ions-doped (1-x)Na_0.5Bi_0.5TiO₃-xCaTiO₃ (Pr-NBT-xCTO) (x?=?0, 0.04, 0.06, 0.08, 0.1, 0.12, and 0.16) thin films on Pt/Ti/SiO₂/Si and fused silicon substrates. The structure phase of thin films was evolving from rhombohedral (R3c) to orthorhombic (Pnma) with increasing CTO content. Owing to the morphotropic phase boundary (MPB), the improved ferroelectric and dielectric properties were obtained at x?=?0.06–0.1. The MPB was formed from the concomitant phase of rhombohedral (R3c) and orthorhombic (Pnma). The Pr-NBT-0.08CTO thin film showed the best ferroelectric and dielectric properties, as well as strong relaxor behavior (the diffusion factor is γ?=?1.79). In addition, all the films exhibited strong red emission as excited by UV light, and wide optical band-gap (3.44–3.47?eV), which might be influenced by grain size and structural variation. Our results indicate that Pr-NBT-xCTO thin films may have potential applications in ferroelectric-luminescence multifunctional optoelectronic devices.     

Influence of Eu and Sr co-substitution on multiferroic properties of BiFeO3

Pure BiFeO₃ (BFO), and Eu-Sr co-substituted BFO samples were prepared by a sol–gel method. The effects of Eu and Sr codoped on the structural, morphological, magnetic and ferroelectric properties were systematically investigated. The X-ray diffraction and Fourier transform infrared spectroscopy reveal that substitution of Eu and Sr at the Bi site results in structural change and single phase formation. The maximum remnant magnetization of 0.287 emu/g and coercive field of 10.305 kOe are observed in the Bi_0.85Eu_0.05Sr_0.10FeO₃ sample. The suppression of spin cycloid caused from the structural distortion can play an important role in the improvement of magnetic properties. The Eu and Sr co-doped samples also exhibit good ferroelectric properties, which may be attributed to suppressing the formation of oxygen vacancies by Eu substitution.     

Dielectric and ferroelectric properties of Ho-doped BiFeO3 nanopowders across the structural phase transition

We have studied Ho-doped BiFeO₃ nanopowders (Bi_1−xHo_xFeO₃, x = 0–0.15), prepared via sol-gel method, in order to analyse the effect of substitution-driven structural transition on dielectric and ferroelectric properties of bismuth ferrite. X-ray diffraction and Raman study demonstrated that an increased Ho concentration (x ≥ 0.1) has induced gradual phase transition from rhombohedral to orthorhombic phase. The frequency dependent permittivity of Bi_1−xHo_xFeO₃ nanopowders was analysed within a model which incorporates Debye-like dielectric response and dc and ac conductivity contributions based on universal dielectric response. It was shown that influence of leakage current and grain boundary/interface effects on dielectric and ferroelectric properties was substantially reduced in biphasic Bi_1−xHo_xFeO₃ (x > 0.1) samples. The electrical performance of Bi_0.85Ho_0.15FeO₃ sample, for which orthorhombic phase prevailed, was significantly improved and Bi_0.85Ho_0.15FeO₃ has sustained strong applied electric fields (up to 100 kV/cm) without breakdown. Under strong external fields, the polarization exhibited strong frequency dependence. The low-frequency remnant polarization and coercive field of Bi_0.85Ho_0.15FeO₃ were significantly enhanced. It was proposed that defect dipolar polarization substantially contributed to the intrinsic polarization of Bi_0.85Ho_0.15FeO₃ under strong electric fields at low frequencies.     

Effect of Nd and Nb co-doping on the structural,magnetic and optical properties of multiferroic BiFeO3 nanoparticles prepared by sol-gel method

BiFeO₃, BiFe_0.95Nb_0.05O₃ and Bi_1−xNd_xFe_0.95Nb_0.05O₃ (x=0.0, 0.05, 0.10, 0.15 and 0.20) nanoparticles are successfully synthesized via a tartaric acid-assisted sol-gel technique for the first time. The effect of Nd and Nb co-doping on the structure, morphology, and magnetic and optical properties is investigated. X-ray diffraction (XRD) and Raman measurements demonstrate that Nd³⁺ ions and Nb⁵⁺ ions co-doping at A and B-sites of BiFeO₃ can result in a structural transformation (from single rhombohedral phase (R3c) to two coexisting phases (R3c and Pbam)). The morphology of the nanoparticles seems to be approximately cubic, and by the co-doping, the size of the nanoparticles decreases from ~132 to ~35 nm. Notable improvement in the remanent magnetization of the sample with x=0.15 is observed with a value of 0.285 emu/g, being 20 times higher than that of the undoped sample (BiFeO₃). A decrease in the optical band gap is also observed in the Nd and Nb co-doped nanoparticles, indicating their favorable potential in photocatalytic applications.     

Effect of La and Ni substitution on structure,dielectric and ferroelectric properties of BiFeO3 ceramics

In this communication, we present the results on Bi_1−xLa_xFe_1−yNi_yO₃ (x=0.0, 0.1; y=0.0, 0.05) samples processed by solid-state reaction route in order to study crystalline and electronic structure, dielectric and ferroelectric properties. The best refinement was achieved by choosing rhombohedral structure (R3c) for BiFeO₃ and Bi_0.9La_0.1FeO₃ samples. Whereas, the XRD pattern of BiFe_0.95Ni_0.05O₃ and Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ samples were refined by choosing rhombohedral (R3c) and cubic (I23) structure. Raman scattering measurement infers nine Raman active phonon modes for all the as prepared samples. The substitution of Ni ion at Fe-site in BiFeO₃ essentially changes the modes position i.e. all the modes are observed to shift to lower wave number. Dielectric constant (ε′) and dielectric loss (tan δ) as a function of frequency have been investigated and they decreases with increasing frequency of the applied alternating field and become constant at high frequencies. This feature is a characteristic of Maxwell Wagner type of interfacial polarization. The remnant polarization (P_r) for Bi_0.9La_0.1FeO_3, BiFe_0.95Ni_0.05O₃, and Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ are 0.08, 0.11, 0.69 μC/cm², respectively and the value of coercive field for Bi_0.9La_0.1FeO_3, BiFe_0.95Ni_0.05O₃, and Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ are 0.53, 0.67, 0.68 kV/cm, respectively. X-ray absorption spectroscopy (XAS) experiments at Fe L₂,₃ and O K-edges are performed to investigate the electronic structure of well-characterized Bi_1−xLa_xFe_1−yNi_yO₃ (x=0.0, 0.1; y=0.0, 0.05) samples. The presence of reasonable ferroelectric polarization at room temperature in Bi_0.9La_0.1Fe_0.95Ni_0.05O₃ ceramics makes it suitable for technological applications.     

Effect of Zn and Ti Co-doping on structure and electrical properties of BiFeO3 ceramics

In this paper, the effect of Zn and Ti co-doping on the structure, dielectric, ferroelectric and magnetic properties of BiFeO₃ (BFO) ceramics have been studied. Co-doped BFO ceramics with different doping concentrations are prepared by using the sol-gel method. The structure, morphology and electrical properties of co-doped BFO ceramics are studied by using X-ray diffraction (XRD), scanning electron microscopy, impedance analyzer and ferroelectric test analysis system. Furthermore, the vibrating sample magnetometer (VSM) is used to investigate the magnetic properties. The structural transition from rhombohedral to tetragonal-like phase is observed with a high level of co-doping (x = 2.5%). In addition, higher content of Zn and Ti results in enhanced dielectric constant (ε) and reduced dielectric loss (tanδ). The highest remnant polarization of 0.4 μC/cm² and coercive electric field of 15.5?kV/cm is demonstrated at x = 2.5%. The enhanced dielectric and ferroelectric properties can be attributed to the formation of defect complexes due to the Zn and Ti substitution.     

Electric field dependence of ferroelectric stability in BiFeO3 thin films co-doped with Er and Mn

Bi_0.9Er_0.1Fe_1−xMn_xO₃ (BEFM_xO, x = 0.00–0.03) films are synthesized by a sol–gel technique. The BEFO film exhibits a conduction mechanism based on electron tunneling. The high applied electric field causes dissociation of the defect complex, and the resulting oxygen vacancies contribute to fake polarization. Consequently, the BEFO film has poor polarization stability at high applied electric fields. Coexistence of two phases (with space groups R3c:H and R3m:R) and reduced concentrations of oxygen vacancies and Fe²⁺ in BEFM_xO are achieved by co-doping with Er and Mn. The presence of bulk-based conduction in the BEFM_xO films then leads to ferroelectric domain switching contributing to the real polarization and to excellent ferroelectric stability. In addition, the BEFM_0.02O film shows a typical symmetrical butterfly curve, the highest remnant polarization of ~109 μC/cm², and the highest switching current of ~1.66 mA. It also has the smallest oxygen vacancy concentration and thus the smallest amount of defect complex, which means that there are fewer pinning effects on ferroelectric domains and therefore excellent ferroelectric stability. This excellent ferroelectric stability makes the BEFM_xO films obtain good stability and reliability in the application of ferroelectric memory devices.     

Multiferroic properties of La/Er/Mn/Co multi-doped BiFeO3 thin films

Bi_0.9-xLa_xEr_0.1Fe_0.96Co_0.02Mn_0.02O₃ (BLa_xEFMCO) thin films were prepared by sol-gel method. The grain size, grain boundary resistance, oxygen vacancies and the amount of Fe²⁺ of the films were reduced by multi-ion doping to reduce the built-in electric field of the films. An applied voltage was adopted to regulate the effects of the directional alignment of the oxygen vacancies, defects, and defect pairs on the ferroelectric domains at the grain boundaries to control the ferroelectric polarization of the films. Meanwhile, the capacitance peak also reveals the effects of the ferroelectric domains switching, the migration of oxygen vacancies, and the directional alignment of defect pairs on the ferroelectric properties. In addition, the remnant polarization value of the BLa_0.01EFMCO thin film reaches 152?μC/cm², the squareness of the hysteresis loop (R_sq) is calculated to be 1.03, and the maximum switching current is 1.50?mA. The typical butterfly curves under positive and negative electric fields indicate the films with the enhanced ferroelectric properties. Moreover, the BLa_0.01EFMCO thin film exhibits the enhanced ferromagnetic properties, and its saturation magnetization (M_s) is 2.32 emu/cm³. Therefore, the ferroelectric properties of the BFO film can be enhanced by the multi-ion doped BFO film to reduce the grain boundary resistance (Rgb), the interface Schottky barrier formed by the asymmetric electrode material at the top and bottom of the film, and the built-in electric field formed by the film internal defect or defect pairs.     

Energy storage performance of (K,Na)NbO3 ferroelectric thin films with Mn-Ta and Mn-Ti co-doping

The lead-free KNN (K_0.5Na_0.5NbO₃) films were co-doped with Mn-Ta and Mn-Ti via the Sol-Gel method. With Mn doping, the leakage behavior in the KNN films is significantly weakened. Annealed at 700 °C, the KNNM film shows good crystallinity, dense morphology and better energy storage performance. With Mn-Ta and Mn-Ti co-doping, the KNNM4R (K_0.5Na_0.5Nb_0.96-xMn_0.04R_xO₃, R = Ta, Ti) films still show tetragonal phase and the smoothest surface morphology at 6mol% Ta or Ti content, with about 200 nm in thickness. Under the breakdown field strength of 2700 kV/cm and 2800 kV/cm, the (P_m-P_r) of Ta6 and Ti6 films are 20.7 μC/cm² and 22.4 μC/cm², the W_rec and η are up to 20.7 J/cm³, 23.9 J/cm³ and 57%, respectively.     

Modified electrical properties of chemical solution deposited epitaxial BiFeO3 thin films by Mn substitution

The effect of Mn substitution on microstructure and electrical properties of epitaxial BiFeO₃ (BFO) thin films grown by an all-solution approach was investigated. Raman analysis reveals that the Mn atoms substitution at Fe sites can result in Jahn-Teller distortion and thus lead to the weakness of long-range ferroelectric order. In addition, the break-down characteristics of BFO thin films are improved with the increase of Mn atoms content, although the leakage current is gradually increased. Meanwhile, the grain size, the dielectric constant and loss are also increased with the increase of Mn content. The P-E hysteresis loops and PUND results demonstrate that the intrinsic ferroelectric polarization is effectively improved with Mn atoms substitution as the grain size increased and Mn atoms play a role of nucleation sites. However, the ferroelectric properties are deteriorated with the excess substituted Mn content due to the higher leakage current.     

Structure and electrical properties of Zn-doped BiFeO3 films

BiFe_1−xZn_xO₃ (x = 0, 0.5, 1, 1.5, 2 mol%) (BFZO) films were prepared on ITO/glass substrates by a sol-gel method. The effects of different Zn contents on the structures and electrical properties of the BFZO films were investigated. From X-ray diffraction (XRD), microstructure and X-ray spectroscopy (XPS) results, the BFZO films with a Zn content of 1 mol% showed a better crystal structure and grain development, and the Fe²⁺ and oxygen vacancy concentrations in this sample were the lowest among all the evaluated BFZO films. The P-E hysteresis loop indicated that the BFZO films with 1 mol% Zn had the highest remanent polarization (2Pr), which was 82.4 μC/cm², along with a coercive field (2E_c) of 887 kV/cm at the tested electric field of 857 kV/cm. The BFZO film with 1 mol% Zn had the lowest leakage current density, which was 3.54 × 10⁻⁷ A/cm² at the tested electric field of 200 kV/cm. Both at high and low electric fields, the space charge-limited current (SCLC) conduction mechanism was the main leakage mechanism. When the test frequency was 10⁵ Hz, the dielectric constant was 133, and the dissipation factor was 0.015.     

Enhanced dielectric and energy-storage properties in BiFeO3-modified Bi0.5(Na0.8K0.2)0.5TiO3 thin films

Lead free Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ thin films doped with BiFeO₃ (abbreviated as BNKT-xBFO) (x = 0, 0.02, 0.04, 0.08, 0.10) were deposited on Pt(111)/Ti/SiO₂/Si substrates by sol-gel/spin coating technique and the effects of BiFeO₃ content on the crystal structure and electrical properties were investigated in detail. The results showed that all the BNKT-xBFO thin films exhibited a single perovskite phase structure and high-dense surface. Reduced leakage current density, enhanced dielectric and ferroelectric properties were achieved at the optimal composition of BNKT-0.10BFO thin films, with a leakage current density, dielectric constant, dielectric loss and maximum polarization of < 2 × 10⁻⁴ A/cm³, ~ 978^, ~ 0.028 and ~ 74.13 μC/cm² at room temperature, respectively. Moreover, the BNKT-0.10BFO thin films possessed superior energy storage properties due to their slim P-E loops and large maximum polarization, with an energy storage density of 22.12 J/cm³ and an energy conversion efficiency of 60.85% under a relatively low electric field of 1200 kV/cm. Furthermore, the first half period of the BNKT-0.10BFO thin film capacitor was about 0.15 μs, during which most charges and energy were released. The large recoverable energy density and the fast discharge process indicated the potential application of the BNKT-0.10BFO thin films in electrostatic capacitors and embedded devices.     

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.     

Effect of precursor pH value on the structure and electrical properties of Bi1.5Zn1.0Nb1.5O7 thin films

Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thin films are synthesized on a FTO (Fluorine -doped SnO₂ transparent conductive glass) substrate by an improved sol-gel method. The pH of the precursor is controlled by citric acid and ammonia, and the effect of pH on the crystal structure and electrical properties of the BZN films is studied. When the precursor pH is 5, the relative intensity of the diffraction peak of BZN films is the highest, the FWHM is the narrowest, and the crystallization degree is the highest, compared to those of BZN films synthesized from precursors at other pH, the crystallization energy offset of the element binding energy of the thin films is the smallest and the phase difference of the binding energy is the closest to the theoretical value, and the films have excellent electrical properties with permittivity of 120, loss tangent of 0.003, tunability of 14.8%, breakdown voltage of 0.43 MV/cm and energy storage density of 9.82 × 10⁻² J/cm³.     

Tunable multiferroic properties of Mn substituted BiFeO3 thin films

The current study explored the influence of Mn substitution on the electrical and magnetic properties of BiFeO₃ (BFO) thin films synthesized using low cost chemical solution deposition technique. X-ray diffraction analysis revealed that pure rhombohedral phase of BiFeO₃ was transformed to the tetragonal structure with P4mm symmetry on Mn substitution. A leakage current density of 5.7×10⁻⁴ A/cm² which is about two orders of magnitude lower than pure BFO was observed in 3% Mn doped BFO thin film at an external electric field >400 kV/cm. A well saturated (p-E) loops with saturation polarization (P_sat) and remanent polarization (2P_r) as high as 60.34 µC/cm² and 25.06 µC/cm² were observed in 10% Mn substituted BFO thin films. An escalation in dielectric tunability (n_r), figure of merit (K) and quality factor (Q) were observed in suitable Mn doped BFO thin films. The magnetic measurement revealed that Mn substituted BFO thin films showed a large saturation magnetization compared to pure BFO thin film. The highest saturation ~31 emu/cc was observed for 3% Mn substituted BFO thin films.     

Processing of highly oriented (K,Na)NbO3 thin films using a tailored metal-alkoxide precursor solution

Highly oriented lead-free K_0.5Na_0.5NbO₃ (KNN) thin films were synthesized by chemical solution deposition. The analysis of the KNN precursor solution revealed that the KNN precursor consists of complex metal alkoxides of potassium and sodium hexaalkoxy niobates with highly symmetric Nb-O octahedra. KNN precursor films were crystallized in a perovskite phase with a (1 0 0) preferred orientation on Pt(1 0 0)/MgO(1 0 0) substrates at 650 °C. The three-dimensional relationship between KNN(1 0 0) and Pt(1 0 0)/MgO(1 0 0) is confirmed by the fourfold symmetry of the pole figure. The fourfold symmetry indicates that the synthesized films are oriented in both the c and a, b directions on the Pt(1 0 0) surface. Although the insulating resistance was not sufficiently high at room temperature, the (1 0 0)-oriented KNN thin films showed potentially larger polarizations compared to the KNN thin films with no preferred orientation at low temperatures. The 2P_r and 2E_c of the oriented KNN films at −190 °C were 41.0 μC/cm² and 90 kV/cm, respectively.     

Chemical and interfacial design in the visible-light-absorbing ferroelectric thin films

Ferroelectric thin films with switchable polarization and anomalous photoelectric effects have received extensive attention recently. However, the improvement of photoelectric performance is accompanied by the weakening of ferroelectricity. Here, both chemical and interlayer design are used to regulate the polarization and optical properties of BiFeO₃-based ferroelectric films. We achieved an improvement in both ferroelectricity and bandgap by chemical composition. The remanent polarization has been enhanced to 73.8 μC/cm² from 0.2 μC/cm², ascribed to the structural transition. The band gap of Eu-BiFeO₃ films has been reduced to 2.23 eV from 2.42 eV due to the unique energy level from Eu 4f, indicating the enhanced visible-light-absorbing capability. We have designed a "sandwich" interfacial structure of homogeneous Eu-BiFeO₃ films. A clever combination between optimal ferroelectricity and narrow band gap with near Eu contents of BFO films would generate an interfacial layer with a homogeneous gradient component, which should favor the switching of ferroelectric domains. The results show that the remanent polarization improved by 17 % to 86.2 μC/cm² while the band gap has also improved. Intriguingly, the short-circuit current density (J_sc) and open circuit (V_oc) of the photovoltaic signal of the optimal films are 89.0 nA and 0.412 V, respectively. This provides a simple and intelligent way to design the ferroelectric-photoelectric thin films and lays the foundation for optical information storage devices.     

水热法合成(Mn,Zn)Fe_2O_4磁性晶体粉末

采用水热加工技术研究（Ｍｎ，Ｚｎ）Ｆｅ２Ｏ４晶体粉末的合成条件，对水热体系的碱度、温度、时间进行了系统实验．结果表明在ｐＨ＝７～１２．５范围内可有效合成（Ｍｎ，Ｚｎ）Ｆｅ２Ｏ４．当温度在１９０～２２０℃时，完成合成所需的时间为１～２ｈ，提高碱度有利于合成反应．本文还初步探讨了（Ｍｎ，Ｚｎ）Ｆｅ２Ｏ４的合成机理，为扩大实验提供依据．     

Role of (La,Gd) co-doping on the enhanced dielectric and magnetic properties of BiFeO3 ceramics

The effect of the La³⁺ and Gd³⁺ co-doping on the structure, electric and magnetic properties of BiFeO₃ (BFO) ceramics are investigated. For the compositions (x=0 and 0≤y≤0.15) in the perovskite structured La_xGd_yBi_1−(x+y)FeO₃ system, a tiny residual phase of Bi₂Fe₄O₉ is noticed. Such a secondary phase is suppressed with the incorporation of ‘La’ content (x). The magnitude of dielectric constant (ε_r) increases progressively by increasing the ‘La’ content from x=0 to 0.15 with a remarkable decrease of dielectric loss. For x=0.15, the system La_xGd_yBi_1−(x+y)FeO₃ exhibits highest remanent magnetization (M_r) of 0.18 emu/g and coercive magnetic field (H_C) of ~1 T in the presence of external magnetic field of 9 T at 300 K. The origin of enhanced dielectric and magnetic properties of La_xGd_yBi_1−(x+y)FeO₃ and the role of doping elements, La³⁺, Gd³⁺ has been discussed.