Multiferroic properties of BiFeO3/Bi4Ti3O12 double-layered thin films fabricated by chemical solution deposition

Multiferroic BiFeO₃/Bi₄Ti₃O₁₂ (BFO/BTO) double-layered film was fabricated on a Pt(111)/Ti/SiO₂/Si(100) substrate by a chemical solution deposition method. The effect of an interfacial BTO layer on electrical and magnetic properties of BFO was investigated by comparing those of pure BFO and BTO films prepared by the same condition. The X-ray diffraction result showed that no additional phase was formed in the double-layered film, except BFO and BTO phases. The remnant polarization (2P_r) of the double-layered film capacitor was 100 μC/cm² at 250 kV/cm, which is much larger than that of the pure BFO film capacitor. The magnetization-magnetic field hysteresis loop revealed weak ferromagnetic response with remnant magnetization (2M_r) of 0.4 kA/m. The values of dielectric constant and dielectric loss of the double-layered film capacitor were 240 and 0.03 at 100 kHz, respectively. Leakage current density measured from the double-layered film capacitor was 6.1 × 10^− 7 A/cm² at 50 kV/cm, which is lower than the pure BFO and BTO film capacitors.     

Structural and magnetic properties of epitaxial SnFe2O4 thin films

Epitaxial thin films of SnFe₂O₄ are deposited on sapphire substrate by ablating the sintered SnFe₂O₄ target with a KrF excimer laser (λ = 248 nm and pulsed duration of 20 ns). X-ray diffraction study reveals that SnFe₂O₄ films are epitaxial along (222) direction. The optical bandgap of SnFe₂O₄ film is estimated using transmittance vs. wavelength data and is observed to be 2.71 eV. The presence of hysteresis loop at room temperature in magnetization vs. field plot indicates the ferromagnetic behavior of the film. It is observed that the coercive field and remnant magnetization decrease with increase in temperature.     

Improvements of the ferroelectric properties of high-valence Tb-doped Bi4Ti3O12 thin film grown by sol-gel method

High-valence Tb-doped bismuth titanate (Bi_3.6Tb_0.4Ti₃O₁₂) (BTT) ferroelectric film was fabricated on Pt/TiO₂/SiO₂/Si (100) substrate by sol-gel technique. The BTT film had a polycrystalline perovskite structure with uniform and dense surface morphology. At a maximum applied electric field of 540 kV/cm, a remnant polarization of 59.8 μC/cm² and coercive field of 298 kV/cm were observed through ferroelectric measurements. The measured dielectric constant and loss of the film were 490 and 0.047 at a frequency of 1 MHz. The film showed excellent anti-fatigue characteristics with less than 2% degradation in switchable polarization after 1.0 × 10¹⁰ switching cycles. These improved ferroelectric properties may be attributed to the structural distortion and the low concentration of oxygen vacancy associated with Tb substitution for Bi.     

V-doping effects on ferroelectric properties of K0.5Bi4.5Ti4O15 thin films

V-doped K_0.5Bi_4.5Ti₄O₁₅ (K_0.5Bi_{4.5 − x/3}Ti_4 − xV_xO₁₅, KBTiV-x, x = 0.00, 0.01, 0.03, and 0.05) thin films were prepared on a Pt(111)/Ti/SiO₂/Si(100) substrate by a chemical solution deposition method. The thin films were annealed by using a rapid thermal annealing process at 750 °C for 3 min in an oxygen atmosphere. Among them, KBTiV-0.03 thin film exhibited the most outstanding electrical properties. The value of remnant polarization (2P_r) was 75 μC/cm² at an applied electric field of 366 kV/cm. The leakage current density of the thin film capacitor was 5.01 × 10⁻⁸ at 100 kV/cm, which is approximately one order of magnitude lower than that of pure K_0.5Bi_4.5Ti₄O₁₅ thin film capacitor. We found that V doping is an effective method for improving the ferroelectric properties of K_0.5Bi_4.5Ti₄O₁₅ thin film.     

Distribution of pyrochlore phase in Pb(Mg1/3Nb2/3)O3-PbTiO3 films and suppression with a Pb(Zr0.52Ti0.48)O3 interfacial layer

Thin films of the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) on Pt/Ti/SiO₂/Si (Pt/Si) substrates both with and without a Pb(Zr_0.52Ti_0.48)O₃ (PZT) interfacial layer were investigated. Perovskite and pyrochlore coexistence was observed for PMN-PT thin films without a PZT interfacial layer. Interestingly, most of the pyrochlore phase was observed in single-coated films and in the first layer of multi-coated films. The pyrochlore phase exhibited grains with an average size of about 25 nm, which is smaller than those of the perovskite phase (about 90 nm). In contrast, for PMN-PT thin films grown on a PZT interfacial layer, the formation of a pyrochlore phase at the interface between PMN-PT layers and the substrate is completely suppressed. Moreover, small grains are not observed in the films with a PZT interfacial layer. The measured polarization-electric field (P-E) hysteresis loops of PMN-PT films with and without PZT layers indicate that enhanced electrical properties can be obtained when a PZT interfacial layer is used. These enhanced properties include an increase in the value of remanent polarization P_r from 2.7 to 5.8 μC/cm² and a decrease in the coercive field E_c from 60.5 to 28.0 kV/cm.     

Nd-substituted SrBi2Ta2O9 ferroelectric thin films prepared by radio frequency magnetron sputtering

Nd-substituted SrBi₂Ta₂O₉ (SNBT) thin films are sputtered on Pt/Ta/SiO₂/Si substrates. X-ray diffraction and x-ray photoelectron spectroscopy studies indicate that Nd³⁺ is substituted into the bismuth layered perovskite structure, preferentially at the Sr²⁺ site. The annealed thin film is polycrystalline with plate/needle-like grain microstructure. Secondary ion mass spectrometry results show that elements in SNBT thin film homogeneously distribute along film depth and interfacial diffusion takes place during post annealing. The Nd substitution leads to enhanced remnant polarization (2P_r = 18 μC/cm²) and reduced coercivity (2E_c = 64 kV/cm) at 180 kV/cm measured at 25 °C. After 10¹⁰ switching cycles, around 9% remnant polarization is decreased.     

Microstructures, ferromagnetic, and ferroelectric properties in polyvinylpyrrolidone-assisted CoFe2O4/Pb(Zr0.53Ti0.47)O3 multiferroic composite thick films

CoFe₂O₄/Pb(Zr_0.53Ti_0.47)O₃ (CFO/PZT) multiferroic composite thick films with different CFO mass fractions have been prepared onto Pt/Ti/SiO₂/Si substrate by a hybrid sol–gel process and spin coating technique. Polyvinylpyrrolidone (PVP) was employed to be an assistance to the sol–gel solution for enhancing the film thickness and promising a crack-free film surface. After annealing at 650 °C in air for 1 h, phase structure, microstructure, magnetic and ferroelectric properties as well as leakage current of multiferroic thick films were investigated. X-ray diffraction indicated a deeply buried distribution of CFO particles in the PZT matrix. Scanning electronic microscopes showed crack-free surfaces and a decreasing film thickness from 7.2 μm to 6.2 μm with increasing CFO content. Furthermore, the saturated magnetization and remanent magnetization were also hence increased. In addition, mass fraction of CFO in PZT matrix was also estimated from 0.36% to 4.58% according to the relationship between M _s and magnetic content. Ferroelectric hysteresis loops revealed saturated polarization (P _s) and remanent polarization (P _r) were diluted by CFO till its mass fraction rising to 1.8%. After that, polarization was increased with further increasing CFO content. Enhanced leakage was demonstrated to be partially contributed to them. A critical content of 1.8% was hence confirmed, where ferroelectric and magnetic properties can be balanced, indicating a possible stress-transferred magnetoelectric coupling effect in this composite.     

Enhancement on effective piezoelectric coefficient of Bi3.25Eu0.75Ti3O12 ferroelectric thin films under moderate annealing temperature

Bi_3.25Eu_0.75Ti₃O₁₂ (BET) thin films were deposited on Pt/Ti/SiO₂/Si(111) substrates by a metal-organic decomposition method. The effects of annealing temperatures 600-800 °C on microstructure, ferroelectric, dielectric and piezoelectric properties of BET thin films were studied in detail. The spontaneous polarization (87.4 × 10^− 6 C/cm² under 300 kV/cm), remnant polarization (65.7 × 10^− 6 C/cm² under 300 kV/cm), the dielectric constant (992.9 at 100 kHz) and the effective piezoelectric coefficient d₃₃ (67.3 pm/V under 260 kV/cm) of BET thin film annealed at 700 °C are better than those of the others. The mechanisms concerning the dependence of the enhancement d₃₃ are discussed according to the phenomenological equation, and the improved piezoelectric performance could make the BET thin film a promising candidate for piezoelectric thin film devices.     

Enhancement on effective piezoelectric coefficient d33 of Bi3.15Dy0.85Ti3O12 ferroelectric thin films

Effects of annealing temperature (600-800 °C) on microstructure, ferroelectric and piezoelectric properties of Bi_3.15Dy_0.85Ti₃O₁₂ (BDT) thin films prepared by metal-organic decomposition were studied. The remnant polarization 2P_r and spontaneous polarization 2P_s (16.2 µC/cm² and 23.3 µC/cm² under 690 kV/cm), effective piezoelectric coefficient d₃₃ (63 pm/V under the bipolar driving field of 310 kV/cm) of BDT thin film annealed at 700 °C are better than those of others. The higher 2P_s and relatively permittivity ε_r induced by moderate annealing temperature should be responsible for the enhancement of piezoelectric properties. The improved d₃₃ may make BDT a promising candidate for piezoelectric thin film devices.     

Effect of Na/K excess on the electrical properties of Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3 thin films prepared by sol-gel processing

(Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) thin films derived from different amounts of Na/K excess content were fabricated via an aqueous sol-gel method on a Pt(111)/Ti/SiO₂/Si substrate, and the effect of Na/K excess content on the microstructure and electrical properties of the NKBT thin films was investigated. A second phase appears when Na/K excess content is below 20 mol%. Appropriated Na/K excess can enhance the polarization and dielectric properties due to compensation of Na/K loss that occurred during heat treatment. The 20 mol% excess derived NKBT thin film exhibits the best ferroelectric and dielectric properties with a remnant polarization (Pr) of 13.6 μC/cm², and a coercive field (Ec) of 104.8 KV/cm, together with a dielectric constant of 406 and a dissipation factor of 0.064. Similar to the dielectric response change with Na/K excess content, the decreasing concentration of charged defects is the main reason resulting in the increase of the piezoelectric property. The film with a 20 mol% excess content exhibited an effective d₃₃^? of about 56 pm/V. Also, the NKBT with a 20 mol% excess content exhibits the lowest current density of 5.6 × 10^− 5 A/cm² at 10 V.     

Enhanced ferroelectric properties of Pb(Zr0.80Ti0.20)O3/PbO thin films prepared by radio frequency magnetron sputtering

The Pb(Zr_0.80Ti_0.20)O₃ (PZT) thin films with and without a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by radio frequency (rf) magnetron sputtering method. The PbO buffer layer improves the microstructure and electrical properties of the PZT thin films. High phase purity and good microstructure of the PZT thin films with a PbO buffer layer were obtained. The effect of the PbO buffer layer on the ferroelectric properties of the PZT thin films was also investigated. The PZT thin films with a PbO buffer layer possess better ferroelectric properties with higher remnant polarization (P_r = 25.6 μC/cm²), and lower coercive field (E_c = 60.5 kV/cm) than that of the films without a PbO buffer layer (P_r = 9.4 μC/cm², E_c = 101.3 kV/cm). Enhanced ferroelectric properties of the PZT thin films with a PbO buffer layer is attributed to high phase purity and good microstructure.     

Electric and magnetic properties of Pb(Zr0.52Ti0.48)O3-CoFe2O4 particle composite thin film on the SrTiO3 substrate

We deposited a thin epitaxial Pb(Zr_0.52Ti_0.48)O₃ (PZT) layer on the (0 0 1) SrTiO₃ (STO) substrate doped with Nb (0.5 wt.%), then grew composite thin film of CoFe₂O₄ (CFO) and PZT phases on it. X-ray diffraction and high resolution transmission electron microscopy showed that the PZT and CFO phases in the film had perfect epitaxial structures. CFO nanoparticles were embedded in PZT matrix randomly, which was useful to enhance the insulativity of the composite film. The composite thin film exhibited good ferromagnetic and ferroelectric properties. The dielectric constants of the composite thin film kept unchangeable in a wide bias electric field, but increased in a magnetic field, namely, magnetodielectric effect. The possible reasons for the magnetodielectric effect were discussed.     

Effect of processing on the properties of Bi3.15Nd0.85Ti3O12 thin films

Various crystallization parameters were studied during the fabrication of Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT) thin films on Pt/Ti/SiO₂/Si (100) by metal organic solution decomposition method. The effect of crystallization processes, crystallization ambients on the properties of BNdT thin films such as orientation, ferroelectric properties were examined. By adopting different fabrication processes, it is possible to get both highly c-axis oriented as well as randomly oriented thin films. Highly c-axis oriented BNdT thin film showed a large remnant polarization (2P_r) of 70 μC/cm² at an applied voltage of 10 V and exhibited a fatigue free behavior unto 2 × 10⁹ switching cycles. The improved ferroelectric properties of BNdT thin films suggest their suitability for high density ferroelectric random access memory applications.     

Growth and electrical performance of Pb(Mg1/3Nb2/3)O3-PbTiO3-Pb(Fe1/2Nb1/2)O3 single crystals

For the first time, we have grown ferroelectric single crystals Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃-Pb(Fe_1/2Nb_1/2)O₃ (PMN-PT-PFN) from the melt by the simple slow cooling process. The chemical composition of the single crystals PMN-PT-PFN (0.59/0.31/0.10) is near the morphotropic phase boundary (MPB). X-ray diffraction (XRD) was used to study phase structure of the as-grown crystals, energy dispersive X-ray spectrometer (EDS) and electron probe micro-analyzer (EPMA) were employed to confirm the chemical composition and element distribution of the as-grown crystals, respectively. The ferroelectric, dielectric and piezoelectric properties of the as-grown PMN-PT-PFN (0.59/0.31/0.10) single crystal oriented along the (0 0 1) axis were measured, which showed that the remnant polarization (P_r), coercive electric fields (E_c), the Curie temperature (T_c) and the piezoelectric coefficient (d₃₃) were 50.2 μC/cm², 13.9 kV/cm, 158 °C and about 1800 pC/N, respectively. All the results indicated that the PMN-PT-PFN (0.59/0.31/0.10) single crystals are promising for applying to field of high frequency.     

Electrical properties of Pb0.97La0.02(Zr0.95Ti0.05)O3 antiferroelectric thin films on TiO2 buffer layer

Pb_0.97La_0.02(Zr_0.95Ti_0.05)O₃ antiferroelectric thin films with thickness of 500 nm were successfully deposited on TiO₂ buffered Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) and Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates via sol-gel process. Microstructure of Pb_0.97La_0.02(Zr_0.95Ti_0.05)O₃ thin films was studied by X-ray diffraction analyses. The antiferroelectric nature of the Pb_0.97La_0.02(Zr_0.95Ti_0.05)O₃ thin films was confirmed by the double hysteresis behaviors of polarization and double buffer fly response of dielectric constant versus applied voltage at room temperature. The capacitance-voltage behaviors of the Pb_0.97La_0.02(Zr_0.95Ti_0.05)O₃ films with and without TiO₂ buffer layer were studied, as a function of temperature. The temperature dependence of dielectric constant displayed a similar behavior and the Curie temperature (T_c) was 193 °C for films on both substrates. The current caused by the polarization and depolarization of polar in the Pb_0.97La_0.02(Zr_0.95Ti_0.05)O₃ films was detected by current density-electric field measurement.     

The influence of defects on ferroelectric and pyroelectric properties of Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 single crystals

Relaxor-based ferroelectric single crystals 0.72Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (PMN-0.28PT) were grown by a modified Bridgman technique. The direct current (dc) conductivity was investigated and corresponding conduction mechanisms were discussed. V^″Pb−VO defects are dominant from 245 °C to 650 °C. The ferroelectric properties of [1 1 1]-oriented PMN-0.28PT were systematically investigated, with the coercive field (E_c) of 5.2 kV cm⁻¹ and remnant polarization (P_r) of 37.8 μC cm⁻² at room temperature. Moreover, the dielectric and pyroelectric performances of PMN-0.28PT were measured and the integrated pyroelectric performances greatly enhanced after annealing in oxygen at 500 °C for 20 h. This is due to the decrease of oxygen vacancies in the single crystals when being annealed in the oxygen-rich atmosphere. These make [1 1 1]-oriented PMN-0.28PT crystals a promising candidate for infrared detectors and thermal imagers used at room temperature.     

Properties of La-substituted Na0.5Bi4.5Ti4O15 ferroelectric thin films

Ferroelectric Na_0.5La_0.5Bi₄Ti₄O₁₅ (NaLaBTi) thin films were prepared by a chemical solution deposition method. The NaLaBTi thin films annealed at 750 °C under oxygen atmosphere were randomly oriented polycrystalline. Electrical properties of the NaLaBTi thin films were compared to Na_0.5Bi_4.5Ti₄O₁₅ thin films and better properties were observed in the NaLaBTi thin films. Remnant polarization (2P_r) and coercive electric field (2E_c) were 43 µC/cm² and 204 kV/cm at an applied electric field of 478 kV/cm, respectively. Leakage current density was 1.95 × 10^− 6 A/cm² at 100 kV/cm. Dielectric constant and dielectric loss were 805 and 0.05 at 1 kHz, respectively. Switchable polarization was suppressed by 15% after 1.44 × 10¹⁰ switching cycles.     

Dependence of magnetic properties on the Fe ion in Ba-doped BiFeO3 multiferroic films

Ba-doped BiFeO₃ thin films were prepared on Pt/TiO₂/SiO₂/Si (100) substrate at various oxygen pressures by pulsed laser deposition. The influences of oxygen pressure on the crystalline structures, magnetic and electrical properties of these films were studied. Studies showed that the oxygen vacancy content and accordingly the leakage current density of the films decreased with the increase of the oxygen pressure, and the concentration of the Fe²⁺ increased with the decrease of the oxygen pressure. Strong ferromagnetism was observed in these films. The magnetization of the films increased with the decrease of the oxygen pressure and the largest remnant magnetization of 2.46 × 10⁴ A/m was obtained at an oxygen pressure of 0.7 Pa. This enhancement of the magnetization can be attributed to the increase of the Fe²⁺ concentration which may be related closely to the increase of the oxygen vacancy in the films.     

Microstructure, domain structures and ferroelectric properties in (Pb0.8,La0.1,Ca0.1)TiO3/Pb(Zr0.2,Ti0.8)O3 bilayered thin film

Ferroelectric (Pb_0.8,La_0.1,Ca_0.1)TiO₃/Pb(Zr_0.2,Ti_0.8)O₃ (PLCT/PZT) bilayered thin film was prepared on Pt(111)/Ti/SiO₂/Si(100) substrate by RF magnetron sputtering technique. Pure perovskite crystalline phase, determined by X-ray diffraction, was formed in the PLCT/PZT bilayer. The bilayered film exhibited a very dense and smooth surface morphology with a uniform grain size distribution. The ferroelectric domain structures were investigated by a combination of vertical and lateral piezoresponse force microscopy (VPFM and LPFM, respectively). It is demonstrated by both VPFM and LPFM observations that out-of-plane and in-plane lamellar ferroelectric domains coexist in the bilayered thin film. The PLCT/PZT bilayered film possesses good ferroelectric properties with relatively high spontaneous polarization (2P_s = 82 µC/cm²) and remnant polarization (2P_r = 26.2 µC/cm²).     

The influence of annealing temperatures on the properties of Bi2VO5.5/LaNiO3/Si thin films

Bi₂VO_5.5 ferroelectric thin films were fabricated on LaNiO₃/Si(100) substrate via chemical solution deposition. Ferroelectric and dielectric properties of the thin films annealed at 500-700 °C were studied. The thin film annealed at 700 °C exhibited more favorable ferroelectric and dielectric properties than those annealed at lower temperatures. The values of remnant polarization 2P_r and coercive field E_c for the film annealed at 700 °C are 10.62 µC/cm² and 106.3 kV/cm, respectively. The leakage current of the film is about 1.92 × 10^− 8 A/cm² at 6 V. The possible mechanism of the dependence of electrical properties of the films on the annealing temperature was discussed.