Effects of co-substitution on the electrical properties of BiFeO3 thin films prepared by chemical solution deposition

Ferroelectric BiFeO₃ thin films with Nd-Cr (or Sm-Cr) co-substitution (denoted by BNdFCr and BSmFCr, respectively) were deposited on the Pt(2 0 0)/TiO₂/SiO₂/Si(1 0 0) substrates by a chemical solution deposition method. X-ray diffraction patterns revealed the formation of BNdFCr and BSmFCr thin films without any secondary phases. The co-substituted BNdFCr (or BSmFCr) thin films, which were annealed at 550 °C for 30 min in N₂ atmosphere, exhibited enhanced electrical properties compared to BFO thin films with the remanent polarization (2P_r) and coercive electric field (2E_c) of 196, 188 μC/cm² and 600, 570 kV/cm with the electric field of 800 kV/cm, respectively. The leakage current densities of BNdFCr and BSmFCr thin films measured at room temperature were approximately three orders of magnitude lower than that of BFO thin film, and the leakage current at room temperature of the thin films exhibited three distinctive conduction behaviors. Furthermore, the values of pulse polarizations [i.e., +(P*-P^{^}) or −(P*-P^{^})] of BNdFCr and BSmFCr thin films were reasonably unchanged up to 1.4 × 10¹⁰ switching cycles.     

Probing of local ferroelectricity in BiFeO3 thin films and (BiFeO3)m(SrTiO3)m superlattices

Ferroelectric BiFeO₃ thin films and artificial superlattices of (BiFeO₃)_m(SrTiO₃)_m (m∼1-10 unit cells) were fabricated on (0 0 1)-oriented SrTiO₃ substrates by pulsed laser ablation. The variation of leakage current and macroscopic polarization with periodicity was studied. Piezo force microscopy studies revealed the presence of large ferroelectric domains in the case of BiFeO₃ thin films while a size reduction in ferroelectric domains was observed in the case of superlattice structures. The results show that the modification of ferroelectric domains through superlattice could provide an additional control on engineering the domain wall mediated functional properties.     

Magnetic properties of Ni-substituted BiFeO3

BiFe_1−xNi_xO₃ ceramic powders with x up to 0.10 have been prepared by the sol-gel technique. The band gap of BiFeO₃ is 2.23 eV, and decreases to 2.09 eV for BiFe_0.95Ni_0.05O₃ and BiFe_0.90Ni_0.10O₃. The Mössbauer spectra show sextet at room temperature, indicating the magnetic ordering and the presence of only Fe³⁺ ions. Superparamagnetism with blocking temperature of 31 K for BiFe_0.95Ni_0.05O₃ and 100 K for BiFe_0.90Ni_0.10O₃ was observed. Enhanced magnetization at room temperature have been observed (1.0 emu/g for BiFe_0.95Ni_0.05O₃ and 2.9 emu/g for BiFe_0.90Ni_0.10O₃ under magnetic field of 10,000 Oe), which is one order larger than that of BiFeO₃ (0.1 emu/g under magnetic field of 10,000 Oe). The enhanced magnetization was attributed to the suppression of the cycloidal spin structure by Ni³⁺ substitution and the ferrimagnetic interaction between Fe³⁺ and Ni³⁺ ions.     

Magnetic field annealing effects on self-oriented BiFeO3 thin films prepared by chemical solution deposition

Self-oriented BiFeO₃ (BFO) thin films are prepared via chemical solution deposition method with magnetic field in-situ annealing process. The effects of magnetic annealing on the microstructure, magnetic and dielectric properties as well as magnetoelectric coupling effect of the BFO thin films are investigated. With increasing the annealing magnetic field, the crystallization quality, texture, grain boundary connectivity and densification of the films are enhanced, which is attributed to the improvement of connection and diffusion of components. The magnetization of the field-annealing films and dielectric constant as well as remanent polarization increases with increasing the strength of annealing magnetic field. In addition, it is observed that magnetocapacitance value of the magnetic-field-annealing BFO thin film is higher than the non-field-annealing one. Moreover the BFO thin films annealed at 3 kOe magnetic field show the magnetoelectric effect with 4% under 2 kOe at room temperature.     

Enhanced magnetization and suppressed current leakage in BiFeO3 ceramics prepared by spark plasma sintering of sol-gel derived nanoparticles

BiFeO₃ (BFO) ceramics of different grain size have been synthesized by spark plasma sintering of sol-gel derived nanoparticles. It was found that with decreasing grain size there occurs an enhancement in magnetization and a simultaneous suppression in current leakage. According to systematic materials characterization, the enhanced magnetization is attributed to the enriched grain boundaries where the missing structural order perturbs the spin helix structure of BFO and thus generates uncompensated spins, while the reduced current leakage is ascribed to fewer conduction paths provided by the compacted grain structure.     

Mn:BiFeO3/Zn:BiFeO3 bilayered thin films of (1 1 1) orientation

Ferroelectric and fatigue behavior of bilayered thin films consisting of Mn⁴⁺-modified BiFeO₃ and Zn²⁺-modified BiFeO₃, which were deposited on SrRuO₃-buffered Pt coated silicon substrates, were systematically investigated. The (1 1 1) orientation is induced for the BiFe_0.95Mn_0.05O₃/BiFe_0.95Zn_0.05O₃ bilayer, due to the introduction of the bottom BiFe_0.95Zn_0.05O₃ layer. With increasing the thickness ratio of the BiFe_0.95Mn_0.05O₃ layer, their leakage current decreases, and the fatigue endurance is greatly improved owing to the introduction of the BiFe_0.95Mn_0.05O₃ layer with a lower fatigue rate. The BiFe_0.95Mn_0.05O₃/BiFe_0.95Zn_0.05O₃ bilayer with the thickness ratio of 3:1 exhibits a larger remanent polarization of 2P_r ∼ 161.0 μC/cm² than those of bilayers with different thickness ratios, while their coercive field slightly increases with increasing the thickness ratio of the BiFe_0.95Mn_0.05O₃ layer.     

Preparation of Nd-doped BiFeO3 films and their electrical properties

The Nd-doped BiFeO₃ thin films were prepared on SnO₂(FTO) substrates spin-coated by the sol–gel method using Nd(NO₃)₃·6H₂O, Fe(NO₃)₃·9H₂O and Bi(NO₃)₃·5H₂O as raw materials. The microstructure and electric properties of the BiFeO₃ thin films were characterized and tested. The results indicate that the diffraction peak of the Nd-doped BiFeO₃ films is shifted towards right as the doping amounts are increased. The structure is transformed from the rhombohedral to pseudotetragonal phase. The crystal grain is changed from an elliptical to irregular polyhedron. Structure transition occurring in the Bi_0.85Nd_0.15FeO₃ films gives rise to the largest Pr of 64 μC/cm². The leakage conductance of the Nd doped thin films is reduced. The dielectric constant and dielectric loss of Bi_0.85Nd_0.15FeO₃ thin film at 10 kHz are 190 and 0.017 respectively.     

BiFeO3/Zn1−xMnxO bilayered thin films

BiFeO₃/Zn_1−xMn_xO (x = 0-0.08) bilayered thin films were deposited on the SrRuO₃/Pt/TiO₂/SiO₂/Si(1 0 0) substrates by radio frequency sputtering. A highly (1 1 0) orientation was induced for BiFeO₃/Zn_1−xMn_xO. BiFeO₃/Zn_1−xMn_xO thin films demonstrate diode-like and resistive hysteresis behavior. A remanent polarization in the range of 2P_r ∼ 121.0-130.6 μC/cm² was measured for BiFeO₃/Zn_1−xMn_xO. BiFeO₃/Zn_1−xMn_xO (x = 0.04) bilayer exhibits a highest M_s value of ∼15.2 emu/cm³, owing to the presence of the magnetic Zn_0.96Mn_0.04O layer with an enhanced M_s value.     

One-magnon light scattering and spin-reorientation transition in epitaxial BiFeO3 thin films

Raman scattering from one-magnon excitation has been observed for the first time in epitaxial BiFeO₃ thin films grown on (1 1 1) SrTiO₃ substrates. The intensities and the frequency of the magnon mode at 18.9 cm⁻¹ (M₁) showed a discrepancy at the characteristic temperatures of ∼140 and 200 K and the magnon mode at 27.9 cm⁻¹ (M₂) disappeared at ∼200 K suggesting spin-reorientation (SR) transition in the epitaxial BFO film. The dc susceptibility measurement showed a large discrepancy near these two temperatures evidently elucidating the spin-reorientation transition mechanism. The partial spectral weight of the magnon modes is believed to be transferred to the lowest phonon mode appearing at 72.8 cm⁻¹ and higher magnon mode M₂ disappearing near 200 K reveal magnon-phonon coupling near to SR transition.     

Soft-mode driven tetragonal-to-monoclinic phase transition in multiferroic BiFeO3

The phase transition of BiFeO₃ (BFO) from tetragonal to monoclinic induced by pressure was investigated by first-principles method. The sequential monoclinic phase, _Ma$M_{\mathrm{a}}$, which is favorable during low compression with respect to the tetragonal phase, was characterized. The order parameters were calculated in the vicinity of the phase transition, showing that phase transition has a second-order character. The results demonstrated that the pressure-induced tetragonal-to-monoclinic phase transition in BFO is related to the softening behavior of the E mode, which are very helpful in further investigations of the morphotropic phase boundary (MPB) in lead-free materials.     

Investigation on dependence of BiFeO3 dielectric property on oxygen content

The influence of oxygen content on the dielectric property of BiFeO3 ceramics is studied by experiment and firstprinciples calculation.The experimental result demonstrates that the dielectric constant of BiFeO3 is strongly dependent on introduced oxygen and oxygen vacancies.By comparison with BiFeO3,the introduced oxygen and oxygen vacancies can lead to a reduction in dielectric constant of BiFeO δ at a lower frequency.The first-principles calculation also shows a similar result when photon energy is in a range of 2.0-4.1 eV.A likely explanation is that this oxygen content dependence may be ascribed to the distortion of Fe-O octahedron structure due to oxygen vacancies or excess oxygen ions in the crystal structure of BiFeO3.     

溶胶-凝胶法制备MgO/(Ba0.8Sr0.2)TiO3多层薄膜及其介电和漏电特性研究

采用改进的溶胶-凝胶方法在LaNiO₃/Si(100)衬底上制备了MgO/(Ba_{0.8Sr0.2)TiO3多层薄膜.实验结果表明，MgO层的引入改变了(Ba0.8}Sr_0.2)TiO₃的介电特性和漏电流行为，使薄膜的漏电 流降低了3个数量级，但介电常数也有相应降低.漏电流的显著降低是由MgO子层的高阻特性 以及微量Mg向(Ba_0.8关键词： 0.8Sr_0.2)TiO₃多层薄膜')" href="#">MgO/(Ba_0.8Sr_0.2)TiO₃多层薄膜 漏电流 介电常 数     

Influence of heating rate on the crystalline properties of 0.7BiFeO3-0.3PbTiO3 thin films prepared by sol-gel process

0.7BiFeO₃-0.3PbTiO₃ (BFPT7030) thin films were deposited on SiO₂/Si substrates by sol-gel process. The influence of heating rate on the crystalline properties of BFPT7030 thin films were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). XRD patterns of the films showed that a pure perovskite phase exists in BFPT7030 films annealed by rapid thermal annealing (RTA) technique. SEM and AFM observations demonstrated that the BFPT7030 films annealed by RTA at 700 °C for 90 s with the heating rate of 1 °C s⁻¹ could show a dense, crack-free surface morphology, and the films’ grains grow better than those of the films annealed by RTA at the same temperature with other heating rates. XPS results of the films indicated that the ratio of Fe³⁺:Fe²⁺ is about 21:10 and 9:5 for the films annealed by RTA at 700 °C for 90 s with the heating rate of 1 and 20 °C s⁻¹, respectively. That means the higher the heating rate, the higher the concentration of Fe²⁺ in the BFPT7030 thin films.     

Effect of magnetic annealing on magnetization of BiFeO3 ceramics

The effect of magnetic annealing treatment on the magnetization of multiferroic BiFeO₃ was studied systematically. A series of pelletized nano-sized BiFeO₃ powders were annealed at high temperature under different magnetic fields. Typical ferromagnetic hysteresis loops were obtained at room temperature of the ceramics which were derived from ferromagnetic BiFeO₃ precursors. On the other hand, antiferromagnetic behaviors were observed in other samples synthesized from nonmagnetic precursors. The enhanced magnetic properties were ascribed to the magnetic anisotropy which was induced by the strong magnetic fields. This work indicates that the strong magnetic annealing method is an alternative approach to tuning the magnetic properties of high performance multiferroic materials with canted antiferromagnetic ordering.     

Influence of substrate temperature in BiFeO3-CoFe2O4 nanocomposites deposited on SrTiO3 (0 0 1)

BiFeO₃-CoFe₂O₄ epitaxial nanocomposites have been deposited on SrTiO₃ (0 0 1) substrates by pulsed laser deposition. We present here a study of the influence of the deposition temperature (T_S), in the 550-800 °C range, on the film composition, morphology and microstructure. Electron-probe microanalysis shows strong reduction of the Bi content in the films when increasing T_S. Films prepared at T_S=750 °C and above are virtually Bi-free. X-ray diffraction (XRD) data show that, due to the volatility of Bi, there is a progressive reduction in the amount of BiFeO₃. The deposition temperature and the concomitant presence of Fe_xO_y spurious phases in the nanocomposites grown at high temperature promote radical changes in film morphology and magnetization. It thus follows that a temperature range suitable for controlled modification of nanocomposites morphology would be extremely narrow.     

The photoemission study of NdNiO3/NdGaO3 thin films, through the metal-insulator transition

The electronic structure of thin films NdNiO₃/NdGaO₃ with various thicknesses (from 17 nm to 150 nm), have been studied by photoemission spectroscopy at 300 K and 169 K. The XPS results are consistent with the literature ab initio calculations of the NdNiO₃ electronic structure. A noticeable variation attributed to the metal-insulator (MI) transition has been found only for the films with relatively high thickness (150 nm). Furthermore, the photoemission spectra and their temperature dependence have been discussed with regard to the results of dc electrical resistivity measurements which also exhibit large thickness dependence. Finally, these new results support a possible large hetero-epitaxial effect on the thinnest sample (17 nm) which could stress the NdNiO₃ structure and consequently makes its electronic structure nearly stabilized.     

多铁性BiFeO3纳米颗粒的尺寸依赖磁性能研究

采用乙二胺四乙酸杂化溶胶法制备了不同晶粒尺寸的纯相BiFeO₃纳米颗粒,并利用X射线衍射仪、扫描电镜、超导量子干涉仪和Mossbauer 谱系统研究了其结构、形貌以及磁性能.结果表明: BiFeO₃纳米颗粒具有明显的弱铁磁性,并呈现强烈的尺寸依赖特性; 这种弱铁磁性主要源于纳米材料的尺寸限制效应,而非杂质相或Fe²⁺ 的存在所致.     

氧空位缺陷对PbTiO_3铁电薄膜漏电流的调控

基于非平衡格林函数及密度泛函理论第一性原理计算方法,计算了Fe, Al, V和Cu四种阳离子掺杂对氧空位缺陷引起的PbTiO_3铁电薄膜漏电流的调控.研究表明:Fe和Al离子掺杂将会增大由其中氧空位缺陷导致的铁电薄膜的漏电流,而Cu和V离子掺杂对该漏电流的大小具有明显抑制作用.这是因为Cu和V掺杂对氧空位缺陷有明显的钉扎作用.相比于半径更大的Cu离子,由于V的离子半径更小,且更接近于PbTiO_3铁电薄膜中Ti的离子半径,可以预言V离子更可能被掺杂进入薄膜,从而抑制氧空位缺陷引起的漏电流.研究结果对铁电薄膜器件的电学性能控制和优化有一定的理论指导意义.     

Phase transition and thermodynamic properties of BiFeO3 from first-principles calculations

The first-principles projector-augmented wave method employing the quasi-harmonic Debye model,is applied to investigate the thermodynamic properties and the phase transition between the trigonal R3c structure and the orthorhombic Pnma structure.It is found that at ambient temperature,the phase transition from the trigonal R3c phase to the orthorhombic Pnma phase is a first-order antiferromagnetic-nonmagnetic and insulator-metal transition,and occurs at 10.56 GPa,which is in good agreement with experimental data.With increasing temperature,the transition pressure decreases almost linearly.Moreover,the thermodynamic properties including Grneisen parameter,heat capacity,entropy,and the dependences of thermal expansion coefficient on temperature and pressure are also obtained.