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.     

Microstructures and multiferroic properties of textured Bi0.8La0.2FeO3 thin films

La-substituted BiFeO₃, Bi_0.8La_0.2FeO₃, thin films were fabricated on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition. X-ray diffraction and high-resolution transmission electron microscope were used to analyze the structures of the films. The results show the films fabricated under optimized growth condition are (0 1 2) textured. X-ray photoemission spectroscopy results indicate that the oxidation state of Fe ion is Fe³⁺ in the films without detectable Fe²⁺. The films show low leakage current and excellent dielectric characters. Multiferroic properties with a remnant ferroelectric polarization of 5.2 μC/cm² and a remanent magnetization of 0.02 μ_B/Fe were established. These results have some implications for further research.     

The exchange bias in polycrystalline BiFeO3/Ni81Fe19 bilayers on Si substrate with LaNiO3 buffer layer

Single phase polycrystalline BiFeO₃ thin films have been grown on Si substrates using LaNiO₃ as buffer layers by pulsed laser deposition. A transmission electron microscope shows an amorphous thin layer of LaNiO₃ followed by the polycrystalline LaNiO₃, which facilitates the crystallization of a BiFeO₃ layer in R3c structure and suppression of the impurity phases. NiFe layers were deposited on the BiFeO₃ layer by magnetron sputtering. Clear exchange coupling between BiFeO₃ and NiFe (with maximum exchange bias field up to 61 Oe) has been observed at room temperature, and the exchange bias field decreases with increasing the NiFe thickness. Our results clearly demonstrate the potential application of polycrystalline BiFeO₃ in magnetoelectric coupling based spintronics.     

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.     

Application of artificial neural networks for the rapid classification of archaeological ceramics by means of laser induced breakdown spectroscopy (LIBS)

In this work, we report on the growth and characterization of polycrystalline BiFeO₃ (BFO) thin films deposited on indium tin oxide (ITO) coated glass substrates via the chemical solution deposition technique. The as-deposited films were crystallized by heat treatment at various temperatures for 1 h in various atmospheres, i.e. oxygen, air and nitrogen. Grazing angle X-ray diffraction (GIXRD) analysis of the heat treated films revealed that both annealing temperatures as well as the atmosphere, had a pronounced effect on the formation of pure perovskite-type BiFeO₃ phase. Ferroelectric measurements showed that the films heat treated in nitrogen showed superior characteristics to those heat treated in air and oxygen with a remanent polarization of 0.52 μC/cm² and 1.9 μC/cm² for the films annealed at 600 °C and 650 °C respectively at an applied field of 60 kV/cm. BiFeO₃ films annealed in nitrogen also showed maximum dielectric constant (ε_r∼300), superior leakage resistance and magnetic properties over other films. PACS 77.80.-e; 77.84.-S; 75.50.Bb     

The electrical and magnetic properties of epitaxial orthorhombic YMnO3 thin films grown under various oxygen pressures

Orthorhombic YMnO₃ thin films were epitaxially grown on bare and LaNiO₃ buffered (0 0 1)-SrTiO₃ substrates by pulsed laser deposition under various oxygen pressures from 5 to 30 Pa. The crystal structure and microstructure of these films have been characterized by both X-ray diffractions and transmission electron microscopy. The leakage current, modeled as the space charge limited current (SCLC) mechanism, decreased significantly with the increase of oxygen content. It is further found that the magnetic property of films is greatly enhanced in YMnO₃ films grown under high oxygen pressure, which can be explained decreased oxygen vacancies. In addition, bipolar switching behavior was obtained only in the films grown under 30 Pa oxygen pressure, which is attributed to the decrease of voltage-driven oxygen vacancy migration.     

Nonvolatile bipolar resistance switching effects in multiferroic BiFeO3 thin films on LaNiO3-electrodized Si substrates

We report on reversible bipolar resistance switching effects in multiferroic BiFeO₃ thin films without electroforming. The BiFeO₃ thin films with (110) preferential orientation were prepared on LaNiO₃-electrodized Si substrates with a Pt/BiFeO₃/LaNiO₃ device configuration. The resistance ratio of high resistance state (HRS) to low resistance state (LRS) of the devices was as high as three orders of magnitude. The dominant conduction mechanisms of LRS and HRS were dominated by ohmic behavior and trap-controlled space charge limited current, respectively. The resistance switching mechanism of the devices was discussed using a modified Schottky-like barrier model taking into account the movement of oxygen vacancies.     

Effects of BaTiO<Subscript>3</Subscript> and SrTiO<Subscript>3</Subscript> as the buffer layers of epitaxial BiFeO<Subscript>3</Subscript> thin films

BiFeO₃ (BFO) thin films with BaTiO₃ (BTO) or SrTiO₃ (STO) as buffer layer were epitaxially grown on SrRuO₃-covered SrTiO₃ substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.     

Structural, optical and magnetic properties of Nd-doped BiFeO3 thin films prepared by pulsed laser deposition

Nd-doped BiFeO₃ thin films were grown by pulsed laser deposition on quartz substrate and their structural, optical and magnetic properties have been studied. X-ray diffraction analysis revealed that Nd addition caused structural distortion even with 5% of Nd concentration, additional secondary phase appeared in all samples but its intensity was greatly reduced with Nd addition. Doping-induced variations in texture and structure modifying both magnetic and optical properties of BiFeO₃ thin films. The energy band gap decreases while the refractive index increases with addition of Nd³⁺ in BiFeO₃ for Bi³⁺. These variations in energy band gap and refractive index have been explained on the basis of density of states and increase in disorders in the system. All the samples were found to exhibit ferromagnetism at room temperature and the saturation magnetization increases with the increase in structural distortion with addition of Nd. Finally, Nd-doping modifies the physical properties of BiFeO₃ in comparison to undoped BiFeO₃ thin films.     

O2掺杂对SiCOH低k薄膜结构与电学性能的影响

以十甲基环五硅氧烷(D5)和氧气(O₂)作为反应气体，采用电子回旋共振等离子体化学气相沉积(ECR-CVD)方法制备了k＝2.62的SiCOH薄膜.研究了O₂掺杂对薄膜结构与电学性能的影响.结果表明，采用O₂掺杂可以在保持较低介电常数的前提下极大地降低薄膜的漏电流，提高薄膜的绝缘性能，这与薄膜中Si-O立体鼠笼、Si-OH结构含量的提高有关. 关键词： SiCOH薄膜 2掺杂')" href="#">O₂掺杂 介电性能 键结构     

Evolution of phase purity and texture on annealing of BiFeO3 thin film prepared by sol-gel technique

We report preparation of phase pure BiFeO₃ thin films on glass, ITO and Si(100) substrates through chemical route using spin coating technique. Sol-gel process was adopted to prepare the films using bismuth nitrate and iron nitrate as precursors. X-Ray diffraction and Raman spectroscopy studies revealed amorphous nature of the as deposited films. Rhombohedral crystalline phase of BiFeO₃ evolved on annealing the films at 500°C, but with Bi₂Fe₄O₉ and Bi₂₄Fe₂O₃₉ as impurity phases. Increasing the annealing temperature to 550°C caused a drastic reduction of the impurity phases and at 600°C, the films were phase-pure BiFeO₃. Micro Raman spectra showed features consistent with the reported characteristic peaks of BiFeO₃ crystalline phase for films annealed at 500 and 550°C. Crystallite size obtained from X-ray diffraction line width analysis are within 30 to 40 nm. Atomic force microscopy (AFM) however showed grain size of ∼192 nm, indicating polycrystalline nature of the grains.      

Impact of annealing atmosphere on the multiferroic and dielectric properties of BiFeO3/Bi3.25La0.75Ti3O12 thin films

Multiferroic BiFeO₃/Bi_3.25La_0.75Ti₃O₁₂ films annealed in different atmospheres (N₂ or O₂) were prepared on Pt/Ti/SiO₂/Si substrates via a metal organic decomposition method. Based on our experimental results, it is considered that, in the films annealed in N₂, fewer Fe²⁺ ions while more oxygen vacancies are involved. As a result, at room temperature, predominated by the reduced Fe²⁺ fraction, lower leakage current and dielectric loss, better ferroelectric property while reduced magnetization are observed. However, the oxygen vacancies might be thermally activated at elevated temperature; thus, more strongly temperature-dependent leakage current and a higher dielectric relaxation peak are observed for the films annealed in N₂.     

Thickness dependent magnetic and ferroelectric properties of LaNiO3 buffered BiFeO3 thin films

BiFeO₃ (BFO) thin films with thickness increasing from 40 to 480 nm were successfully grown on LaNiO₃ (LNO) buffered Pt/Ti/SiO₂/Si(100) substrate and the effects of thickness evolution on magnetic and ferroelectric properties are investigated. The LNO buffer layer promotes the growth and crystallization of BFO thin films. Highly (100) orientation is induced for all BFO films regardless of the film thickness together with the dense microstructure. All BFO films exhibited weak ferromagnetic response at room temperature and saturation magnetization is found to decrease with increase in film thickness. Well saturated ferroelectric hysteresis loops were obtained for thicker films; however, the leakage current dominated the ferroelectric properties in thinner films. The leakage current density decreased by three orders of magnitude for 335 nm film compared to 40 nm film, giving rise to enhanced ferroelectric properties for thicker films. The mechanisms for the evolution of ferromagnetic and ferroelectric characteristics are discussed.     

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.     

The coexistence of ferroelectricity and ferromagnetism in Mn-doped BaTiO3 thin films

Due to the fault of the first author, this article entitled “The coexistence of ferroelectricity and ferromagnetism in Mn-doped BaTiO3 thin films”, published in “Chinese Physics B”, 2011,Vol.20, Issue 12, Article No. 127701, has been found to copy from the article entitled“Decisive role of oxygen vacancy in ferroelectric versus ferromagnetic Mn-doped BaTiO3 thin films”, published in “Journal of Applied Physics”, 2011,Vol.109, Issue 8, article No. 084105. So the above article in “Chinese Physics B” has been withdrawn from the publication.<     

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     

Effect of non-magnetic doping on leakage and magnetic properties of BiFeO3 thin films

In this paper, we show that the leakage current properties of BiFeO₃ (BFO) thin films have been greatly improved by Zr-doping. In contrast, the magnetic properties of Zr-doped BFO films are affected as a weak ferromagnetism. Beyond the double-exchange interactions arising from the creation of Fe²⁺, we propose another simple model considering the replacement of the magnetically active Fe³⁺, time to time, by a non-active Zr⁴⁺, which is expected to induce a local ferromagnetic coupling rather than an antiferromagnetic one.     

Effect of KNbO<Subscript>3</Subscript> modification on structural,electrical and magnetic properties of BiFeO<Subscript>3</Subscript>

The polycrystalline samples of (Bi_1?x K _x ) (Fe_1?x Nb _x ) O₃ (BKFN) for x = 0.0, 0.1, 0.2 and 0.3 were synthesized by a solid-state reaction method. The X-ray diffraction patterns of BKFN exhibit that the addition of KNbO₃ in BiFeO₃ gradually changes its structure from rhombohedral to pseudocubic. The analysis of scanning electron micrograph clearly showed that the sintered samples have well-defined and uniformly distributed grains. Addition of KNbO₃ to BiFeO₃ enhances the dielectric, ferroelectric and ferromagnetic properties of BiFeO₃. Detailed studies of impedance and related parameters of BKFN using the complex impedance spectroscopic technique exhibit the significant contributions of grain and grain boundaries in the resistive and transport properties of the materials. Some oxygen vacancies created in the ceramic samples during high-temperature processing play an important role in the conduction mechanism. The leakage current or tangent loss of BiFeO₃ is greatly reduced on addition of KNbO₃ to the parent compound BiFeO₃. Preliminary studies of ferroelectric and magnetic characteristics of the samples reveal the existence of ferroelectric, and weak ferromagnetic ordered ceramics.     

Off-stoichiometry effects on BiFeO3 thin films

The effects of Bi and Fe-excess on the structure, ferroelectric, leakage current and magnetic properties of BiFeO₃ (BFO) thin films are reported. BFO with 5% excess exhibits no change in the structure with an improvement in leakage current properties in comparison to stoichiometric BFO. Raman spectroscopy of 10% Bi excess suggests a structural change from monoclinic to rhombohedral accompanied with an improvement of resistivity and ferroelectric polarization switching. A higher Fe-excess leads to the formation of pyrochlore Bi₂Fe₄O₉ and gamma-Fe₂O₃ that cause an increase in conductivity at the macroscopic scale. The results are discussed in terms of Fe and Bi-excess effects on the defect structure of BFO.     

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.