BiFeO3 Matrix Research Articles

Destructive Interactions between Pore Forming Agents and Matrix Phase during the Fabrication Process of Porous BiFeO3 Ceramics

Porous bismuth ferrite ceramics were synthesized by sacrificial pore former method. A mixture of BiFeO3 and 20 wt% of various pore formers including high density polyethylene, polyethylene glycol, polyvinyl alcohol, urea and graphite was intensively milled for 10 h in a planetary ball mill, uniaxially cold pressed and then subjected to the multi-stage heat treatment. The results revealed that urea and polyvinyl alcohol are appropriate candidates for maintaining the strength of the final porous structure. Density and porosity measurements showed that by employing 20 wt% of high density polyethylene and graphite, a porous sample with a maximum porosity of nearly 40% could be obtained. Mercury porosimetry results showed that using urea as a pore former gives porous bismuth ferrite with a mean pore diameter of 7 μm, uniform pore distribution as well as interconnected pores. Moreover, reactions between BiFeO3 matrix phase and thermal decomposition products of pore formers can lead to degradation of the BiFeO3 phase in the final porous samples. Analysis of X-ray diffraction patterns illustrated that in the samples processed with graphite, high density polyethylene and polyvinyl alcohol as pore former, BiFeO3 matrix phase partially or completely decomposed to intermediate phases of Bi2Fe4O9 and Bi25FeO40. Using of urea did not damage the matrix phase and porous BiFeO3 within the original perovskite structure could be prepared. Furthermore, thermodynamic investigation was carried out for prediction of possible interactions between matrix phase and pore former at elevated temperatures.

BiFeO3 thin films via aqueous solution deposition: a study of phase formation and stabilization

This paper reports a thorough microstructural investigation of bismuth ferrite (BFO) thin films subjected to various processing conditions and discusses their influence on the stability of the BiFeO3 perovskite phase. The formation of secondary phases in BFO thin films is studied as a function of annealing temperature and time, film thickness, Bi excess, and Ti substitution. While films annealed at 600 °C consist of the desired BiFeO3 phase, higher temperatures induce the decomposition leading to a significant amount of secondary phases, particularly the iron-rich Bi2Fe4O9 phase. A longer annealing time at 700 °C further enhances the decomposition of BiFeO3. Qualitative microstructural analysis of the films is performed by electron backscattered diffraction which provides phase analysis of individual grains. The morphology of the single-crystalline Bi2Fe4O9 grains that are embedded in the BiFeO3 matrix drastically changes as a function of the film thickness. Nucleation of these Bi2Fe4O9 grains probably occurs at the film/substrate interface, after which grain growth continues toward the surface of the film through the depletion of the BFO phase. Addition of Bi excess or the substitution of Fe with Ti in the precursor solutions significantly reduces the formation of an iron-rich secondary phase. Influence of the secondary phases as well as Ti substitution on magnetic properties of BFO films was investigated.

Improved dielectric and magnetic properties of multiferroic BiFeO3–NiFe2O4 nanocomposite thin films

Multiferroic (1−x)BiFeO3(BFO)–xNiFe2O4(NFO) (x = 0, 0.1, 0.2, 0.3) nanocomposite thin films were prepared by sol–gel technique and their structural, electrical and magnetic properties were studied. X-ray diffraction and transmission electron microscopy examinations confirmed that NFO nanoparticles were well distributed in BFO matrix. The magnetic and dielectric properties were significantly improved by incorporation of NFO nanoparticle in matrix of BFO. The saturation magnetization (M s ) and remnant magnetization (M r ) increased as high as ~34 and ~7 emu/cm3 respectively for x = 0.1. The dielectric constant of the films increased from 160 (for x = 0) to 280 (for x = 0.3). However, the values of ferroelectric polarization were decreased with increasing x. Importantly, the sample with (x = 0.1), the best sample in our study demonstrates improved ferroelectric as well as magnetic properties and thus provides a great opportunities for many passive electronic devices for future potential applications.

Multiferroic behavior of templated BiFeO3-CoFe2O4 self-assembled nanocomposites.

Self-assembled BiFeO3-CoFe2O4 nanocomposites were templated into ordered structures in which the ferrimagnetic CoFe2O4 pillars form square arrays of periods 60-100 nm in a ferroelectric BiFeO3 matrix. The ferroelectricity, magnetism, conductivity, and magnetoelectric coupling of the ordered nanocomposites were characterized by scanning probe microscopy. The insulating BiFeO3 matrix exhibited ferroelectric domains, whereas the resistive CoFe2O4 pillars exhibited single-domain magnetic contrast with high anisotropy due to the magnetoelasticity of the spinel phase. Magnetoelectric coupling was observed in which an applied voltage led to reversal of the magnetic pillars.

Pillar shape modulation in epitaxial BiFeO3–CoFe2O4 vertical nanocomposite films

Self-assembled epitaxial CoFe2O4-BiFeO3 nanocomposite films, in which pillars of CoFe2O4 grow within a single crystal BiFeO3 matrix, show both ferrimagnetism and ferroelectricity. The pillars typically have a uniform cross-section, but here two methods are demonstrated to produce a width modulation during growth by pulsed laser deposition. This was achieved by growing a blocking layer of BiFeO3 to produce layers of separated pillars or pillars with constrictions, or by changing the temperature during growth to produce bowling-pin shaped pillars. Modulated nanocomposites showed changes in their magnetic anisotropy compared to nanocomposites with uniform width. The magnetic anisotropy was interpreted as a result of magnetoelastic and shape anisotropies.

Integration of Self‐Assembled Epitaxial BiFeO3–CoFe2O4 Multiferroic Nanocomposites on Silicon Substrates

Perovskite‐spinel epitaxial nanocomposite thin films are commonly grown on single crystal perovskite substrates, but integration onto a Si substrate can greatly increase their usefulness in devices. Epitaxial BiFeO3–CoFe2O4 nanocomposites consisting of CoFe2O4 pillars in a BiFeO3 matrix are grown on (001) Si with two types of buffer layers: molecular beam epitaxy (MBE)‐grown SrTiO3‐coated Si and pulsed‐laser‐deposited (PLD) Sr(Ti0.65Fe0.35)O3/CeO2/yttria‐stabilized ZrO2/Si. The nanocomposite grows with the same crystallographic orientation and morphology as that observed on single crystal SrTiO3 when the buffered Si substrates are smooth, but roughness of the Sr(Ti0.65Fe0.35)O3 promoted additional CoFe2O4 pillar orientations with 45° rotation. The nanocomposites on MBE‐buffered Si show very high magnetic anisotropy resulting from magnetoelastic effects, whereas the hysteresis of nanocomposites on PLD‐buffered Si can be understood as a combination of the hysteresis of the Sr(Ti0.65Fe0.35)O3 film and the CoFe2O4 pillars.

Raman scattering spectra, magnetic and ferroelectric properties of BiFeO3–CoFe2O4 nanocomposite thin films structure

Multiferroic (1−x)BiFeO3(BFO)–xCoFe2O4(CFO) (x=0 and 0.1) nanocomposite thin films were deposited on ITO coated glass using sol–gel spin coating technique. X-ray diffraction and transmission electron microscopy examinations confirm the coexistence of both perovskite BFO and spinel CFO phases. The effect of addition of CFO in BFO matrix has been studied on Raman spectra, magnetic and ferroelectric properties. BFO/CFO nanocomposite showed good magnetic behavior (Ms~40.3emu/cm3, Mr~12.9emu/cm3, Hc~90Oe) with no change in ferroelectric properties. The strain analysis carried out by Raman spectroscopy reveals that both BFO and CFO bands are found to be strained in BFO/CFO composite nanostructure. The strain of the bands is discussed on the basis of lattice mismatch (interfacial stress) between CFO (a=0.839nm) and BFO (a=0.396nm) phases.

Electrical and Magnetic Properties of Multiferroic $(1\hbox{-}{\rm x})\ {\hbox{BiFeO}}_{3}\hbox{-}{\rm x}\ {\hbox{CoFe}}_{2}{\hbox{O}}_{4}$ Nanocomposite Thin Films Derived by Sol-gel Process

In the present work, multiferroic (1-x) BiFeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -x (CoFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) ( x=0, 0.3, 0.5) nanocomposite thin films were prepared by sol-gel spin coating process. Structural, electrical and magnetic properties of nanocomposite thin films have been studied. X-ray diffraction study confirmed the co-existence of both perovskite BiFeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (BFO) and spinel CoFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (CFO) phases. Atomic force microscopy shows CFO particles distributed in BFO matrix. These nanocomposite thin films exhibit both ferroelectric and ferromagnetic switching at room temperature. The magnetic and dielectric properties are improved by incorporation of CFO grains in matrix of BFO. The saturation magnetization (M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) increases as x varies from 0 to 0.5. For x=0.5, M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> is found as high as ~ 196 emu/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Coercivity is increased with CFO concentration and found maximum ( H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ~ 1.20 kOe) for x=0.50. This large increase in coercivity indicates strong magnetoelastic coupling. The dielectric constant of the films is increased from 240 (for x=0) to 370 (for x=0.5). All composite films show ferroelectric behavior, however the ferroelectric properties are found to decrease with incorporation of magnetic phase.

EELS tomography in multiferroic nanocomposites: from spectrum images to the spectrum volume.

Electron Energy Loss Spectroscopy (EELS) in a transmission electron microscope offers the possibility of extracting high accuracy maps of composition and electronic properties through EELS spectrum images (EELS-SI). Acquiring EELS-SI for different tilt angles, a 3D tomographic reconstruction of EELS information can be achieved. In the present work we show that an EELS spectrum volume (EELS-SV), a 4D dataset where every voxel contains a full EELS spectrum, can be reconstructed from the EELS-SI tilt series by the application of multivariate analysis. We apply this novel approach to characterize a nanocomposite material consisting of CoFe2O4 nanocolumns embedded in a BiFeO3 matrix grown on a LaNiO3 buffered LaAlO3 (001) substrate.

Compositionally Modulated Magnetic Epitaxial Spinel/Perovskite Nanocomposite Thin Films

There is great interest in self‐assembled oxide vertical nanocomposite films consisting of epitaxial spinel pillars in a single crystal perovskite matrix, due to their tunable electronic, magnetic, and multiferroic properties. Varying the composition or geometry of the pillars in the out‐of‐plane direction has not been previously reported but can provide new routes to tailoring their properties in three dimensions. In this work, ferrimagnetic epitaxial CoFe2O4, MgFe2O4, or NiFe2O4 spinel nanopillars with an out‐of‐plane modulation in their composition and shape are grown in a BiFeO3 matrix on a (001) SrTiO3 substrate using pulsed laser deposition. Changing the pillar composition during growth produces a homogeneous pillar composition due to cation interdiffusion, but this can be suppressed using a sufficiently thick blocking layer of BiFeO3 to produce bi‐pillar films containing for example a layer of magnetically hard CoFe2O4 pillars and a layer of magnetically soft MgFe2O4 pillars, which form in different locations. A thinner blocking layer enables contact between the top of the CoFe2O4 and the bottom of the MgFe2O4 which leads to correlated growth of the MgFe2O4 pillars directly above the CoFe2O4 pillars and provides a path for interdiffusion. The magnetic hysteresis of the nanocomposites is related to the pillar structure.

Nucleation-Induced Self-Assembly of Multiferroic BiFeO3–CoFe2O4 Nanocomposites

Large areas of perfectly ordered magnetic CoFe2O4 nanopillars embedded in a ferroelectric BiFeO3 matrix were successfully fabricated via a novel nucleation-induced self-assembly process. The nucleation centers of the magnetic pillars are induced before the growth of the composite structure using anodic aluminum oxide (AAO) and lithography-defined gold membranes as hard mask. High structural quality and good functional properties were obtained. Magneto-capacitance data revealed extremely low losses and magneto-electric coupling of about 0.9 μC/cmOe. The present fabrication process might be relevant for inducing ordering in systems based on phase separation, as the nucleation and growth is a rather general feature of these systems.

Self-assembled growth and magnetic properties of a BiFeO3-MgFe2O4 nanocomposite prepared by pulsed laser deposition

The fabrication and magnetic properties are described for a nano-composite epitaxial thin film system consisting of perovskite phase BiFeO3 and spinel phase MgFe2O4 grown on a (001) SrTiO3 substrate by combinatorial pulsed laser deposition. The MgFe2O4 phase formed well-defined vertical pillars in a BiFeO3 matrix, and both phases were epitaxial with the substrate. The BiFeO3-MgFe2O4 composite films showed soft magnetic behavior and magnetically isotropic properties which is explained by the weak magnetoelastic effects in MgFe2O4.

Self-assembled perovskite-spinel heterostructure on a highly distorted substrate

The pattern configuration and interface structure in epitaxial BiFeO3-CoFe2O4 heterostructures grown on (010)pc NdGaO3 substrates have been investigated systematically by transmission electron microscopy and chemical analysis. The crystal orientation of CoFe2O4 variants was tuned to [111]CFO, while BiFeO3 kept [010]pc matching the substrate. Triangular prism-shaped CoFe2O4 embedded in the BiFeO3 matrix grew as an equilibrium island mode with {111}CFO as its surfaces and interfaces. Two types of BiFeO3-CoFe2O4 orientation relationships were determined as (001)[010]BFO//(0-22)[111]CFO and (101)[010]BFO//(−220)[111]CFO. The results reveal that the dominant factors controlling the growth orientation of the present vertical heterostructures are surface energy anisotropy and atomic structure continuity.

Strong magnetic enhancement in self-assembled multiferroic-ferrimagnetic nanostructures

In the past decade, self-assembled vertical nano-heterostructures have drawn considerable attention because a high interface-to-volume ratio can be used to tailor or create functionalities. We have systematically investigated the magnetic properties of oxide heterostructures consisting of the CoFe2O4 nanopillars embedded in the BiFeO3 matrix using macroscopic magnetization measurements and element-selective soft X-ray absorption magnetic circular dichroism (XMCD) at the Co- and Fe-L2,3 edge. The magnetization and XMCD data show that the total ordered magnetic moment of Co(2+) in CoFe2O4-BiFeO3 nano-heterostructures is strongly enhanced. This study clearly indicates that the high interface-to-volume ratio vertical nanostructure creates a strong ferromagnetic and antiferromagnetic magnetic coupling via an interface. Furthermore, the magnetic coupling can be tuned in the multiferroic-ferrimagnetic self-assembled heterostructures by controlling the spacing between nanopillars.

Thermoelectric properties of Co-doped BiFeO&lt;sub&gt;3&lt;/sub&gt; and Bi&lt;sub&gt;24&lt;/sub&gt;CoO&lt;sub&gt;37&lt;/sub&gt;–BiFeO&lt;sub&gt;3&lt;/sub&gt; compound systems

We investigated the thermoelectric properties of doped multiferroic material: Co-doped BiFeO3 (BFO) and related materials: Bi24CoO37 and Bi24CoO37–BFO composite systems. Because of their insulating nature, the BFO-based samples showed high Seebeck coefficients compared with other oxides being explored as candidates for new thermoelectric materials such as NaCo2O4, and La0.9Sr0.99Ca0.11CoO4. On the other hand, the Bi24CoO37 sample showed high conductivity. The Bi24CoO37 (20%)–BFO composite sample showed a high power factor, because of its high conductivity and high Seebeck coefficient. These results indicated that the BFO matrix plays an important role in maintaining a high Seebeck coefficient, and that Bi24CoO37 works as a conductive pass on this system.

Ferroic states and phase coexistence in BiFeO3-BaTiO3 solid solutions

In this paper structural, electric, magnetic, and Mössbauer spectroscopy studies were conducted in (x)BiFeO3–(1-x)BaTiO3, 0.9 ≥ x ≥ 0.3, solid solutions. X-ray diffraction and Rietveld refinement studies indicated the formation of single-phased materials crystallized in a distorted perovskite structure with the coexistence of rhombohedral and monoclinic symmetries. Room temperature ferroelectric hysteresis loops showed that the electric polarization increases with the increase of the BaTiO3 content due to the singular structural evolution of the studied solid solutions. All samples presented weak ferromagnetic ordering, which indicates that the BaTiO3 substitution in the BiFeO3 matrix released the latent magnetization. Mössbauer studies revealed a magnetic spectral signature corresponding to ordered Fe3+ ions, and a decrease of the magnetic hyperfine magnetic fields with the increase of the BaTiO3 content. The composition 0.3BiFeO3–0.7BaTiO3 presented a spectral signature corresponding to a paramagnetic behavior, which strongly suggests that the observed magnetization in this sample is due to the Ti3+ ions.

Novel Nanorod Precipitate Formation in Neodymium and Titanium Codoped Bismuth Ferrite

AbstractThe discovery of unusual nanorod precipitates in bismuth ferrite doped with Nd and Ti is reported. The atomic structure and chemistry of the nanorods are determined using a combination of high angle annular dark field imaging, electron energy loss spectroscopy, and density functional calculations. It is found that the structure of the BiFeO3 matrix is strongly modified adjacent to the precipitates; the readiness of BiFeO3 to adopt different structural allotropes in turn explains why such a large axial ratio, uncommon in precipitates, is stabilized. In addition, a correlation is found between the alignment of the rods and the orientation of ferroelastic domains in the matrix, which is consistent with the system's attempt to minimize its internal strain. Density functional calculations indicate a finite density of electronic states at the Fermi energy within the rods, suggesting enhanced electrical conductivity along the rod axes, and motivating future investigations of nanorod functionalities.

Position-controlled functional oxide lateral heterostructures consisting of artificially aligned (Fe,Zn)3O4 nanodots and BiFeO3 matrix

We demonstrate an advanced fabrication method for perfectly position-controlled ferromagnetic semiconductor (Fe,Zn)3O4 nanodot arrays down to several hundred nanometers in size surrounded by a ferroelectric BiFeO3 matrix. By performing position-selective crystal growth of perovskite BiFeO3 on the position-controlled epitaxial spinel (Fe,Zn)3O4 nanodot-seeding template, which is prepared using a hollow molybdenum mask lift-off nanoimprint lithography process on a perovskite La-doped SrTiO3(001) substrate, we produce functional oxide three-dimensional lateral heterojunctions. The position-selectivity can be explained based on standard surface diffusion theory with a critical nucleation point. Establishing this fabrication process could lead to innovative nanointegration techniques for spintronic oxide materials.

Magnetic properties of CoFe2O4 nanoparticles distributed in a multiferroic BiFeO3 matrix

BiFeO3-CoFe2O4 composite thin films were formed on Pt/Ti/SiO2/Si(100) substrates by chemical solution deposition from a mixed precursor solution. X-ray diffraction and transmission electron microscopy analyses confirmed that CoFe2O4 nanoparticles less than 10 nm were uniformly distributed in the BiFeO3 matrix. The BiFeO3-CoFe2O4 composite films exhibited the same ferroelectric switching charge as BiFeO3 thin films, although a larger applied electric field was necessary. However, the magnetic properties were significantly improved by incorporation of CoFe2O4 nanoparticles into BiFeO3; a saturated magnetization of 80 emu/cm3 and a magnetic coercive field of 450 Oe were attained at 300 K. Furthermore, the composite films did not show superparamagnetic behavior in zero-field-cooling and field-cooling measurements, which suggest that the thermal fluctuation of CoFe2O4 nanoparticles was suppressed by exchange coupling with BiFeO3.

Directed Self-Assembly of Epitaxial CoFe2O4–BiFeO3 Multiferroic Nanocomposites

CoFe2O4 (CFO)–BiFeO3 (BFO) nanocomposites are an intriguing option for future memory and logic technologies due to the magnetoelectric properties of the system. However, these nanocomposites form with CFO pillars randomly located within a BFO matrix, making implementation in devices difficult. To overcome this, we present a technique to produce patterned nanocomposites through self-assembly. CFO islands are patterned on Nb-doped SrTiO3 to direct the self-assembly of epitaxial CFO–BFO nanocomposites, producing square arrays of CFO pillars.