BiFeO3 CoFe2O4 Research Articles

Highly ordered core–shell CoFe2O4–BiFeO3 nanocomposite arrays from dimension confined phase separation and their interfacial magnetoelectric coupling properties

With dimension confinement, highly ordered core–shell CoFe2O4–BiFeO3 nanocomposite arrays were obtained from the self-assembly phase separation.

Hierarchical templating of a BiFeO3-CoFe2O4 multiferroic nanocomposite by a triblock terpolymer film.

A process route to fabricate templated BiFeO3/CoFe2O4 (BFO/CFO) vertical nanocomposites is presented in which the self-assembly of the BFO/CFO is guided using a self-assembled triblock terpolymer. A linear triblock terpolymer was selected instead of a diblock copolymer in order to produce a square-symmetry template, which had a period of 44 nm. The triblock terpolymer pattern was transferred to a (001) Nb:SrTiO3 substrate to produce pits that formed preferential sites for the nucleation of CFO crystals, in contrast to the BFO, which wetted the flat regions of the substrate. The crystallographic orientation and magnetic properties of the templated BFO/CFO were characterized.

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.

Deep data analysis of conductive phenomena on complex oxide interfaces: physics from data mining.

Spatial variability of electronic transport in BiFeO3-CoFe2O4 (BFO-CFO) self-assembled heterostructures is explored using spatially resolved first-order reversal curve (FORC) current voltage (IV) mapping. Multivariate statistical analysis of FORC-IV data classifies statistically significant behaviors and maps characteristic responses spatially. In particular, regions of grain, matrix, and grain boundary responses are clearly identified. k-Means and Bayesian demixing analysis suggest the characteristic response be separated into four components, with hysteretic-type behavior localized at the BFO-CFO tubular interfaces. The conditions under which Bayesian components allow direct physical interpretation are explored, and transport mechanisms at the grain boundaries and individual phases are analyzed. This approach conjoins multivariate statistical analysis with physics-based interpretation, actualizing a robust, universal, data-driven approach to problem solving, which can be applied to exploration of local transport and other functional phenomena in other spatially inhomogeneous systems.

Multiferroic CoFe2O4–BiFeO3 core–shell nanofibers and their nanoscale magnetoelectric coupling

Abstract

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.

Local probing of magnetoelectric coupling and magnetoelastic control of switching in BiFeO3-CoFe2O4 thin-film nanocomposite

We report on the combination of piezoresponse force microscopy (PFM), magnetic force microscopy, and local ferroelectric switching with magnetic field for the study of a thin-film magnetoelectric (ME) nanocomposite. The collection of PFM under an applied variable magnetic field within a polycrystalline perovskite-spinel BiFeO3-CoFe2O4 (BFO-CFO) 0-3 type thin-film nanocomposite enables quantitative and proximal measurement of magnetoelastic strain-driven ME response. Combination of measurement of the as-grown strain state with local measurements of microstructure and macroscopic magnetization permits local mapping of ME coupling.

Synthesis, structure, and magnetic studies on the CoFe2O4–BiFeO3 nanocomposite films with different number of CoFe2O4 layers

The multiferroic heterostructures consisting of CoFe2O4 (CFO)–BiFeO3 (BFO) layers with increasing the number of CFO layers (2, 4, 6 and 8) have been grown on LaNiO3 buffered Si (100) substrate by a simple sol–gel spin-coating route. X-ray diffraction (XRD) shows that the CFO and BFO phases have been successfully retained in the heterostructures and the films are polycrystalline. The high resolution transmission electron microscope (HRTEM) images show the clear interplanar distances and the interface between the two phases. The magnetization (M) versus field (H) loops reveal that, with increasing the CFO layers, the magnetization of CFO–BFO films will be enhanced. The Ms, Mr and Hc of the composite film with 8 CFO layers are measured to be the largest values of 1304emu/cm3, 562emu/cm3 and 2074Oe, respectively.

Engineered Magnetic Shape Anisotropy in BiFeO3–CoFe2O4 Self-Assembled Thin Films

We report growth of various phase architectures of self-assembled BiFeO3-CoFe2O4 (BFO-CFO) thin films on differently oriented SrTiO3 (STO) substrates. CFO forms segregated square, stripe, and triangular nanopillars embedded in a coherent BFO matrix on (001)-, (110)-, and (111)-oriented STO substrates, respectively. Nanostructures with an aspect ratio of up to 5:1 with a prominent magnetic anisotropy were obtained on both (001) and (110) STO along out-of-plane and in-plane directions. Magnetic easy axis rotation from in-plane to out-of-plane directions was realized through aspect ratio control. An intractable in-plane anisotropy was fixed in CFO on (111) STO due to the triangular shape of the ferromagnetic phase nanopillars. These studies established a detailed relationship of magnetic anisotropy with specific shape and dimensions of ordered magnetic arrays. The results suggest a way to effectively control the magnetic anisotropy in patterned ferromagnetic oxide arrays with tunable shape, aspect ratio, and elastic strain conditions of the nanostructures.

Influence of the thickness of BiFeO3 on the ferromagnet properties of layered multiferroic CoFe2O4–BiFeO3 films

Multiferroic layered films consisting of CoFe2O4–BiFeO3 (CFO–BFO) with increasing the thickness of BFO layer have been prepared on the LaNiO3 (LNO)-buffered Si (100) substrate via a simple spin-coating process. The thickness effects of BFO layer on the magnetic properties of CFO–BFO films have been investigated in detail. The X-ray diffraction results testify that magnetic CFO and ferroelectric BFO phases coexist in the heterostructured films. The HRTEM images show the well-defined microstructure of the CFO layer on LNO buffer. The magnetic measurement reveal that, with increasing the BFO thickness, the Ms of CFO–BFO films will be enhanced due to the stress. The film with 400nm thickness BFO layers demonstrates the highest Ms value of 230emu/cm3, which exhibits 137.1% enhancement than that of the film without BFO layers.

Simulation of inhomogeneous magnetoelastic anisotropy in ferroelectric/ferromagnetic nanocomposites

The magnetic response of CoFe2O4/BiFeO3 (CFO/BFO) nanocomposite thin films, in which ferromagnetic CFO nanopillars are embedded in a ferroelectric BFO matrix, has been modeled by including the position-dependent magnetoelastic anisotropy of the CFO. A finite element simulation of the strain state of an arrangement of CFO pillars was performed in which the BFO matrix surrounding one or all of the pillars was subject to a piezoelectric strain. The strain transferred to the CFO pillars was calculated and transformed into a spatially varying magnetoelastic anisotropy in the CFO, and a micromagnetic model was then used to calculate the hysteresis of the pillar, which differed significantly from a macrospin model. The position-dependent anisotropy led to a complex reversal process and to a reorientation of the easy axis to the in-plane direction at sufficient applied electric fields.

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.

Magnetic anisotropy in composite CoFe2O4-BiFeO3 ultrathin films grown by pulsed-electron deposition

Many works have demonstrated perpendicular magnetic anisotropy in CoFe2O4-BiFeO3 (CFO-BFO) composites, which is commonly believed to originate from out-of-plane compressive strain in the CFO pillars due to the lattice mismatch with the BFO matrix. Others have shown that the pillar−matrix interface in similar NiFe2O4-BFO composites is fully relaxed. To study the origin of the magnetic anisotropy, composite films were grown on SrTiO3 with thicknesses ranging from 13 to 150 nm via pulsed electron deposition. In-plane compressive strain in the pillars is found for thinner samples, which induces in-plane magnetoelastic anisotropy. A model for the origin of this previously unreported strain is proposed and the results are contrasted with the thicker composite films found in the literature.

Deposition of epitaxial BiFeO3/CoFe2O4 nanocomposites on (001) SrTiO3 by combinatorial pulsed laser deposition

BiFeO3/CoFe2O4 (BFO/CFO) nanocomposites were grown on SrTiO3 by pulsed laser deposition using a combinatorial method in which Bi1.2FeO3 and CoFe2O4 targets are alternately ablated. The films had the same vertically nanostructured morphology as thin films prepared by ablation of a single target, consisting of epitaxial CoFe2O4 pillars in a BiFeO3 matrix. In a series of samples synthesized with a compositional spread, the out-of-plane magnetic anisotropy and the out-of-plane compressive strain of the CoFe2O4 pillars increased with decreasing volume fraction, and the anisotropy agreed with the value predicted from the strain state and magnetoelastic coefficients of CoFe2O4. These results show the dominant effect of magnetoelastic anisotropy in determining the magnetic hysteresis of the nanocomposite.

Depth and material sensitivity in magneto-optic nanostructures

Magneto–optic (MO) methods are used for characterisation of magnetic ultrathin lms and nanostructures. The paper deals with the sensitivity of the complex MO effect to lms at different depths and to different materials in periodic multilayers and self assembled nanostructures. It is shown, that the basic principle and separation methods are the same for depth sensitive selectivity and the material sensitivity in nanostructures. Method of MO signal separation from different lms and phases and the gure of merit used to quantify the MO ellipsometric selectivity of magnetic nanostructures is dened on the basis of linear matrix algebra. We review applicability of the method to separate MO contributions from (Au/Co)N, N = 2, 3; (Co/Au/NiFe/Au)10 multilayers; BiFeO3–CoFe2O4 self–assembled nanostructure; cobalt lm on self–organised Au islands; and magnetically hard Fe nanoparticles near the surface of soft FeNbB amorphous ribbons.

Tunable magnetic anisotropy of CoFe2O4 nanopillar arrays released from BiFeO3 matrix

AbstractWe deposited epitaxial BiFeO3–CoFe2O4 (BFO–CFO) self‐assembled thin films on (001) SrTiO3 (STO) substrates. We find that a combined annealing and etching process could remove the BFO matrix, thereby resulting in free‐standing CFO nanopillar arrays. Scanning electron and atomic force microscopies showed well separated CFO nanopillars, which were very similar to the original CFO ones in the self‐assembled structure. Finally, comparison of the magnetic hysteresis loops before and after removal of the BFO matrix showed a significant decrease of the coercive field and a dramatic decrease in the strain dominated magnetic anisotropy. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Strain analysis of multiferroic BiFeO3-CoFe2O4 nanostructures by Raman scattering

We report a Raman scattering investigation of columnar BiFeO3-CoFe2O4 (BFO-CFO) epitaxial thin film nanostructures, where BFO pillars are embedded in a CFO matrix. The feasibility of a strain analysis is illustrated through an investigation of two nanostructures with different BFO-CFO ratios. We show that the CFO matrix presents the same strain state in both nanostructures, while the strain state of the BFO pillars depends on the BFO/CFO ratio with an increasing tensile strain along the out-of-plane direction with decreasing BFO content. Our results demonstrate that Raman scattering allows monitoring strain states in complex 3D multiferroic pillar/matrix composites.

Quantitative determination of anisotropic magnetoelectric coupling in BiFeO3–CoFe2O4 nanostructures

The transverse and longitudinal magnetoelectric susceptibilities (MES) were quantitatively determined for (001) heteroepitaxial BiFeO3–CoFe2O4 nanostructures. Both of these MES values were sharply enhanced at magnetic fields below 6 kOe and revealed asymmetric line shapes with respect to the dc magnetic field, demonstrating the strain-induced magnetoelectric effect. The maximum transverse MES, which reached as high as ∼60 mV/cm Oe, was about five times larger than the longitudinal MES. This observation signifies that transverse magnetostriction of the CoFe2O4 nanopillars is enhanced more than the bulk value due to preferred magnetic domain alignment along the [001] direction coming from compressive, heteroepitaxial strain.

Magnetoelectric and multiferroic properties of variously oriented epitaxial BiFeO3–CoFe2O4 nanostructured thin films

We report the ferroelectric, ferromagnetic, and magnetoelectric (ME) properties of self-assembled epitaxial BiFeO3–CoFe2O4 (BFO–CFO) nanostructure composite thin films deposited on (001), (110), and (111) SrTiO3 (STO) single crystal substrates. These various properties are shown to depend on orientation. The maximum values of the relative dielectric constant, saturation polarization, longitudinal piezoelectric coefficient, saturation magnetization, and ME coefficient at room temperature were 143, 86 μm/cm2, 50 pm/V, 400 emu/cc, and 20 mV/cm Oe, respectively.

Review of magnetoelectric perovskite–spinel self-assembled nano-composite thin films

Two-phase multiferroic nano-composite thin films have been a topic of research interests in the last few years. This is because of their expected magnetoelectric coupling, as well as potential applications. This review focuses on recent findings in self-assembled nano-structure composite thin films, and various efforts to realize and improve their magnetoelectricity. Topics include: (i) nano-pillar and maze structures, and their formation mechanisms, and a nano-belt structure oriented in-plane found by our research group; (ii) the ferroelectric properties of composite thin films, and how they can be enhanced by epitaxial engineering; (iii) a magnetic anisotropy that is induced by constraint stress, and by the nano-structures of the ferromagnetic phase; and (iv) a magnetoelectric coupling that was first observed via a change in magnetization near the Curie temperature of the ferroelectric phase, a magnetization switching assisted by electric field, and recently direct measurements using a magnetic cantilever method yielding values of 18 mV/cm Oe in BiFeO3–CoFe2O4.