Enhanced Magnetodielectric Effect in Graded CoFe2O4/Pb(Zr0.52Ti0.48)O3 Particulate Composite Films

Graded particulate composite films in which ferromagnetic CoFe₂O₄ nanoparticles are gradedly distributed in the ferroelectric Pb(Zr_0.52Ti_0.48)O₃ matrix along the thickness direction have been prepared on Pt/Ti/SiO₂/Si wafers via sol–gel spin‐coating method and rapid annealing process. Compared with the homogenous films, the graded ones not only exhibited magnetic anisotropy, but also showed evident increase in both ferroelectric polarization and dielectric tunability. Moreover, great enhancement of magnetodielectric effect was observed in such graded films. We elucidated the origin of enhanced magnetodielectric coupling and attributed it to the combined influence of two factors, that is, the enhanced ferroelectric polarization caused by CoFe₂O₄ distribution gradient and flexoelectric polarization induced by strain gradient under external magnetic field. This work presents a feasible way to modulate the magnetoelectric coupling in ferromagnetic–ferroelectric composite films for developing high‐performance multiferroic materials at nanoscale.     

Magnetoelectricity in CoFe2O4 nanocrystal-P(VDF-HFP) thin films

Transition metal ferrites such as CoFe₂O₄, possessing a large magnetostriction coefficient and high Curie temperature (T_c > 600 K), are excellent candidates for creating magnetic order at the nanoscale and provide a pathway to the fabrication of uniform particle-matrix films with optimized potential for magnetoelectric coupling. Here, a series of 0–3 type nanocomposite thin films composed of ferrimagnetic cobalt ferrite nanocrystals (8 to 18 nm) and a ferroelectric/piezoelectric polymer poly(vinylidene fluoride-co-hexafluoropropene), P(VDF-HFP), were prepared by multiple spin coating and cast coating over a thickness range of 200 nm to 1.6 μm. We describe the synthesis and structural characterization of the nanocrystals and composite films by XRD, TEM, HRTEM, STEM, and SEM, as well as dielectric and magnetic properties, in order to identify evidence of cooperative interactions between the two phases. The CoFe₂O₄ polymer nanocomposite thin films exhibit composition-dependent effective permittivity, loss tangent, and specific saturation magnetization (M_s). An enhancement of the effective permittivity and saturation magnetization of the CoFe₂O₄-P(VDF-HFP) films was observed and directly compared with CoFe₂O₄-polyvinylpyrrolidone, a non-ferroelectric polymer-based nanocomposite prepared by the same method. The comparison provided evidence for the observation of a magnetoelectric effect in the case of CoFe₂O₄-P(VDF-HFP), attributed to a magnetostrictive/piezoelectric interaction. An enhancement of M_s up to +20.7% was observed at room temperature in the case of the 10 wt.% CoFe₂O₄-P(VDF-HFP) sample.     

Magnetoelectric coupling in CoFe2O4-Pb(Zr0.2Ti0.8)O3 coaxial nanofibers

In this work, we reported the fabrication and magnetoelectric coupling properties of the multiferroic CoFe₂O₄-PbZr_0.2Ti_0.8O₃ (CFO-PZT) coaxial nanofibers synthesized by electrospinning technique. The coaxial structure of nanofibers was demonstrated by magnetic force microscope and transmission electron microscope. The multiferroic properties of coaxial nanofibers have been revealed by magnetic hysteresis loops and piezoresponse amplitude butterfly curves and phase hysteresis loops. The as-prepared coaxial nanofibers show an effective piezoelectric coefficient d₃₃ of 30 pm/V and a saturated magnetization of 12 emu/g. Their magnetoelectric response has been probed by means of the localized changes in magnetization after poling the domains of the composite system. A static, large converse magnetoelectric coupling coefficient of 1.2 × 10⁻⁸ s/m was obtained in a single CFO-PZT nanofiber.     

Lead-free BLTO/NMFO magnetoelectric composite films prepared by the sol-gel method

The lead-free ferroelectric films of Bi_4?xLa_xTi₃O₁₂(BLTO) and ferromagnetic films of Ni_1?xMn_xFe₂O₄(NMFO) were prepared on Pt/Ti/SiO₂/Si substrate by means of the sol-gel and spin-coating method. The lead-free magnetoelectric composite films with the structure of Bi_3.4La_0.6Ti₃O₁₂/Ni_0.7Mn_0.3Fe₂O₄/substrate (BN) and Ni_0.7Mn_0.3Fe₂O₄/Bi_3.4La_0.6Ti₃O₁₂/ substrate (NB) were also deposited on Pt/Ti/SiO₂/Si substrate. The X-ray diffraction results show that two composite films possess BLTO and NMFO phases without any intermediate phase. The SEM images show that two composite films exhibit layered structure, clear interface and no transition layer between BLTO and NMFO films. Two composite films exhibit both good ferromagnetic and ferroelectric properties, as well as magnetoelectric coupling effect. The deposition sequence of ferroelectric and ferromagnetic films in the composite films has significant influence on the ferroelectric, ferromagnetic and magnetoelectric coupling properties of the composite films. The values of magnetoelectric voltage coefficient of the BN composite films are higher than those of the NB composite films at any fixed H_bias.     

Microstructural, electrical and magnetic properties of multiferroic CoFe2O4/0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 nanocomposite thin films

Bilayered CoFe₂O₄/0.68Pb(Mg_1/3Nb_2/3)O₃-0.32PbTiO₃ nanocomposite films are successfully prepared on Pt/Ti/SiO₂/Si substrate via simple sol-gel process. X-ray diffraction result reveals that there exists no chemical reaction or phase diffusion between the CoFe₂O₄ and 0.68Pb(Mg_1/3Nb_2/3)O₃-0.32PbTiO₃ phases. The microstructure is characterized by scanning/transmission electron microscopy (STEM). The composite thin films exhibit both strong ferroelectric and ferromagnetic responses at room temperature. The maximal magnetoelectric coupling coefficient of the nanocomposite films reaches up to 25 mV/cm Oe, occurs at a lower bias magnetic field (H_dc) of 550 Oe.     

Fabrication and magnetoelectric effect of 1–3 type composites with microscale (K,Na)NbO3-based Pb-free piezoceramic pillars embedded in CoFe2O4-epoxy matrix

Microscale magnetoelectric composites with 1–3 connectivity structure were fabricated by a dice-and-fill method. Mechanically sliced microscale pillar arrays of (K,Na)NbO₃-based ceramics were used as the piezoelectric phase, which were embedded in a hybrid matrix composed of ferromagnetic CoFe₂O₄ particles bonded with epoxy. Commercial CoFe₂O₄ nano-powder and CoFe₂O₄ particles derived from a sol–gel method during the filling process were used in two sets of samples, respectively. Obvious ME response signals were detected to be switched with the applied magnetic field. It was deduced that the different ME coefficient values of the two composites were mainly due to the interface morphology between the piezoelectric pillars and the matrix.     

Dielectric,ferroelectric and magnetoelectric properties of in-situ synthesized CoFe2O4/BaTiO3 composite ceramics

Magnetoelectric composite materials have attracted more and more attention because of their coupling of ferroelectricity and ferromagnetism. It is a hotspot to realize the combination of ferromagnetic phase and ferroelectric phase. In this work, we used a new strategy to prepare CoFe₂O₄/BaTiO₃ composite ceramics: firstly, porous ferromagnetic CoFe₂O₄ phase was prepared by annealing of MOFs (metal organic frameworks) precursor Fe₃[Co(CN)₆]₂. And then, the ferroelectric BaTiO₃ phase in-situ grew in the pores of CoFe₂O₄ by a hydrothermal method. In the end, the CoFe₂O₄/BaTiO₃ composite ceramics sintered at different temperatures have been synthesized. The effects of sintering temperature on the structure, dielectric and ferroelectric properties have also been studied. Because the crystallinity and density increase with the increase of sintering temperature, the composite ceramic sintered at 1200 °C shows the best dielectric properties. It is found that sintering temperature has little effect on the ferroelectric and magnetic properties of ceramics. Taking the CoFe₂O₄/BaTiO₃ composite ceramic sintered at 1200 °C as an example, derived from the interaction between the ferromagnetic CoFe₂O₄ phase and ferroelectric BaTiO₃ phase, the applied magnetic field lead to the reduction of P_r and E_c.     

Effect of CoFe2O4 mole percentage on multiferroic and magnetoelectric properties of Na0.5Bi0.5TiO3/CoFe2O4 particulate composites

Na_0.5Bi_0.5TiO₃ (NBT), CoFe₂O₄ (CFO) as well as particulate composites containing different mole percentages of NBT and CFO were synthesized by the solid-state sintering route and characterized for their ferroelectric and ferrimagnetic hysteresis loops, magnetostriction and magnetoelectric (ME) output. The mole% of CFO was found to influence the ferroelectric and ferrimagnetic hysteresis loops as well as magnetostriction and piezomagnetic coefficients which in turn had a significant effect on the magnetoelectric voltage coefficient. The highest magnetoelectric voltage coefficient (α) of 0.5 mV/cm/Oe was recorded in (65) NBT–(35) CFO composite.     

Hybrid nanocomposites based on superparamagnetic and ferromagnetic particles: A comparison of their magnetic and dielectric properties

Nanocomposites of magnetic nanoparticles and polymer matrices combine the properties of their components, and as such are good examples of functional nanomaterials with excellent application potential. Against this background, experimental and theoretical studies of such composites are of great interest. In this study we aim to provide insight into the static and dynamic magnetic response, as well as the dielectric response, of magnetic nanocomposites subjected to external magnetic and electric fields. We directly compare the behavior of polyurethane films doped with superparamagnetic Fe₃O₄, and blocked ferromagnetic CoFe₂O₄ nanoparticles. While a reversible, Langevin magnetization curve is observed for Fe₃O₄@PU films, hysteretic magnetic behavior is found in case of CoFe₂O₄@PU films. The hysteresis observed for CoFe₂O₄ nanoparticles can be explained by interactions at the interface between particles and polymer matrix in conjunction with its ferromagnetic nature. The results of dielectric spectroscopy experiments revealed different effects of Fe₃O₄ and CoFe₂O₄ nanoparticles on polymer dynamics.     

Multiferroic and magnetoelectric properties of lead-free Ba0.8Sr0.2Ti0.9Zr0.1O3-Ni0.8Zn0.2Fe2O4 composite films with different deposition sequence

Ba_0.8Sr_0.2Ti_0.9Zr_0.1O₃/Ni_0.8Zn_0.2Fe₂O₄(BN) and Ni_0.8Zn_0.2Fe₂O₄/Ba_0.8Sr_0.2Ti_0.9Zr_0.1O₃ (NB) composite film were deposited on Pt/Ti/SiO₂/Si substrates by the sol-gel method and spin-coating method. The results show that the deposition sequences of the composite films have significant influence on the ferroelectric, ferromagnetic and magnetoelectric properties of the composite films. Two composite films possess not only good ferroelectric and ferromagnetic properties but good magnetoelectric properties as well. The NB composite film has clear interface between the ferroelectric film and ferromagnetic film and possesses greater magnetoelectric coupling effect than the BN composite film under the same H_bias. The maximum value of α_E is 70.14?mV?cm^?1 Oe^?1 was obtained in the NB composite film when H_bias is 638?Oe.     

Magnetoelectric composite thick films of PZT–PMnN + NiZnFe2O4 by aerosol-deposition

Highly dense magnetoelectric composite films with 10 μm-thick of high piezoelectric voltage coefficient material, 0.9Pb(Zr₅₇Ti₄₃)O₃–0.1Pb(Mn_1/3Nb_2/3)O₃ (PZT–PMnN) and magnetostrictive material, Ni_0.8Zn_0.2Fe₂O₄ (NZF), were fabricated on a platinized Si substrate using aerosol deposition (AD). With increasing magnetic NZF content, dielectric and ferroelectric properties were gradually decreased while magnetizations were improved. The 20% NZF added composite thick film were found to exhibit the maximum ME coefficient. This optimal NZF content is the same as that of bulk ME composite materials. It is noticeable that AD can control the content ratio of ME composite films by controlling the powder composition. The fabricated ME composite films have high ME voltage coefficient coupling because of high density without severe inter-reactions of two phases.     

Flexible magnetoelectric PVDF–CoFe2O4 fiber films for self-powered energy harvesters

Integrating the concept of magnetoelectric in the mechanical energy harvesters through the magneto-mechano-electrical (MME) nanogenerators has been explored to realize the self-powered devices. The magnetoelectric interaction enabled the output performance of the MME nanogenerator under magnetic stimulus of the active components of the energy harvesters. In this perspective, we fabricated a flexible biomechanical and MME nanogenerator using PVDF/CoFe₂O₄ fibers composite films. CoFe₂O₄ fibers were synthesized by the electrospinning technique and the process parameters were optimized to achieve uniform and bead-free fibers. The structural and morphological properties were investigated through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The structural and morphology revealed the fibers calcined at 800 °C with a heating rate of 2 °C/min produced bead-free continuous fibers with a fiber diameter of 210 nm with cubic spinel crystalline structure with a crystallite size of 34 nm. These highly crystalline fibers were used to fabricate PVDF/CoFe₂O₄ fibers composite films. The magnetoelectric behaviour of the films verified through polarization vs. electric field (P-E) loops under magnetic field. The leakage current density and mechanism of the composite films were investigated, and it was discovered that the mechanism was due to Schottky emission. Further the energy harvesting performance of the composite films were estimated where the nanogenerator achieved an output voltage of 13 V under biomechanical tapping force while the MME nanogenerator produced 3.5 V under a low frequency stray magnetic field of 6 Oe with a power density of 28 μW/m².     

Magnetoelectric coupling effect of Ga0.8Fe1.2O3/Bi0.5(Na0.85K0.15)0.5TiO3 multiferroic composite films

Lead-free Ga_0.8Fe_1.2O₃/Bi_0.5(K_0.15Na_0.85)_0.5TiO₃ (GFO/BKNT) bilayer multiferroic composite films were fabricated on Pt(100)/Ti/SiO₂/Si substrates via sol-gel methods. The microstructure, domain structure, ferroelectric, piezoelectric, magnetic properties as well as magnetoelectric coupling effect were investigated for the composite films at room temperature. Well-defined interfaces between GFO and BKNT layers and clear electric domain structures are observed. A strong magentoelectric effect is obtained with magnetoelectric voltage coefficient of α_E=30.89 mV/cm Oe, which is attributed to excellent ferroelectric, piezoelectric, and magnetic properties, as well as the coupling interaction between ferromagnetic GFO and ferroelectric BKNT phases for lead-free bilayer composite films. Besides, GFO and BKNT demonstrate the similar perovskite structure with well lattice matching, which endows the outstanding coupling and fascinating magnetoelectric properties. The present work opens up the opportunity of lead-free magnetoelectric composite films for both further fundamental studies and practical device applications such as sensors, transducers and multistate memories.     

Microstructure and multiferroic properties of BaTiO3/CoFe2O4 films on Al2O3/Pt substrates fabricated by electrophoretic deposition

BaTiO₃/CoFe₂O₄ bilayer films on high temperature resistant Al₂O₃/Pt substrates were fabricated using the electrophoretic deposition (EPD) method. Powder synthesis, suspension preparation and kinetics of deposition were investigated in detail. Two different sintering temperatures were tested and the resultant microstructure and properties of ceramic films were investigated. The composite films sintered at a temperature of 1200 °C/2 h yielded optimal microstructures and properties. The obtained bilayer films had a dense structure with no phase diffusion or passive layer. Besides to ferroelectric and magnetic properties, magnetoelectric (ME) coupling properties were also confirmed by the magnetic anisotropy and magnetic field induced polarization investigation. The composite films showed a increased P_s from 16.2 to 16.85 μC/cm² with variations ΔP_s of 4% when the magnetic field was applied in parallel direction and a decreased P_s from 16.2 to 14.9 μC/cm² with variations ΔP_s of ?8% when a normal direction field was applied.     

Study of crystal structure,dielectric, magnetic and magnetoelecrtic properties of xCoFe2O4-(1-x)Na0.5Bi0.5TiO3 composites

Multiferroic composites of spinel ferrite and ferroelectric xCoFe₂O₄ – (1-x)Na_0.5Bi_0.5TiO₃ (with x = 0.10,0.30,0.50) were efficiently prepared by standard solid state reaction mechanism. X-ray diffractometer was used to analyze crystal structure of the prepared composites. The observed XRD patterns of the composites comprise peaks of both the phases i.e. ferrite and ferroelectric, with no sign of secondary peaks. Rietveld refinement of XRD data further confirms the coexistence of these two phases with cubic (Fd3m) and rhombohedral (R3c) symmetry corresponding to ferrite and ferroelectric phase respectively. The 3-dimensional overview of crystal structure of pure CoFe₂O₄ and Na_0.5Bi_0.5TiO₃ and of composite 0.50CoFe₂O₄?0.50Na_0.5Bi_0.5TiO₃ is generated by using refined parameters. The dielectric constant (ε´) and dielectric loss (tanδ) values were recorded as a function of frequency ranging from 100?Hz to 7?MHz and at different temperatures. Both ε´ and tanδ follow dispersion pattern at lower frequencies while show frequency independent behavior at higher frequencies. The magnetic evaluation carried by analyzing M-H hysteresis loop reveals the ferrimagnetic characteristics of these composites. The highest value of magnetic moment is 1.12μ_B observed for composite 0.50CoFe₂O₄ – 0.50Na_0.5Bi_0.5TiO_3. Magnetoelectric (ME) voltage coefficient (α) was also demonstrated to observe the interaction between ferrite and ferroelectric phases. The highest value of α (72.72μV/Oe cm) is obtained for low ferrite composition 0.10CoFe₂O₄ – 0.90Na_0.5Bi_0.5TiO₃, which suggests the dependence of magnetoelectric response on the resistivity of the composites.     

Fabrication and characterization of magnetic cobalt ferrite/polyacrylonitrile and cobalt ferrite/carbon nanofibers by electrospinning

An electrospinning process was used to fabricate cobalt ferrite (CoFe₂O₄)-embedded polyacrylonitrile (PAN) nanofibers. Oleic acid-modified CoFe₂O₄ nanoparticles were dispersed in the PAN before spinning. The surface morphologies and structures of the nanofibers were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM observation showed that the average diameter of the CoFe₂O₄/PAN nanofibers was 110 nm, and the magnetic CoFe₂O₄ nanoparticles were embedded in the PAN nanofibers. X-ray photoelectron spectroscopy was used to characterize the CoFe₂O₄/PAN and CoFe₂O₄/carbon nanofibers. Fiber magnetic properties were measured by vibrating sample magnetometry, showing that the saturation magnetization of the CoFe₂O₄/PAN nanofibers was 45 emu/g and that the fibers demonstrated superparamagnetic behavior.     

The Direct and the Converse Magnetoelectric Effect in Multiferroic Cobalt Ferrite–Barium Titanate Ceramic Composites

We report on a systematic study of the magnetoelectric effect in cobalt ferrite (CoFe₂O₄)—barium titanate (BaTiO₃) ceramic composites with (0‐3) connectivity. Both the converse magnetoelectric coefficient, α_C, and the direct voltage magnetoelectric coefficient, α^E, were measured in dependence on composition and electric and magnetic bias fields. The strongest ME effect was observed in the composition (1?x) CoFe₂O₄–xBaTiO₃ with x = 0.5 yielding values α_C = 25 psm^?1 and α^E = 3.2 mV/(cm·Oe). We show that the proper conversion between these two coefficients demands knowledge about the dielectric permittivity of the sample. For low BaTiO₃ content the dielectric coefficient of the composite yields a better correspondence, whereas for high BaTiO₃ content the sample's average dielectric coefficient yields a better match. The influence of mutual orientation of polarization and magnetization on the ME effect is addressed. We found that for measurements performed parallel to the polarization direction (longitudinal effect), the ME coefficient is approximately twice as large and of opposite sign in comparison to the measurements perpendicular to the polarization direction (transverse effect). This difference has been rationalized in terms of the different contributions of the material coefficient tensor components to the ME effect, the demagnetizing factor, and losses. The obtained results provide a better understanding of peculiarities of the ME effect in bulk ceramic composites.     

Influence of Zr/Ti atomic ratio and seed layer on the magnetoelectric coupling of Pb(ZrxTi1−x)O3 film-on-CoFe2O4 bulk ceramic composites

Lead zirconate titanate Pb(Zr_xTi_1−x)O₃ films with various Zr/Ti ratios of 20/80, 40/60, 52/48, 60/40 and 80/20 are deposited on highly dense CoFe₂O₄ ceramics using a simple chemical solution deposition. All Pb(Zr_xTi_1−x)O₃ films are polycrystalline and have no preferential orientations. The dielectric, ferroelectric, piezoelectric and magnetoelectric properties strongly depend on the Zr/Ti ratio. And the Pb(Zr_xTi_1−x)O₃ films with a Zr/Ti ratio close to morphotropic phase boundary exhibit best properties, whose magnetoelectric coefficient is over 1.5 times larger than those of other Zr/Ti ratios. The introduction of a PbO seeding layer between the Pb(Zr_0.52Ti_0.48)O₃ films and CoFe₂O₄ substrates facilitates the (100)-texture. Therefore, the magnetoelectric coefficient was enhanced by 1.5 times. The further improvement of the magnetoelectric coupling could be anticipated by fabricating Pb(Zr_0.52Ti_0.48)O₃ films with more or absolute (100)-texture and using conductive interfacial layer between two phases.     

Sonochemical synthesis of CoFe2O4 nanoparticles and their application in magnetic polystyrene nanocomposites

CoFe₂O₄ (CoFe) nanoparticles were synthesized via a facile surfactant-free sonochemical reaction. For preparation of magnetic polymeric films, CoFe₂O₄ nanoparticles were added to polystyrene (PS). Nanoparticles were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Magnetic properties of the samples were investigated using an alternating gradient force magnetometer (AGFM). CoFe₂O₄ nanoparticles exhibit a ferromagnetic behaviour with a saturation magnetization of 62 emu/g and a coercivity of 640 Oe at room temperature. By preparing magnetic films the coercivity is increased. The coercivity of PS/CoFe₂O₄ (10%) nanocomposites is higher than that obtained for PS/CoFe₂O₄ (30%).     

Processing‐Property Relationships in Electrospun Janus‐Type Biphasic Ceramic Nanofibers

Electrospinning provides a promising route to quickly produce high‐purity ceramic fibers from readily available sol–gel precursors. Recently, techniques have been developed to enable the production of biphasic nanocomposites on a single fiber, enabling the development of novel composites with connectivities that are difficult to obtain using existing methods. These “composites on a fiber” can be used as a unique anisotropic building block for creating more complex ordered structures. This work examines links between processing, structure, and properties for Janus‐type composite nanofibers for the magnetoelectric BaTiO₃–CoFe₂O₄ system. Specifically, we show how altering the viscosity and conductivity of precursor solutions can provide a unique route to synthesize biphasic composites of a range of sizes and compositions. Furthermore, we show how altering these properties can impact the magnetic and magnetoelectric behavior of these fibers. While this work focuses specifically on the BaTiO₃‐CoFe₂O₄ system, lessons learned can be applied to electrospinning biphasic ceramic materials from a variety of material systems and applications.