Magnetoelectricity in CoFe2O4 nanocrystal-P(VDF-HFP) thin films
© Liu et al.; licensee Springer. 2013
Received: 1 May 2013
Accepted: 24 August 2013
Published: 3 September 2013
Transition metal ferrites such as CoFe2O4, possessing a large magnetostriction coefficient and high Curie temperature (Tc > 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 (Ms). 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 Ms up to +20.7% was observed at room temperature in the case of the 10 wt.% CoFe2O4-P(VDF-HFP) sample.
KeywordsThin film Magnetoelectric nanocomposite Magnetostrictive P(VDF-HFP) CoFe2O4
Magnetoelectric materials, possessing spontaneous electric and magnetic ordering, show applications in multiple-state memory elements, magnetic field sensors, phase shifters, and microwave frequency transducers. Single-phase multiferroics, such as BiFeO3, YMnO3, and CdCr2S4, exhibit intrinsic magnetoelectric (ME) effect with inherent cross-coupling between magnetic and electric orders. However, such materials are empirically rare and magnetoelectrically weak due to the contraindication between ferroelectricity and magnetism. In addition, the observed ME effect is far below room temperature, which severely limits practical use in device fabrication. An alternative approach is two-phase composite films, in which the ferri/ferromagnetic and ferro/piezoelectric phases are combined. With various connectivity schemes (e.g., 0–3 particulate type, 2–2 laminate type, and 1–3 fiber/rod type), these heterostructures have offered the opportunity to tune ferroelectric and magnetic properties independently, and the ME coefficient is 3 orders of magnitude higher than their single-phase counterparts.
The magnetoelectric effect in most multiferroic composites is known as strain-mediated, in which the ME coupling is a concerted result of the piezoelectric effect from the piezoelectric phase and magnetostrictive effect from the magnetic phase. An electric field induces a distortion of the piezoelectric phase, which in turn distorts the magnetostrictive phase, generating a magnetic field and vice versa. Substantial ME coupling requires the ferroelectric phase to be in possession of a high piezoelectric coefficient, while the magnetic phase possess both high magnetostriction and resistivity, with an intimate mechanical contact between the two. Ceramic composites have a combination of ferroelectric and magnetic oxides; polymer composites have the magnetic oxides embedded in ferroelectric polymer matrix. The former is limited by high dielectric loss resulting from the interface; the latter offers mechanical flexibility with facile processing. For instance, with high strength and good stability, polyvinylidene difluoride (PVDF) and its copolymers such as poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) and poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP))[11–13] are well known for their ferroelectricity and piezoelectricity, which make them ideal candidates for multiferroic film fabrication and ME effect exploration. Transition metal ferrites such as CoFe2O4, possessing a large magnetostriction coefficient (λ ≈ 10−4) and high Curie temperature (Tc > 600 K), serve as excellent candidates for the magnetic phase.
Although the mechanism of the magnetoelectric coupling is straightforward, complications arise when quantifying the details of polymer-based nanocomposites. The presence of polymorphism (e.g., α, β, γ, δ phases in PVDF), domain walls, grain boundaries, residual stain/magnetization, surface charge, and voids can significantly hinder the ME effect. Andrew and Clarke found that the inclusion of well-dispersed Ni0.5Zn0.5Fe2O4 nanoparticles in a PVDF matrix can enhance the ferroelectric phase content. Liu et al.. reported epitaxial BaTiO3-CoFe2O4 nanocomposite thin films (thickness, 100 nm) with phase transition mediated by tensile strain. Recently, a magnetoelectric coupling coefficient of 12 V/cm · Oe was obtained for P(VDF-HFP)/Metglas laminates. Martins et al. fabricated ferrites/PVDF nanocomposites films with thickness of 40 to 50 μm by solvent casting and melt processing. Guo and co-workers prepared particulate Ni0.5Zn0.5Fe2O4/P(VDF-TrFE) films (thickness, 50 to 60 μm) by wet chemistry routes, with observable magnetodielectric coefficients represented as a percentage change of dielectric constant with and without an external magnetic field.
In this work, we report a novel approach to fabricate 0–3 type particulate nanocomposite thin films composed of dispersed CoFe2O4 (CFO) nanoparticles embedded in P(VDF-HFP) matrix. Prepared through spin/cast-coating techniques, such films exhibit homogenous thickness ranging from 200 nm to 1.6 μm. With a focus on the potential for magnetoelectric coupling, the morphology, microstructure, dielectric, magnetic, and magnetoelectric properties are investigated systematically.
The CoFe2O4 nanocrystals were synthesized by a hydrothermal route. In a typical reaction, 2 mmol Co(NO3)2 · 6H2O (Aldrich, 98+%) and 4 mmol Fe(NO3)3 · 9H2O (Aldrich, 98+%) were first dissolved in deionized water. Ethanolamine was dropwise added in the solution until precipitation completed. The obtained precipitate was collected by centrifugation and washed with deionized water. Ammonium hydroxide was then added to re-dissolve the solids. The reaction mixture was transferred into a stainless steel autoclave, with 80% volume filled with the ammonium hydroxide solution. The autoclave was then heated at 200°C for 10 to 30 h. The resultant CoFe2O4 nanopowders were washed, collected, and dried in air at 60°C overnight.
The CoFe2O4/polymer nanostructured films were prepared via multiple spin coating and cast coating followed by thermal treatment. N,N-dimethylformamide was first used to dissolve CoFe2O4 nanoparticles and P(VDF-HFP) pallets or polyvinylpyrrolidone (PVP) powder separately, with concentration of 20 mg/ml. Then, the two suspensions were mixed under ultrasonification, according to the weight ratio of CFO versus polymer, and spin-coated or cast-coated on Si or glass substrates and dried at 90°C under vacuum. The thickness of the obtained thin films (200 nm to 1.6 μm) was controlled by the times and/or rotation speed (300 to 1000 rpm) of the spin coating. To measure film thickness, scanning electron microscopy (SEM) cross-sectional analysis was applied. The Si substrate was scored and cut/fractured in order to observe film cross sections, which were then easily analyzed by SEM. Correct instrumental calibration and review of the film over several regions confirmed thin film uniformity, expected for spin/cast coating, and thicknesses could be determined to within ±7%. For dielectric measurements, the glass substrates were pre-deposited with rectangular (1 mm × 5 mm) Ag bottom electrodes by a thermal evaporator. Top electrodes were deposited (5 mm × 1 mm) after the films were coated and dried, leaving the composite sandwiched between two electrodes with square crossed area of 1 mm × 1 mm.
The phase purity and crystal structure of the CoFe2O4 particles was analyzed by X-ray diffraction (XRD) with a PANalytical powder X-ray diffractometer (Almelo, The Netherlands) with Ni-filtered Cu Kα radiation (λ = 1.54056 Å). The data were collected in step scanning mode from 20° to 80°. Microstructural characterization of the CFO powders was performed by transmission electron microscopy (TEM) with a JEOL 3000 F (Akishima-shi, Japan) with an accelerating voltage of 300 kV. We used a JEOL ARM 200CF equipped with cold field emission gun and spherical aberration correctors for both scanning transmission electron microscopy (STEM) and high-resolution transmission electron microscopy (HRTEM). Surface morphology, nanoparticle distribution, and film thickness of the CFO/polymer composite were evaluated by a Zeiss Supra 55VP SEM (Oberkochen, Germany). Dielectric measurements including frequency dependence of ϵ′, dielectric constant and tan δ, and dielectric loss were measured by an Agilent 4294A precision impedance analyzer. Magnetic measurements including zero field-cooled and field-cooled (ZFC/FC) low field magnetization versus temperature and room temperature hysteresis loops were carried out using a Quantum Design MPMS XL-5 SQUID magnetometer (San Diego, CA, USA), with applied fields up to 5 T and temperatures from 1.84 to 400 K.
Results and discussion
Comparison of effective permittivity of the CFO/polymer films at 100 kHz from experimental and modified Kerner model
ϵeff(calculated from Kerner equation)
10 wt.% CFO
20 wt.% CFO
30 wt.% CFO
10 wt.% CFO
20 wt.% CFO
30 wt.% CFO
The effective permittivity of the CFO/P(VDF-HFP) films shows a distinctive and continuous increase relative to the theoretical value estimated by the Kerner model, contrary to the expectations based solely on a composited effective dielectric constant. This can be contrasted with CFO/PVP, which shows significantly less deviation between experiment and theory, and follows expected behavior for a simple combination of two components for ϵ eff . This observation, of deviating behavior in the case of CFO/P(VDF-HFP), is interesting and strongly suggests additional interactions between the polymer and nanoparticle. The phenomenon is ascribed to interfacial interactions between the magnetic filler and the piezoelectric matrix. P(VDF-HFP) undergoes lattice distortion under an applied electric field due to the piezoelectric effect, which introduces local stresses and strain at the ferrite-copolymer interface. Since the thermal shrinkage nature of the P(VDF-HFP) makes complete mechanical coverage of the copolymer over the CFO nanocrystals, and both CFO and P(VDF-HFP) are mechanically hard phases, with Young’s modulus of 141.6 and 237 GPa, respectively, the interfacial stress will be inversely applied to the copolymer phase via strong elastic interactions. As a result of the piezoelectric effect, the spatial charges and electric dipoles within the copolymer matrix are redistributed, manifested as variation of effective permittivities from the Kerner model. With higher ferrite contents, the interfacial elastic effect is stronger and leads to a more pronounced departure from the theoretical value.
Saturation magnetization (M s ) and normalized percentage change of saturation magnetization (Δ M s %) values for CFO/P(VDF-HFP) and CFO/PVP films with various CFO contents
10 wt.% CFO
30 wt.% CFO
50 wt.% CFO
10 wt.% CFO
30 wt.% CFO
50 wt.% CFO
where Ms is the saturation magnetization of a film with certain CFO weight fraction, f is the corresponding weight percentage, Ms0 is the saturation magnetization of pure CFO, and ΔMs% is the normalized percentage change of the Ms value of each polymer-based film relative to the comparative weighted, pure cobalt ferrite films. The ΔMs% values for both P(VDF-HFP) and PVP films are summarized in Table 2. The ΔMs% for the CFO/PVP films is close to zero for all three samples, indicating that the net magnetic moments of the thin films is equivalent to the sum of the contributions from each individual CFO grain inside the PVP matrix (volume fraction contribution only). In contrast, all CFO/P(VDF-HFP) films exhibit positive values of ΔMs%, with a gradual increase as the copolymer fraction increases. This observation was carefully calibrated against possible errors in the calculation of the volume fraction, most notably through measurement of the film thickness by SEM (see ‘Methods’ section). This observation strongly suggests a contribution to the ΔMs due to the presence of P(VDF-HFP) in the form of an enhancement of the saturation magnetization of the composite, with the enhancement stronger for samples with a lower CFO:P(VDF-HFP) ratio.
where E is the magnetic strain energy density, λs is the magnetostrictive expansion at saturation, θ is the angle between the saturation magnetization, and σ is the stress applied on a single magnetic domain. With limited expansion allowed by intimate contact of two hard phases, when compression is applied to CFO phase, the energy is minimized when magnetization is parallel to σ (θ = 0). Consequently, Ms is increased by tension. Moreover, in a sample of pure CFO nanoparticles (Ms = 66 emu/g) each Co2+ ion exhibits a magnetic moment of 2.7 μB, while in the 10 wt.% CFO/P(VDF-HFP)) films (Ms = 8.0 emu/g), the Co2+ ion shows a net magnetic moment of 3 μB, which equals the maximum magnetic moment a Co2+ ion can offer in the inverse spinel structure. This observation indicates that by intimate mechanical coverage of the CFO particles, P(VDF-HFP) reduces the nanocrystals’ degree of surface disorder and surface anisotropy via redistributing charges and dipoles within the copolymer matrix, which allows the magnetization of the cobalt ions to completely follow the external magnetic field. Additionally, as the content of cobalt ferrite nanoparticles increases, the particles’ tendency towards agglomeration increases. The interfacial area is reduced due to the formation of small clusters of nanoparticles, and therefore, the interfacial interaction is weakened. This explains why the Ms enhancement is strongest in the 10 wt.% sample (+20.7%), in which the nanoparticles are more completely dispersed, compared to 30 and 50 wt.% samples (+9.6% and +8.6%, respectively).
The magnetoelectric effect associated with the magnetostrictive/piezoelectric coupling typically can only be observed under high magnetic fields at a very low temperature. In this work, the nanocomposite thin films show substantial magnetoelectric coupling at room temperature. The piezoelectric properties of P(VDF-HFP) and ferrimagnetic properties of CoFe2O4 nanocrystals are ideal and complimentary in this respect, resulting an observable magnetoelectric coupling.
Crystalline ultrafine CFO with a relatively narrow size distribution from 8 to 18 nm were dispersed in a P(VDF-HFP) copolymer host, forming 0–3 particulate type magnetoelectric nanocomposite thin films. The resulting films exhibit composition-dependent effective permittivity and loss. Following full structural characterization, the magnetic properties of the pure CoFe2O4 nanoparticles were studied and it was confirmed that the saturation magnetization and ZFC/FC curves demonstrate typical ferrimagnetic behavior. By comparing the P(VDF-HFP) and PVP samples, a clear difference in the behavior of the nanocomposite films with respect to effective permittivity and saturation magnetization is observed, highlighting the difference between the use of the ferroelectric polymer and the non-ferroelectric polymer. A magnetoelectric coupling is believed to be observed in the case of CFO/P(VDF-HFP). The origin of the magnetoelectric coupling is attributed to strong elastic interactions between the electric and magnetic phases. The nanocomposite, given its room temperature properties, is an interesting candidate magnetoelectric material with applications in smart devices such as sensors.
Energy-dispersive X-ray spectroscopy
High angle annular dark field scanning transmission electron microscopy
High-resolution transmission electron microscopy
Scanning electron microscopy
Scanning transmission electron microscopy
Transmission electron microscopy
This project was supported by the Advanced Research Project Agency for Energy (ARPA-e), ADEPT DE-AR0000114 and the National Science Foundation under award NSF CMMI #1014777. The work was partially funded by the Center for Exploitation of Nanostructures in Sensors and Energy Systems, City College of New York, under NSF Cooperative Agreement award number 0833180. TEM work was supported by the US Department of Energy’s Office of Basic Energy Science, Division of Materials Science and Engineering under contract number DE-AC02-98CH10886 and was carried out, in part, at the Center for Functional Nanomaterials, Brookhaven National Laboratory supported by the US Department of Energy, Office of Basic Energy Sciences. Stephen O’Brien acknowledges support from the Columbia-CCNY NSF MIRT, #1122594.
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