Mediated Magneto-electric Coupling Research Articles

Magnetoelectric behavior and magnetic field-tuned energy storage capacity of SrFe12O19 nanofiber reinforced P(VDF-HFP) composite films

• Fabrication of flexible magnetoelectric (ME) P(VDF-HFP)/SrFe 12 O 19 composite films. • Inclusion of SrFe 12 O 19 nanofiber enhances the polar β-phase in P(VDF-HFP) matrix. • P(VDF-HFP)/SrFe 12 O 19 films exhibit enhanced dielectric and ferroelectric properties . • A strong strain mediated ME coupling exists in the P(VDF-HFP)/SrFe 12 O 19 films. • P(VDF-HFP)/SrFe 12 O 19 films' energy storage capacity is tuned by magnetic fields. Flexible, self-standing magnetoelectric (ME) polymer composite films were prepared using the solution casting method by reinforcing one-dimensional ferromagnetic strontium ferrite (SrFe 12 O 19 ) nanofibers, synthesized using the electrospinning method, into poly(vinylidene fluoride–hexafluoropropylene) (P(VDF-HFP)) matrix. The effects of loading of SrFe 12 O 19 nanofibers on the functional, structural, magnetic, dielectric, and ferroelectric properties of P(VDF-HFP) were investigated. The loading enhanced the electroactive β-phase of the films, as confirmed by XRD and FTIR analyses. The ME coupling that existed in the composite films was confirmed by the changes in ferroelectric parameters under various magnetic fields. The energy storage capacity and dielectric constant of the composite films were enhanced with the addition of SrFe 12 O 19 nanofibers, with the maximum values of 56 at 100 Hz and 1678 mJ/cm 3 at 444 kV/cm, respectively, in the composite film with 20 wt% SrFe 12 O 19 nanofibers. The energy storage capacity was further enhanced and tuned by applying an external magnetic field. Thus, this work reports an innovative approach to tuning the energy storage capacity of ME polymer composite films through a magnetic field and also describes use of these films for a wide range of applications, such as energy storage and memory devices and magnetic sensors.

FEM modelling of magnetoelectric coupling in (2-2)LSMO/ P(VDF-TrFE) polymer composite

Strain mediated magnetoelectric (ME) coupling effect has been investigated in bilayer structure of LSMO/P (VDF-TrFE) nanocomposite in the transverse ME mode. In the transverse mode, Finite Element Method (FEM) based small signal analysis has been performed by the COMSOL Multiphysics 6.0 software. La0.7Sr0.3MnO3(LSMO)/P (VDF-TrFE) nanocomposite (2-2) has been prepared by gluing LSMO pellet with P(VDF-TrFE) polymer film. This bilayer structure shows multiferroic property at room temperature. Simulated FEM modelverifies the magneto-strictive property of LSMO layer and explained the experimental results.Experimental and simulated ME coupling coefficient are found to be very close to each other.

Magneto-electric interactions in composites of self-biased Y- and W-type hexagonal ferrites and lead zirconate titanate: Experiment and theory

This report is on the observation and theory for strong mechanical strain mediated magneto-electric (ME) coupling in composites of lead zirconate titanate (PZT) and self-biased Y- or W-type hexagonal ferrites. Polycrystalline Y-type (Ni1−xZnx)2 Y, and W-type (Co1−xZnx)2 W, hexagonal ferrites for x = 0–0.4 prepared by ceramic processing techniques showed a large remanent magnetization due to uniaxial or in-plane magneto-crystalline anisotropy. The strength of ME coupling in symmetric trilayer composites of the ferrites and PZT was measured by the ME voltage coefficient (MEVC) at low-frequencies and at longitudinal electromechanical resonance. The bias magnetic field H-dependence of MEVC at low-frequencies in the composites with (Ni, Zn) Y showed hysteresis with its value under self-bias 90% or more of the value for the optimum bias field. In the case of composites with W-type ferrites, the MEVC under zero-external bias was 60%–80% of its value for the optimum bias field. Both types of composites when subjected to an ac magnetic field at the EMR frequency showed an order of magnitude enhancement in the MEVC compared to low-frequencies and the peak value at EMR for zero-bias was 90% of its value under the optimum bias. A model has been developed for the large ME response under the self-bias provided by the remanent magnetization and estimated values of MEVC are in good agreement with the data. The hexaferrite-ferroelectric composites showing ME response without the need for an external magnetic bias are of importance for use as sensors and sensor arrays of magnetic fields.

Magnetic field control of polarization/capacitance/voltage/resistance through lattice strain in BaTiO3-CoFe2O4 multiferroic nanocomposite

Magnetoelectric (ME) nanocomposites is a topic of intensive research due to their superficial potential in spintronic applications. In the present work, the magnetic field controlled electrical polarization is studied in hydrothermally synthesized multiferroic 0.25BaTiO3-0.75CoFe2O4 (BTO-CFO) nanocomposite. This multiferroic heterostructure is combined ferrimagnetic (CFO) with ferroelectric/piezoelectric (BTO) and achieved strain-mediated ME effect, which can effectively mediate magnetic anisotropy. The X-ray diffraction pattern confirmed polycrystalline phases of spinel CFO and tetragonal BTO, for which the compressive lattice strain is made. The microstructural study has evaluated BTO-CFO nanoparticles formation and the value of d-lattice spacing is calculated. The room temperature magnetic hysteresis is arising which depends on CFO inversion degree, and the change in bond-angle/length along A and B-sites. The ferroelectric hysteresis is measured at room temperature, which changed with applied magnetic field, i.e., the phenomenon of reduction in domain wall pinning related with oxygen vacancies and grain boundaries effect. The magnetic field is also influenced impedance spectra to induce magnetoimpedance effect and the positive value of magnetoresistance is obtained. A giant magnetodielectric coefficient up to −27% is obtained at 1 kOe of field. A strain mediated ME coupling enhancement is obtained.

Self-assembly of multiferroic core-shell composites using DNA functionalized nanoparticles

Ferrite-ferroelectric core-shell nanoparticles were prepared by deoxyribonucleic acid (DNA) assisted self-assembly and the strained mediated magneto-electric (ME) interactions between the ferroic phases were studied. The nanoparticle type and size were varied and the DNA linker sequence was also varied. Two kinds of particles, one with 600 nm barium titanate (BTO) core and 200 nm nickel ferrite (NFO) shell and another with 200 nm BTO core and 50 nm nickel cobalt ferrite (NCFO) shell were prepared. The particles were linked by three different oligomeric DNA containing 19, 18 or 30 base pairs. The core–shell structure was evident from electron microscopy and scanning microwave microscopy images. Films and disks of the core-shell particles were assembled in a magnetic field and used for measurements of low frequency ME voltage coefficient (MEVC) and magnet-dielectric effect. The MEVC data on films indicate that particles assembled with DNA with 30 base pairs exhibit the strongest ME coupling suggesting a more fully integrated heterogenous nanocomposite and the weakest interaction for DNA with 18 base pairs. These results indicate that the longer linker region in DNA is the key factor for forming better composites. This result may be due to the irregular shape of the nanoparticles. Longer DNA strands would be able to bridge better generating more linkages. Shorter strands would not able to bridge the irregularly shaped particles as well and therefore result in linkages and less heterogeneity in the composites.

Magnetoelectric coupling studies on (x) (0.5BZT-0.5BCT) – (100-x) NiFe2O4 [x=90−70 wt%] particulate composite

In this article, the effect of composition on magnetoelectric properties of lead-free (x) [0.5BZT – 0.5BCT] – (100-x) NiFe2O4 (x=90, 80, 70wt%) particulate composites synthesized by sol-gel method was studied. Presence of ferroelectric and ferromagnetic phases in the composites was confirmed using x-ray diffraction technique. The microstructure of the composites displayed uniform grains distribution and dispersion of NFO phase in BCZT matrix. The co-existence of ferroelectric and magnetic orderings at room temperature was confirmed by measuring the P-E and M-H loops respectively, of the particulate composites. Frequency dependence of admittance measurements exhibited high electro-mechanical coupling coefficient (kp) in the range of 0.41 to 0.37. The magneto-electric (ME) properties of the composites were studied both in longitudinal (αE,33) and transverse (αE,31) modes at low frequency of 1kHz under external bias field (Hdc). High strain mediated ME coupling responses, αE,31=13.3mV/cmOe, 8mV/cmOe and 4.3mV/cmOe were recorded for samples with x=80, 90, and 70wt% BCZT respectively.

Strain mediated magnetoelectric coupling in a NiFe2O4–BaTiO3 multiferroic composite

In this paper we demonstrate significant magnetoelectric coupling in ferrimagnetic, NiFe2O4, and ferroelectric, BaTiO3, multiferroic composite bulk materials by measuring temperature dependent magnetization. X-ray diffraction, scanning electron microscopy and high resolution transmission electron microscopy data show that the two phases coexist with a highly crystalline and sharp interface without any detectable impurities, which enables significant magnetoelectric (ME) coupling. The temperature dependent magnetization data of the composite clearly show the jumps in magnetization curves at the structural phase transitions of BaTiO3, thereby indicating their origin in ME coupling. The change in coercivity of composite sample in different ferroelectric phases of BaTiO3 has been observed compared to the NiFe2O4 sample. The different lattice strains corresponding to different ferroelectric phases of BaTiO3 could be the driving force for modulating the magnetization and coercivity of the composite material. This is clear evidence of strain mediated ME coupling in ferrimagnetic and ferroelectric composite materials.

Magneto-electric interactions at bending resonance in an asymmetric multiferroic composite: Theory and experiment on the influence of electrode position

In magnetostrictive-piezoelectric bilayers the strength of mechanical strain mediated magneto-electric (ME) interactions shows a resonance enhancement at bending modes. Such composites when operating under frequency modulation at bending resonance have very high ME sensitivity and are of importance for ultrasensitive magnetometers. This report provides an avenue for further enhancement in the ME sensitivity by strategic positioning of the electrodes in the bilayer. We discuss the theory and measurements on the dependence of ME coupling on the position of electrodes in a lead zirconate titanate-permendur bilayer. Samples of effective length L with full electrodes and partial electrodes of length l = L/3 are studied. A five-fold increase in ME voltage coefficient (MEVC) at bending resonance and a 75% increase in low-frequency MEVC are measured as the partial electrode position is moved from the free-end to clamped-end of the bilayer. When the partial electrode is close to the clamped end, the low-frequency and resonance MEVC are 22% and 45% higher, respectively, than for fully electroded bilayer. According to the model discussed here these observations could be attributed to non-uniform stress along the sample length under flexural deformation. Such deformations are stronger at the free-end than at the clamped-end, thereby reducing the stress produced by applied magnetic fields and a reduction in MEVC. Estimates of MEVC are in good agreement with the data.

Magnetic field directed assembly of superstructures of ferrite-ferroelectric core-shell nanoparticles and studies on magneto-electric interactions

Composites of ferromagnetic and ferroelectric are of interest for studies on mechanical strain mediated magneto-electric (ME) interactions and for useful technologies. Here, we report on magnetic-field-assisted-assembly of barium titanate (BTO)-nickel ferrite (NFO) core-shell particles into linear chains and 2D/3D arrays and measurements of ME effects in such assemblies. First, we synthesized the core-shell nano-particles with 50–600 nm BTO and 10–200 nm NFO by chemical self-assembly by coating the ferroic particles with complementary coupling groups and allowing them to self-assemble in the presence of a catalyst via the “click” reaction. The core-shell structure was confirmed with electron microscopy and scanning probe microscopy. We obtained superstructure of the core-shell particles by subjecting them to a magnetic field gradient that exerts an attractive force on the particles and align them toward the regions of high field strengths. At low particle concentration, linear chains were formed and they evolved into 2D and 3D arrays at high particle concentrations. Magnetoelectric characterization on unassembled films and assembled arrays has been performed through measurements of low-frequency ME voltage coefficient (MEVC) by subjecting the sample to a bias magnetic field and an ac magnetic field. The MEVC is higher for field-assembled samples than for unassembled films and is found to be sensitive to field orientation with a higher MEVC for magnetic fields parallel to the array direction than for magnetic fields perpendicular to the array. A maximum MEVC of 20 mV/cm Oe, one of the highest reported for any bulk nanocomposite, is measured across the array thickness. A model is provided for ME coupling in the superstructures of BTO-NFO particulate composites. First, we estimated the MEVC for a free-standing BTO-NFO core-shell particle and then extended the model to include an array of linear chains of the particles. The theoretical estimates are in qualitative agreement with the data.

Superstructures of self-assembled multiferroic core-shell nanoparticles and studies on magneto-electric interactions

Superstructures of linear chains and arrays of chemically self-assembled core-shell nanoparticles of nickel ferrite and barium titanate have been obtained by magnetic-field-assisted assembly (MFAA). Studies on strain mediated magneto-electric (ME) effects by ME voltage coefficient (MEVC) measurements reveal higher MEVC for MFAA samples than for unassembled films and is higher for magnetic fields parallel to the array orientation than for transverse fields. The strongest ME effect is measured in MFAA-films with a maximum MEVC of 8 mV/cm Oe, one of the highest reported for as-assembled nanocomposite. A model is discussed for the ME coupling in the MFA samples.

Shear strain mediated magneto-electric effects in composites of piezoelectric lanthanum gallium silicate or tantalate and ferromagnetic alloys

Shear strain mediated magneto-electric (ME) coupling is studied in composites of piezoelectric Y-cut lanthanum gallium silicate (LGS) or tantalate (LGT) and ferromagnetic Fe-Co-V alloys. It is shown that extensional strain does not result in ME effects in these layered composites. Under shear strain generated by an ac and dc bias magnetic fields along the length and width of the sample, respectively, strong ME coupling is measured at low-frequencies and at mechanical resonance. A model is discussed for the ME effects. These composites of Y-cut piezoelectrics and ferromagnetic alloys are of importance for shear strain based magnetic field sensors.

Effect of Ni0.5Zn0.5Fe2O4 (NZFO) layer thickness on the magneto-electric properties of BaTiO3 (BTO)-NZFO composite bilayer thin films

Structural, magnetic, ferroelectric and magneto-electric (ME) studies of bilayer thin films of BaTiO3 (BTO)-Ni0.5Zn0.5Fe2O4(NZFO) prepared using pulsed laser deposition on platinized silicon substrate are reported. Thickness of BTO layer(≈400 nm) is kept the same and the thickness of the NZFO layer is varied from 25–150 nm to study the effects. The prepared films are studied using grazing incidence x-ray diffraction (GIXRD), Raman spectroscopy, atomic force microscopy (AFM), magnetization and ferroelectric loop tracer measurements. From GIXRD, all the films are found to be polycrystalline in nature. Raman data confirms the ferroelectric (tetragonal) and cubic phases of BTO and NZFO, respectively in the bilayers. It is observed from the AFM data that the growth of NZFO layer is correlated with that of bottom BTO layer for thickness 75 nm resulting in stress on the BTO phase evidenced from shifting of Raman mode corresponding to BTO. Room temperature magnetic and ferroelectric properties are shown by recording the room temperature M-H and P-E loops. Further, PUND (positive up and negative down) measurements are carried out to capture the true ferroelectric polarization of the films. ME properties of the films was probed by recording P-E measurements and PUND in the presence of external magnetic field and it is observed that the ME coupling is maximum for the film with NZFO layer thickness of 75 nm. The observed results are explained in terms of stress mediated ME coupling.

Magnetic field assisted self-assembly of ferrite-ferroelectric core-shell nanofibers and studies on magneto-electric interactions

Core-shell nanofibers of nickel ferrite and lead zirconate titanate have been synthesized by electrospinning, assembled into superstructure in uniform or non-uniform magnetic fields, and have been characterized in terms of ferroic order parameters and strain mediated magneto-electric (ME) coupling. The core-shell structure was confirmed by electron microscopy and scanning probe microscopy. Studies on magnetic field induced polarization P in assembled samples showed a decrease or increase in P, depending on the nature of fibers and strengthening of ME coupling with change in remnant-P as high as 32%. Strong ME interactions were evident from H-induced variation in permittivity at 20–22 GHz.

Thickness-dependent converse magnetoelectric coupling in bi-layered Ni/PZT thin films

The converse magnetoelectric (ME) effect was investigated in bi-layered Ni/Pb(Zr0.52Ti0.48)O3 (PZT) thin films grown on Si substrates, with different thicknesses of Ni thin layers. By using an AC-mode magneto-optical Kerr effect method, it was revealed that the electric-voltage induced magnetization variation was dependent on the thickness of the Ni thin layer. The results showed that the Ni/PZT bilayered films with thick (about 40 nm) Ni films presented a dominative strain mediated ME coupling, whereas an interface-charge and strain co-mediated ME coupling might coexist in the Ni/PZT bilayered films with decreasing thickness (e.g., about 10 nm) of Ni films.

Low-frequency and resonance magnetoelectric effects in piezoelectric and functionally stepped ferromagnetic layered composites

The strain mediated magnetoelectric (ME) coupling is studied in bilayers of lead zirconate titanate (PZT) and a ferromagnetic layer that is functionally stepped. Nickel with negative piezomagnetic coefficient $q$ and Metglas with positive $q$ are bonded to achieve the desired step in $q$ for the ferromagnetic phase. Samples of PZT-Ni-Metglas and PZT-Metglas-Ni are used for measurements of ME voltage coefficient (MEVC) at low frequencies and at frequencies corresponding to bending resonance. It is shown that at low frequencies the bending moment due to stepped-$q$ counteracts the asymmetry-related flexural strain in the sample and enhances the strength of ME coupling. The MEVC is found to be as high as in symmetric trilayers in which flexural deformation is absent, and it ranges from 0.4 to 3.4 V/cm Oe depending on the grading scheme and the thickness of Metglas. A resonance enhancement of the MEVC to 40--220 V/cm Oe is measured at bending modes for samples clamped at one end. Samples of PZT-Ni-Metglas show a higher MEVC than for PZT-Metglas-Ni both at low frequencies and at bending resonance. The $q$-stepped composites are of importance for ultra-sensitive magnetic field sensors.

Tunable magnetoresistance devices based on multiferroic heterostructures

Surface-effect induced strong magnetoelectric (ME) coupling was investigated in ultrathin Ni80Fe20/PZN-PT (lead zinc niobate-lead titanate) multiferroic heterostructures. Giant electric field (E-field) tuning of the magnetic anisotropy, ferromagnetic resonance field, coercive field and anisotropic magnetoresistance were demonstrated through strain mediated ME coupling in 10 nm thick Ni80Fe20/PZN-PT multiferroic heterostructures. E-field dynamic modulation of magnetoresistance was also displayed. These provide great opportunities for electrostatic tunable MR devices.

Voltage control of the magnetic coercive field: Multiferroic coupling or artifact?

The ability to dynamically tune the coercive field of magnetic thin films is a powerful tool for applications, including in magnetic recording disk technologies. Recently, a number of papers have reported the electrical voltage control of the coercive field of various magnetic thin films in multiferroic composites. Theoretically, this is possible in magneto-electric (ME) multiferroics due to the piezoferroelectric component that can be electrically activated to dynamically modify the properties of the magnetic component of the composite via a direct or strain mediated ME coupling. In this paper we fabricated and examined such structures and we determined that the magnetic coercive field reduction is most likely due to a heating effect. We concluded that this effect is probably an artifact that cannot be attributed to a multiferroic coupling.

Flexural deformation in a compositionally stepped ferrite and magnetoelectric effects in a composite with piezoelectrics

The nature of strain mediated magnetoelectric (ME) coupling is investigated in laminates of lead zirconate titanate (PZT) and compositionally stepped ferrite with grading of piezomagnetic coefficient. ME effects that could only be attributed to grading related bending strain are observed in a trilayer of ferrite and oppositely poled PZT. It is shown that in a bilayer, grading induced flexural strain counteracts bending moment due to structural asymmetry and enhances ME coupling by a factor of 2. A zero-bias field ME effect is observed in such laminates. The graded composites are of interest for self-biased magnetic field sensors.