Sabita Acharya

Ferromagnetic–ferroelectric bilayers capable of strong zero-bias magnetoelectric (ME) coupling are critical for device miniaturization. Here, we report a self-biased ME response achieved in a bilayer structure consisting of lead zirconate titanate (PZT) and a dual-phase ferromagnetic composite. The composite consisted of hard barium hexaferrite (BaFe₁₂O₁₉) (BaM), which provides a high built-in magnetocrystalline anisotropy field, and soft nickel zinc spinel ferrite (Ni_0.8 Zn_0.2 Fe₂O₄) (NZFO) to ensure high magnetostriction compliance. Samples of (100 − x) wt.% BaM − x wt.% NZFO (BNZx) were synthesized via conventional solid-state sintering, with phase purity confirmed by X-ray diffraction. Magnetic characterization revealed substantial in-plane anisotropy fields (H_A) ranging from 4.8 kOe to 8.3 kOe and a high saturation magnetostriction λ₁₁ = −25 ppm. The ME voltage coefficient (MEVC) of the bilayers increased with higher spinel content, peaking at x = 90. Under zero external DC bias field, the BNZ90-PZT bilayer showed a low-frequency (=100 Hz) MEVC of 114 mV/cm Oe and a resonant (10.6 kHz) MEVC of 3.6 V/cm Oe. This strong self-biased coupling is driven by the BaM-provided internal magnetic flux, eliminating the need for external biasing elements and offering a practical framework for compact, bias-free sensors and energy harvesters.

This work is on magneto-electric (ME) interactions in multiferroic composites of electro-spun triaxial nanofibers composed of lead zirconate titanate (PZT), strontium ferrite, SrFe₁₂O₁₉, (SrM), and nickel ferrite NiFe₂O₄ (NFO). M-type hexagonal ferrite, SrM, with a high uniaxial magneto-crystalline anisotropy field, was chosen to achieve a self-magnetic bias, and NFO with high magnetostriction and piezomagnetic coefficient was chosen to strengthen the ME coupling in the composites. By integrating these three distinct ferroic phases, the triaxial design optimizes interfacial strain transfer to enhance the ME coupling strength in the absence of an external magnetic bias. Fibers with PZT core (Sample A), SrM core (Sample B), or NFO core (Sample C) were synthesized and annealed at 750 C for crystallization of the ferroic phases. X-ray diffraction and electron and scanning probe microscopy confirmed the production of continuous, defect-free fibers with well-defined boundaries and coexisting crystalline phases free of impurities. Magnetic, ferroelectric, and magnetostrictive characterization verified ferroic ordering across all samples. Measurements of the ME voltage coefficients (MEVC) were carried out at low frequencies and at electromechanical resonance (EMR) on rectangular platelets of the fibers. A strong zero-bias ME response was measured, indicating an efficient strain-mediated coupling that bypasses the need for magnetic bias fields. Sample-C with NFO core, PZT inner shell, and SrM outer shell showed the highest low-frequency MEVC of 26.7 mV/cm Oe, and that increased to 161 mV/cm Oe at EMR, both values at zero bias. The stacking order of the core and shell phases dictated the resulting ME interactions. The results indicate the potential of these triaxial fibers as candidates for miniature magnetic sensors and arrays, multifunctional devices, and energy harvesters.

This report is on Co and Ti substituted M-type barium and strontium hexagonal ferrites that are reported to be single phase multiferroics due to a transition from Neel type ferrimagnetic order to a spiral spin structure that is accompanied by a ferroelectric polarization in an applied magnetic field. The focus here is the nature of magnetoelectric (ME) interactions in the bilayers of ferroelectric PZT and Co and Ti substituted BaM and SrM. The ME coupling in the ferrite-PZT bilayers arise due to the transfer of magnetostriction-induced mechanical deformation in a magnetic field in the ferrite resulting in an induced electric field in PZT. Polycrystalline Co and Ti doped ferrites, Ba (CoTi)_x Fe_12−2xO_19, (BCTx), and Sr (CoTi)_x Fe_12−2xO₁₉ (SCTx) (x = 0–4) were found to be free of impurity phases for all x-values except for SCTx, which had a small amount of α-Fe₂O₃ in the X-ray diffraction patterns for x ≤ 2.0. The magnetostriction for the ferrites increased with applied filed H to a maximum value of around 2 to 6 ppm for H~5 kOe. BCTx/SCTx samples showed ferromagnetic resonance (FMR) for x = 1.5–2.0, and the estimated anisotropy field was on the order of 5 kOe. The magnetization increased with the amount of Co and Ti doping, and it decreased rapidly with x for x > 1.0. Measurements of ME coupling strengths were conducted on the bilayers of BCTx/SCTx platelets bonded to PZT. The bilayer was subjected to an AC and DC magnetic field H, and the magnetoelectric voltage coefficient (MEVC) was measured as a function of H and frequency of the AC field. For BCTx-PZT, the maximum value of MEVC at low frequency was ~5 mV/cm Oe, and a 40-fold increase at electromechanical resonance (EMR). SCTx–PZT composites also showed a similar behavior with the highest MEVC value of ~14 mV/cm Oe at low frequencies and ~200 mV/cm Oe at EMR. All the bilayers showed ME coupling for zero magnetic bias due to the magnetocrystalline anisotropy field in the ferrite that provided a built-in bias field.

This report is on Co- and Ti-substituted M-type barium and strontium hexagonal ferrites that are reported to be single phase multiferroics due to transition from Neel type ferrimagnetic order to a spiral spin structure that is accompanied by a ferroelectric polarization in an applied magnetic field. The focus here is the nature of magnetoelectric (ME) interactions in bilayers of ferroelectric PZT and Co- and Ti-substituted BaM and SrM. The ME coupling in the ferrite-PZT bilayers arise due to the transfer of magnetostriction induced mechanical deformation in a magnetic field in the ferrite resulting in an induced electric field in PZT. Polycrystalline Co and Ti doped ferrites, Ba (CoTi)x Fe12-2xO19, (BCTx), and Sr (CoTi)x Fe12-2xO19 (SCTx) (x = 0-4) were found to be free of impurity phases free all x-values except for SCTx that had a small amount of α-Fe2O3 in the x-ray diffraction patterns for x ≤ 2.0. The magnetostriction for the ferrites increased with applied filed H to a maximum value of around 2 to 6 ppm for H ~ 5 kOe. BCTx/SCTx samples showed ferromagnetic resonance (FMR) for x = 1.5 - 2.0 and the estimated anisotropy field was on the order of 5 kOe. The magnetization increased with the amount of Co and Ti doping and it decreased rapidly with x for x > 1.0. Measurements of ME coupling strengths were done on bilayers of BCTx/SCTx platelets bonded to PZT. The bilayer was subjected to an AC and DC magnetic field H and the magnetoelectric voltage coefficient (MEVC) was measured as a function of H and frequency of the AC field. For BCTx-PZT the maximum value of MEVC at low-frequency was ~5 mV/cm Oe and a 40-fold increase at electromechanical resonance (EMR). SCTx-PZT composites also showed a similar behavior with the highest MEVC value of ~14 mV/cm Oe at low frequencies and ~200 mV/cm Oe at EMR. All the bilayers showed ME coupling for zero magnetic bias due to the magneto-crystalline anisotropy field in the ferrite that provided a built-in bias field.

Composites of ferromagnetic and ferroelectric phases are of interest for studies on mechanical strain-mediated coupling between the two phases and for a variety of applications in sensors, energy harvesting, and high-frequency devices. Nanocomposites are of particular importance since their surface area-to-volume ratio, a key factor that determines the strength of magneto-electric (ME) coupling, is much higher than for bulk or thin-film composites. Core–shell nano- and microcomposites of the ferroic phases are the preferred structures, since they are free of any clamping due to substrates that are present in nanobilayers or nanopillars on a substrate. This review concerns recent efforts on ME coupling in coaxial fibers of spinel or hexagonal ferrites for the magnetic phase and PZT or barium titanate for the ferroelectric phase. Several recent studies on the synthesis and ME measurements of fibers with nickel ferrite, nickel zinc ferrite, or cobalt ferrite for the spinel ferrite and M-, Y-, and W-types for the hexagonal ferrites were considered. Fibers synthesized by electrospinning were found to be free of impurity phases and had uniform core and shell structures. Piezo force microscopy (PFM) and scanning microwave microscopy (SMM) measurements of strengths of direct and converse ME effects on individual fibers showed evidence for strong coupling. Results of low-frequency ME voltage coefficient and magneto-dielectric effects on 2D and 3D films of the fibers assembled in a magnetic field, however, were indicative of ME couplings that were weaker than in bulk or thick-film composites. A strong ME interaction was only evident from data on magnetic field-induced variations in the remnant ferroelectric polarization in the discs of the fibers. Follow-up efforts aimed at further enhancement in the strengths of ME coupling in core–shell composites are also discussed in this review.

This report is on magneto-electric (ME) interactions in bulk composites with coaxial fibers of nickel–zinc ferrite and PZT. The core–shell fibers of PZT and Ni_1−xZn_xFe₂O₄ (NZFO) with x = 0–0.5 were made by electrospinning. Both kinds of fibers, either with ferrite or PZT core and with diameters in the range of 1–3 μm were made. Electron and scanning probe microscopy images indicated well-formed fibers with uniform core and shell structures and defect-free interface. X-ray diffraction data for the fibers annealed at 700–900 °C did not show any impurity phases. Magnetization, magnetostriction, ferromagnetic resonance, and polarization P versus electric field E measurements confirmed the ferroic nature of the fibers. For ME measurements, the fibers were pressed into disks and rectangular platelets and then annealed at 900–1000 °C for densification. The strengths of strain-mediated ME coupling were measured by the H-induced changes in remnant polarization P_r and by low-frequency ME voltage coefficient (MEVC). The fractional change in P_r under H increased in magnitude, from +3% for disks of NFO–PZT to −82% for NZFO (x = 0.3)-PZT, and a further increase in x resulted in a decrease to a value of −3% for x = 0.5. The low-frequency MEVC measured in disks of the core–shell fibers ranged from 6 mV/cm Oe to 37 mV/cm Oe. The fractional changes in P_r and the MEVC values were an order of magnitude higher than for bulk samples containing mixed fibers with a random distribution of NZFO and PZT. The bulk composites with coaxial fibers have the potential for use as magnetic field sensors and in energy-harvesting applications.

This report is on magneto-electric (ME) interactions in bulk composites with coaxial nanofibers of nickel zinc ferrite and PZT. Core-shell fibers of PZT and Ni1-x Znx Fe2 O4 (NZFO) with x = 0-0.5 were made by electrospinning. Both kind of fibers, either with ferrite- or PZT-core and with diameters in the range 1-3 m were made. Electron- and scanning probe microscopy images indicted well-formed fibers with uniform core and shell structures and defect free interface. X-ray diffraction data for fibers annealed at 700-900 C did not show any impurity phases. Magnetization, magnetostriction, ferromagnetic resonance, and polarization P versus electric field E measurements confirmed the ferroic nature of the fibers. For ME measurements the fibers were pressed into disks and rectangular platelets and then annealed at 900-1000 C for densification. Strengths of strain mediated ME coupling were measured by H-induced changes in remnant polarization Pr and by low-frequency ME voltage coefficient (MEVC). The fractional change in Pr under H increased in magnitude, from +3% for disks of NFO– PZT to -82% for NZFO (x=0.3)-PZT, and further increase in x resulted in a decrease to a value of -3% for x=0.5. Low-frequency MEVC measured in disks of the core-shell fibers ranged from 6 mV/cm Oe to 37 mV/cm Oe. The fractional changes in Pr and the MEVC values were an order of magnitude higher than for bulk samples containing mixed fibers with random distribution of NZFO and PZT. The bulk composites with coaxial fibers have the potential for use as magnetic field sensors and in energy harvesting applications.

The magnetoelectric effect (ME) is an important strain mediated-phenomenon in a ferromagnetic-piezoelectric composite for a variety of sensors and signal processing devices. A bias magnetic field, in general, is essential to realize a strong ME coupling in most composites. Magnetic phases with (i) high magnetostriction for strong piezomagnetic coupling and (ii) large anisotropy field that acts as a built-in bias field are preferred so that miniature, ME composite-based devices can operate without the need for an external magnetic field. We are able to realize such a magnetic phase with a composite of (i) barium hexaferrite (BaM) with high magnetocrystalline anisotropy field and (ii) nickel ferrite (NFO) with high magnetostriction. The BNx composites, with (100 − x) wt.% of BaM and x wt.% NFO, for x = 0–100, were prepared. X-ray diffraction analysis shows that the composites did not contain any impurity phases. Scanning electron microscopy images revealed that, with an increase in NFO content, hexagonal BaM grains become prominent, leading to a large anisotropy field. The room temperature saturation magnetization showed a general increase with increasing BaM content in the composites. NFO rich composites with x ≥ 60 were found to have a large magnetostriction value of around −23 ppm, comparable to pure NFO. The anisotropy field H_A of the composites, determined from magnetization and ferromagnetic resonance (FMR) measurements, increased with increasing NFO content and reached a maximum of 7.77 kOe for x = 75. The BNx composite was cut into rectangular platelets and bonded with PZT to form the bilayers. ME voltage coefficient (MEVC) measurements at low frequencies and at mechanical resonance showed strong coupling at zero bias for samples with x ≥ 33. This large in-plane H_A acted as a built-in field for strong ME effects under zero external bias in the bilayers. The highest zero-bias MEVC of ~22 mV/cm Oe was obtained for BN75-PZT bilayers wherein BN75 also has the highest H_A. The Bilayer of BN95-PZT showed a maximum MEVC ~992 mV/cm Oe at electromechanical resonance at 59 kHz. The use of hexaferrite–spinel ferrite composite to achieve strong zero-bias ME coupling in bilayers with PZT is significant for applications related to energy harvesting, sensors, and high frequency devices.

The magnetoelectric effect (ME) is an important strain mediated phenomenon in a ferromagnetic-piezoelectric composite for a variety of sensors and signal processing devices. A bias magnetic field, in general, is essential to realize a strong ME coupling in most composites. Magnetic phases with (i) high magnetostriction for strong piezomagnetic coupling and (ii) large anisotropy field that acts as a built-in bias field are preferred so that miniature, ME composite-based devices can operate without the need for an external magnetic field. We are able to realise such a magnetic phase with a composite of (i) barium hexaferrite (BaM) with high magnetocrystalline anisotropy field and (ii) nickel ferrite (NFO) with high magnetostriction. The BNx composites, with (1-x) wt.% of BaM and x wt.% NFO, for x = 0-100, were prepared. X-ray diffraction analysis shows that the composites did not contain any impurity phases. Scanning electron microscopy images revealed that with an increase in NFO content, hexagonal BaM grains become prominent, leading to a large anisotropy field. The NFO rich composites with x ≥60 show a large magnetostriction value of around -23 ppm, comparable to pure NFO. The anisotropy field HA of the composites, determined from ferromagnetic resonance (FMR) measurements, increased with increasing NFO content and reached a maximum of 12.39 kOe for x= 75. This large in-plane HA acted as a built-in field for strong ME effects under zero external bias in a bilayer with PZT. The BN composite was cut into rectangular platelets and bonded with PZT to form the bilayers. ME voltage coefficient (MEVC) measurements at low frequencies and at mechanical resonance showed strong coupling at zero bias for samples with x ≥33. The highest zero-bias MEVC of ~22 mV/cm Oe was obtained for BN75-PZT bilayers wherein BN75 also has the highest HA. Bilayer of BN41-PZT showed a amximum MEVC ~800 mV/cm Oe at electromechanical resonance at 68.4 kHz. The use of hexaferrite-spinel ferrite composite to achieve strong zero-bias ME coupling in bilayers with PZT is significant for applications related to energy harvesting, sensors, and high frequency devices.