Magnetoelectric Research Articles

Modeling the Converse Magnetoelectric Effect in the Low-Frequency Range.

This article is devoted to the theory of the converse magnetoelectric (CME) effect for the longitudinal, bending, longitudinal-shear, and torsional resonance modes and its quasi-static regime. In contrast to the direct ME effect (DME), these issues have not been studied in sufficient detail in the literature. However, in a number of cases, in particular in the study of low-frequency ME antennas, the results obtained are of interest. Detailed calculations with examples were carried out for the longitudinal mode on the symmetric and asymmetric structures based on Metglas/PZT (LN); the bending mode was considered for the asymmetric free structure and structure with rigidly fixed left-end Metglas/PZT (LN); the longitudinal-shear and torsional modes were investigated for the symmetric and asymmetric free structures based on Metglas/GaAs. For the identification of the torsion mode, it was suggested to perform an experiment on the ME structure based on Metglas/bimorphic LN. All calculation results are presented in the form of graphs for the CME coefficients.

Magnetic-Field-Assisted Electric-Field-Induced Domain Switching of a Magnetic Single Domain in a Multiferroic/Magnetoelectric Ni Nanochevron/[Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (PMN-PT) Layered Structure.

We report the magnetic-field-assisted electric-field-controlled domain switching of a magnetic single domain in a multiferroic/magnetoelectric Ni nanochevrons/[Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (PMN-PT) layered structure. Initially, a magnetic field was applied in the transverse direction across single-domain Ni nanochevrons to transform each of them into a two-domain state. Subsequently, an electric field was applied to the layered structure, exerting the converse magnetoelectric effect to transform/release the two-domain Ni nanochevrons into one of two possible single-domain states. Finally, the experimental results showed that approximately 50% of the single-domain Ni nanochevrons were switched permanently after applying our approach (i.e., the magnetization direction was permanently rotated by 180 degrees). These results mark important advancements for future nanoelectromagnetic systems.

Array Study of VLF Thin-Film Magnetoelectric Antenna with a Microbridge Structure.

Recently, magnetoelectric (ME) antennas have become a hot topic in the field of antenna miniaturization in the very-low-frequency (VLF) band because their size can be reduced to one-ten-thousandth of the size of conventional electric antennas. However, they still suffer from narrow transmission/reception bandwidth and weak radiation intensity. To address these issues, VLF thin-film ME antennas with a microbridge structure are designed, and the method of array connection is used. Test results show that the detection limit of the ME antenna unit is 636 pT/√Hz at 23 kHz and the radiant magnetic field intensity at 0.12 m is 0.87 nT (input power of 10 mW). By series-connecting three ME antenna units with the same resonance frequency, the output response has been increased to 1.72 times and the EM wave radiation intensity is increased to 1.9 times compared to a single unit. By parallel-connecting two ME antenna units with different resonance frequencies, the output response bandwidth has been expanded to 1.56 times compared to a single unit, and the signal radiation bandwidth has been expanded to 1.47 times. This work provides a valuable reference for the future larger-scale arraying of ME antennas.

Directed Self-Assembly of Oxide Nanocomposites by Ion-Beam Lithography.

Vertically aligned self-assembled nanocomposite films have provided a unique platform to study magnetoelectric effects and other forms of coupling between complex oxides. However, the distribution in the locations and sizes of the phase-separated nanostructures limits their utility. In this work, we demonstrate a process to template the locations of the self-assembled structure using ion lithography, which is effective for general insulating substrates. This process was used to produce a nanocomposite consisting of fin-shaped vertical nanostructures of ferroelectric BiFeO3 and ferrimagnetic CoFe2O4 with a feature size of 100 nm on (111)-oriented SrTiO3 substrates. Cross-sectional imaging of the three-phase perovskite-spinel-substrate epitaxial interface reveals the selective nucleation of CoFe2O4 in the trenches of the patterned substrate, and the magnetic domains of CoFe2O4 were manipulated by applying an external magnetic field.

Characterization of In‐Plane Piezoelectric Strain of Ferroelectric Thin Films by the Magnetoelectric Coupling Effect

Piezoelectric response of the ferroelectric/piezoelectric thin films is crucial for actuation of micro‐electromechanical systems (MEMS) devices as well as strain‐mediated electronic devices. Among many piezoelectric modes, longitudinal and transverse piezostrain is the most widely used. Various methods have been well established for quantitative characterization of the longitudinal piezostrain, such as laser interferometer and piezoelectric force microscopy. However, it is still a great challenge to characterize transverse piezostrain. Herein, a new method is established to characterize the transverse (in‐plane) piezostrain of ferroelectric/piezoelectric thin films by the inverse magnetoelectric (ME) coupling effect. A ferromagnetic thin film is patterned onto the ferroelectric thin film to form an artificial multiferroic heterostructure. The transverse piezostrain can transfer from the ferroelectric layer to the ferromagnetic layer and shift its ferromagnetic resonance field through the ME coupling effect. By comparing with a control sample of “bulk piezoceramic/ferromagnetic thin film” whose in‐plane piezostrain can be easily measured, the in‐plane piezostrain and piezoelectric coefficient of the ferroelectric thin films are then estimated. The in‐plane piezoelectric coefficient d31 and piezoelectric strain S31 of Pb(Zr,Ti)O3 (PZT) thin films are measured by this method. This new method will promote to fully understand the piezoelectric properties of the ferroelectric/piezoelectric thin films.

A Novel Spinel Ferrite-Hexagonal Ferrite Composite for Enhanced Magneto-Electric Coupling in a Bilayer with PZT.

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 HA 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 HA 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 HA. 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.

Enhanced magnetoelectric coupling effect in Mg-doped Y-type hexaferrite BaSrCo2-xMgxFe11.1Al0.9O22 ceramics

The influences of Mg doping on the magnetic order structure and magnetoelectric (ME) effect were investigated in Y-type hexaferrite BaSrCo2-xMgxFe11.1Al0.9O22 (x = 0.00, 0.25, 0.50, 0.75, 1.00) ceramics. The results revealed that the Mg doping could increase the magnetic phase transition temperature. The optimal magnetic-driven electric polarization could be obtained at x = 0.25 ceramic, exhibiting a maximum electric polarization of 16.22 μC/m2 and a ME coefficient of 203 ps/m at 100 K. The stability of reversible electric polarization was demonstrated via the measurement of continuous magnetic field cycles. Moreover, the ME phase diagram was established by measuring the magnetic dielectric properties. Finally, a possible Mg-tune magnetic structure was proposed to explain the enhanced ME effect.

Microstructural and physical characterizations of (1-x)(0.67BiFeO3+ 0.33BaTiO3+ 1%MnO2)− x(CoFe2O4+ 3%ZnO) magnetoelectric composites

BiFeO3 (BFO) was mixed with BaTiO3 (BTO) to form a solid solution of large piezoelectric coefficient (d33) for the fabrication of magnetoelectric (ME) composites with the Zn-doped CoFeO4 (CFO). The 0.67BFO+0.33BTO solid solution, which was doped with 1 at% MnO2 for lowering leakage current, showed a high d33 of 121 pC/N. However, in the composites made with CFO, a much lower d33 (<42 pC/N) was retained. The microstructure of the composites was featured by the large CFO grains (∼5 μm) embedded in the 0.67BFO+0.33BTO matrix of much finer grains (∼0.5 μm). The CFO grains each had a well-defined morphology with clear boundary discriminating them from the neighboring piezoelectric phase. Both the CFO and piezoelectric grains showed a high crystallinity as indicated by the appearance of clear Kikuchi patterns in electron backscattered diffraction. After reducing DC conductivity of CFO by Zn-doping and the optimization of fractional ratio of the Zn-doped CFO in the composites, a ME voltage coefficient of 81.8 mV/cm-Oe was achieved, which was high among the lead-free particulate ME composites prepared by the conventional solid-state sintering.

Resonance magnetoelectric effect in a three-layer non-symmetric ring structure Ni/PZT/Metglas

The resonant magnetoelectric (ME) effect in a ring-type structure of nickel–lead zirconate titanate–Metglas amorphous alloy (Metglas 2605SA1) has been studied. The use of ring geometry and magnetic layers with different signs of magnetostriction and comparable saturation fields led to a noticeable enhancement in the ME coefficient and a decrease in the optimal bias field, as compared with two-layer planar heterostructures. At the frequency of radial acoustic oscillations of the structure and under circular magnetization, the ME coefficient of 6.4 V (Oe·cm)−1 and ac field sensitivity of 0.6 V Oe−1 were obtained for a permanent bias field of 2 Oe.

Multiferroic and magnetic characterization of nickel ferrite- barium titanate nanocomposites

Nickel ferrite (NiFe2O4) samples were prepared by the modified sol–gel technique which is an effective, simple, and rapid synthesis method. Multiferroic composites of nickel ferrite and barium titanate (NiFe2O4-BaTiO3) in the ratio 1:1, 1:2, 2:1 as well as composites of nickel zinc ferrite and barium titanate (Ni1-xZnxFe2O4-BaTiO3) in the ratio 2:1 were synthesized by solid-state reaction method. The formation of nickel ferrite and barium titanate heterogeneous composites was identified using X-ray diffraction analysis (XRD). Magnetic properties were studied using vibrating sample magnetometer at 300 K. The coexistence of ferroelectric and ferromagnetic order confirmed through Magneto-electric (ME) coupling measurement.

Metastable Co3Mn/Fe/Pb(Mg1/3Nb2/3)O3–PbTiO3 multiferroic heterostructures

Using a molecular beam epitaxy technique, we experimentally demonstrate a multiferroic heterostructure consisting of metastable ferromagnetic Co3Mn on piezoelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT). Inserting a 2-nm-thick Fe layer between Co3Mn and PMN-PT(001) allows the formation of bcc Co3Mn layers even at an extremely low growth temperature of ∼80°C. Upon increasing this temperature to 200 °C, a bcc Co3Mn/Fe/PMN-PT(001) multiferroic heterostructure with a relatively large saturation magnetization of ∼1680 kA/m and an atomically flat interface is obtained, resulting in an obvious converse magnetoelectric (CME) effect. The large CME effect originates mainly from the strain-induced modulation of the magnetic anisotropy energy, supported by the first-principles calculations.

Cylindrical conformation and miniaturization of cavity-backed magnetoelectric antenna with an outer Γ-shaped probe

Abstract In this article, the cylindrical conformation of a linearly polarized cavity-backed magnetoelectric (ME) antenna is studied. Starting from a planar ME antenna presenting a wide bandwidth due to a specific design of its feeding probe, the impact of conformation is shown; the coupling between the ME dipole and the cavity walls is demonstrated to be the key element to keep a wideband behavior. Conformal antennas offering the same impedance bandwidth as the planar antenna are presented operating at Global Navigation Satellite System frequencies (1.164–1.61 GHz). As a result of the conformation, the antenna size has to be reduced to maintain the coupling and a wideband behavior. A prototype conformed to a 44-mm radius cylinder was built using low-cost additive manufacturing. External dimensions of 62 × 62 × 35 mm3 (0.285 × 0.285 × 0.16λ03, where λ0 is the wavelength at 1.38 GHz) were obtained, showing a ground plane area reduction of 46% compared to the planar antenna with the same materials. The conformal antenna also exhibits very steady radiation properties with a gain of around 4.5 dBi and a very similar and stable 3 dB beamwidth around 113° in E- and H-planes. A relatively good agreement is found between measurements and simulation.

Detecting Magnetoelectric Effect in a Metallic Antiferromagnet via Nonreciprocal Rotation of Reflected Light.

Certain types of media breaking both space-inversion (P) and time-reversal (T) symmetries but preserving their combination PT exhibit the polarization rotation of reflected light even when that of transmitted light is prohibited. Such an effect is termed nonreciprocal rotation of reflected light (NRR). Although NRR shows nearly the same phenomenon as the magnetooptical Kerr effect or, equivalently, the Hall effect at optical frequencies, its origin is distinct and ascribed to a magnetoelectric (ME) effect at optical frequencies, i.e., the optical ME effect. Here we show the observation of NRR in a metallic antiferromagnet TbB_{4}. The result demonstrates that the ME effect in a metallic system, which is considered to be ill defined, can be detected using reflected light. Furthermore, we spatially resolve antiferromagnetic domains in TbB_{4} by microscope observations of NRR. Our work offers a unique way to probe the ME effect in metallic systems.

Resonant Magnetoelectric Coupling of Fe-Si-B/Pb(Zr,Ti)O3 Laminated Composites with Surface Crystalline Layers.

The resonant magnetoelectric (ME) effect of Fe78Si9B13/Pb(Zr,Ti)O3 (FeSiB/PZT) composites with a surface-modified Fe78Si9B13 amorphous alloy has been studied. The surface-modified FeSiB can improve the ME coefficient at the resonant frequency by optimizing the magnetomechancial power conversion efficiency. The maximum ME coefficient of the surface-modified ribbons combined with soft PZT (PZT5) is two-thirds larger than that of the composites with fully amorphous ribbons. Meanwhile, the maximum value of the ME coefficient with surface-modified FeSiB ribbons and hard PZT (PZT8) is one-third higher compared with the fully amorphous composites. In addition, experimental results of magnetomechanical coupling properties of FeSiB/PZT composites with or without piezoelectric layers indicate that the power efficiency of the composites first decreases and then increases with the increase in the number of FeSiB layers. When the surface crystalline FeSiB ribbons are combined with a commercially available hard piezoelectric ceramic plate, the maximum magnetoelectric coupling coefficient of the ME composite reaches 5522 V/(Oe*cm), of which the electromechanical resonant frequency is 23.89 kHz.

High SNR magnetoelectric sensor with dual working modes for wideband magnetic field detection

Magnetoelectric (ME) composites exhibit extremely high sensitivity for magnetic field at the fixed resonant frequency, due to the strong strain-mediated ME coupling. However, wideband magnetic field detection is still challenging, as the ME coefficient depends strongly on the frequency. In this work, we propose a magnetoelectric sensor composed of three ME units in series/parallel connections, which works at dual modes to extend the frequency range with high signal-to-noise ratio (SNR). The SNR is enhanced by about 22.68 dB and the bandwidth is broadened from 100 Hz to 1 kHz, with a high sensitivity for frequency conversion mode by series/parallel connections. Limits of detections (LoDs) as low as 207 pT and 460 pT are demonstrated at 1 kHz and 100 kHz, respectively, showing promising applications in wideband magnetic field detection.

Probing magnetoelectric effect in the spin-modulated magnet Fe2GeO4

The distinct spin amplitude wave was reported in a highly frustrated magnetic compound Fe2GeO4, which is very different from observations on other members of the M2GeO4 (M = Fe, Co, and Ni) family, raising interest in this compound for some additional emergent phenomena. In particular, this non-uniform spin order allows the intrinsic connection between ferroelectric polarization and magnetically gradient structure to probe the potential linear magnetoelectric (ME) effect. In this work, we address this issue and investigate the magnetism of Fe2GeO4 single crystal that hosts two successive anomalies at antiferromagnetic (AFM) Néel temperatures T N1 ∼ 7.5 K and T N2 ∼ 6.7 K, respectively. Our results reveal a remarkable metamagnetic transition in the magnetization as a function of the magnetic field, occurring at a critical magnetic field H c ∼ 4.1 T when applied along the [110] and [1–10] directions, while such transition along the [001] direction is pointedly absent. Further exploration uncovers two predominant off-diagonal ME coefficients αyz and αzy in the incommensurate AFM phase between T N1 and T N2. Additionally, all components of the linear ME tensor remain non-vanishing in the canting AFM phase below T N2. This indicates the ME mechanisms for the two phases that may be driven by different magnetic structures. All these presented results are sufficient for us to draw a non-trivial ME phase diagram, which is beneficial to understanding the ME behavior of Fe2GeO4. Therefore, our study implies that Fe2GeO4, an unusual frustrated magnet, provides a platform for manipulating the fascinating ME effect in the spinel structure.

Polarization-induced magnetoelectric effect in Fe3Ga/HfO2/Fe3Ga heterojunction

Fe3Ga/HfO2/Fe3Ga heterojunction possesses reasonable lattice mismatch and good ferroelectric at the nanoscale. However, its magnetoelectric coupling is unexplored. Based on the first-principles calculations, we demonstrate that the magnetoelectric coupling in Fe3Ga/HfO2/Fe3Ga heterojunction is induced by polarization, which is different from the common strain-mediated magnetoelectric effect. The polarization-induced magnetoelectric effect of heterojunction is explained by the analyses of orbital-resolved density of states and spin densities, finding that the interfaces between Fe3Ga and HfO2 play an important role in magnetoelectric coupling, offering an alternative pathway for generating magnetoelectric coupling at room temperature.

Inducing a magnetic morphotropic phase boundary in (0.7) BiNdxFeO3-(0.3)BaTiO3 (0 ≤ x ≤ 0.05) system

Multiferroics materials processing ferroelectric and magnetic properties are being explored widely for device applications around the world. Herein, a composite multiferroic material was designed which is expected to show enhanced magnetoelectric (ME) multiferroic properties. In this unique idea, a lead free (0.7)BiFeO3-(0.3)BaTiO3 system with ferroelectric morphotropic phase boundary (MPB) was chosen and a magnetic MPB is induced through Nd incorporation. The MPB was confirmed through X-ray diffraction and Raman studies. The hysteresis loops confirmed the ferroelectric nature of solid solutions and the increase in polarization is related to the induced structural changes. A consistent increase in magnetization with Nd content indicates weaking of R3c symmetry of BiFeO3 due to continuous suppression of the spiral spin structure leading to transition from antiferromagnetic to ferromagnetic response. The presence of ferroelectric and ferromagnetic nature along with the existence of MPB indicates that solid solutions are promising candidates for magnetoelectric multiferroic applications.

Room temperature ferroelectricity and ferromagnetism in Ni-substituted Bi5Ti3FeO15

Aurivillius-type bismuth layer-structured ferroelectric (BLSF) Bi5Ti3FeO15 (BTF) has recently attracted considerable attention as a typical multiferroic material because ferroelectric and magnetic orders coexist, but bulk BTF exhibits antiferromagnetic (AFM) orders and negligible intrinsic magnetoelectric (ME) coupling effects. In this study, nickel-substituted Bi5Ti3FeO15 (Bi5Ti3Fe0.5Ni0.5O15, abbreviated as BTF-Ni) was synthesized using a solid-state reaction method to explore and enhance both the magnetic and ferroelectric properties of BTF. Polarization-electric field P-E loops indicate that the BTF-Ni exhibits considerable maximum polarization P m of 11.9 μC/cm2 and remnant polarization P r of 5.8 μC/cm2, but still keeps a very high ferroelectric Curie temperature (FE T c) of 1029 K, which are much superior to those of pure BTF. Moreover, magnetization-magnetic field M-H loops indicate that BTF-Ni exhibits significant ferromagnetic properties with a large saturation magnetization M s of 60 memu/g, low coercive field H c of 31 Oe at room temperature, and a high ferromagnetic Curie temperature (FM T c) of 698 K, whereas pure BTF has an antiferromagnetic Néel temperature (T N) of 80 K. Our work suggests that nickel-substituted BTF is a potential room-temperature magnetoelectric multiferroic material.

Electrical detection and modulation of magnetism in a Dy-based ferroelectric single-molecule magnet

Electrical control of magnetism in single-molecule magnets with peculiar quantum magnetic behaviours has promise for applications in molecular electronics and quantum computing. Nevertheless, such kind of magnetoelectric effects have not been achieved in such materials. Herein, we report the successful realization of significant magnetoelectric effects by introducing ferroelectricity into a dysprosium-based single-molecule magnet through spatial cooperation between flexible organic ligands and halide ions. The stair-shaped magnetization hysteresis loop, alternating current susceptibility, and magnetic relaxation can be directly modulated by applying a moderate electric field. Conversely, the electric polarization can be modulated by applying a small magnetic field. In addition, a resonant magnetodielectric effect is clearly observed, which enables detection of quantum tunnelling of magnetization by a simple electrical measurement. The integration of ferroelectricity into single-molecule magnets not only broadens the family of single-molecule magnets but also makes electrical detection and modulation of the quantum tunnelling of magnetization a reality.