Magnetoelectric Polarization Research Articles

Consistent c-axis orientation and reinforced magnetoelectric coupling performance in bulk BaSrCo2Fe11AlO22

Consistent c-axis orientation characteristics toward the direction of applied pressure of Y-type BaSrCo2Fe11AlO22 (BSCFAO-θ) were successfully prepared by the strategy of hot pressing. The samples’ magnetoelectric coupling performance was greatly enhanced by the strategy of hot pressing than that of the traditional solid reaction method. The magnetoelectric current of BSCFAO-θ (θ = 0°) (12.06 μA/m2) was nearly 5 times higher than BSCFAO-SS (2.24 μA/m2). The magnetoelectric polarization of BSCFAO-θ (θ = 0°) (32.82 μC/m2) was nearly 4 times higher than that of BSCFAO-SS (8.31 μC/m2). The magnetoelectric coupling coefficients of BSCFAO-θ (θ = 0°) (1880 ps/m) were nearly 4 times higher than those of BSCFAO-SS (404 ps/m). The enhancement of magnetoelectric polarization of BSCFAO-θ (θ = 0°) can be attributed to the uniform c-axis orientation, which reinforced magnetoelectric polarization compared with none orientation that eliminated the magnetoelectric polarization on grain boundaries. The magnetoelectric coupling performance of BSCFAO-θ presented a gradually decreasing trend with the angle changing from 0° to 90° for the changing of orientation. Compared with traditional solid reaction methods, the strategy of hot pressing inhibits grain growth and increases grain boundaries, thus facilitating the enhancement of grain boundaries’ resistivity, and the electrical resistivity of BSCFAO-θ (3.08 × 1010 Ω cm) was nearly 10 000 times higher than that of BSCFAO-SS (2.5 × 106 Ω cm), which also benefits the magnetoelectric polarization performance of BSCFAO-θ. Therefore, the hot-pressing strategy can contribute to the forming of oriented ceramics and enhance the grain boundaries’ resistivity to improve magnetoelectric coupling performance.

Strain-induced modulation of magnetodielectric and magnetoelectric coupling in pzt/cfo thin films grown by rf-sputtering

The miniaturization of systems devices based on the magnetoelectric effect requires a better understanding of the factors that rule the coupling at the interface between different materials in magnetoelectric composites. In this context, this work presents the influence of stress generated at the interface PZT-CFO on multiferroic properties and the magnetodielectric coupling of the PbZr0,2Ti0,8O3/CoFe2O4 heterostructures fabricated by RF-Sputtering technique. It was found that tensile stress generated in the PZT/CFO interface modifies both XRD patterns and ferromagnetic hysteresis loop, inducing an additional contribution to the magnetic anisotropy compared with CFO and PZT layers. In addition, the dielectric properties of the PZT/CFO system are ruled by interfacial charges that accumulate in the PZT-CFO interface. On the other hand, changes in dielectric permittivity as a function of magnetic field (i.e., the magnetodielectric effects) result of the association between the ferroelectric domain movement modulated by stress on the PZT phase due to magnetostriction generated by the CFO phase and the magnetoelectric polarization contribution properly characterized by changes in the remnant polarization due to a magnetic field.

Low magnetic-field induced high temperature dynamic magnetoelectric coupling performances in Z-type Sr3Co2Fe24O41

Magnetic-field induced dynamic magnetoelectric coupling effects and polarization performance of Z-type Sr3Co2Fe24O41 (SCFO) ceramic has been investigated. Results found that SCFO’s transverse tapered magnetic structure can induce electric polarization, and its electric polarization direction will not change under external magnetic effects. First-order dynamic magnetoelectric coupling coefficient (α) and second-order dynamic magnetoelectric coupling coefficient (β) of SCFO exhibited strong response main in magnetic structural phase transition region. The magnetoelectric structural phase transition position appeared in low magnetic field, and the magnetic moment vector and its corresponding electric polarization vector of SCFO exhibited the most unstable state near its equilibrium position, which is beneficial for inducing strong dynamic magnetoelectric coupling response. When the applied magnetic fields to SCFO increased, the magnetic moment stability near the equilibrium position increased, and the dynamic magnetoelectric coupling response decreased. Results showed that the dynamic magnetoelectric coupling response of SCFO can bear T 1 = 370 K high temperature. The dynamic magnetoelectric coupling response induced by low magnetic fields in SCFO contributes to its actual application in next generation magnetoelectric information storage devices.

Probing magnetic symmetry in antiferromagnetic Fe4Nb2O9 single crystals by linear magnetoelectric tensor

In the present study, we investigated magnetodielectric, magnetoelectric (ME), and angular-dependent polarization in single-crystal Fe4Nb2O9. The magnetodielectric effects in epsilon(x) (x//[100]), epsilon(y) (y//[120]), and epsilon(z) (z//[001]) are found to be significant only around T-N approximate to 95 K when magnetic fields are applied along three orthogonal x-, y- (y//[120]), and z directions. The finite polarization P-x, P-y, and P-z of 70, 100, and 30 mu C/m(2), respectively, can be induced in the antiferromagnetic phase when a finite magnetic field up to 9 T is applied along the three orthogonal directions. The angular-dependent polarization measurements verify the dominating linear ME effects below T-N. From the above experimental results, a linear ME tensor a(ij) with all nine nonzero components can be inferred, demonstrating a much lower magnetic point group of -1' for the canted antiferromagnetic configuration.

Hollow double-shell structured Void@SiO2@Co-C composite for broadband electromagnetic wave absorption

Current electromagnetic wave (EMW) absorbing materials generally face the problem of narrow absorption bandwidth, which is the bottleneck in practical applications. Exploring new strategies and designing new architectures to overcome this problem is urgent but challenging. Here, a hollow double-shell structured magneto-electric composite, composed of an internal hollow cavity, a middle SiO2 matching shell and an outer Co-C porous shell ([email protected]2@Co-C), was prepared by assembling metal–organic-framework particles on hollow SiO2 cavity and pyrolysis. The multiple magnetic resonances, magneto-electric interface polarization, defect dipole polarization and conductivity of the composite can be well tuned by controlling the pyrolysis temperature. In particular, the hollow SiO2 cavity plays an essential role in improving impedance matching. Benefitting from the synergistic effect between hollow SiO2 cavity, porous carbon shell and Co particles, the [email protected]2@Co-C composite exhibits a broad effective absorption bandwidth above 7 GHz at a thin thickness of 2.2 mm, which is superior to other Co-C composites (less than 6 GHz). This work provides an effective solution for broadening the EMW absorption bandwidth.

Magnetoelectric coupling in the mixed erythrosiderite [(NH4)1−xKx]2[FeCl5(H2O)]

We present a study of the dielectric, structural, and magnetic properties of the multiferroic or linear magnetoelectric substitution series [(NH$_4$)$_{1-x}$K$_x$]$_2$[FeCl$_5$(H$_2$O)]. Pyroelectric currents, magnetic susceptibilities, and thermodynamic properties were examined on large single crystals of the erythrosiderite compounds and detailed magnetic-field versus temperature phase diagrams are derived for three different substitution levels. With increasing potassium concentration the material is tuned from a multiferroic ($x \le 0.06$) to a linear magnetoelectric ($x \ge 0.15$) ground state. In contrast to the respective pure parent compounds with $x=0$ or 1, however, the ferroelectric or linear magnetoelectric polarization in none of the substituted samples is switchable by external electric fields, because these samples exhibit a significant electric polarization already above the magnetic ordering transition. The polarization arises at a higher-lying structural phase transition that is examined by THz spectroscopy and, on a deuterated pure single crystal, by comprehensive neutron-diffraction experiments. The structural phase transition is attributed to an ordering of NH$_4^+$ tetrahedra but does not break inversion symmetry in the pure material, while a finite K content causes pyroelectricity.

Topological insulators and higher-order topological insulators from gauge-invariant one-dimensional lines

In this manuscript, we study the interplay between symmetry and topology with a focus on the $Z_2$ topological index of 2D/3D topological insulators and high-order topological insulators. We show that in the presence of either a two-fold-rotational symmetry or a mirror symmetry, a gauge-invariant quantity can be defined for arbitrary 1D lines in the Brillouin zone. Such 1D quantities provide a new pathway to compute the $Z_2$ index of topological insulators. In contrast to the generic setup, where the $Z_2$ index generally involves 2D planes in the Brillouin zone with a globally-defined smooth gauge, this 1D approach only involves some 1D lines in the Brillouin zone without requiring a global gauge. Such a simplified approach can be used in any time-reversal invariant insulators with a two-fold crystalline symmetry, which can be found in 30 of the 32 point groups. In addition, this 1D quantity can be further generalized to higher-order topological insulators to compute the magnetoelectric polarization $P_3$.

Constructing Asymmetrical Ni-Centered {NiN2O4} Octahedra in Layered Metal-Organic Structures for Near-Room-Temperature Single-Phase Magnetoelectricity.

Layered metal-organic structures (LMOSs) as magnetoelectric (ME) multiferroics have been of great importance for realizing new functional devices in nanoelectronics. Until now, however, achieving such room-temperature and single-phase ME multiferroics in LMOSs have proven challenging due to low transition temperature, poor spontaneous polarization, and weak ME coupling effect. Here, we demonstrate the construction of a LMOS in which four Ni-centered {NiN2O4} octahedra form in layer with asymmetric distortions using the coordination bonds between diphenylalanine molecules and transition metal Ni(II). Near room-temperature (283 K) ferroelectricity and ferromagnetism are observed to be both spontaneous and hysteretic. Particularly, the multiferroic LMOS exhibits strong magnetic-field-dependent ME polarization with low-magnetic-field control. The change in ME polarization with increasing applied magnetic field μ0H from 0 to 2 T decreases linearly from 0.041 to 0.011 μC/cm2 at the strongest ME coupling temperature of 251 K. The magnetic domains can be manipulated directly by applied electric field at 283 K. The asymmetrical distortion of Ni-centered octahedron in layer spurs electric polarization and ME effect and reduces spin frustration in the octahedral geometry due to spin-charge-orbital coupling. Our results represent an important step toward the production of room-temperature single-phase organic ME multiferroics.

Fractional charge bound to a vortex in two-dimensional topological crystalline insulators

We establish the correspondence between the fractional charge bound to a vortex in a textured lattice and the relevant bulk band topology in two-dimensional (2D) topological crystalline insulators. As a representative example, we consider the Kekule textured graphene whose bulk band topology is characterized by a 2D $\mathbb{Z}_{2}$ topological invariant $\nu_{\rm 2D}$ protected by inversion symmetry. The fractional charge localized at a vortex in the Kekule texture is shown to be related to the change in the bulk topological invariant $\nu_{\rm 2D}$ around the vortex, as in the case of the Su-Schriefer-Heeger model in which the fractional charge localized at a domain wall is related to the change in the bulk charge polarization between degenerate ground states. We show that the effective three-dimensional (3D) Hamiltonian, where the angle $\theta$ around a vortex in Kekule-textured graphene is a third coordinate, describes a 3D axion insulator with a quantized magnetoelectric polarization. The spectral flow during the adiabatic variation of $\theta$ corresponds to the chiral hinge modes of an axion insulator and determines the accumulated charge localized at the vortex, which is half-quantized when chiral symmetry exists. When chiral symmetry is absent, electric charge localized at the vortex is no longer quantized, but the vortex always carries a half-quantized Wannier charge as long as inversion symmetry exists. For the cases when magnetoelectric polarization is quantized due to the presence of symmetry that reverses the space-time orientation, we classify all possible topological crystalline insulators whose vortex defect carries a fractional charge.

Magnetodielectric and magnetoelectric correlation in (1-x)PMN-PT/xCFO 0–3 particulate composites

In this work, the relationship between dielectric, ferroelectric, piezoelectric properties and the magnetoelectric coupling in (1-x)PMN-PT/xCFO multiferroic particulate composites were investigated. The magnetodielectric response showed strong frequency and CFO concentration dependence, which for composites with CFO concentration higher than 30 M%, it was reported a magnetodielectric response up to 10% around the electromechanical resonance regime for magnetic fields of 15 kOe. Additionally, at low magnetic fields a positive magnetodielectric coefficient (∼2%) associated to the magnetoelectric coupling was observed. The MD effects for particulate composites were attributed to the co-contribution of the magnetoelectric polarization, at low magnetic fields, and a compressive stress on ferroelectric phase due to magnetostriction effect, at high magnetic field.

Distorted Monolayer ReS2 with Low-Magnetic-Field Controlled Magnetoelectricity.

Two dimensional (2D) materials possessing ferroelectric/ferromagnetic orders and especially low-magnetic-field controlled magnetoelectricity have great promise in spintronics and multistate data storage. However, ferroelectric and magnetoelectric (ME) dipoles in the atom-thick 2D materials are difficult to be realized due to structural inversion symmetry, thermal actuation, and depolarized field. To overcome these difficulties, the monolayer structure must possess an in-plane inversion asymmetry in order to provide out-of-plane ferroelectric polarization. Herein, crystal chemistry is adopted to engineer specific atomic displacement in monolayer ReS2 to change the crystal symmetry to induce out-of-plane ferroelectric polarization at room temperature. The cationic Re vacancy in the atom-displaced ReS2 monolayer causes spin polarization of two immediate neighbor sulfur atoms to generate magnetic ordering, and the ferroelectric distortion near the Re vacancy locally tunes the ferromagnetic order thereby triggering low-magnetic-field controlled ME polarization at about 28 K. As a result, 2D ME coupling multiferroics is achieved. Our results not only reveal a design methodology to attain coexistence of ferroelectric and ferromagnetic orders in 2D materials but also provide insights into magnetoelectricity in 2D materials.

Magnetic Field Control of Cycloidal Domains and Electric Polarization in Multiferroic BiFeO3

The magnetic field induced rearrangement of the cycloidal spin structure in ferroelectric monodomain single crystals of the room-temperature multiferroic BiFeO_{3} is studied using small-angle neutron scattering. The cycloid propagation vectors are observed to rotate when magnetic fields applied perpendicular to the rhombohedral (polar) axis exceed a pinning threshold value of ∼5 T. In light of these experimental results, a phenomenological model is proposed that captures the rearrangement of the cycloidal domains, and we revisit the microscopic origin of the magnetoelectric effect. A new coupling between the magnetic anisotropy and the polarization is proposed that explains the recently discovered magnetoelectric polarization perpendicular to the rhombohedral axis.

Interaction of a hydrogenlike ion with a planar topological insulator

An electric charge near the surface of a topological insulator (TI) induces an image magnetic monopole. Here we study the spectra of hydrogenlike ions near the surface of a planar TI, taking into account the modifications which arise due to the presence of the image monopole magnetic fields. In fact, the atom-TI interaction provides additional contributions to the Casimir-Polder potential while the ion-TI interaction modifies the energy shifts in the spectrum, which now became distance dependent. We show that the hyperfine structure is sensitive to the image magnetic monopole fields in states with nonzero angular momentum, and that circular Rydberg ions can enhance the maximal energy shifts. We discuss in detail the energy splitting of the $n$P$_{1/2}$ and $n$P$_{3/2}$ states in hydrogen. We also analyze the Casimir-Polder potential and find that this magnetic interaction produces a large distance repulsive tail for some particular atomic states. A sizable value of the maximum of the potential requires TIs with very low values of the permittivity together with high values of the topological magnetoelectric polarization.

Magnetic and magnetoelectric properties of the Tb0.75Ho0.25Fe3(BO3)4 ferroborate

Single crystals of the Tb0.75Ho0.25Fe3(BO3)4 ferroborate have been grown by the group method from a solution–melt based on bismuth trimolybdate. The magnetic and magnetoelectric properties of the ferroborate single crystals have been investigated in the temperature range from 4.2 to 300 K and in magnetic fields up to 9 T. Magnetically, this material is an antiferromagnet with the Neel temperature TN = 38.8 K and easy-axis anisotropy. The magnitude of the magnetoelectric polarization has been found to be more than 1.5–2.0 times greater than the sum of the polarizations induced by the magnetic field for the ferroborates TbFe3(BO3)4 and HoFe3(BO3)4 taken in the corresponding shares.

Magnetoelectric effects in the skyrmion host material Cu2OSeO3.

Insulating helimagnetic Cu2OSeO3 shows sizeable magnetoelectric effects in its skyrmion phase. Using magnetization measurements, magneto-current analysis and dielectric spectroscopy, we provide a thorough investigation of magnetoelectric coupling, polarization and dielectric constants of the ordered magnetic and polar phases of single-crystalline Cu2OSeO3 in external magnetic fields up to 150 mT and at temperatures below 60 K. From these measurements we construct a detailed phase diagram. Especially, the skyrmion phase and the metamagnetic transition of helical to conical spin order are characterized in detail. Finally we address the question if there is any signature of polar order that can be switched by an external electric field, which would imply multiferroic behaviour of Cu2OSeO3.

A mode III moving interfacial crack based on strip magneto-electric polarization saturation model

This paper deals with a mode III Yoffe-type interfacial crack propagating subsonically under the moving strip magneto-electric saturation model. Nonlinear effects are characterized by different magnetic and electric saturation strips around the crack tip. Employing the extended Stroh method, we obtain generalized moving interfacial dislocation densities analytically under impermeable magneto-electric crack boundary conditions. The generalized intensity factor and local energy release rate with nonlinear effects are derived as fracture parameters for the moving magneto-electro-elastic (MEE) interfacial crack. Numerical results are presented to show the characteristics of fracture dominant parameters with respect to the loading as well as the propagation velocity. In addition, a dimensionless parameter defined by the ratio of the volume fraction of the composite constituents is proposed to evaluate the influences of the MEE bimaterial properties. This research will give us ideas on material selection for optimizing the fracture toughness of MEE composites.

Magnetoelectric and magnetic properties of aluminum borates Ho1 − x Nd x Al3(BO3)4

The magnetoelectric and magnetic properties of substituted aluminum borates Ho1 − xNdxAl3(BO3)4 have been studied experimentally and theoretically. A large magnetoelectric effect exceeding all known values in isostructural compounds except for HoAl3(BO3)4 has been found. The magnetoelectric polarization of Ho0.8Nd0.2Al3(BO3)4 and Ho0.5Nd0.5Al3(BO3)4 at T = 5 K in a field of 9 T is ΔPab(Bb) ≈ −2630 and 1380 μC/m2, respectively. A theoretical consideration based on the crystal field model for the rare-earth ion made it possible to interpret all measured properties within the unified approach. The crystal field parameters have been determined. The temperature (3–300 K) and field (up to 9 T) dependences of the magnetization and the temperature (5–100 K) and field (up to 9 T) dependences of the polarization have been described. The studied properties of Ho1 − xNdxAl3(BO3)4 have been compared with those of HoAl3(BO3)4 demonstrating record-high polarization values.

Quantized electromagnetic response of three-dimensional chiral topological insulators

Protected by the chiral symmetry, three dimensional chiral topological insulators are characterized by an integer-valued topological invariant. How this invariant could emerge in physical observables is an important question. Here we show that the magneto-electric polarization can identify the integer-valued invariant if we gap the system without coating a quantum Hall layer on the surface. The quantized response is demonstrated to be robust against weak perturbations. We also study the topological properties by adiabatically coupling two nontrivial phases, and find that gapless states appear and are localized at the boundary region. Finally, an experimental scheme is proposed to realize the Hamiltonian and measure the quantized response with ultracold atoms in optical lattices.

Ferroelectric-ferromagnetic multilayers: A magnetoelectric heterostructure with high output charge signal

Multiferroic composites and heterostructures comprising ferroelectric and ferromagnetic materials exhibit room-temperature magnetoelectric (ME) effects greatly exceeding those of single-phase magnetoelectrics known to date. Since these effects are mediated by the interfacial coupling between ferroic constituents, the ME responses may be enhanced by increasing the density of interfaces and improving their quality. A promising material system providing these features is a ferroelectric-ferromagnetic multilayer with epitaxial interfaces. In this paper, we describe theoretically the strain-mediated direct ME effect exhibited by free-standing multilayers composed of single-crystalline ferroelectric nanolayers interleaved by conducting ferromagnetic slabs. Using a nonlinear thermodynamic approach allowing for specific mechanical boundary conditions of the problem, we first calculate the polarization states and dielectric properties of ferroelectric nanolayers in dependence on the lattice mismatch between ferroic constituents and their volume fractions. In these calculations, the ferromagnetic component is described by a model which combines linear elastic behavior with magnetic-field-dependent lattice parameters. Then the quasistatic ME polarization and voltage coefficients are evaluated using the theoretical strain sensitivity of ferroelectric polarization and measured effective piezomagnetic coefficients of ferromagnets. For Pb(Zr0.5Ti0.5)O3−FeGaB and BaTiO3−FeGaB multilayers, the ME coefficients are calculated numerically as a function of the FeGaB volume fraction and used to evaluate the output charge and voltage signals. It is shown that the multilayer geometry of a ferroelectric-ferromagnetic nanocomposite opens the way for a drastic enhancement of the output charge signal. This feature makes biferroic multilayers advantageous for the development of ultrasensitive magnetic-field sensors for technical and biomedical applications.

Magnetoelectric Polarization of Paramagnetic HoAl<sub>3-X</sub>Ga<sub>X</sub>(BO<sub>3</sub>)<sub>4</sub> Single Crystals

HoAl3-xGax(BO3)4 (0 ≤ х ≤ 3) single crystals were grown from Bi2Mo3O12Li2MoO4-based fluxes. Structural inversion twinning depending on crystal composition and heat treatment conditions is revealed. It is established that upon substitution of Ga3+ ions for Al3+ the magnetic characteristics of the crystals do not significantly change, while the magnetoelectric polarization decreases with increasing gallium content. The measured value and sign of polarization charges depend on a ratio between the left and right isomers.