Magnetoelectric Susceptibility Research Articles

Signatures of nonlinear magnetoelectricity in the second harmonic spectra of SU(2) symmetry-broken quantum many-body systems

Quantum mechanical perturbative expressions for second-order dynamical magnetoelectric (ME) susceptibilities have been derived and calculated for a small molecular system using the Hubbard Hamiltonian with SU(2) symmetry breaking in the form of spin–orbit coupling (SOC) or spin–phonon coupling. These susceptibilities will have signatures in second harmonic generation spectra. We show that SU(2) symmetry breaking is the key to generate these susceptibilities. We have calculated these ME coefficients using the full many-body basis and solved the Hamiltonian matrix for low-lying excited states using Lanczos method. Varying the Hubbard term along with SOC strength, we find spin and charge and both spin–charge dominated spectra of dynamical ME coefficients. We have shown that intensities of the peaks in the spectra are the highest when the magnitudes of Hubbard and SOC strengths are in a similar range.

Magnetoelectric coupling on fused azulene oligomers

The global magnetic phase diagram for fused azulene oligomers is obtained by using a fermionic Hubbard model, which is an intermediate between the molecular Pariser-Parr-Pople empiric Hamiltonian and the spin-$1/2$ antiferromagnetic Heisenberg model. We employ the density matrix renormalization group (DMRG) approach to explore the ground state properties of azulene-like molecules as a function of the electronic correlation and the oligomer size. It is shown that, depending on the length of the oligomer, fused azulene transitions from a singlet ($S=0$) to a higher-spin ($S=1,2$) ground state. Near the quantum magnetic phase transition the electric dipole moment, present on fused azulene molecules, couples with the magnetic moment leading to a divergent magnetoelectric susceptibility at the boundary lines of the magnetic phase diagram. These spontaneous electric and magnetic polarizations, together with the magnetoelectric coupling between them, indeed corroborate that these fused azulene oligomers can be viewed as a purely organic multiferroic material, being a magnetoelectric molecule.

Optical signature of magnetoelectric polarizability of topological insulators

The discovery of topological insulators has opened a new chapter of condensed matter physics. Recently, several experiments have confirmed that the magnetoelectric susceptibility of a topological insulator with time-reversal symmetry is quantized in terms of the fine-structure constant. To realize such measurements, a number of challenges have been overcome; particularly, high-precision time-domain terahertz polarimetry is devised. Here an experiment is proposed to observe the topological magnetoelectric effect. Based on the generalized multisphere Mie theory, it is shown that the optical activity of a cluster of topological insulator spheres elevates as metal spheres are deliberately brought near to them: The destructive interference of fields of the induced electric dipoles allows fields of the induced magnetic dipoles, which originate from the topological magnetoelectric effect, to play a prominent role. For example, for clusters of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and $\mathrm{InSb}$ spheres, the elevated azimuth and ellipticity angles are about $\ifmmode\pm\else\textpm\fi{}{20}^{\ensuremath{\circ}}$. Such a giant optical activity serves as an optical bridge to the determination of the fine-structure constant.

Magnetoelectric coupling susceptibility in novel lead-free 0–3 type multiferroic particulate composites of (1-x)Na0.5Bi0.5TiO3 -(x)CoCr0.4Fe1.6O4

Lead-free 0–3 particulate composites of chromium substituted cobalt nano-ferrite CoCr0.4Fe1.6O4 (CCFO) and sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT) having molar ratio of (1-x)Na0.5Bi0.5TiO3 -(x)CoCr0.4Fe1.6O4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5& 1) have been investigated. XRD patterns reveal that the composites crystallized with binary rhombohedraland cubic phase, equivalent to room temperature crystal symmetry of host ferroelectric NBT and magnetic CCFO materials, respectively. SEM micrographs depict the presence of both parent phases with different micro-structural symmetries. The observed ferroic properties, derived from isothermal magnetization and polarization measurements, emphasize that all the (1-x)NBT - (x)CCFO compositions exhibit significant sensitivity towards the magnetic field as well as the electric field. To evaluate the degree of magnetoelectric coupling in CCFO-NBT composites, we employed the magneto-dielectric characterization technique as a sensitive probing tool. We note that magneto-dielectric property initially increases with x and is found to be maximum ∼ −0.78% (x = 0.4) and further decreases with x. This unconventional product behaviour of particulate composite system is availed through magnetostrictive nature of magnetic subsystem - CFO and piezoelectric characteristics of ferroelectric- NBT. These lead-free multiferroic composites may provide an opportunity for the miniaturization of environmental-friendly magnetoelectric (ME) devices.

Observation of Anomalously Large Magnetoelectric Coupling in the Hexagonal Z‐Type Ferrite Films

AbstractAlthough the bulk hexaferrites are known to exhibit strong magnetoelectric (ME) effects near room temperature, the same effects are not realized in a thin film form. Herein, the growth of nearly epitaxial Co2Z‐type hexaferrite Ba0.3Sr2.7Co2Fe24O41 (BSCFO) thin films on a SrTiO3 (111) substrate by a chemical solution deposition method is reported. The reciprocal space mapping on the annealed BSCFO films reveals broadening along the qx directions in the (0014) and (0018) peaks, indicating that the oxygen planes belonging to the off‐centered octahedra have randomly distributed out‐of‐plane tilt to indicate significant disorder in the crystallographic orientation. Quantitative investigations reveal that both ME susceptibility and modulated electric polarization (ΔP) by magnetic field up to 370 K in the BSCFO films are always larger than those of the Co2Z‐type single crystal by ≈2–15 times and further increase upon having enhanced qx broadening in the (0014) and (0018) peaks. As the off‐centered octahedral sites can be a primary source of ΔP as predicted by the p–d hybridization mechanism, these results indicate that effective off‐centering could be further enhanced by the orientational disorder. This study identifies the potential of applying the ME hexaferrite films for multifunctional devices.

InSitu Electric-Field Control of THz Nonreciprocal Directional Dichroism in the Multiferroic Ba_{2}CoGe_{2}O_{7}.

Nonreciprocal directional dichroism, also called the optical-diode effect, is an appealing functional property inherent to the large class of noncentrosymmetric magnets. However, the insitu electric control of this phenomenon is challenging as it requires a set of conditions to be fulfilled: Special symmetries of the magnetic ground state, spin excitations with comparable magnetic- and electric-dipole activity, and switchable electric polarization. We demonstrate the isothermal electric switch between domains of Ba_{2}CoGe_{2}O_{7} possessing opposite magnetoelectric susceptibilities. Combining THz spectroscopy and multiboson spin-wave analysis, we show that unbalancing the population of antiferromagnetic domains generates the nonreciprocal light absorption of spin excitations.

Poling magnetic field dependence and memory effect of magnetoelectric coupling in honeycomb antiferromagnet cobalt niobate

We have firstly studied the poling magnetic field dependence of magnetoelectric (ME) coupling in a honeycomb antiferromagnet Co4Nb2O9, and found that the ME susceptibilities do not increase monotonously with increasing the poling magnetic field strength (HPole) as the usual case, but reach maxima around 10 kGs. This phenomenon results from opposite HPole dependences of domain wall thickness and energitically preferred domain's proportion. More interestingly, if the sample returns to paramagnetic phase after being poled and then reenters antiferromagnetic phase without being poled, ME coupling still exists. This memory effect is attributed to the presence of ME domain nucleation centers in paramagnetic phase.

Magnetoelectric effect in the honeycomb-lattice antiferromagnet BaNi2(PO4)2

We report a comprehensive investigation of the structural, magnetic, and electrical properties in a quasi-two-dimensional planar antiferromagnet ${\mathrm{BaNi}}_{2}{({\mathrm{PO}}_{4})}_{2}$, which crystallizes in the rhombohedral structure (space group $R\overline{3}$) consisting of honeycomb layers of ${\mathrm{Ni}}^{2+}$ ions. Magnetic susceptibility and heat capacity data reveal a long-range antiferromagnetic ordering of ${\mathrm{Ni}}^{2+}$ ions at ${T}_{N}=24$ K. Interestingly, an applied magnetic field induces a dielectric anomaly and an electric polarization at ${T}_{N}$ with the polarization proportional to the applied magnetic fields, demonstrating the linear magnetoelectric effect in ${\mathrm{BaNi}}_{2}{({\mathrm{PO}}_{4})}_{2}$ with a coupling coefficient of 1.67 ps/m. It is interesting to note that the magnetic symmetry associated with the magnetoelectric effect is ${\overline{1}}^{\ensuremath{'}}$, which allows all tensor elements of the magnetoelectric susceptibility.

Electrically tunable and reversible magnetoelectric coupling in strained bilayer graphene

The valleys in hexagonal two-dimensional systems with broken inversion symmetry carry an intrinsic orbital magnetic moment. Despite this, such systems possess zero net magnetization unless additional symmetries are broken, since the contributions from both valleys cancel. A nonzero net magnetization can be induced through applying both uniaxial strain to break the rotational symmetry of the lattice and an in-plane electric field to break time-reversal symmetry owing to the resulting current. This creates a magnetoelectric effect whose strength is characterized by a magnetoelectric susceptibility, which describes the induced magnetization per unit applied in-plane electric field. Here, we predict the strength of this magnetoelectric susceptibility for Bernal-stacked bilayer graphene as a function of the magnitude and direction of strain, the chemical potential, and the interlayer electric field. We estimate that an orbital magnetization of ~5400 $\mu_{\text{B}}/\mu\text{m}^2$ can be achieved for 1% uniaxial strain and a 10 $\mu\text{A}$ bias current, which is almost three orders of magnitude larger than previously probed experimentally in strained monolayer MoS$_2$. We also identify regimes in which the magnetoelectric susceptibility not only switches sign upon reversal of the interlayer electric field but also in response to small changes in the carrier density. Taking advantage of this reversibility, we further show that it is experimentally feasible to probe the effect using scanning magnetometry.

Magnetoelectric Induced Switching of Perpendicular Exchange Bias Using 30-nm-Thick Cr2O3 Thin Film

Magnetoelectric (ME) effect is a result of the interplay between magnetism and electric field and now, it is regarded as a principle that can be applied to the technique of controlling the antiferromagnetic (AFM) domain state. The ME-controlled AFM domain state can be read out by the magnetization of the adjacent ferromagnetic layer coupled with the ME AFM layer via exchange bias. In this technique, the reduction in the ME layer thickness is an ongoing challenge. In this paper, we demonstrate the ME-induced switching of exchange bias polarity using the 30-nm thick ME Cr2O3 thin film. Two typical switching processes, the ME field cooling (MEFC) and isothermal modes, are both explored. The required ME field for the switching in the MEFC mode suggests that the ME susceptibility (α33) is not deteriorated at 30 nm thickness regime. The isothermal change of the exchange bias shows the hysteresis with respect to the electric field, and there is an asymmetry of the switching field depending on the switching direction. The quantitative analysis of this asymmetry yields α33 at 273 K of 3.7 ± 0.5 ps/m, which is comparable to the reported value for the bulk Cr2O3.

Influence of XY anisotropy on a magnetoelectric effect in spin-1/2 XY chain in a transverse magnetic field

A magnetoelectric effect according to Katsura-Nagaosa-Balatsky mechanism in spin-1/2 XY chain in transverse magnetic field is considered. A spatial orientation of the electric field is chosen to provide an exact solution of the model in terms of free spinless fermions. The simplest model of quantum spin chain demonstrating a magnetoelectric effect, a zero temperature case of the spin-1/2 XX chain in a transverse magnetic field with Katsura-Nagaosa-Balatsky mechanism, is considered. The model has the simplest possible form of the magnetization, polarization and susceptibility functions, depending on electric and magnetic fields in a most simple form. For the case of arbitrary XY anisotropy, a non-monotonous dependence of magnetization on the XY anisotropy parameter is figured out. This non-uniform behaviour is governed by the critical point which is connected with the possibility to drive the system gapless or gapped by the electric field. Singularities of the magnetoelectric susceptibility at the critical value of system parameters are shown.

Inverse and direct magnetoelectric effects in orthorhombic DyMnO3 manganite single crystals

Correlations between the direct and inverse magnetoelectric effects in orthorhombic DyMnO3 single crystals have been investigated. In the inverse magnetoelectric effect, the magnetic moment of the crystal appears to have a contribution that is sinusoidal oscillating in ac electric fields below the temperature TFE of ferroelectric phase transition. The first and the second harmonics of the inverse magnetoelectric effect are clearly detected. The magnetoelectric susceptibilities α and β, corresponding to first and second harmonics, are found to correlate with the derivative of polarization with respect to the magnetic field. The influence of the magnetic and electric regimes during cooling on magnetoelectric effects has also been studied. The maximum change in the magnetic moment of a sample under the action of electric fields is observed under the same (T, H) conditions as the rotation of the spontaneous polarization vector from the crystallographic c direction to the a axis in the magnetic field H || b.

Jacobi-Maupertuis Randers-Finsler metric for curved spaces and the gravitational magnetoelectric effect

In this paper, we return to the subject of Jacobi metrics for timelike and null geodesics in stationary spacetimes, correcting some previous misconceptions. We show that not only null geodesics but also timelike geodesics are governed by a Jacobi-Maupertuis type variational principle and a Randers-Finsler metric for which we give explicit formulas. The cases of the Taub-NUT and Kerr spacetimes are discussed in detail. Finally, we show how our Jacobi-Maupertuis Randers-Finsler metric may be expressed in terms of the effective medium describing the behavior of Maxwell’s equations in the curved spacetime. In particular, we see in very concrete terms how the gravitational electric permittivity, magnetic permeability, and magnetoelectric susceptibility enter the Jacobi-Maupertuis Randers-Finsler function.

Controlling of light with electromagnons

Abstract Magnetoelectric coupling in multiferroic materials opens new routes to control the propagation of light. The new effects arise due to dynamic magnetoelectric susceptibility that cross-couples the electric and magnetic fields of light and modifies the solutions of Maxwell equations in media. In this paper, two major effects will be considered in detail: optical activity and asymmetric propagation. In case of optical activity the polarization plane of the input radiation rotates by an angle proportional to the magnetoelectric susceptibility. The asymmetric propagation is a counter-intuitive phenomenon and it represents different transmission coefficients for forward and backward directions. Both effects are especially strong close to resonance frequencies of electromagnons, i. e. excitations in multiferroic materials that reveal simultaneous electric and magnetic character.

Magnetoelectric spectroscopy of spin excitations in LiCoPO4

We have studied spin excitations in a single-domain crystal of antiferromagnetic LiCoPO4 by THz absorption spectroscopy. By analyzing the selection rules and comparing the strengths of the absorption peaks in the different antiferromagnetic domains, we found electromagnons and magnetoelectric spin resonances besides conventional magnetic-dipole active spin-wave excitations. Using the sum rule for the magnetoelectric susceptibility we determined the contribution of the spin excitations to all the different off-diagonal elements of the static magnetoelectric susceptibility tensor in zero as well as in finite magnetic fields. We conclude that the magnetoelectric spin resonances are responsible for the static magnetoelectric response of the bulk when the magnetic field is along the x-axis, and the symmetric part of the magnetoelectric tensor with zero diagonal elements dominates over the antisymmetric components.

Sign change of polarization rotation under time or space inversion in magnetoelectric YbAl3(BO3)4

Materials with optical activity can rotate the polarization plane of transmitted light. The most typical example is the natural optical activity, which has the symmetry property of changing sign after space inversion but being invariant to time inversion. Faraday rotation exhibits the opposite: it is invariant to space inversion but changes sign after time reversal. Here, we demonstrate that in a magnetoelectric material, another type of polarization rotation is possible. This effect is investigated in magnetoelectric YbAl3(BO3)4 under the viewpoint of time and space inversion symmetry arguments. We observe the sign change of the rotation sense under either time or space reversal. This investigation proves that the polarization rotation in YbAl3(BO3)4 must be classified as gyrotropic birefringence, which has been discussed within the idea of time-reversal breaking in underdoped cuprates. The diagonal terms in the magnetoelectric susceptibility are responsible for the observed signal of gyrotropic birefringence. Further analysis of the experimental spectra reveals a substantial contribution of the natural optical activity to the polarization rotation. We also demonstrate that the observed activity originates from the magnetoelectric susceptibility.

Renovation of Interest in the Magnetoelectric Effect in Nanoferroics

Recent theoretical studies of the influence of the magnetoelectric effect on the physical properties of nanosized ferroics and multiferroics have been reviewed. Special attention is focused on the description of piezomagnetic, piezoelectric, and linear magnetoelectric effects near the ferroid surface in the framework of the Landau–Ginzburg–Devonshire phenomenological theory, where they are considered to be a result of the spontaneous surface-induced symmetry reduction. Therefore, nanosized particles and thin films can manifest pronounced piezomagnetic, piezoelectric, and magnetoelectric properties, which are absent for the corresponding bulk materials. In particular, the giant magnetoelectric effect induced in nanowires by the surface tension is possible. A considerable influence of size effects and external fields on the magnetoelectric coupling coefficients and the dielectric, magnetic, and magnetoelectric susceptibilities in nanoferroics is analyzed. Particular attention is paid to the influence of a misfit deformation on the magnetoelectric coupling in thin ferroic films and their phase diagrams, including the appearance of new phases absent in the bulk material. In the framework of the Landau–Ginzburg–Devonshire theory, the linear magnetoelectric and flexomagnetoelectric effects induced in nanoferroics by the flexomagnetic coupling are considered, and a significant influence of the flexomagnetic effect on the nanoferroic susceptibility is marked. The manifestations of size effects in the polarization and magnetoelectric properties of semiellipsoidal bismuth ferrite nanoparticles are discussed.

Magnetoelectric coupling in iron oxide nanoparticle—barium titanate composites

Ferrimagnetic iron oxide nanoparticle monolayers on top of ferroelectric BaTiO3 substrates were prepared and a magnetoelectric coupling effect was observed. We employed hereby a magnetoelectric AC susceptibility setup as modification of a commercial superconducting quantum interference device magnetometer. The magnetoelectric coefficient shows two jumps at the BaTiO3 phase transition temperatures. Moreover, the magnetic depth profile of the nanoparticle monolayer was probed by polarized neutron reflectivity. The data recorded at various electric field values show that the electric field is able to alter the magnetism of the nanoparticle monolayer by a strain mediated magnetoelectric coupling effect.

A generalized thermodynamic frame of magneto-electric-caloric coupling effects of single phase epitaxial multiferroic thin films

This paper presents a generalized thermodynamic frame of the magneto-electric-caloric coupling effects of a single phase ferroic thin film. The magneto-electric-caloric effects under external fields in single phase multiferroic thin films have been investigated theoretically based on Landau theory. Due to the lack of some important material parameters, especially, magnetostrictive coefficients, we develop a generalized normalized free energy function of single phase multiferroic thin films, in which we construct a clear link between dimensionless normalized parameters and specific material coefficients. The numerical simulations present some important conclusions, including the shifts of the normalized Curie temperature under external fields and the enhancement of magnetoelectric susceptibility and multicaloric effects near the phase transition temperatures as the magnetoelectric coupling becomes stronger, and the suppression of the normalized magnetoelectric susceptibility under both electric and magnetic fields. Most importantly, both multicaloric isothermal entropy change Δs and electrocaloric isothermal entropy change Δsp have two peak values at two phase transition points, and these two temperatures can be controlled by stress and strain to approach the same work temperature in principle.

Theory of orbital magnetic quadrupole moment and magnetoelectric susceptibility

We derive a quantum-mechanical formula of the orbital magnetic quadrupole moment (MQM) in periodic systems by using the gauge-covariant gradient expansion. This formula is valid for insulators and metals at zero and finite temperature. We also prove a direct relation between the MQM and magnetoelectric (ME) susceptibility for insulators at zero temperature. It indicates that the MQM is a microscopic origin of the ME effect. Using the formula, we quantitatively estimate these quantities for room-temperature antiferromagnetic semiconductors BaMn$_2$As$_2$ and CeMn$_2$Ge$_{2 - x}$Si$_x$. We find that the orbital contribution to the ME susceptibility is comparable with or even dominant over the spin contribution.