Optical Magnetoelectric Effect Research Articles

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.

Confirming the trilinear form of the optical magnetoelectric effect in the polar honeycomb antiferromagnet Co2Mo3O8

Magnetoelectric phenomena are intimately linked to relativistic effects and also require the material to break spatial inversion symmetry and time-reversal invariance. Magnetoelectric coupling can substantially affect light–matter interaction and lead to non-reciprocal light propagation. Here, we confirm on a fully experimental basis, without invoking either symmetry-based or material-specific assumptions, that the optical magnetoelectric effect in materials with non-parallel magnetization (M) and electric polarization (P) generates a trilinear term in the refractive index, δn ∝ k ⋅ (P × M), where k is the propagation vector of light. Its sharp magnetoelectric resonances in the terahertz regime, which are simultaneously electric and magnetic dipole active excitations, make Co2Mo3O8 an ideal compound to demonstrate this fundamental relation via independent variation of M, P, and k. Remarkably, the material shows almost perfect one-way transparency in moderate magnetic fields for one of these magnetoelectric resonances.

Selection rules and dynamic magnetoelectric effect of the spin waves in multiferroic BiFeO3

We report the magnetic field dependence of the THz absorption and non-reciprocal directional dichroism spectra of BiFeO$_3$ measured on the three principal crystal cuts for fields applied along the three principal directions of each cut. From the systematic study of the light polarization dependence we deduced the optical selection rules of the spin-wave excitations. Our THz data, combined with small-angle neutron scattering results showed that i) an in-plane magnetic field rotates the $\mathbf{q}$ vectors of the cycloids perpendicular to the magnetic field, and ii) the selection rules are mostly determined by the orientation of the $\mathbf{q}$ vector with respect to the electromagnetic fields. We observed a magnetic field history dependent change in the strength and the frequency of the spin-wave modes, which we attributed to the change of the orientation and the length of the cycloidal $\mathbf{q}$ vector, respectively. Finally, we compared our experimental data with the results of linear spin-wave theory that reproduces the magnetic field dependence of the spin-wave frequencies and most of the selection rules, from which we identified the spin-polarization coupling terms relevant for the optical magnetoelectric effect.

Nonreciprocal directional dichroism in multiferroics

Nonreciprocal directional dichroism in multiferroics, namely magnetoelectric coupling in the dynamic regime, is endowed with rich physics and promising applications, which are entangled with fundamental physical components, such as spin, orbital, lattice, charge, and topology. Such a linear nonreciprocal response behavior in the GHz-THz frequency range, represented by optical magnetoelectric effect and magnetochiral dichroism, occurs ubiquitously in material systems with the spontaneous breaking of space-time symmetry, and is subject to Onsager’s reciprocal theorem in the thermodynamic limit. Microscopically, these nonreciprocal responses are usually encoded by toroidization (chirality) and electromagnon (quasiparticle), thus establishing a comprehensive understanding of magnetoelectric coupling and irreversible dynamics. Herein, the basic mechanisms and emergent nonreciprocal directional dichroism in single-phase multiferroics are summarized. We expect that the present review will stimulate diverse possibilities toward nonreciprocal directional dichroism within and beyond multiferroics.

Imaging switchable magnetoelectric quadrupole domains via nonreciprocal linear dichroism

Parity-odd magnetoelectric multipoles such as magnetic quadrupoles and toroidal dipoles contribute to various symmetry-dependent magnetic phenomena and formation of exotic ordered phases. However, the observation of domain structures emerging due to symmetry breaking caused by these multipoles is a severe challenge because of their antiferromagnetic nature without net magnetization. Here, we report the discovery of nonreciprocal linear dichroism for visible light (~4% at 1.8 eV) in a magnetic quadrupole ordered phase of antiferromagnetic Pb(TiO)Cu4(PO4)4, which enables the identification of magnetic quadrupole domains of opposite signs. Symmetry considerations indicate that nonreciprocal linear dichroism is induced by the optical magnetoelectric effect, i.e., the linear magnetoelectric effect for electromagnetic waves. Using the nonreciprocal linear dichroism, we successfully visualize spatial distributions of quadrupole domains and their isothermal electric-field switching by means of a transmission-type polarized light microscope. The present work exemplifies that the optical magnetoelectric effect efficiently visualizes magnetoelectric multipole domains responding to external perturbations.

Orientation-Dependent Optical Magnetoelectric Effect in Patterned BaTiO3/La0.67Sr0.33MnO3 Heterostructures.

The optical magnetoelectric effect has been widely investigated, but obtaining the large and tunable optical magnetoelectric effect at room temperature is still a big challenge. We here design ferroelectric/ferromagnetic heterostructures with various orientations, which are composed of titanate BaTiO3 and manganese oxide La0.67Sr0.33MnO3. This artificial bilayer structure presents room-temperature ferroelectric and ferromagnetic properties. After patterning a 4 μm grating structure on the bilayer thin film, the optical magnetoelectric effect for near-infrared light is investigated systematically through the Bragg diffraction method. The relative change of diffracted light intensity of the order n = 1 has a strong dependence on the magnetization and polarization of the thin films, whether the superlattice is irradiated in reflection or transmission geometries. For (100)- and (111)-oriented samples, both show the room-temperature optical magnetoelectric effect, while the (111)-oriented thin film has a stronger optical magnetoelectric effect. These results pave the way for designing next-generation optical magnetoelectric devices based on the ferroelectric/ferromagnetic structure.

The tunable optical magneto-electric effect in patterned manganese oxide superlattices

The optical magneto-electric (OME) effect has been widely investigated in magnetic materials, but obtaining the large and tunable OME effect is an ongoing challenge. We here design a tri-color superlattice composed of manganese oxides, Pr0.9Ca0.1MnO3, La0.9Sr0.1MnO3, and La0.9Sb0.1MnO3, where the space-inversion and time-reversal symmetries are broken. With the aid of the grating structure, the OME effect for near-infrared light in tri-color superlattices is investigated systematically through the Bragg diffraction method. The relative change of diffracted light intensity of the order n = ±1 has a strong dependence on the magnetization and polarization of the tri-color superlattice, whether the superlattice is irradiated in reflection or transmission geometries. Otherwise, the relative change of diffracted light intensity increases with the increase in the superlattice period and with the decrease in the grating period. The maximum relative change of diffracted light intensity in tri-color superlattices with the grating structure patterned is as large as 8.27%. These results pave the way for designing next-generation OME devices based on manganese oxides.

Role of commensurability of spin order for optical magnetoelectric effect with electromagnons in multiferroic YMn2O5

Multiferroic phases of materials like YM${}_{2}$O${}_{5}$ feature an optical magnetoelectric effect on the resonance of the electromagnon. The effect manifests itself in nonreciprocal directional dichroism. Here, the authors examine the role of the commensurability of noncollinear spin order for the optical magnetoelectric effect for two types of electromagnon. For the electromagnon driven by the exchange striction, the crucial role of the commensurability is indicated by the suppression of the directional dichroism by the magnetic phase transition. On the other hand, the gapped electromagnon due to the spin-current mechanism is identified by the directional dichroism, regardless of (in)commensurability. These results provide new insights into the light-matter interaction driven by dynamical magnetoelectric coupling.

Versatile optical magnetoelectric effects by electromagnons inMnWO4with canted spin-spiral plane

The optical magnetoelectric (OME) effect based on electromagnon resonance has been investigated in a multiferroic helimagnet ${\mathrm{MnWO}}_{4}$ with the canted spin-spiral plane by the terahertz spectroscopy. Versatile OME effects emerge from the canted spin-spiral state, including the magnetochiral effect as manifested by the nonreciprocal directional dichroism. In addition to the transverse fluctuation mode of the spin-driven ferroelectric polarization, the electromagnon with a character of the longitudinal amplitude mode was found to cause the OME effect characteristic of the canted spin-spiral plane.

One-way transparency of four-coloured spin-wave excitations in multiferroic materials

The coupling between spins and electric dipoles governs magnetoelectric phenomena in multiferroics. The dynamical magnetoelectric effect, which is an inherent attribute of the spin excitations in multiferroics, drastically changes the optical properties of these compounds compared with conventional materials where light-matter interaction is expressed only by the dielectric permittivity or magnetic permeability. Here we show via polarized terahertz spectroscopy studies on multiferroic Ca2CoSi2O7, Sr2CoSi2O7 and Ba2CoGe2O7 that such magnetoeletric spin excitations exhibit quadrochroism, that is, they have different colours for all the four combinations of the two propagation directions (forward or backward) and the two orthogonal polarizations of a light beam. We demonstrate that one-way transparency can be realized for spin-wave excitations with sufficiently strong optical magnetoelectric effect. Furthermore, the transparent and absorbing directions of light propagation can be reversed by external magnetic fields. This magnetically controlled optical-diode function of magnetoelectric multiferroics may open a new horizon in photonics.

Symmetry conditions for nonreciprocal light propagation in magnetic crystals

Recent studies demonstrated the violation of reciprocity in optical processes in low-symmetry magnetic crystals. In these crystals the speed of light can be different for counter-propagating beams. Correspondingly, they can show strong directional anisotropies such as direction dependent absorption also called directional dichroism[S. Bord\'acs et al., Nat. Phys. 8, 734 (2012); M. Saito et al., J. Phys. Soc. Jpn. 77, 013705 (2008)]. Based on symmetry considerations, we identify the magnetic point groups of materials which can host such directional anisotropies and also provide a list of possible candidate materials to observe these phenomena. In most of these cases, the symmetry of the crystal allows directional anisotropy not only for optical processes but also for the propagation of beams of particles and scalar waves. We also predict new types of directional optical anisotropies -- besides the optical magnetoelectric effect and the magnetochiral dichroism investigated so far -- and specify the magnetic point groups of crystals where they can emerge.

Electronic structure, linear, and nonlinear optical responses in magnetoelectric multiferroic material BiFeO3

BiFeO(3) has attracted great interest for its multiferroic property. The spontaneous electric polarization, multiferroism, and static magnetoelectric coupling have been widely studied both experimentally and theoretically. Here, in this paper, we report on the effects of magnetic ordering, spin fluctuation, and external magnetic field on the linear dielectric function and second-harmonic generation (SHG) in multiferroic BiFeO(3). First, our generalized gradient approximation plus Hubbard U calculations reproduce very well experimental data of linear dielectric function. In the mean time, it is revealed that SHG susceptibilities differ dramatically between antiferromagnetic configuration and ferromagnetic one, which is due to the enhanced contrast in the double-photon resonance absorption. Further Monte Carlo simulation of the coupling between electric order and spin-pair correlation function is presented for the spin fluctuation dependence of SHG. The significant nonlinear optical magnetoelectric effect around the Neel temperature provides novel route to enhancing magnetoelectric coupling in multiferroic materials.

Nonreciprocal Directional Dichroism in Ferroelectric Nd2Ti2O7

The nonreciprocal directional dichroism derived from the optical magnetoelectric effect has been investigated for the f – f transitions of Nd 3+ ion in a ferroelectric Nd 2 Ti 2 O 7 single crystal under magnetic field, in which both space-inversion and time-reversal symmetries are viewed as simultaneously broken. The magnitude of nonreciprocity in each absorption band of Nd 3+ was semi-quantitatively elucidated by taking account of the interference between electric dipole and magnetic dipole transitions.

Optical magnetoelectric effect at CaRuO3–CaMnO3 interfaces as a polar ferromagnet

A correlated electron interface between paramagnetic CaRuO3 and antiferromagnetic CaMnO3 has been characterized with optical magnetoelectric (OME) effect as an interface-selective probe for spin and charge states. To detect the OME effect, i.e., nonreciprocal directional dichroism for visible or near-infrared light, we have constructed a “tricolor” superlattice with artificially broken inversion symmetry by stacking CaRuO3, CaMnO3, and CaTiO3, and patterned a grating structure with 4μm period on the superlattice. The observed intensity modulation (0.3% at 50K) in the Bragg diffraction verifies a charge transfer and concomitant ferromagnetism at the CaRuO3–CaMnO3 interface.

Gigantic Optical Magnetoelectric Effect in CuB2O4

Although it has been well known that materials in which both space inversion and time reversal symmetries are broken can host optical magneto-electric effect, i.e., change in optical constants with the reversal of propagating direction of light, the largest change in absorption ever reported on this effect was 0.2%. Here we show that optical absorption in noncentrosymmetric weak ferromagnetic material CuB 2 O 4 changes by more than 100% with reversal of a low magnetic field of 300 Oe. The gigantic optical magneto-electric effect is ascribed to the canted antiferromagnetic spin ordering of square-coordinated Cu 2+ sites, where the local inversion is slightly broken.

Optical Magnetoelectric Effect of Patterned Oxide Superlattices with Ferromagnetic Interfaces

Nonreciprocal directional dichroism, termed the optical magnetoelectric (OME) effect, has been observed in patterned superlattice (SL) composed of perovskite oxides, LaMnO3, SrMnO3, and LaAlO3. Such a tricolor SL with ferromagnetic interfaces is expected to artificially break both space-inversion and time-reversal symmetries and hence to show the OME effect. The Bragg diffraction from the grating structure with a period of 4 microm fabricated on the SL was employed to sensitively detect the OME effect, yielding the relative change of the diffracted light intensity (~0.2%-0.5%) upon a reversal of either the in-plane magnetization or the propagation vector of the diffracted light.

Optical magnetoelectric effect of patterned oxide superlattices with ferromagnetic interfaces

This is a copy of the slides presented at the meeting but not formally written up for the volume. Recent development of pulsed laser deposition technique makes it possible to build up different kinds of perovskite oxides and to create new electric and magnetic properties of heterointerfaces, which can neither be realized in bulk properties of the constitute oxides and nor be treated as the simple combination of them. The extraction of unique properties of interfaces thus crucially relies on the development of a new method to selectively detect their electronic properties accompanied with magnetism. Here we show a new approach with use of the optical magnetoelectric (ME) effect to address the unique properties of ferromagnetic oxide superlattices (SLs). The ME effect, that is, the control of the polarization P by a magnetic field H or inversely the control of the magnetization M by an electric field E, can be considered as a typical manifestation of cross-correlation phenomena in solids. Even at optical frequencies, such a cross-correlation response coming from the ME effect is known to show up in materials with a lack of both space-inversion and time-reversal symmetries. This is referred to as the optical ME effect. The optical ME response emerges as a change of reflection and transmission when the wavevector k of light is set to parallel or antiparallel to the toroidal moment T defined as P X M, which in turn enables us to control the intensity of light by changing the directions of E and/or H. In this work, we fabricated SLs composed of LaMnO3, SrMnO3, and LaAlO3 and exploited the use of their unique properties of heterointerfaces as a medium for optical ME effect. Such ’tricolor’ SLs are expected to artificially break both space-inversion and time-reversal symmetries, which are induced by asymmetric stacking sequences of three different oxides and by the charge-transfer-induced magnetism at LaMnO3/SrMnO3 interfaces, respectively. We patterned the grating structure with a period of 4 im on SLs and employed the Bragg diffraction geometry to sensitively detect the optical ME effect. The optical ME effect was clearly observed when the diffracted light was used as a probe. The optical ME response depending on PABC X Minterface is a direct consequence of the symmetry breaking at interfaces. Its magnitude per interface was thus estimated to be tilde operator = varies with = similar to 0.01% in H of 2 kOe, which are relatively large as compared to previously reported values in bulk materials showing the optical ME effect. Our data provide that the present method would be used as a tool for the study of oxide heterointerfaces.

Optical magnetoelectric effect induced by a moving medium

In this Brief Report, we report a theoretical investigation of the possibility of realizing the optical magnetoelectric effect with a moving medium. This unique approach is quite distinct from other methods using conventional multiferroic materials or artificial materials with broken space inversion and time reversal symmetries simultaneously. Its advantage lies in the controllability of the optical properties, which change with the change of velocity and intrinsic refractive index of the moving medium. This work in the present frame offers a route to tune this phenomenon.

Magnetoelectric emission in rare-earth doped ferroelectric crystalsLa2Ti2O7:R3+(R=Er, Eu, and Nd)

The optical magnetoelectric (OME) effect, i.e., the change of optical response on the reversal of light propagation vector $(\mathbit{k})$, has been investigated for the emission of rare earth ${R}^{3+}$ ion ($R=\text{Nd}$, Eu, Er) doped in a ferroelectric ${\text{La}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ single crystal under magnetic field $(\mathbit{H})$. The symmetry condition for the appearance of the OME effect for $\mathbit{H}\ensuremath{\perp}\mathbit{k}$ was confirmed by varying the relative angle between the electric polarization and magnetic field. Another tensor component of the second-order magnetolectric tensor ${\ensuremath{\beta}}_{ijk}$ for $\mathbit{H}\ensuremath{\parallel}\mathbit{k}$, i.e., the magnetochiral effect, is allowed in the Faraday configuration but found to be small compared with the OME effect in the Voigt configuration. The importance of the spin-orbit coupling, the magnetic dipole transition, and the noncentrosymmetric crystal structure is discussed as the origin of the OME effect on the basis of the observed signal magnitude depending on the species of the rare-earth ion and its optical transition moment.

Multiferroics—toward strong coupling between magnetization and polarization in a solid

Multiferroics, in which magnetization and polarization can coexist, are potential candidates of the new spintronic materials which may enable the electrical control of magnetic state. The materials design of the multiferroics and the strategy to enhance the DC and optical magnetoelectric effect are argued in terms of the toroidal moment, helical spin structure, and critical-state phase control.