Origin of Electromagnon Excitations in MultiferroicRMnO3

Abstract

Electromagnon excitations in multiferroic orthorhombic RMnO3 are shown to result from the Heisenberg coupling between spins despite the fact that the static polarization arises from the much weaker Dzyaloshinskii-Moriya (DM) exchange interaction. We present a model incorporating the structural characteristics of this family of manganites that is confirmed by far infrared transmission data as a function of temperature and magnetic field and inelastic neutron scattering results. A deep connection is found between the magnetoelectric dynamics of the spiral phase and the static magnetoelectric coupling in the collinear E-phase of this family of manganites. PACS numbers:The coupling between the magnetic and ferroelectric order in a diverse set of materials termed multiferroics is currently a topic of intense study [1,2]. The interplay between these two orders is particularly striking in materials where ferroelectricity appears as a consequence of spontaneously breaking of inversion symmetry of the magnetic ordering. In many such magnetic ferroelectrics the spins order in an incommensurate cycloidal spiral state [3]. The microscopic origin of ferroelectricity for this case has been discussed by a number of authors [4,5,6], leading to a consensus, generically termed the spiral mechanism. It relies on the lowering of the energy of the antisymmetric Dzyaloshinskii-Moriya (DM) exchange in the spiral state by a polar lattice distortion, which induces an electric polarization, P ∝ Q×R, where R ∝ S i ×S i+1 is the spin rotation axis, and Q is the wave vector of the spiral. These ideas have been of central importance in the recent discovery of new multiferroic compounds.Another apparent consequence of multiferroicity is the existence of novel coupled magnon-phonon excitations called electromagnons [7,8]. A magnon that gives rise to oscillations of electric polarization can be excited by electric fields, thereby coupling much more strongly to light than the usual magnetic dipole excitation of magnons corresponding to antiferromagnetic resonance (AFMR). The resulting electric dipole spectral weight has been transferred from the phonons down to the magnon frequency. The dynamic magnetoelectric effects resulting from the coupling between spin and polarization waves were discussed theoretically at an early stage of the research on multiferroic materials [9]. More recently, Katsura, Balatsky and Nagaosa (KBN) noted that the magnetoelectric coupling of the spiral mechanism, also gives rise to an electromagnon [10]. When the spiral plane rotates around Q, so does the induced electric polarization, which couples this magnetic excitation to electric field e of a light wave normal to the spiral plane: e R. The first observation of the electromagnon peak for e a in TbMnO 3 with the bc-plane spiral spin ordering (Q b) seemed to confirm this selection rule [7]. However, recent measurements on other spiral multiferroics from the same family of materials, Eu 0.75 Y 0.25 MnO 3 [11] and DyMnO 3 [12], showed that this selection rule is violated...

High-temperature electromagnons in the magnetically induced multiferroic cupric oxide driven by intersublattice exchange

“…The strength DE ¼ 0.075 of this electromagnon is weaker than the E o 8a electromagnons reported in TbMnO 3 (DE ¼ 2), thought to arise from the Heisenberg exchange mechanism 14 , and comparable to that assigned to an eigenmode of the spin cycloid (DE ¼ 0.05) 12 . Previously, the application of an optical sum rule has shown that electromagnons in RMn 2 O 5 account for step-like changes in the static dielectric constant 9 .…”

“…Magnetoelectric coupling creates a new quasiparticle excitation-the electromagnon-at terahertz (THz) frequencies [7][8][9][10][11][12][13] . Electromagnons have been discovered in rareearth-doped compounds such as RMnO 3 and RMn 2 O 5 at low temperature (o70 K), and are thought to result from modifications of the Heisenberg exchange interaction 14,15 , which has a S i Á S j term in the spin Hamiltonian. Single-ion anisotropy (SIA) and biquadratic exchange (BE) are required to capture fully the spectral features of electromagnons in the THz range 16 .…”

Long-wavelength magnetic and magnetoelectric excitations in the ferroelectric antiferromagnet BiFeO3

“…In antiferromagnets, magnon resonance frequencies and dispersion are determined by the strength of the antiferromagnetic exchange and by magnetic anisotropy 18 . In multiferroic manganites, far-infrared spectroscopic studies of magnetic excitations helped unravel the underlying microscopic ME interactions that were found to mix magnons with phonons 19,20 . In this article, we present a terahertz (THz) spectroscopic study of long-wavelength magnetic excitations in BFO.…”

Terahertz-frequency magnetoelectric effect in Ni-doped CaBaCo4O7

“…Detailed investigations of spin excitations in high magnetic field provide the experimental basis for the construction of accurate microscopic magnetic Hamiltonians [7][8][9][10] . THz spectroscopy has also provided the observations of electromagnons [11][12][13][14][15][16][17][18] which are electric-dipole-active spin excitations that receive their electric-dipole activity via microscopic magnetoelectric interactions.…”

“…In multiferroic manganites RMnO 3 , THz-frequency electromagnons provided a detailed microscopic mechanism for exchange striction that governs the dynamic magnetoelectricity in these materials 14 . THz optical properties of undoped CBCO have been previously reported by Bordács et al 19 , who found sharp absorption lines in the 1-2 THz range due to spin and optical phonon excitations.…”