Session T30: Focus Session: Excitations in Multiferroics

Electromagnon spectra in multiferroic manganites $R$MnO$_3$

The magnetoelectric (ME) effect, in which electric polarization (magnetization) are controlled by magnetic (electric) field, have recently attracted intensive attention. One of the materials which exhibits large ME effects is the manganites $R$MnO$_3$. In this presentation I focus on {\em electromagnon} excitations (in a wide sense). In those manganites, cycloidal spin structures create ferroelectricity through spin-orbit couplings. Originally, electromagnon is defined as a collective excitation of spins in such a way that rotation of the cycloidal plane is driven by the {\em electrical field} of light coupled to the ferroelectric moment. Spectra for electrically excited magnons are indeed reported in TbMnO$_3$, GdMnO$_3$, (Eu,Y)MnO$_3$ and DyMnO$_3$. However, the selection rule observed in these compounds ($E^\omega \parallel a$, irrespective of the cycloidal planes) is inconsistent with the theoretical prediction. Alternatively, couplings between electric field and spins through spin-dependent local polarizations have been proposed. I show both one- and two-magnon excitation spectra, and clarify consistencies and discrepancies of the model with experimental data. Roles of phonons and orbital orders in these compounds are also discussed. \\\\ References: Y. Takahashi {\em et al.}, PRL{\bf 101}, 187201 (2008). Also, Katsura {\em et al.}, PRL{\bf98}, 027203 (2007); A. Pimenov {\em et al.}, Nature Phys. {\bf 2}, 97 (2006); R. Valdes Aguilar {\em et al.}, PRB{\bf 76}, 060404(R) (2007); Kida {\em et al.}, PRB{\bf 78}, 104414 (2008); R. Valdes Aguilar {\em et al.}, arXiv:0811.2966.   

Selection rules for electromagnons in anharmonic cycloidal multiferroics

The coexistence of magnetic and ferroelectric phases in multiferroic materials gives rise to hybrid excitations with mixed magnetic and electric character. These excitations, so-called electromagnons, have been observed in several oxide materials, but their origin and optical selection rules are central questions that are not yet understood. We present a theory of electromagnon excitation in magnets with anharmonic cycloidal ground state. In contrast to previous theories, we show that multiple electromagnons are excited by light polarized along the cycloid plane as well as perpendicular to it. Our results allow the distinction between different magnetoelectric couplings and have important implications for the interpretation of optical experiments in rare earth manganites as well as other materials with anharmonic spiral magnetic order.   

Low-magnetic field control of electromagnon

Since the 1960s, there has been intense debate concerning the presence of the spin excitation driven by the electric field of light $E^\omega$ in ferroelectric magnets. This is recently revived by the observation of the absorption peak structure around 3 meV in a ferroelectric magnet TbMnO$_3$ [1]. As an origin of this excitation, the hybridized spin excitation with electric polarization (now called electromagnon) was considered [2]. However, this issue remains controversial by our THz spectroscopic studies on a family of $R$MnO$_3$ [3] ($R$ = Tb, Dy, and Gd$_{0.7}$Tb$_{0.3}$) in a variety of spin phases tuned by temperature and magnetic field. Here we report an optical investigation of the low-energy (2$-$10 meV) spin dynamics for other ferroelectric magnets, hexaferrite, by using THz time-domain spectroscopy. We find the signature of the genuine electromagnon at THz frequencies. As a manifestation of the strong magnetoelectric coupling inherent to the electromagnon, we demonstrate the low-magnetic field ($\sim$ 100 Oe) control of the optical constants at THz frequencies. \noindent [1] A. Pimenov {\it et al.}, Nat. Phys. {\bf 2}, 97 (2006). [2] D. Senff {\it et al.}, PRL {\bf 98}, 137206 (2007). [3] N. Kida {\it et al.}, PRB {\bf 78}, 104414 (2008); Y. Takahashi {\it et al.}, PRL {\bf 101}, 187201 (2008); N. Kida {\it et al.}, J. Phys. Soc. Jpn. Dec. issue (2008).   

Theory of novel one-magnon excitation induced by electric fields in cycloidal spin magnets

We propose a new mechanism to induce a novel one-magnon excitation by electric-field component of light in cycloidal spin states. We calculated optical spectra in the ycloidal spin structures as observed in multiferroic perovskite manganites $R$MnO$_{3}$ where novel magnetic excitations induced by electric-field component of light are observed. When symmetric pairs of spins dependent electric polarizations are introduced, we have light absorptions at terahertz frequencies with one- and two-magnon excitations driven by electric-field components. Our results show that some parts of optical absorption peaks observed experimentally at terahertz frequencies are one-magnon excitation absorptions.   

Origin of electromagnons in multiferroic manganites

The interest in multiferroic materials has increased in the last few years due to the fundamental physics of strong interaction between ferroelectric and magnetic orders, as well as for the promise of novel applications in future electronics. From powerful symmetry arguments and with modeling of the microscopic coupling mechanism, these efforts have led to the discovery of a vast set of multiferroic compounds. An important recent step in this regard was the discovery of a new kind of magnetic excitation that couples strongly to light by acquiring electric dipole activity from the infrared active phonons, called electromagnon, which is a hybrid excitation of magnon and phonon character \footnote{A. Pimenov, et al. Nature Physics 2, 97 (2006)}$^{,}$\footnote{A.B. Sushkov, et al. PRL 98, 027202 (2007)}. These discoveries have highlighted the importance of the dynamical aspects of the magnetoelectric coupling. Even though a wide consensus has been reached regarding the origin of the static ferroelectric polarization, the mechanism of the magnetoelectric dynamic effect of electromagnons was not clear. In this talk a combination of theory and experiment is presented that clarifies the origin of the electromagnon excitations in RMnO$_3$. This model is based on symmetric exchange striction and takes into account the lattice and magnetic symmetry of this family of perovskite manganites. It reproduces the fact that the observed selection rule for electromagnons in RMnO$_3$ is independent of the spin plane\footnote{R. Vald\'{e}s Aguilar, et al. PRB 76, 060404 (2007)}$^{,}$\footnote{R. Vald\'{e}s Aguilar, et al. arxiv:0811.2966}. This result is due to the effective modulation of the exchange interaction between Mn spins induced by the electric field of light. The proposed mechanism is also related to the origin of static polarization in the E-phase of this RMnO$_3$ multiferroic family. The model and experiments carried out so far demonstrate that the symmetric exchange interaction is responsible for all the observed dynamical magnetoelectric effects, and opens a new avenue for study of these multiferroic compounds.   

Infrared-active excitations related to the $R^{3+}$ ligand-field splitting in $R$Mn$_{2}$O$_{5 }(R$=Ho, Dy, Tb)

Optical transitions between ligand-field split states of $R^{3+}$ ions in $R$Mn$_{2}$O$_{5}$ multiferroic single crystals have been studied at the phase transitions in the external magnetic field up to 13 T and uniaxial pressure up to 5 kbar. Spectra of the ligand field excitations change significantly in external magnetic field and correlate with the reversal of electric polarization induced by magnetic field. The oscillator strength and selection rules for ligand field excitations change with external uniaxial pressure. We discuss the connection between the ligand field on $R^{3+}$ with the magnetism and dielectric properties of this compounds.   

THz magneto-optical study of multiferroic compound TbMnO$_3$

We present results of magneto-optical absorption measurements in the THz region on multiferroic TbMnO$_3$ in magnetic fields up to 12\,T. The temperature range of our studies covers the paramagnetic phase ($T>42$\,K), the collinear incommensurate spin density wave phase of Mn$^{3+}$ $S=2$ spins (28\,K$<42$\,K), the ferro-electric phase with incommensurate elliptic spin order ($T<28$\,K) and the phase where the magnetic moments of Tb are ordered, $T<7$\,K. It is known that in the FE phase the magnetic field ${\mathbf B_0}\parallel {\mathbf a}$ equal to 10.5\,T flops the electric polarization from ${\mathbf P}\parallel {\mathbf c}$ to ${\mathbf P}\parallel {\mathbf a}$. The polarization ${\mathbf P}$ is in the plane of elliptical spiral and perpendicular to spiral order vector, ${\mathbf k}\parallel {\mathbf b}$ in TbMnO$_3$. It is expected that the selection rule for the electric-dipole active spin excitation in the spiral phase changes from ${\mathbf E_1}\parallel {\mathbf a}$ to ${\mathbf E_1}\parallel {\mathbf c }$. Polarization-sensitive absorption measurements are performed to distinguish between magnetic- or electric-dipole active spin excitations, i.e. magnons or electromagnons, respectively, in the whole temperature and magnetic field range.   

Magneto-optical THz study in multiferroic Ni$_3$V$_2$O$_8$

We present results of absorption measurements in THz region, from 3 to 220\,cm$^{-1}$, in multiferroic Ni$_3$V$_2$O$_8$ in magnetic fields up to 12\,T and temperatures above 2\,K. Ni$_3$V$_2$O$_8$ is a magnetic insulator with Ni$^{2+}$ spin-1 ions arranged in a kagom\'{e} staircase lattice. The phase diagram of Ni$_3$V$_2$O$_8$ is complicated - ground state changes from paramagnetic (PM) to high temperature incommensurate (HTI) helical state, at 9.1\,K in zero field. In low temperature incommensurate (LTI) phase below 6.3\,K spontaneous electric polarization $\mathbf{P}$ appears along the $b$ axis. $\mathbf{P_b}$ can be suppressed by further cooling below 3.9\,K or by application of external magnetic field. Low energy optical excitations are discussed for all low-$T$ phases of Ni$_3$V$_2$O$_8$. Polarization sensitive absorption measurements are performed to distinguish between magnetoand electroactive spin excitations i.e. magnons and electromagnons. Different optical selection rules for magnon and electromagnon excitations enable us to search for evidence of the spin helix plane orientation and its changes at critical magnetic fields and temperatures.   

Evolution of magnetic exchange interactions in the multiferroic Mn$_{1-x}$Fe$_{x}$WO$_{4}$

The experimental investigation of the spin dynamical properties in the multiferroic material Mn$_{1-x}$Fe$_{x}$WO$_{4}$ is essential to the understanding of the interplay between the magnetic and ferroelectric phenomena. We have systematically studied the low temperature magnetic excitations in the Fe-doped Mn$_{1-x}$Fe$_{x}$WO$_{4}$. The spin wave dispersion relations in the commensurate (CM) phase are well described by a model that accounts for the magnetic exchange coupling of up to nine nearest neighbors. Our results indicate that these magnetic systems are highly frustrated and the CM spin structures result from the competing interactions. The evolution of the spin dynamics reveals the role of the magnetic impurities and the influence of Fe-doping to the multiferroic properties are discussed. This work was partially supported by Division of Scientific User Facilities of the Office of Basic Energy Sciences, U.S. DOE.   

Long-wavelength magnetic excitations in multiferroic BiFeO$_{3}$

Magnetic and lattice vibrations play a central role in the properties of multiferroics. This low-energy electrodynamics can help unravel the fundamental interactions between magnetic and lattice degrees of freedom. BiFeO$_{3}$ is a multiferroic material with robust room temperature ferroelectricity and antiferromagnetism and promising technological potential. The interaction between the ferroelectric and antiferromagnetic order parameters leads to the modification of the isotropic Heisenberg-antiferromagnet ground state that becomes an incommensurate cycloid with a very long period. The cycloidal magnetic structure results in a complex spectrum of zero-wavevector magnetic excitations; these magnetic modes were detected using Raman scattering. Here, we report a far-infrared spectroscopic study of a BiFeO$_{3}$ single crystal. We detected magnetic resonances at energies close to those reported in the Raman spectroscopy studies. The magnetic character of these excitations is supported by their characteristic temperature dependence. We will discuss our results in the context of possible electric-dipole activity of the observed resonances.   

Low energy excitations in multiferroic Ca3CoMnO6 in the far infrared.

Ferroelctricity was recently discovered in Ca3CoMnO6 (Y.J.Choi et al., PRL 100 (2008) 047601) which is a quasi 1 D spin up-up- down-down system. We report the results of infrared (5-250 cm- 1) transmission study of multiferroic Ca3CoMnO6 as a function of temperature T (3-300~K) and magnetic field H (0-8 T). Two peaks are observed at and below T$_N=17$ K. Narrow peak at 35 cm-1 is observed at low T and is identified as the ground state feature. This feature is suppressed with raising T or in magnetic field and the other broad feature at 25 cm-1 emerges. Neither peak is split or shifted by magnetic field. We will discuss the possible origin of these two excitations.