Session V3: Electronic, Magnetic, and Magnetoelectric Excitations in Multiferroics

Magnetic and magnetoelectric excitations in multiferroic manganites

Multiferroics are materials simultaneously showing ferromagnetic and ferroelectric order. Two order parameters are coupled in these materials, which leads to such unusual effects like magnetic switching of electric polarization and dielectric constant. As can be expected already from the first principles, changes in the static properties of multiferroics must be accompanied by dynamic effects like characteristic magnetoelectric excitations. Indeed, such excitations could be recently observed in the spectra and were called electromagnons. Contrary to the conventional magnons, the electromagnons are excited by the electric component of the electromagnetic wave and contribute to the static dielectric permittivity. The suppression of electromagnons in external magnetic fields provides a natural explanation for the magnetoelectric effects in broad frequency range between dc and terahertz.   

Dynamical investigations of multiferroics: hexagonal manganites and a hexaferrite.

The electrodynamic response of magnetoelectric multiferroics can provide significant insight in the microscopic origin of multiferroicity. Hexagonal manganite HoMnO$_{3}$ is a robust room temperature ferroelectric with frustrated triangular antiferromagnetic order of Mn spins setting in at 72 K. Strong magnetoelectric effects were observed in HoMnO$_{3}$ and related hexa-manganites, the most intriguing of which is the control of magnetization by an applied electric field. The magnetic exchange interaction between the Ho and Mn ions was identified as a possible mechanism responsible for the observation, even though the detailed knowledge about this interaction was lacking. To fill this void, we studied magnetic excitations in HoMnO$_{3}$ by far-infrared spectroscopy and elucidated the ferromagnetic nature of the rare-earth/Mn exchange. Hexaferrites that display room-temperature magnetic order are also good candidates for room temperature multiferroics. We present a study of magnetic excitations in the hexaferrite Ba$_{0.6}$Sr$_{1.4}$Zn$_{2}$Fe$_{12}$O$_{22}$ using optical pump-probe spectroscopy. Pump-probe spectroscopy is known as an excellent tool for manipulating and probing magnons and phonons and for studying dynamic magnetoelectric effects. In Ba$_{0.6}$Sr$_{1.4}$Zn$_{2}$Fe$_{12}$O$_{22}$, we have observed a magnetic resonance using time domain pump-probe reflectance spectroscopy, revealing a modulation of the dielectric tensor by magnetization precession. The magnetic motion in the hexaferrite modifies the dielectric constant at visible wavelengths, providing a novel manifestation of the dynamic magnetoelectric coupling and a new way of detecting magnetic motion in multiferroics. Our results highlight that magnetoelectric dynamics manifests from the far-infrared through the visible and that both time-integrated and time-resolved spectroscopy are important tools in elucidating the microscopic properties of multiferroics.   

Optical spectroscopic study of multiferroic BiFeO$_3$ and LuFe$_2$O$_4$

Iron-based multiferroics such as BiFeO$_3$ and LuFe$_2$O$_4$ exhibit the highest magnetic and ferroelectric ordering temperatures among known multiferroics. LuFe$_2$O$_4$ is a frustrated system with several phase transitions that result in electronically driven multiferroicity. To understand how this peculiar multiferroic mechanism correlates with magnetism, we studied electronic excitations by optical spectroscopy and other complementary techniques. We show that the charge order, which determines the dielectric properties, is due to the ``order by fluctuation'' mechanism, evidenced by the onset of charge fluctuation well below the charge ordering transition. We also find a low temperature monoclinic distortion driven by both temperature and magnetic field, indicating strong coupling between structure, magnetism and charge order. BiFeO$_3$ is the only known single phase multiferroics with room temperature magnetism and ferroelectricity. To investigate the spin-charge coupling, we measured the optical properties of BiFeO$_3$. We find that the absorption onset occurs due to on-site Fe$^{3+}$ excitations at 1.41 and 1.90 eV. Temperature and magnetic-field-induced spectral changes reveal complex interactions between on-site crystal-field and magnetic excitations in the form of magnon sidebands. The sensitivity of the magnon sidebands allows us to map out the magnetic-field temperature phase diagram which demonstrates optical evidence for spin spiral quenching above 20 T and suggests a spin domain reorientation near 10 T. Work done in collaboration with T.V. Brinzari, R.C. Rai, M. Angst, R.P. Hermann, A.D. Christianson, J.-W. Kim, Z. Islam, B.C. Sales, D. Mandrus, S. Lee, Y.H. Chu, L. W. Martin, A. Kumar, R. Ramesh, S.W. Cheong, S. McGill, and J.L. Musfeldt.   

Electromagnons in multiferroics

Recent spectroscopic studies at THz frequencies for a variety of multiferroics endowed with both ferroelectric and magnetic orders have revealed the emergence of a new collective excitation, referred to as electromagnon.$^1$ It is magnetic origin, but it uniquely becomes active in response to the electric field component of light. Here we show our recent advance in the terahertz time-domain spectroscopy of electromagnons in multiferroics.$^{2}$ First, we extract general optical features in a variety of the spin ordered phases of perovskite manganites, $R$MnO$_3$ ($R$=Gd, Tb, Dy, Eu, Y, and their solutions),$^{3-8}$ which are realized by tuning R, temperature, and magnetic field. In addition to the antiferromagnetic resonances driven by the magnetic field component of light, we clarify that the electromagnon appears only for light polarized along the a-axis, but independent of the direction of the spiral spin plane. A possible origin of the electromagnon is discussed with theoretical considerations based on Heisenberg model. Second, we show the recent finding of the electromagnon in Ba$_2$Mg$_2$Fe$_{12}$O$_{22}$ with conical (i.e., ferromagnetic plus spiral) spin order.$^9$ Reflecting the ferromagnetic nature of the compound, the conical spin state is completely modified to a large extent by magnetic fields, leading to a remarkable change (terahertz magneto-chromism) of the electromagnon spectrum. On the basis of the optical investigations presented here, we emphasize the particular role of the non-collinear spin order rather than the ferroelectric order as a source of electromagnons. This work was done in collaboration with S. Kumakura, Y. Takahashi, J. S. Lee, Y. Ikebe, R. Shimano, D. Okuyama, S. Ishiwata, M. Tokunaga, Y. Kaneko, Y. Yamsaki, Y. Taguchi, K. Iwasa, T. Arima, N. Nagaosa, and Y. Tokura $^1$A. Pimonov {\it et al.,} Nat. Phys. {\bf 2}, 97 (2006). $^2 $N. Kida {\it et al.,} J. Opt. Soc. Am. B {\bf 26}, A35 (2009). $^3$N. Kida {\it et al.,} Phys.Rev. B {\bf 78}, 104414 (2008). $^4$N. Kida {\it et al.,} J. Phys. Soc. Jpn. {\bf 77}, 123704 (2008). $^5$Y. Takahashi {\it et al.,} Phys. Rev. Lett. {\bf 101}, 187201 (2008). $^6$J. S. Lee {\it et al.,} Phys. Rev. B {\bf 79}, 180403(R) (2009). $^7$Y. Takahashi {\it et al.,} Phys. Rev. B {\bf 79}, 214431 (2009). $^8$J. S. Lee {\it et al.,} Phys. Rev. B {\bf 80}, 134409 (2009). $^9$N. Kida {\it et al.,} Phys. Rev. B (in press).   

Magnetic excitations and optical transitions in the multiferroic spin-$\frac{1} {2}$ system LiCu$_2$O$_2$

A class of materials where multiferroicity can exist are spin cycloidal compounds. LiCu$_2$O$_2$ is the first copper-based multiferroic material where spin cycloidal and polar orders develop simultaneously and is also one of the very few spin-$\frac{1}{2}$ multiferroics. Coupled ferroelectric and magnetic orders in multiferroics can lead to strong mixing between phonons and magnons rendering some magnons electric dipole active. These electroactive magnons or electromagnons can give, due to their low resonance frequency, significant contribution to the static dielectric constant. We explore magnetic excitations in LiCu$_2$O$_2$ using THz absorption spectroscopy in magnetic fields up to 30\,T. Below the cycloidal ordering temperature, $T$=24\,K, eight optically active transitions are observed in the spin system of LiCu$_2$O$_2$ in the range from 4 to 30\,cm$^{-1}$. In magnetic field the number of modes increases, some modes anticross and the electric polarization flop is seen as a change in magnetic field dependence of mode energies. The polarization dependence of two of the modes fits the selection rules for the spin cycloid tilted from the $bc$ plane. For the remaining six modes electric and magnetic dipole approximations cannot explain the observed polarization dependence. The electromagnon is not directly seen in the explored energy range although there is evidence that it could exist below 4\,cm$^{-1}$. Comparisons will be made to non-multiferroic isostructual spin cycloidal compound NaCu$_2$O$_2$.