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Electrically driven spin excitation in the ferroelectric magnet DyMnO3

N. Kida, Y. Ikebe, Y. Takahashi, J. P. He, Y. Kaneko, Y. Yamasaki, R. Shimano, T. Arima, N. Nagaosa, and Y. Tokura
Phys. Rev. B 78, 104414 – Published 18 September 2008
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  • INTRODUCTION
  • METHODS
  • RESULTS
  • DISCUSSION
  • SUMMARY
  • ACKNOWLEDGEMENTS
  • APPENDICES
    • References

    Abstract

    Temperature (5–250 K) and magnetic-field (0–70 kOe) variations of the low-energy (1–10 meV) electrodynamics of spin excitations have been investigated for a complete set of light-polarization configurations for a ferroelectric magnet DyMnO3 by using terahertz time-domain spectroscopy. We identify the pronounced absorption continuum (1–8 meV) with a peak feature around 2 meV, which is electric-dipole active only for the light E vector along the a axis. This absorption band grows in intensity with lowering temperature from the spin-collinear paraelectric phase above the ferroelectric transition but is independent of the orientation of spiral spin plane (bc or ab), as shown on the original Ps (ferroelectric polarization) ∥c phase as well as the magnetic-field induced Ps∥a phase. The possible origin of this electric-dipole active band is argued in terms of the large fluctuations of spins and spin current.

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    • Received 4 July 2008

    DOI:https://doi.org/10.1103/PhysRevB.78.104414

    ©2008 American Physical Society

    Synopsis

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    Electrically driven spin excitations

    Published 22 September 2008

    Researchers in Japan have identified spin excitations in multiferroics that can be driven by electric fields.

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    Authors & Affiliations

    N. Kida1, Y. Ikebe2, Y. Takahashi1, J. P. He1, Y. Kaneko1, Y. Yamasaki3, R. Shimano1,2, T. Arima4, N. Nagaosa3,5, and Y. Tokura1,3,5

    • 1Multiferroics Project (MF), ERATO, Japan Science and Technology Agency (JST), c/o Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
    • 2Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
    • 3Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
    • 4Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
    • 5Cross-Correlated Materials Research Group (CMRG), ASI, RIKEN, 2-1 Hirosawa, Wako, 351-0198, Japan

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    Issue

    Vol. 78, Iss. 10 — 1 September 2008

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    Images

    • Figure 1
      Figure 1
      (Color online) Schematic illustration of the experimental setup for terahertz time-domain spectroscopy in a transmission geometry.Reuse & Permissions
    • Figure 2
      Figure 2
      (Color online) Low-energy electrodynamics of a complete set of available configurations of single crystals of DyMnO3 measured around 10 K in zero magnetic field. Upper and lower panels show the real and imaginary parts of ϵμ spectrum (closed circles) when Eω and Hω were set parallel to the crystallographic axes of ac, ab, and bc surface crystal plates. The solid lines in (a) and (b) are results of a least-squares fit to reproduce low-lying and high-lying peak structures by assuming two Lorentz oscillators for ϵ. On the contrary, ϵμ spectra in (d), (e), and (f) can be reproduced by two Lorentz oscillators for ϵ and μ as indicated by solid lines.Reuse & Permissions
    • Figure 3
      Figure 3
      (Color online) Temperature dependence of the real (upper panels) and imaginary (lower panels) parts of the selected ϵμ spectra (closed circles) of DyMnO3 for (a) Eω∥a, (b) Hω∥a, and (c) Hω∥c. Note that the scale of the vertical axis is different in respective figures. The Im[ϵμ] spectra shown in (b) and (c) are vertically offset for clarity. We also plot the extracted Im[ϵ] and Im[μ] spectra [at (b) 5 and (c) 4.2 K] indicated by dotted and gray lines, respectively, by assuming that Im[ϵμ] spectrum can be represented by two Lorentz oscillators for ϵ and μ (solid lines). The Im[μ] spectrum is multiplied by 5. On the contrary, ϵμ spectra for Eω∥a [(a)] can be fitted by two Lorentz oscillators.Reuse & Permissions
    • Figure 4
      Figure 4
      (Color online) Temperature (T) dependence of the integrated spectral weight per Mn-site, Neff, as defined by Eq. (1) in the text of the ac surface crystal plate of DyMnO3 for (a) Hω∥a and (b) Eω∥a. The upper panel of (a) shows the phase diagram with variation of T. PE and PM stand for paraelectric and paramagnetic, respectively. We also show the schematic illustrations of the bc spiral and collinear spin states induced by T. Néel temperature TNMn (39 K) and ferroelectric transition temperature Tc (19 K) are indicated by vertical solid lines in (a) and (b). The data measured at 75 and 249 K were also plotted at the 50 K position in (b) with square and triangle symbols, respectively. The horizontal dashed line in (a) represents the estimated contribution of the optical-phonon absorption at 5 K.Reuse & Permissions
    • Figure 5
      Figure 5
      (Color online) Magnetic-field (H) dependence of the real (upper panels) and imaginary (lower panels) parts of the selected ϵμ spectra (closed circles) of the ac surface crystal plate of DyMnO3 at 7 K when (a) Eω or (b) Hω was set parallel to the a axis. The ferroelectric polarization flop occurs at 20 kOe for H∥b. Note that Im[ϵμ] spectra in (b) are vertically offset for clarity and that the scale of the vertical axis is different in (a) and (b).Reuse & Permissions
    • Figure 6
      Figure 6
      (Color online) Magnetic-field (H) dependence of the integrated spectral weight per Mn-site, Neff, as defined by Eq. (1) in the text of the ac surface crystal plate of DyMnO3 for (a) Hω∥a and (b) Eω∥a, measured at 7 K. The upper panel of (a) shows the phase diagram with variation of H. We also show the schematic illustrations of the bc and ab spiral spin planes induced by H. H of 20 kOe at which the ferroelectric polarization flop occurs at 7 K, is indicated by the vertical solid line in (a) and (b). The difference of Neff at 7 K with [in (a) and (b)] and without H (in Fig. 4), arises from the difference of the integrated energy range (see text).Reuse & Permissions
    • Figure 7
      Figure 7
      (Color online) (a) Sample geometry with the complex Fresnel transmission coefficients from vacuum to the sample tv→s and from the sample to vacuum ts→v. The complex transmission coefficient t was obtained by Eq. (B1). d and ñ are the thickness and the complex refractive index of the sample, respectively. (b) Typical example of the measured terahertz wave form in time-domain with and without the sample (ac surface crystal plate of DyMnO3) measured at 5.2 K in zero magnetic field. Eω and Hω were set parallel to the c and a axes, respectively. The amplitude of the transmitted terahertz wave form with the sample was vertically offset for clarity. (c) Power transmission spectrum of DyMnO3.Reuse & Permissions
    • Figure 8
      Figure 8
      (Color online) (a) Validity of using Eqs. (B2a, B2b) instead of Eqs. (B3a, B3b). The Im[ϵμ] spectra of DyMnO3 for Eω∥c and Hω∥a at 5.2 K was almost unchanged even if we used Eqs. (B3a, B3b). The fixed parameters of Re[μ] and Im[μ] are described in the right-hand side of the figure, which were confirmed to be reasonable by assuming that ϵ can be represented by a single Lorentz oscillator [see (b)]. (b) The estimated Im[μ] spectrum (closed circles) by assuming that ϵ can be represented by a single Lorentz oscillator. We also plot the Im[μ] spectrum (a solid line) as extracted from Eq. (B1) combined with Eqs. (B2a, B2b).Reuse & Permissions
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