Electromagnon excitation in the field-induced noncollinear ferrimagnetic phase of ${\mathrm{Ba}}_{2}{\mathrm{Mg}}_{2}{\mathrm{Fe}}_{12}{\mathrm{O}}_{22}$ studied by polarized inelastic neutron scattering and terahertz time-domain optical spectroscopy

Electromagnon excitation in the field-induced noncollinear ferrimagnetic phase of ${Ba}_{2} {Mg}_{2} {Fe}_{12} O_{22}$ studied by polarized inelastic neutron scattering and terahertz time-domain optical spectroscopy

Taro Nakajima, Youtarou Takahashi, Shunsuke Kibayashi, Masaaki Matsuda, Kazuhisa Kakurai, Shintaro Ishiwata, Yasujiro Taguchi, Yoshinori Tokura, and Taka-hisa Arima

Phys. Rev. B 93, 035119 – Published 19 January 2016

Abstract

We have studied magnetic excitations in a field-induced noncollinear commensurate ferrimagnetic phase of ${Ba}_{2} {Mg}_{2} {Fe}_{12} O_{22}$ by means of polarized inelastic neutron scattering (PINS) and terahertz (THz) time-domain optical spectroscopy under magnetic field. A previous THz spectroscopy study reported that the field-induced phase exhibits electric-dipole-active excitations with energies of around 5 meV [Kida et al., Phys. Rev. B 83, 064422 (2011)]. In the present PINS measurements, we observed inelastic scattering signals around 5 meV at the zone center in the spin-flip channel. This directly shows that the electric-dipole-active excitations are indeed of magnetic origin, that is, electromagnons. In addition, the present THz spectroscopy confirms that the excitations have oscillating electric polarization parallel to the $c$ axis. In terms of the spin-current model (Katsura-Nagaosa-Balatsky model), the noncollinear magnetic order in the field-induced phase can induce static electric polarization perpendicular to the $c$ axis, but not dynamic electric polarization along the $c$ axis. We suggest that the electromagnon excitations can be explained by applying the magnetostriction model to the out-of-phase oscillations of the magnetic moments, which is deduced from the present experimental results.

Received 26 August 2015
Revised 25 November 2015

DOI:https://doi.org/10.1103/PhysRevB.93.035119

Physics Subject Headings (PhySH)

Electric polarization Ferrimagnetism Ferroelectricity

Ferrimagnets Multiferroics

Inelastic neutron scattering Optical spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Taro Nakajima^1,*, Youtarou Takahashi^2,3, Shunsuke Kibayashi², Masaaki Matsuda⁴, Kazuhisa Kakurai^1,5, Shintaro Ishiwata², Yasujiro Taguchi¹, Yoshinori Tokura^1,2, and Taka-hisa Arima^1,6

¹RIKEN Center for Emergent Matter Science (CEMS), Saitama 351-0198, Japan
²Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan
³PRESTO, Japan Science and Technology Agency, Chiyoda, Tokyo 102-8666, Japan
⁴Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁵Quantum Beam Science Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
⁶Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8561, Japan

^*taro.nakajima@riken.jp

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Vol. 93, Iss. 3 — 15 January 2016

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Figure 1
(a) The crystal structure of ${Ba}_{2} {Mg}_{2} {Fe}_{12} O_{22}$ , the hexagonal basis $(a, b, c)$ , and rhombohedral basis $(a_{R}, b_{R}, c_{R})$ . Schematic models of magnetic structures in the (b) LC and (c) FE3 phases, depicted on the basis of the block approximation.
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Figure 2
Schematics showing the relationships among the directions of the incident light wave, electric-field, and magnetic-field components of the light, magnetic moments, and the dc magnetic field for the (a) $E^{ω} ∥ [001]$ and (b) $E^{ω} ∥ [100]$ configurations. Spectra of the imaginary part of the dielectric constant obtained from the time-domain THz spectroscopy under the magnetic field along [100] with (c) 0, (d) 1, and (e) 7 T at 4.6 K. Red and blue lines show the spectra measured in the $E^{ω} ∥ [001]$ and $E^{ω} ∥ [100]$ configurations, respectively. The single and double black arrows show the positions of the peaks (see the main text for details).
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Figure 3
Magnetic field dependence of the imaginary part of the dielectric constant between 0 and 1 T measured by the THz optical spectroscopy with the $E^{ω} ∥ [001]$ configuration.
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Figure 4
Unpolarized inelastic neutron scattering spectra measured by constant- $Q$ scans at reciprocal lattice points along the $(0, 0, L)$ line, at $H^{dc} = 1$ T and $T = 1.5$ K. The monitor counts of $323 000$ correspond to $320 \sim 400$ s in the energy range of $0 \sim 14$ meV. Note that the data are shifted by 200 for each, to enhance the visibility. The dashed lines are guides to the eyes.
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Figure 5
(a) The polarized neutron diffraction profile along the $(0, 0, L)$ line at $H^{dc} = 1$ T and $T = 1.5$ K. (b), (c) The polarized inelastic scattering spectra at (b) (0,0,10.5) and (c) (0,0,9). (d) A schematic showing the relationship among the Cartesian coordinate $x y z$ , the directions of the magnetic moments, dc magnetic field, neutron polarization, and crystallographic axes. (e) The reciprocal lattice map of the $(0, K, L)$ scattering plane. (f) The polarized neutron diffraction profile along the $(0, \bar{1}, L)$ line. (g), (h) The polarized inelastic scattering spectra at (b) (0, $\bar{1}$ ,2.5) and (c) (0, $\bar{1}$ ,1). In Figs. (a)–(c) and (f)–(h), open and filled symbols show the intensities measured in the SF and NSF channels, respectively. The monitor counts of 1.3 million correspond to $1200 \sim 1600$ s in the energy range of $0 \sim 14$ meV. The dashed lines are guides to the eyes. The vertical single and doubled arrows show the peak positions corresponding to the two peaks around 3 and 5 meV observed in the THz absorption spectra, respectively.
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Figure 6
Schematic pictures of (a) in-phase and (b) out-of-phase oscillations of the $y$ $(c$ -axis) component of the magnetic moments. Horizontal orange dotted arrows denote the local electric dipole moments $(p_{local})$ embedded in the crystal structure. (c) Calculated values of $δ P_{c}$ for the in-phase and out-of-phase oscillations.
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Physical Review B

covering condensed matter and materials physics

Electromagnon excitation in the field-induced noncollinear ferrimagnetic phase of ${Ba}_{2} {Mg}_{2} {Fe}_{12} O_{22}$ studied by polarized inelastic neutron scattering and terahertz time-domain optical spectroscopy

Taro Nakajima, Youtarou Takahashi, Shunsuke Kibayashi, Masaaki Matsuda, Kazuhisa Kakurai, Shintaro Ishiwata, Yasujiro Taguchi, Yoshinori Tokura, and Taka-hisa Arima

Phys. Rev. B 93, 035119 – Published 19 January 2016

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Physical Review B

covering condensed matter and materials physics

Electromagnon excitation in the field-induced noncollinear ferrimagnetic phase of Ba2Mg2Fe12O22 studied by polarized inelastic neutron scattering and terahertz time-domain optical spectroscopy

Taro Nakajima, Youtarou Takahashi, Shunsuke Kibayashi, Masaaki Matsuda, Kazuhisa Kakurai, Shintaro Ishiwata, Yasujiro Taguchi, Yoshinori Tokura, and Taka-hisa Arima

Phys. Rev. B 93, 035119 – Published 19 January 2016

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Electromagnon excitation in the field-induced noncollinear ferrimagnetic phase of ${Ba}_{2} {Mg}_{2} {Fe}_{12} O_{22}$ studied by polarized inelastic neutron scattering and terahertz time-domain optical spectroscopy