Abstract-Magnetoelectric spectroscopy of spin excitations in LiCoPO 4

V. Kocsis, S. Bordács, Y. Tokunaga, J. Viirok, L. Peedu, T. Rõõm, U. Nagel, Y. Taguchi, Y. Tokura, and I. Kézsmárki

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.100.155124

We have studied spin excitations in a single-domain crystal of antiferromagnetic ${\mathrm{LiCoPO}}_4$ by terahertz absorption spectroscopy. By analyzing the selection rules and comparing the strengths of the absorption peaks in the different antiferromagnetic domains, we found electromagnons and magnetoelectric (ME) spin resonances in addition to conventional magnetic dipole active spin-wave excitations. Using the sum rule for the ME susceptibility, we determined the contribution of the spin excitations to all the different off-diagonal elements of the static ME susceptibility tensor in zero and finite magnetic fields. We conclude that the ME spin resonances are responsible for the static ME response of the bulk when the magnetic field is along the $x$ axis, and the symmetric part of the ME tensor with zero diagonal elements dominates over the antisymmetric components.

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Abstract-Optical magnetoelectric resonance in a polar magnet (Fe,Zn)2Mo3O8 with axion-type coupling

T. Kurumaji, Y. Takahashi, J. Fujioka, R. Masuda, H. Shishikura, S. Ishiwata, and Y. Tokura

https://journals.aps.org/prl/accepted/e4078Y6fQc815163827c0719a8372934e33e6b2ac

We report the polarization rotation of terahertz light resonant with the magnetoelectric (ME) spin excitation in the multiferroic (Fe, Zn)2Mo3O8. This resonance reflects the frequency dispersion of the diagonal ME susceptibility (axion term), with which we quantitatively reproduce the thermal and magnetic-field evolution of the observed polarization rotation spectra. The application of the sum rule on the extrapolated DC value of the spectral weight of the ME oscillator provides insight into the DC linear ME effect. The present finding highlights a novel optical functionality of spin excitations in multiferroics that originates from diagonal ME coupling.

Abstract-Role of commensurability of spin order for optical magnetoelectric effect with electromagnons in multiferroic YMn 2 O 5

R. Masuda, Y. Kaneko, Y. Yamasaki, Y. Tokura, and Y. Takahashi

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.041117

The optical magnetoelectric effect, which produces the nonreciprocal directional dichroism, on the electromagnon resonances is investigated for multiferroic phases of ${\mathrm{YMn}}_2{O}_5$ by terahertz spectroscopy. For the electromagnon driven by the exchange striction, the crucial role of the commensurability of spin order in the magnetoelectric coupling is manifested by the suppression of the directional dichroism in the incommensurate spin phase. Furthermore, the gapped electromagnon via the spin-current mechanism is identified in terms of the directional dichroism, irrespective of commensurability/incommensurability in the cycloidal spin order.

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Abstract-Role of commensurability of spin order for optical magnetoelectric effect with electromagnons in multiferroic YMn2O5

R. Masuda, Y. Kaneko, Y. Yamasaki, Y. Tokura, and Y. Takahashi

https://journals.aps.org/prb/accepted/8e071Y8bOb21675532797ee972b87bc76cc0a7fb6


Optical magnetoelectric effect, which produces the nonreciprocal directional dichroism, on the electromagnon resonances is investigated for multiferroic phases of YMn2O5 by terahertz spectroscopy. For the electromagnon driven by the exchange striction, a crucial role of the commensurability of spin order in the magnetoelectric coupling is manifested by the suppression of the directional dichroism in the incommensurate spin phase. Furthermore, the gapped electromagnon via the spin-current mechanism is identified in term of the directional dichroism, being irrespective of commensurability/incommensurability in the cycloidal spin order.

Abstracts-Electromagnon resonance in a collinear spin state of the polar antiferromagnet Fe2Mo3O8

T. Kurumaji, Y. Takahashi, J. Fujioka, R. Masuda, H. Shishikura, S. Ishiwata, and Y. Tokura

Phys. Rev. B 95, 020405(R) – Published 5 January 2017

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.020405

Magnetic excitations are investigated for a hexagonal polar magnet Fe2Mo3O8 by terahertz spectroscopy. We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the c axis. We unravel the nature of these excitations by investigating the correlation between the evolution of the mode profile and the magnetic transition from antiferromagnetic to ferrimagnetic order induced by a magnetic field or Zn doping. We propose that the observed electromagnon mode involves the electric polarization oscillating within the c plane induced by the collective precession of the spins through the same mechanism as producing the linear magnetoelectric effect.

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Abstract-Unidirectional terahertz light absorption in the pyroelectric ferrimagnet CaBaCo4O7

S. Bordács, V. Kocsis, Y. Tokunaga, U. Nagel, T. Rõõm, Y. Takahashi, Y. Taguchi, Y. Tokura

http://arxiv.org/abs/1601.00444

Spin excitations were studied by absorption spectroscopy in CaBaCo4O7 which is a type-I multiferroic compound with the largest magnetic-order induced ferroelectric polarization ({\Delta}P=17mC/m2) reported, so far. We observed two optical magnon branches: a solely electric dipole allowed one and a mixed magnetoelectric resonance. The entangled magnetization and polarization dynamics of the magnetoelectric resonance gives rise to unidirectional light absorption, i.e. that magnon mode absorbs the electromagnetic radiation for one propagation direction but not for the opposite direction. Our systematic study of the magnetic field and temperature dependence of magnon modes provides information about the energies and symmetries of spin excitations, which is required to develop a microscopic spin model of CaBaCo4O7.

Abstract-Unidirectional terahertz light absorption in the pyroelectric ferrimagnet CaBaCo4O7

S. Bordács, V. Kocsis, Y. Tokunaga, U. Nagel, T. Rõõm, Y. Takahashi, Y. Taguchi, and Y. Tokura

Phys. Rev. B 92, 214441 – Published 31 December 2015

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.92.214441

ABSTRACT 

Spin excitations were studied by absorption spectroscopy in CaBaCo4O7 which is a type-I multiferroic compound with the largest magnetic-order induced ferroelectric polarization (ΔP=17 mC/m2) reported, so far. We observed two optical magnon branches: a solely electric dipole allowed one and a mixed magnetoelectric resonance. The entangled magnetization and polarization dynamics of the magnetoelectric resonance gives rise to unidirectional light absorption, i.e., that magnon mode absorbs the electromagnetic radiation for one propagation direction but not for the opposite direction. Our systematic study of the magnetic field and temperature dependence of magnon modes provides information about the energies and symmetries of spin excitations, which is required to develop a microscopic spin model of CaBaCo4O7.

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Abstract-Unidirectional terahertz light absorption in the pyroelectric ferrimagnet CaBaCo4O7

S. Bordács, V. Kocsis, Y. Tokunaga, U. Nagel, T. Rõõm, Y. Takahashi, Y. Taguchi, and Y. Tokura

https://journals.aps.org/prb/accepted/87070Y68Wc51f14ff95a2790416e16664585554e4

Spin excitations were studied by absorption spectroscopy in CaBaCo4O7 which is a type-I multiferroic compound with the largest magnetic-order induced ferroelectric polarization (DP=17 mC/m2) reported, so far. We observed two optical magnon branches: a solely electric dipole allowed one and a mixed magnetoelectric resonance. The entangled magnetization and polarization dynamics of the magnetoelectric resonance gives rise to unidirectional light absorption, i.e. that magnon mode absorbs the electromagnetic radiation for one propagation direction but not for the opposite direction. Our systematic study of the magnetic field and temperature dependence of magnon modes provides information about the energies and symmetries of spin excitations, which is required to develop a microscopic spin model of CaBaCo4O7.

Colossal optical isolator effect driven by spin helix -Demonstration of the new gigahertz and terahertz optical devices

Colossal optical isolator effect driven by spin helix -Demonstration of the new gigahertz and terahertz optical devices- ： Project Associate Professor Youtaro Takahashi,　Quantum-Phase Electronics Center

http://www.t.u-tokyo.ac.jp/etpage/release/2014/2014080401.html

  Y. Takahashi (associate professor, University of Tokyo), S. Kibayashi (graduated student, University of Tokyo), S. Seki (unit leader, CEMS Riken) and Y. Tokura (professor, University of Tokyo, director, CEMS Riken) discovered new optical functionality of the helical spin structure, which differentiates the optical absorption between the counter-propagating light beams.

   They found that the emergence of the electromagnon, which is a kind of collective spin motion, in the frequency range of gigahertz to terahertz, when the helical spin structure shows up.  Due to the presence of the both “magnetism” and “chirality” by the helical spin structure, the electromagnon exhibits the colossal magnetochiral effect, leading to the changes of the extinction coefficient up to 400 % depending on the propagation direction of light beams.

   In the frequency region including the higher gigahertz and terahertz, which is expected for future high capacity communication etc., the optical device is developing.  The current results will be used for the possible optical devices, such as an isolator, and the electrically and magnetically controllable optical elements.

Abstract-Terahertz Magnetoelectric Resonance Enhanced by Mutual Coupling of Electromagnons

Y. Takahashi¹, Y. Yamasaki², and Y. Tokura^1,3 
1Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan
²Photon Factory and Condensed Matter Research Center, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan
³RIKEN Center for Emergent Matter Science (CEMS), Wako, 351-0198, Japan

http://prl.aps.org/abstract/PRL/v111/i3/e037204

Received 30 March 2013; published 17 July 2013

Both electric- and magnetic-dipole active spin excitations, i.e., electromagnons, which mediate the dynamical magnetoelectric effect, have been investigated for a multiferroic perovskite of manganite by optical spectroscopy at terahertz frequencies. Upon the magnetoelectric resonance at 1 meV in the multiferroic phase with the bc-plane spin cycloidal order, a gigantic dynamical magnetoelectric effect has been observed as a nonreciprocal directional dichroism or birefringence. The light k-vector-dependent difference (Δκ=κ₊-κ_-) of the extinction coefficient (κ_±) is as large as Δκ∼1 or 2Δκ/(κ₊+κ_-)∼0.7 at the lowest-lying electromagnon energy. We clarified the mutual coupling of theE^ω∥a-polarized electromagnons of the different origins, leading to the enhancement of the magnetoelectric resonance.

© 2013 American Physical Society

Abstract-Terahertz magnetoelectric resonance enhanced by mutual coupling of electromagnons

Y. Takahashi, Y. Yamasaki, and Y. Tokura
http://prl.aps.org/accepted/a307cY05P3d16856702e05a55632c8825b5eb5713
Both electric- and magnetic-dipole active spin excitations, $i.e.$ electromagnons, which mediate the dynamical magnetoelectric effect, have been investigated for a multiferroic perovskite of manganite by the optical spectroscopy at terahertz frequencies. Upon the magnetoelectric resonance at 1 meV in the multiferroic phase with the $bc$-plane spin cycloidal order, a gigantic dynamical magnetoelectric effect has been observed as a non-reciprocal directional dichroism/birefringence. The light $k$-vector dependent difference ($\Delta \kappa =\kappa_+-\kappa_-$) of the extinction coefficient ($\kappa_{\pm}$) is as large as $\Delta \kappa \sim 1$ or $2\Delta \kappa/(\kappa_++\kappa_-) \sim 0.7$ at the lowest-lying electromagnon energy. We clarified the mutual coupling of the $E^{\omega}\| a$-polarized electromagnons of the different origins, leading to the enhancement of the magnetoelectric resonance.