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Magnon circular birefringence: polarization rotation of spin waves and its applications

An international team of researchers from Thailand, USA and Japan, has conducted a thorough study of an exotic behavior of material called "noncentrosymmetric antiferromagnet."The team, monitoring the behavior of the propagation of spin waves in magnetic material, has reported its findings [1], which show, for the first time, direct evidence of the nonreciprocal magnons.

A "circular birefringence" effect, where photons travelling inside a certain kind of crystal have different speeds depending on their circular polarization is fairly common. In other words, left-handed photons might travel faster than right-handed photons. Such an effect specifically appearing under a finite external magnetic field is the Faraday effect, where light polarization rotates as it propagates along the crystal with the rotation angle linearly depending on the field. There have been tremendous applications of this effect in modern optical and photonic technology. Optical isolator is one of such devices using the Faraday effect, whereas magneto-optical recording is based on its reflection variant, the Kerr effect.

Fig. 1: Linearly polarized states of observed antiferromagnetic spin waves. The polarization angle changes in space, which indeed is an analogous effect to the "circular birefringence" of light.

Other systems also exhibit behaviors that resemble the circular birefringence effect. In an ordered magnetic material, a spin excitation can also propagate along the crystal. This excitation is called a "magnon." Similar to the polarization states of photons, magnons in an antiferromagnet also have two distinct states: left-circular and right-circular state. In most magnetic material, these two states have the same energy and are therefore indistinguishable. However, in a certain type of magnetic material, these two states of magnons behave differently due to a lack of spatial inversion symmetry in the crystal structure.

Fig. 2: Observed spin-wave dispersion relations and corresponding spin fluctuations in the circularly polarized states.

This phenomenon, called nonreciprocal magnons, has been predicted by Hayami et al. [2] However, there has been no direct observation of these nonreciprocal magnons until this work.

The research team performed neutron scattering experiments on single-crystal α-Cu2V2O7 and showed clear evidences of different energy-momentum dispersion relations between the left-circular and right-circular magnon propagation. The experimental data is confirmed by linear spin-wave calculations.

This work opens up a new regime of magnetic material which might find applications in magnon-based electronics (magnonics) such as the spin-wave field-effect transistor [3].

[1] G. Gitgeatpong, Y. Zhao, P. Piyawongwatthana, Y. Qiu, L. W. Harriger, N. P. Butch, T. J. Sato, and K. Matan, Phy. Rev. Lett. 119, 047201 (2017).
[2] S. Hayami, H. Kusunose, and Y. Motome, J. Phys. Soc. Jpn. 85, 053705 (2016).
[3] R. Cheng, M. W. Daniels, J.-G. Zhu, and D. Xiao, Sci. Rep. 6, 24223 (2016).

Press release in Japanese

Publication Details:

Title: Nonreciprocal magnons and symmetry-breaking in the noncentrosymmetric antiferromagnet
Authors: G. Gitgeatpong, Y. Zhao, P. Piyawongwatthana, Y. Qiu, L. W. Harriger, N. P. Butch, T. J. Sato, and K. Matan
Journal: Physical Review Letters
DOI: https://doi.org/10.1103/PhysRevLett.119.047201

Contact:

Kittiwit Matan
Department of Physics,
Mahidol University
Email: kittiwit.mat@mahidol.ac.th

Taku J Sato
Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University
Email: taku@tohoku.ac.jp
Website: http://www2.tagen.tohoku.ac.jp/en/index.html

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