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

01.08.2017

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.


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.

Credit: Taku J Sato


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

Credit: Taku J Sato

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.

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.

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).

Taku J Sato | EurekAlert!

Further reports about: crystal structure magnetic field magnetic material photons polarization waves

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