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

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.

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. However, there has been no direct observation of these nonreciprocal magnons until this work.

The research team performed neutron scattering experiments on single-crystal ?-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.