Magnonic activity of ferromagnetic nanocylinders

The optical activity refers to the rotation of the vibration plane of linearly polarized light traversing certain substances. Here we report an analogous phenomenon for spin waves (SWs) in cylindrical nanowires, whose azimuthal distribution undergoes a continuous rotation as traveling along the wire. This effect, in line with the Fresnel model of the optical activity, is attributed to different propagating speeds of two circular states (left and right handed) composing the SW modes. The SW circular birefringence observed here originates from the overall curvilinear shape of the waveguide. Moreover, the fact that an external field is able to alter the rotatory power of the nanowires is reminiscent of the Faraday magneto-optical effect.