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Directional dichroism (DD) is the effect when material absorbs light beams travelling in opposite direction differently. At THz frequencies DD has been found in multiferroics with magnetic and charge order. In these materials the spin waves are coupled to the oscillations of electric polarization. A spin wave acquires electric dipole activity due to optical magnetoelectric (ME) coupling and therefore interacts with the electric component of the THz radiation in addition to magnetic component.
We studied spin excitations in Sr₂CoSi₂O₇ below 2 THz between 3 and 100 K and in magnetic fields up to 30 T. Almost one-way transparency is seen in some spin wave modes. What is more, the DD increases above the Neel temperature (7 K) in high magnetic field. This is unusual in multiferroics but is explained by the ME coupling on a single spin site [1]. This study demonstrates that DD exists in multiferroics with the single spin site ME interaction even above the magnetic ordering temperature when sufficiently strong polarizing external magnetic field is applied [2].
[1] Akaki et al., PRB 86, 060413 (2012)
[2] arXiv:1809.10207 (2018)