Magneto-electric control of toroidic moments in multiferroic LiCoPO4

In addition to the three widely known forms of ordering, elastic, electric and magnetic orders, a new so called ferrotoroidic phase has been recently observed.[1] The toroidic moment is asymmetric under both time reversal and space inversion symmetries allowing ferrotoroidic materials to exhibit intrinsic magneto-electric effect. Possibility to control magnetic properties using electric field makes such materials desirable for applications. We discuss the magneto-electric control of toroidic moments in the multiferroic material, LiCoPO$_4$. Based on symmetry arguments we derive microscopic model for induced polarization and 'toroidization'. Using multiboson approach we investigate the experimentally observed magnon absorption spectrum following different magneto-electric poling processes. We reproduce the mono-domain ferrotoroidic state established by magneto-electric poling, as well as unconventional optical properties, such as the unidirectional light transmission, emerging in magnon spectrum of LiCoPO$_4$. [1] Bas B. Van Aken et al, Nature 449, 702-705 (11 October 2007), Anne S. Zimmermann et al, Nature Communications 5, Article number: 4796