  We theoretically find that in a multiferroic chiral magnet Cu2OSeO3, resonant
magnetic excitations are coupled to collective oscillation of electric
polarization, and thereby attain simultaneous activity to ac magnetic field and
ac electric field. Because of interference between these magnetic and electric
activation processes, this material hosts gigantic magnetochiral dichroism on
microwaves, that is, the directional dichroism at gigahertz frequencies in
Faraday geometry. The absorption intensity of microwave differs by as much as
30% depending on whether its propagation direction is parallel or antiparallel
to the external magnetic field.
