  In contrast to well studied multiferroic manganites with a spiral structure,
the electric polarization in multiferroic borates is induced within collinear
antiferromagnetic structure and can easily be switched by small static fields.
Because of specific symmetry conditions, static and dynamic properties in
borates are directly connected, which leads to giant magnetoelectric and
magnetodielectric effects. Here we prove experimentally that the giant
magnetodielectric effect in samarium ferroborate SmFe3(BO3)4 is of intrinsic
origin and is caused by an unusually large electromagnon situated in the
microwave range. This electromagnon reveals strong optical activity exceeding
120 degrees of polarization rotation in a millimeter thick sample.
