Colossal magnetodielectric effect in SmFe$_3$(BO$_3$)$_4$ multiferroic

The colossal (more than threefold) decrease in the dielectric constant $\varepsilon$; in the easy-plane SmFe$_3$(BO$_3)_4$ ferroborate in a magnetic field of ${\sim}\,5\,$kOe applied in the basal $ab$-plane of the crystal has been found. A close relation of this effect to anomalies in the field dependence of the electric polarization has been established. It has been shown that this magnetodielectric effect is due to the contribution to $\varepsilon$ from the electric susceptibility, which is related to the rotation of spins in the $ab$-plane, arises in the region of the antiferromagnetic ordering $T<T_N=33\,$K, and is suppressed by the magnetic field. A theoretical model describing the main features of the behavior of $\varepsilon$ and electric polarization in the magnetic field has been proposed, taking into account the additional anisotropy in the basal plane induced by the magnetoelastic stresses.