Magnetic-order-induced ferroelectric polarization in the polar antiferromagnets RFeWO6 ( R = Sm, Gd, Er, and Tm)

Abstract

We report the synthesis, structure, and magnetism-induced multiferroic properties of the polar magnets $R\mathrm{FeW}{\mathrm{O}}_{6}$ (R = Tm, Sm, Gd, and Er). All these compounds crystallize in the orthorhombic structure with the polar symmetry $Pna{2}_{1},$ which results from the ordering of ${\mathrm{Fe}}^{3+}$ and ${\mathrm{W}}^{6+}$ ions at different crystallographic sites. DC magnetization and specific heat measurements confirm the antiferromagnetic order of ${\mathrm{Fe}}^{3+}$ spins at ${T}_{\mathrm{N}1}=14\ensuremath{-}18\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and magnetic ordering of R ions at low temperatures. The magnetic order of ${\mathrm{Fe}}^{3+}$ ions in these compounds is accompanied by a dielectric anomaly and a change in electric polarization. Intriguingly, a second ferroelectric transition occurs at the magnetic ordering temperature (${T}_{\mathrm{N}2}=5.5\phantom{\rule{4pt}{0ex}}\mathrm{K}$) of ${\mathrm{Tm}}^{3+}$ ions in $\mathrm{TmFeW}{\mathrm{O}}_{6}$. The magnetic field dependent behavior of electric polarization varies with R ion, indicating the coupling between $4f\ensuremath{-}3d$ spins. The emergence of change in ferroelectric polarization at the magnetic ordering temperatures demonstrates the multiferroic nature of the polar magnets $R\mathrm{FeW}{\mathrm{O}}_{6}$ (R = Tm, Sm, Gd, and Er). Our study indicates that the aeschynite type family of compounds with polar symmetry can be an excellent platform to understand the role of $4f\ensuremath{-}3d$ coupling on multiferroicity.