Origin of the enhanced ferroelectricity in multiferroic SmMn2O5

Abstract

In orthorhombic $\mathrm{SmM}{\mathrm{n}}_{2}{\mathrm{O}}_{5}$ single crystals, $\mathrm{S}{\mathrm{m}}^{3+}$ crystal-field (CF) excitations are studied by infrared transmission as a function of temperature and under applied magnetic field up to 10 T. These measurements are complemented with the study of Raman-active phonon frequency shifts as a function of temperature between 300 and 5 K. The frequencies of all $^{6}H_{j}$ crystal-field levels of $\mathrm{S}{\mathrm{m}}^{3+}$ were determined as well as those of $^{6}F_{j}$. At high temperatures, the evolutions of $\mathrm{S}{\mathrm{m}}^{3+}$ CF excitations exhibit anomalies around the characteristic temperatures, ${T}^{*}\ensuremath{\sim}60\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}^{\mathrm{S}}\ensuremath{\sim}120\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and reflect the thermal disorder induced by splitting of the Sm--O bonds in $\mathrm{SmM}{\mathrm{n}}_{2}{\mathrm{O}}_{5}$ that contribute to the frequency and linewidth phonon shifting. At low temperatures, the degeneracy of the ground-state Kramers doublet is lifted (${\mathrm{\ensuremath{\Delta}}}_{0}\ensuremath{\sim}36\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) due to the $\mathrm{S}{\mathrm{m}}^{3+}\ensuremath{-}\mathrm{M}{\mathrm{n}}^{3+}$ interaction in the ferroelectric phase and strongly enhanced below ${T}_{\mathrm{C}}\ensuremath{\sim}26\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The Sm-Mn exchange interaction ${J}_{6}$ is determined and compared to that of Gd-Mn interaction in $\mathrm{GdM}{\mathrm{n}}_{2}{\mathrm{O}}_{5}$. The Sm magnetic moment ${m}_{\mathrm{Sm}}(T)$ and the Sm contribution to the magnetic susceptibility are also evaluated from ${\mathrm{\ensuremath{\Delta}}}_{0}(T)$, indicating that the Sm-Mn interaction is strongly implicated in the magnetic and the ferroelectric orderings below $\ensuremath{\sim}26\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The study of the $\mathrm{S}{\mathrm{m}}^{3+}$ CF excitations as a function of magnetic field reveals twinning in $\mathrm{SmM}{\mathrm{n}}_{2}{\mathrm{O}}_{5}$. This twinning could affect its electric polarization behavior versus magnetic field.