Modifying the magnetoelectric coupling in TbMnO3 by low-level Fe3+ substitution

Abstract

We report a comprehensive study of the low-level substitution of ${\mathrm{Mn}}^{3+}$ by ${\mathrm{Fe}}^{3+}$ effect on the static and dynamic magnetoelectric coupling in $\mathrm{Tb}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3}$ ($x=0$, 0.02, and 0.04). The cationic substitution has a large impact on the balance between competitive magnetic interactions and, as a result, on the stabilization of the magnetic structures and ferroelectric phase at low temperatures. Low-lying electromagnon excitation is activated in the cycloidal modulated antiferromagnetic and ferroelectric phase in $\mathrm{TbMn}{\mathrm{O}}_{3}$, while it is observed up to ${T}_{N}$ in the Fe-substituted compounds, pointing at different mechanisms for static and dynamic magnetoelectric coupling. A second electrically active excitation near 40 $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ is explained by means of ${\mathrm{Tb}}^{3+}$ crystal-field effects. This excitation is observed up to room temperature, and exhibits a remarkable $15\phantom{\rule{4pt}{0ex}}\text{c}{\mathrm{m}}^{\ensuremath{-}1}$ downshift on cooling in Fe-substituted compounds. Both electromagnon and crystal-field excitations are found to be coupled to the polar phonons with frequencies up to 250 $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$. Raman spectroscopy reveals a spin-phonon coupling below ${T}_{N}$ in pure $\mathrm{TbMn}{\mathrm{O}}_{3}$, but the temperature where the coupling starts to be relevant increases with Fe concentration and reaches 100 K in $\mathrm{Tb}{\mathrm{Mn}}_{0.96}{\mathrm{Fe}}_{0.04}{\mathrm{O}}_{3}$. The anomalies in the $T$ dependence of magnetic susceptibility above ${T}_{N}$ are well accounted for by spin-phonon coupling and crystal-field excitation, coupled to oxygen motions.