Modifying the magnetoelectric coupling in TbMnO${}_3$ by low-level Fe${}^{3+}$ substitution

Abstract

We report a comprehensive study of the low-level substitution of Mn${}^{3+}$ by Fe${}^{3+}$ effect on the static and dynamic magnetoelectric coupling in TbMn${}_{1-x}$Fe${}_x$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 TbMnO${}_3$, while it is observed up to TN in the Fe-substituted compounds, pointing for different mechanisms for static and dynamic magnetoelectric coupling. A second electrically active excitation near 40 cm${}^{-1}$ is explained by means of Tb3+ crystal-field effects. This excitation is observed up to room temperature, and exhibits a remarkable 15 cm${}^{-1}$ 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 cm${}^{-1}$. Raman spectroscopy reveals a spin-phonon coupling below TN in pure TbMnO${}_3$, but the temperature where the coupling start to be relevant increases with Fe concentration and reaches 100 K in TbMn${}_{0.96}$Fe${}_{0.04}$O${}_3$. The anomalies in the T-dependence of magnetic susceptibility above TN are well accounted by spin-phonon coupling and crystal-field excitation, coupled to oxygen motions.