Electromagnons, magnons, and phonons in Eu$_{1-x}$Ho$_x$MnO$_3$

TL;DR

This study investigates the THz and FIR responses of Eu$_{1-x}$Ho$_x$MnO$_3$, revealing how magnetic phase transitions affect excitations and electromagnon spectral weight, with implications for understanding multiferroic mechanisms.

Contribution

It provides new insights into the magnetic excitations and electromagnon behavior across different phases in Eu$_{1-x}$Ho$_x$MnO$_3$, highlighting unexpected spectral weight transfer mechanisms.

Findings

Significant change in excitation intensity at phase transitions.

Discovery of a new excitation near 40 cm$^{-1}$ in the collinear phase.

Spectral weight transfer from phonons varies with polarization direction.

Abstract

Here we present a detailed study of the THz and FIR response of the mixed perovskite manganite system Eu$_{1-x}$Ho$_x$MnO$_3$ for holmium concentrations x = 0.1 and 0.3. We compare the magnetic excitations of the four different magnetically ordered phases (A-type antiferromagnetic, sinusoidally modulated collinear, helical phases with spin planes perpendicular to the crystallographic a and c axes). The transition between the two latter phases goes hand in hand with a switching of the ferroelectric polarization from P||a to P||c. Special emphasis is paid to the temperature dependence of the excitations at this transition. We find a significant change of intensity indicating that the exchange striction mechanism cannot be the only mechanism to induce dipolar weight to spin-wave excitations. We also focus on excitations within the incommensurate collinear antiferromagnetic phase and find a so far unobserved excitation close to 40 cm$^{-1}$. A detailed analysis of optical weight gives a further unexpected result: In the multiferroic phase with P||c all the spectral weight of the electromagnons comes from the lowest phonon mode. However, for the phase with the polarization P||a additional spectral weight must be transferred from higher frequencies.