Temperature-Dependent Infrared Reflectivity Studies of Multiferroic TbMnO_{3}: Evidence for Spin-Phonon Coupling

TL;DR

This study investigates the infrared reflectivity of TbMnO₃ across temperatures, revealing phonon mode softening below the magnetic transition, which indicates spin-phonon coupling affecting the material's lattice dynamics.

Contribution

It provides experimental evidence of spin-phonon coupling in TbMnO₃ through temperature-dependent infrared spectroscopy and phonon mode analysis.

Findings

Observation of 15 phonon modes in the infrared spectra.

Anomalous phonon softening below T_N (~46K).

Increase in oxygen effective charge below T_N.

Abstract

We have measured near normal incidence far infrared (FIR) reflectivity spectra of a single crystal of TbMnO3 from 10K to 300K in the spectral range of 50 cm$^{-1}$ to 700 cm$^{-1}$. Fifteen transverse optic (TO) and longitudinal optic (LO) modes are identified in the imaginary part of the dielectric function $\epsilon_2$($\omega$) and energy loss function Im(-1/$\epsilon$($\omega$)), respectively. Some of the observed phonon modes show anomalous softening below the magnetic transition temperature T$_N$ (~ 46K). We attribute this anomalous softening to the spin-phonon coupling caused by phonon modulation of the super-exchange integral between the Mn$^{3+}$ spins. The effective charge of oxygen (Z$_O$) calculated using the measured LO-TO splitting increases below T$_N$.