Colossal electromagnon excitation in the non-cycloidal phase of TbMnO3 under pressure

Ian Aupiais,Yann Gallais,Romain Grasset,Hideaki Sakata,Maximilien Cazayous,Masahito Mochizuki,Alain Sacuto

doi:10.1038/s41535-018-0130-3

Abstract

The magnetoelectric coupling, i.e., cross-correlation between electric and magnetic orders, is a very desirable property to combine functionalities of materials for next-generation switchable devices. Multiferroics with spin-driven ferroelectricity presents such a mutual interaction concomitant with magneto-active and electro-active excitations called electromagnons. TbMnO3 is a paradigmatic material in which two electromagnons have been observed in the cycloidal magnetic phase. However, their observation in TbMnO3 is restricted to the cycloidal spin phase and magnetic ground states that can support the electromagnon excitation are still under debate. Here, we show by performing Raman spectroscopy measurements under pressure that the lower-energy electromagnon (4 meV) disappears when the ground state enters from a cycloidal phase to an antiferromagnetic phase (E-type). On the contrary, the magneto-electric activity of the higher-energy electromagnon (8 meV) increases in intensity by one order of magnitude. Using microscopic model calculations, we demonstrate that the lower-energy electromagnon, observed in the cycloidal phase, originates from a higher harmonic of the magnetic cycloid, and we determine that the symmetric exchange-striction mechanism is at the origin of the higher-energy electromagnon which survives even in the E-type phase. The colossal enhancement of the electromagnon activity in TbMnO3 paves the way to use multiferroics more efficiently for generation, conversion and control of spin waves in magnonic devices.

Highlights

In the so-called improper multiferroics such as perovskite manganites RMnO3 with R being a rare-earth ion, the electric polarization is induced by a spin order breaking the spatial inversion symmetry via the spin-orbit interaction[1,2,3,4,5] or the magnetic exchange striction.[6,7]
Tuning magnetic properties of multiferroics is the first step to build a new technology based on spin waves called magnonics using these promising materials.[16,17]
We investigate the dynamical part of the magnetism in TbMnO3 under hydrostatic pressure to determine the exact spin ground state responsible for the electromagnon activity

Summary

INTRODUCTION

In the so-called improper multiferroics such as perovskite manganites RMnO3 with R being a rare-earth ion, the electric polarization is induced by a spin order breaking the spatial inversion symmetry via the spin-orbit interaction[1,2,3,4,5] or the magnetic exchange striction.[6,7] TbMnO3 is one of the most studied multiferroic materials of this class Such a compound is fundamental to study novel coupling between microscopic degrees of freedom such as spin and charge.[8] In 2006, Pimenov et al.[9] succeeded in exciting spin waves with the electric-field component of terahertz (THz) light in TbMnO3.

Experiments

DISCUSSION

METHODS