Reduced crystal symmetry as origin of the ferroelectric polarization within the incommensurate magnetic phase of TbMn2O5

TL;DR

This study reveals that reduced crystal symmetry in TbMn2O5, detected via neutron diffraction, is key to understanding its ferroelectric polarization, highlighting the roles of exchange-striction and spin-spiral interactions in its multiferroic behavior.

Contribution

The paper provides direct experimental evidence of reduced symmetry in TbMn2O5 and clarifies the mechanisms behind its ferroelectric polarization, emphasizing the importance of exchange interactions and spin-spiral structures.

Findings

Detection of forbidden Bragg reflections indicating reduced symmetry

Exchange-striction as the dominant polarization mechanism in the commensurate phase

Presence of spin-spiral order contributing to incommensurate magnetic phase

Abstract

The precise crystal symmetry and hence the emergence of the electric polarization still remains an open question in the multiferroic materials $R$Mn$_2$O$_5$ ($R$ = rare-earth, Bi, Y). While previous diffraction studies have indicated that $R$Mn$_2$O$_5$ possesses the centro-symmetric space group P$bam$, an atomic displacement allowing for the electric polarization would require a non-centrosymmetric crystal symmetry. Our single crystal neutron diffraction experiments on TbMn$_2$O$_5$ provide direct evidence of a reduced crystallographic symmetry already above the magnetic and ferroelectric phase transitions and a change in magnetic order upon entering the ferroelectric phase. This is indicated through the presence of additional nuclear Bragg reflections that are otherwise forbidden for the space group P$bam$ but are in good agreement with the polar space group P$12_11$. It implies that the exchange-striction, which arises from a symmetric $S_i \cdot S_j$ spin coupling, is the dominating mechanism for the generation of the electric polarization in the commensurate magnetic phase of TbMn$_2$O$_5$. Furthermore, the commensurate magnetic reflections are in accordance with a quartile step spin-spiral along the $c$-axis. Therefore, the antisymmetric $S_i \times S_j$ exchange via the inverse Dzyaloshinskii-Moriya interaction contributes as well and becomes the leading term in the low temperature incommensurate spin-spiral magnetic phase. These new findings provide important information for the understanding of the complex interplay between the magnetic and the structural order throughout the $R$Mn$_2$O$_5$ series of type-II multiferroics.