Collinear-to-Spiral Spin Transformation without Changing Modulation Wavelength upon Ferroelectric Transition in Tb1-xDyxMnO3

TL;DR

This study investigates the magnetic and lattice structures of Tb1-xDyxMnO3 around its ferroelectric transition, revealing a collinear-to-spiral spin transformation that occurs without changing the modulation wavelength.

Contribution

It demonstrates that ferroelectricity in Tb1-xDyxMnO3 is linked to a spin structure transformation, not a modulation wavelength change, providing new insights into multiferroic mechanisms.

Findings

Modulation vectors remain temperature-independent through T_C.

Ferroelectric transition involves a transition from sinusoidal to spiral magnetic order.

Spiral structure below T_C explains the polarization along the c axis.

Abstract

Lattice modulation and magnetic structures in magnetoelectric compounds Tb1-xDyxMnO3 have been studied around the ferroelectric (FE) Curie temperature T_C by x-ray and neutron diffraction. Temperature-independent modulation vectors through T_C are observed for the compounds with 0.50< x < 0.68. This indicates that ferroelectricity with a polarization (P) along the c axis in the RMnO3 series cannot be ascribed to such an incommensurate-commensurate transition of an antiferromagnetic order as was previously anticipated. Neutron diffraction study of a single crystal with x=0.59 shows that the FE transition is accompanied by the transformation of the Mn-spin alignment from sinusoidal (collinear) antiferromagnetism into a transverse spiral structure. The observed spiral structure below T_C is expected to produce P along the c axis with the `inverse' Dzialoshinski-Moriya interaction, which is consistent with the observation.