Evolution of magnetic and crystal structures in the multiferroic FeTe2O5Br

TL;DR

This study investigates the magnetic and crystal structure evolution in FeTe2O5Br, revealing incommensurate magnetic order with amplitude modulation in both non-ferroelectric and multiferroic states, and explaining the differences in ferroelectric behavior.

Contribution

It provides detailed insights into the magnetic ordering and symmetry properties of FeTe2O5Br across different phases, highlighting the role of phase shifts in ferroelectric responses.

Findings

Incommensurate magnetic order with amplitude modulation exists in both states.

Symmetry restrictions for electric polarization are absent in both phases.

Differences in ferroelectric response are due to phase shifts in modulation waves.

Abstract

Neutron diffraction and nuclear quadrupole resonance (NQR) measurements were employed to investigate magnetic order in the non-ferroelectric phase preceding the low-temperature multiferroic state in FeTe2O5Br. Refnement of the neutron diffraction data and simulations of 79,81Br NQR spectra reveal that the incommensurate magnetic ordering in the non-ferroelectric state comprises amplitude-modulated magnetic moments, similarly as in the multiferroic state. The two ordered states differ in the orientation of the magnetic moments and phase shifts between modulation waves. Surprisingly, all symmetry restrictions for the electric polarization are absent in both states. The different ferroelectric responses of the two states are thus argued to arise from the differences in the phase shifts between certain modulation waves, which cancel out in the non-ferrolectric state.