Coupling of frustrated Ising spins to magnetic cycloid in multiferroic TbMnO3

TL;DR

This study reveals how magnetic orders of Tb and Mn in multiferroic TbMnO3 are coupled and locked at specific wave vectors below the ferroelectric transition, explained by a frustrated ANNNI model with domain wall arrangements.

Contribution

It demonstrates the coupling and locking of Tb and Mn magnetic propagation vectors in TbMnO3 and explains this behavior using a frustrated ANNNI model with external periodic fields.

Findings

Tb and Mn magnetic orders are coupled below 28 K.

At low temperatures, Tb and Mn wave vectors lock into rational fractions.

The locking behavior is explained by a frustrated ANNNI model with domain walls.

Abstract

We report on diffraction measurements on multiferroic TbMnO3 which demonstrate that the Tb- and Mn-magnetic orders are coupled below the ferroelectric transition TFE = 28 K. For T < TFE the magnetic propagation vectors (tau) for Tb and Mn are locked so that tauTb = tauMn, while below TNTb = 7 K we find that tauTb and tauMn lock-in to rational values of 3/7 b* and 2/7 b*, respectively, and obey the relation 3tauTb - tauMn = 1. We explain this novel matching of wave vectors within the frustrated ANNNI model coupled to a periodic external field produced by the Mn-spin order. The tauTb = tauMn behavior is recovered when Tb magnetization is small, while the tauTb = 3/7 regime is stabilized at low temperatures by a peculiar arrangement of domain walls in the ordered state of Ising-like Tb spins.