Imitation of spin density wave order in Cu$_3$Nb$_2$O$_8$

TL;DR

This paper investigates the magnetic phases of Cu$_3$Nb$_2$O$_8$, revealing that what appears as a spin density wave is actually a decoupled magnetic component, leading to a novel mechanism for symmetry breaking without structural inversion symmetry.

Contribution

The study demonstrates that the reported spin density wave in Cu$_3$Nb$_2$O$_8$ is an imitation phase caused by decoupled magnetic components, not a true density wave, offering new insights into magnetic symmetry breaking in insulators.

Findings

Confirmed a single magnetic phase transition to helicoidal order.

Identified the intermediate phase as an imitation of spin density wave.

Showed magnetic structure can break inversion symmetry without structural change.

Abstract

Spin density waves, based on modulated local moments, are usually associated with metallic materials, but have recently been reported in insulators which display coupled magnetic and structural order parameters. We discuss one such example, the multiferroic Cu$_3$Nb$_2$O$_8$, which is reported to undergo two magnetic phase transitions, first to a spin density wave phase at $T_N \approx 26.5K$, and then to a helicoidal structure coupled to an electric polarization below $T_2 \approx 24K$ [R. D. Johnson, et al., Phys. Rev. Lett., 107, 137205 (2011)] which breaks the crystallographic inversion symmetry. We apply spherical polarimetry to confirm the low-temperature magnetic structure, yet only observe a single magnetic phase transition to helicoidal order. We argue that the reported spin density wave originates from a decoupling of the components of the magnetic order parameter, as allowed by symmetry and driven by thermal fluctuations. This provides a mechanism for the magnetic, but not nuclear, structure to break inversion symmetry thereby creating an intermediate phase where the structure imitates a spin density wave. As the temperature is reduced, this intermediate structure destabilizes the crystal such that a structural chirality is induced, as reflected by the emergence of the electric polarization, and the imitation spin density wave relaxes into a generic helicoid. This provides a situation where the magnetic structure breaks inversion symmetry while the crystal structure remains centrosymmetric.