Magnetic Excitations of the Hybrid Multiferroic (ND4)2FeCl5D2O

TL;DR

This study uses inelastic neutron scattering and spin-wave theory to analyze magnetic excitations in the multiferroic compound (ND4)2FeCl5D2O, revealing robust magnetic interactions across different phases and mechanisms.

Contribution

It provides a detailed quantitative analysis of magnetic interactions and dynamics in (ND4)2FeCl5D2O, clarifying the role of multiferroic mechanisms and their influence on magnetic couplings.

Findings

Magnetic interactions are well-described by a Heisenberg model with easy-plane anisotropy.

No significant change in exchange parameters between phases.

Magnetic interactions are more robust than electric polarization changes.

Abstract

We report a comprehensive inelastic neutron scattering study of the hybrid molecule-based multiferroic compound (ND4)2FeCl5D2O in the zero-field incommensurate cycloidal phase and the high-field quasi-collinear phase. The spontaneous electric polarization changes its direction concurrently with the field-induced magnetic transition, from mostly aligned with the crystallographic a-axis to the c-axis. To account for such change of polarization direction, the underlying multiferroic mechanism was proposed to switch from the spin-current model induced via the inverse Dzyalloshinskii-Moriya interaction to the p-d hybridization model. We perform a detailed analysis of the inelastic neutron data of (ND4)2FeCl5D2O using linear spin-wave theory to quantify magnetic interaction strengths and investigate possible impact of different multiferroic mechanisms on the magnetic couplings. Our result reveals that the spin dynamics of both multiferroic phases can be well-described by a Heisenberg Hamiltonian with an easy-plane anisotropy. We do not find notable differences between the optimal model parameters of the two phases. The hierarchy of exchange couplings and the balance among frustrated interactions remain the same between two phases, suggesting that magnetic interactions in (ND4)2FeCl5D2O are much more robust than the electric polarization in response to delicate reorganizations of the electronic degrees of freedom in an applied magnetic field.