  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.
