Magnetic Excitations of the Hybrid Multiferroic (ND₄)₂[FeCl₅(D₂O)]

(ND₄)₂[FeCl₅(D₂O)] is a molecular magnet exhibiting direct coupling between magnetism and electric polarization. Here we report a comprehensive inelastic neutron scattering study of the magnetic excitations in the zero-field incommensurate cycloidal phase and the high-field quasi-collinear phase. Accompanying with the field-induced magnetic phase transition, the spontaneous electric polarization direction changes from mostly aligned with the crystallographic a-axis to the c-axis, and the underlying multiferroic mechanism is believed to switch from the inverse Dzyalloshinskii-Moriya model to the p-d hybridization model. Our detailed data analysis using linear spin wave theory reveals that the spin dynamics of both multiferroic phases can be well-described by a Heisenberg Hamiltonian with an easy-plane anisotropy. However, the optimal model parameters show small but clear difference between the two phases, suggesting that delicate reorganizations of electronic degrees of freedom occurs in (ND₄)₂[FeCl₅(D₂O)] in response to external magnetic fields.