  The molecular compound (NH4)2[FeCl5(H2O)] is a type-II multiferroic material,
in which incommensurate cycloidal order directly induces ferroelectric
polarization. The multiferroic domain kinetics in (NH4)2[FeCl5(H2O)] were
studied by time-resolved neutron diffraction experiments utilizing neutron
polarization analysis. The temperature and electric-field dependent
multiferroic relaxation obeys the simple combined Arrhenius-Merz law, which was
reported to describe domain kinetics in the prototype multiferroics TbMnO3 and
NaFeGe2O6. However, the characteristic time scale of the multiferroic
relaxation is considerably larger than those in TbMnO3 or NaFeGe2O6.
Temperature-dependent diffraction on (NH4)2[FeCl5(H2O)] reveals the emergence
of higher-order and commensurate magnetic contributions upon cooling in the
multiferroic phase in zero field. The good agreement with studies of
higher-harmonic contributions in the deuterated material indicates that the
isotopes only posses a minor impact on the magnetic ordering. But in contrast
to similar observations in multiferroic MnWO4, this anharmonic modification of
magnetic ordering does not depin multiferroic domain walls or alter the
temperature dependence of the multiferroic relaxation.
