  The magnetic order, spin dynamics, and crystal structure of the multiferroic
Sr0.56Ba0.44MnO3 have been investigated using neutron and x-ray scattering.
Ferroelectricity develops at TC=305 K with a polarization of 4.2 microC/cm2
associated with the displacements of the Mn ions, while the Mn4+ spins order
below TN = 200 K into a simple G-type commensurate magnetic structure. Below TN
the ferroelectric order decreases dramatically demonstrating that the two order
parameters are strongly coupled. The ground state spin dynamics are
characterized by a spin gap of 4.6(5) meV and the magnon density of states
peaking at 43 meV. Detailed spin wave simulations with a gap and isotropic
exchange of J=4.8(2) meV describe the excitation spectrum well. Above TN strong
spin correlations coexist with robust ferroelectric order.
