  Iron pnictides and selenides display a variety of unusual magnetic phases
originating from the interplay between electronic, orbital, and lattice degrees
of freedom. Using powder inelastic neutron scattering on the two-leg ladder
BaFe2Se3, we fully characterize the static and dynamic spin correlations
associated with the Fe4 block state, an exotic magnetic ground state observed
in this low-dimensional magnet and in Rb0.89Fe1.58Se2. All the magnetic
excitations of the Fe4 block state predicted by an effective Heisenberg model
with localized spins are observed below 300 meV and quantitatively reproduced.
However, the data only account for 16 mub^2 per Fe2+, approximatively 2/3 of
the total spectral weight expected for localized S=2 moments. Our results
highlight how orbital degrees of freedom in iron-based magnets can conspire to
stabilize an exotic magnetic state.
