  Magnetic excitations in a weakly coupled spin dimers and chains compound
Cu2Fe2Ge4O13 are measured by inelastic neutron scattering. Both structure
factors and dispersion of low energy excitations up to 10 meV energy transfer
are well described by a semiclassical spin wave theory involving interacting
Fe$^{3+}$ ($S = 5/2$) chains. Additional dispersionless excitations are
observed at higher energies, at $\hbar \omega = 24$ meV, and associated with
singlet-triplet transitions within Cu$^{2+}$-dimers. Both types of excitations
can be understood by treating weak interactions between the Cu$^{2+}$ and
Fe$^{3+}$ subsystems at the level of the Mean Field/ Random Phase
Approximation. However, this simple model fails to account for the measured
temperature dependence of the 24 meV mode.
