Magnetic excitations in weakly coupled spin dimers and chains material Cu2Fe2Ge4O13

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.