Indirect magnetic interaction mediated by spin dimer in Cu${}_2$Fe${}_2$Ge${}_4$O${}_{13}$

Cu${}_2$Fe${}_2$Ge${}_4$O${}_{13}$ is a bicomponent compound that consists of Cu dimers and Fe chains with separate energy scale. By inelastic neutron scattering technique with high-energy resolution we observed the indirect Fe - Fe exchange coupling by way of the Cu dimers. The obtained parameters of the effective indirect interaction and related superexchange interactions are consistent with those estimated semi-statically. The consistency reveals that the Cu dimers play the role of nonmagnetic media in the indirect magnetic interaction.