Cooperative ordering of gapped and gapless spin networks in Cu$_2$Fe$_2$Ge$_4$O$_{13}$

The unusual magnetic properties of a novel low-dimensional quantum ferrimagnet Cu$_2$Fe$_2$Ge$_4$O$_{13}$ are studied using bulk methods, neutron diffraction and inelastic neutron scattering. It is shown that this material can be described in terms of two low-dimensional quantum spin subsystems, one gapped and the other gapless, characterized by two distinct energy scales. Long-range magnetic ordering observed at low temperatures is a cooperative phenomenon caused by weak coupling of these two spin networks.