Cooperative order and excitation spectra in the bicomponent spin networks

A ferrimagnetic spin model composed of S = 1/2 spin-dimers and S = 5/2 spin-chains is studied by combining the bond-operator representation (for S = 1/2 spin-dimers) and Holstein-Primakoff transformation (for S = 5/2 spins). A finite interaction J _DF between the spin-dimer and the spin chain makes the spin chains ordered antiferromagnetically and the spin dimers polarized. The effective interaction between the spin chains, mediated by the spin dimers, is calculated up to the third order. The staggered magnetization in the spin dimer is shown proportional to J _DF. Due to the triplon-magnon interaction, the degeneracy of the triplons is lifted and the hybridized triplon-like excitations show different behaviors near the vanishing J _DF. A mode with longitudinal polarization is identified. The hybridized magnon-like excitations are also studied. These results are compared with the experiments on Cu₂Fe₂Ge₄O₁₃.