Cooperative order and excitation spectra in the bicomponent spin networks

TL;DR

This paper investigates the magnetic excitations and order in a complex ferrimagnetic spin system combining spin dimers and chains, revealing how interactions influence excitation spectra and magnetic properties, consistent with experimental observations.

Contribution

It introduces a combined theoretical approach using bond-operator and Holstein-Primakoff transformations to analyze the spin dynamics and order in a bicomponent spin network.

Findings

Hybridized triplon-magnon excitations exhibit different behaviors near zero interaction.

The gap in magnon-like excitations scales with the interaction strength $J_{DF}$.

The model's results agree with experimental data on Cu$_{2}$Fe$_{2}$Ge$_{4}$O$_{13}$.

Abstract

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 $J_{\rm DF}$ 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 $J_{\rm DF}$. It presents an effective staggered field reacting on the spin chains. The degeneracy of the triplons is lifted due to the chain magnetization and a mode with longitudinal polarization is identified. Due to the triplon-magnon interaction, the hybridized triplon-like excitations show different behaviors near the vanishing $J_{\rm DF}$. On the other hand, the hybridized magnon-like excitations open a gap $\Delta_A\sim J_{\rm DF}$. These results consist well with the experiments on Cu$_{2}$Fe$_{2}$Ge$_{4}$O$_{13}$.