Transverse and Longitudinal Excitation Modes in Interacting Multispin Systems

概要

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Magnetic excitation in coupled multispin system is studied theoretically focusing on Cu2Fe2Ge4O13 and Cu2CdB2O6 as typical examples of such system. These compounds consist of spin dimer and spin monomer parts and show an antiferromagnetic phase transition at low temperatures due to the spin monomer part. A multispin containing a spin dimer and spin monomers is treated as a basis unit. The multispin forms a spin multiplet and its energy levels are separated into high and low regions reflecting the characteristic energies of the dimer and monomer parts. We regard the system as interacting multispins and apply an extended Holstein–Primakoff theory by introducing bosons for each energy level of a spin multiplet. In the low-energy region, the obtained magnon dispersion and dynamical spin correlation function agree quantitatively with experimental results of inelastic neutron scattering performed in Cu2Fe2Ge4O13. Analyzing dynamical spin correlation functions, we find several high-energy modes with longitudinal spin fluctuations peculiar to the coupled spin dimer and spin monomer system. One-magnon Raman scattering is studied to probe such longitudinal modes. Magnetic excitation in Cu2CdB2O6 is also studied in the same way.
2010-08-15