Magnetic double structure for S=1 and S=1/2 mixed-spin systems

We investigate the zero-temperature and the finite-temperature properties o f the two-dimensional antiferromagnetic quantum spin system composed of the s=1/2 and s=1 spins. The spin excitation spectrum as well as the thermodyn amic quantities are computed by means of the Schwinger-boson mean-herd theo ry. We discuss how the magnetic double structure with the gapful and the ga pless excitations is generated, and apply the results systematically to the Haldane gap system in a staggered magnetic held and also to the weakly cou pled ferrimagnetic mixed-spin chains. It is confirmed that the results obta ined are consistent with the experiments for the quasi-one-dimensional Hald ane compounds R2BaNiO5.