  We investigate the zero-temperature and the finite-temperature properties of
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 thermodynamic
quantities are computed by means of the Schwinger-boson mean-field theory. We
discuss how the magnetic double structure with the gapful and the gapless
excitations is generated, and apply the results systematically to the Haldane
gap system in a staggered magnetic field and also to the weakly coupled
ferrimagnetic mixed-spin chains. It is confirmed that the results obtained are
consistent with the experiments for the quasi-one-dimensional Haldane compounds
$R_2 \rm BaNiO_5$.
