  Understanding the dynamics of interacting quantum spins has been one of the
active areas of condensed matter physics research. Recently, extensive
inelastic neutron scattering measurements have been carried out in an
interesting class of systems, Cr_2(W, Te, Mo)O_6. These systems consist of
bilayers of Cr^{3+} spins (S=3/2) with strong antiferromagnetic inter-bilayer
coupling (J) and tuneable intra-bilayer coupling (j) from ferro (for W and Mo)
to antiferro (for Te). In the limit when J>|j|, the system reduces to weakly
interacting quantum spin-3/2 dimers. In this paper we discuss the
low-temperature magnetic properties of Cr_2TeO_6 systems where both intra-layer
and inter-layer exchange couplings are antiferromagnetic, i.e. J,j>0. Using
linear spin-wave theory we obtain the magnon dispersion, sublattice
magnetization, two-magnon density of states, and longitudinal spin-spin
correlation function.
