  We consider a two-dimensional geometrically frustrated integer-spin
Heisenberg system that admits an exact ground state. The system corresponds to
a decorated square lattice with two coupling constants J1 and J2, and it can be
understood as a generalized Shastry-Sutherland model. Main elements of the spin
model are suitably coupled antiferromagnetic spin trimers with integer spin
quantum numbers s and their ground state Phi will be the product state of the
local singlet ground states of the trimers. We provide exact numerical data for
finite lattices as well as analytical considerations to estimate the range of
the existence in dependence on the ratio of the two couplings constants J2 and
J1 and on the spin quantum number s. Moreover, we find that the magnetization
curves as a function of the applied magnetic field shows plateaus and jumps. In
the classical limit s \to \infty the model exhibits phases of three- and
two-dimensional ground states separated by a one-dimensional (collinear)
plateau state at 1/3 of the saturation magnetization.
