  The effects of spin-1 impurities introduced in dimerized antiferromagnetic
spin chains are studied using Lanczos diagonalization and quantum Monte Carlo
simulations. Firstly, the magnitude of the interaction between a Ni-ion
replacing a Cu-ion is derived for the case study system Cu-O-Cu. Then, it is
shown that the introduction of S=1 impurities leads to the suppression of the
original spin gap of the dimerized system and to the enhancement of the
antiferromagnetic correlations close to the impurity. These features are
interpreted in terms of a valence bond picture of the ground state of both the
pure and the doped system. The similitudes and differences between the
underlying mechanisms that produce these effects in the cases of nonmagnetic
and S=1 impurities are analyzed. It is stressed that a singlet formed between
the spin-1 impurity site and the two spin-1/2 neighbors spins behaves as an
effective nonmagnetic vacancy which may be partially responsible for the
enhancement of the antiferromagnetic correlations on the whole system.
Implications of these results for experiment are also discussed.
