  We consider the properties of finite isotropic antiferromagnetic Heisenberg
chains with S=1/2, 1, 3/2 spins when a weak magnetic field is applied on a few
sites, using White's density matrix renormalization group (DMRG) method. For
the S=1 chain there exists only one length scale in the system which determines
the behavior of the one- and two-point correlation functions both around the
local perturbation and near the free boundary. For the critical,
half-odd-integer spin cases the exponent of the spin-spin correlation function
was found to be $\eta=1$, and the exponent of the decay of the site
magnetization around the perturbed site is $x_m =\eta /2 $. Close to a free
boundary, however, the behavior is completely different for S=1/2 and $S >
1/2$.
