  The two-dimensional quantum Heisenberg antiferromagnet at zero temperature
with two kinds of locally frustrating defects - isolated ferromagnetic bonds
and impurity spins, is studied. For a quantum spin, coupled
antiferromagnetically to the two sublattices, we find a local ground state
transition. In the case of a ferromagnetic bond, the local magnetization is
found to remain finite even for strong coupling, contrary to previous results,
derived within the linear spin-wave approximation. Our results are based on an
analytic approach, suitable to treat strong defect-environment interaction, as
well as exact diagonalization studies. Technical details and some extension of
this work can be found in cond-mat/9712281.
