  We study the impurity-induced phase transitions in a quasi-one-dimensional
Heisenberg antiferromagnet doped with magnetic spin-1/2 impurities and
non-magnetic ones. The impurity-induced transition temperature determined by
the quantum Monte Carlo method with the continuous-time loop algorithm is
monotonically increasing as a function of the magnitude of the impurity spin.
To these results, we give discussions based on the valence-bond solid-like
picture for the pure system and the inspection of the local magnetic structure
around the impurities.
