  We study the temperature dependence of the low temperature spin
configurations, investigating the magnetization profile of the local states due
to the impurities and the two point correlation function centered in one of the
impurities. This correlation is found to be weak against temperature effects
although the magnetization profile in the triplet state is visible up to higher
temperatures. Here we introduce a loop cluster quantum Monte-Carlo method with
a fixed magnetization Mz in order to study the correlations in the ground state
of a given value of Mz. From the population distribution of magnetization, the
very small energy gap between the quasi degenerate states due to the impurities
is obtained.
