We study the magnetic properties of S = 1/2 antiferromagnetic Heisenberg chains with inhomogeneity of interaction. Using a quantum Monte Carlo method and an exact diagonalization method, we study bond-impurity effect in the uniform S = 1/2 chain and also in the bond-alternating chain. Here 'bond impurity' means a bond with strength different from those in the bulk or a defect in the alternating order. Local magnetic structures induced by bond impurities are investigated both in the ground state and at finite temperatures, calculating the local magnetization, the local susceptibility and the local field susceptibility. We also investigate the force acting between bond impurities and find the force generally attractive.

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 M z in order to study the correlations in the ground state of a given value of M z . From the population distribution of magnetization, the very small energy gap between the quasi degenerate states due to the impurities is obtained.