Spin models that have been proposed to describe dimerized chains, ladders, two dimensional antiferromagnets, and other compounds are here studied when some spins are replaced by spinless vacancies, such as it occurs by Zn doping. A small percentage of vacancies rapidly destroys the spin gap, and their presence induces enhanced antiferromagnetic correlations near those vacancies. The study is performed with computational techniques which includes Lanczos, world-line Monte Carlo, and the Density Matrix Renormalization Group methods. Since the phenomenon of enhanced antiferromagnetism is found to occur in several models and cluster geometries, a common simple explanation for its presence may exist. It is argued that the resonating-valence-bond character of the spin correlations at short distances of a large variety of models is responsible for the presence of robust staggered spin correlations near vacancies and lattice edges. The phenomenon takes place regardless of the long distance properties of the ground state, and it is caused by a "pruning" of the available spin singlets in the vicinity of the vacancies. The effect produces a broadening of the low temperature NMR signal for the compounds analyzed here. This broadening should be experimentally observable in the structurally dimerized chain systems Cu(N O3)2 · 2.5H2O, CuW O4, (V O)2P2O7, and Sr14Cu24O41, in ladder materials such as SrCu2O3, in the spin-Peierls systems CuGeO3 and N aV2O5, and in several others since it is a universal effect common to a wide variety of models and compounds.

We study the formation of antiferromagnetic correlations induced by impurity doping in anisotropic twodimensional spin-Peierls systems. Using a mean-field approximation to deal with the interchain magnetic coupling, the intrachain correlations are treated exactly by numerical techniques. The magnetic coupling between impurities is computed for both adiabatic and dynamical lattices and is shown to have an alternating sign as a function of the impurity-impurity distance, hence suppressing magnetic frustration. An effective model based on our numerical results supports the coexistence of antiferromagnetism and dimerization in this system.

The effects of magnetic and nonmagnetic impurities in spin-Peierls systems are investigated allowing for lattice relaxation and quantum fluctuations. We show that, in isolated chains, strong bonds form next to impurities, leading to the appearance of magnetoelastic solitons. Generically, these solitonic excitations do not bind to impurities. However, interchain elastic coupling produces an attractive potential at the impurity site which can lead to the formation of bound states. In addition, we predict that small enough chain segments do not carry magnetic moments at the ends. ͓S0163-1829͑99͒01821-4͔