From enhanced to reduced quantum antiferromagnetism by tuning a magnetic impurity

Based on quantum Monte Carlo simulations, we study the local-order-parameter modulations induced by a magnetic impurity that couples to a single moment of a two-dimensional quantum antiferromagnet. A strong antiferromagnetic coupling of the impurity spin reduces the antiferromagnetic order in the host beyond the next-nearest-neighbor sites of the impurity spin, whereas on the next-nearest neighbors the order increases, similar as for an embedded vacancy. We show that for weak and intermediate couplings to the impurity spin, the local antiferromagnetic order is enhanced throughout the host magnet. We compare our numerical findings to analytical results from a recent perturbative approach based on spin-wave theory.