Abstract. Two-dimensional spin-1/2 Heisenberg antiferromagnets with nonmagnetic or ferromagnetic impurities are studied using an improved Green's function theory and quantum Monte Carlo simulations over the whole temperature region. The antiferromagnetic spin correlation functions at the bonds closest to the impurities are found to be enhanced in both the nonmagnetic and ferromagnetic impurity cases at all temperatures. For the nonmagnetic impurity case, our numerical results for spin correlation functions and ground-state energies agree with those from previous Monte Carlo simulations. For the ferromagnetic impurity case, we found that the spin-wave excitations are strongly influenced by the impurities. Discrete local modes emerge above the continuous spin-wave excitations. The effects of the distance between two ferromagnetic impurities on the local modes are also investigated.

Print publication: Issue 24 (19 June 2000)
Received 20 December 1999, in final form 4 April 2000