Green's function theory of the two-dimensional antiferromagnetic Heisenberg model with local magnetic impurities

Two-dimensional spin-in Heisenberg antiferromagnets with nonmagnetic or fer romagnetic 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 i mpurities are found to be enhanced in both the nonmagnetic and ferromagneti c impurity cases at all temperatures. For the nonmagnetic impurity case, ou r numerical results for spin correlation functions and ground-state energie s agree with those from previous Monte Carlo simulations. For the ferromagn etic impurity case, we found that the spin-wave excitations are strongly in fluenced by the impurities. Discrete local modes emerge above the continuou s spin-wave excitations. The effects of the distance between two ferromagne tic impurities on the local modes are also investigated.