Isolated ferromagnetic bonds in the two-dimensional spin-1/2 Heisenberg antiferromagnet

An isolated ferromagnetic bond replacing an antiferromagnetic link in the spin-1/2 Heisenberg antiferromagnet on a square lattice is investigated within the linearized spin-wave theory. As a function of the ferromagnetic coupling K, quantum fluctuations on neighboring sites are first reduced, which increases the sublattice magnetization. Larger couplings suppress the sublattice magnetization and increase the transversal correlations between nearest neighbors close to the impurity link. Our simple approach breaks down for K->-J- (J being the antiferromagnetic coupling). The interaction between two such ferromagnetic links depends on their relative position and its sign oscillates as a function of distance.