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

Several mechanisms for high-temperature superconductivity invoke the strong antiferromagnetic correlations within the CuO planes. In particular, properties of the high-T_c compounds are believed to be related to defects in the planes, e.g., static vacancies, ferromagnetic bonds, and mobile holes. We consider the effects of isolated ferromagnetic links on an otherwise antiferromagnetic square lattice. The addition of holes in La₂CuO₄ (by doping with Sr) introduces a local ferromagnetic exchange coupling between Cu spins. We describe the Heisenberg antiferromagnet within the linearized spin-wave theory; the problem of a single ferromagnetic bond embedded in an otherwise antiferromagnetic square lattice can then be solved exactly. The procedure is similar to the one employed in Ref. 1 for a vacancy. The longitudinal terms, involving S_z, tend to enhance the ordered staggered magnetic moment, competing in this way with the transverse terms, involving S_x and S_y, which represent the quantum fluctuations and suppress the sublattice magnetization. The interplay between these two interactions is discussed as a function of the ferromagnetic coupling strength.