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

Abstract

An isolated ferromagnetic bond of coupling constants ${\mathit{K}}_{\mathit{z}}$ and ${\mathit{K}}_{\mathit{x}\mathit{y}}$ replacing an antiferromagnetic link in the spin-1/2 anisotropic Heisenberg antiferromagnet of coupling constants ${\mathit{J}}_{\mathit{z}}$\ensuremath{\ge}${\mathit{J}}_{\mathit{x}\mathit{y}}$ on a square lattice is investigated within the linearized spin-wave approximation. Two competing interactions affect the local ordered magnetic moment 〈${\mathit{S}}_{\mathit{i}}^{\mathit{z}}$〉 and the correlation 〈${\mathit{S}}_{\mathit{i}}^{\mathit{x}}$${\mathit{S}}_{\mathit{j}}^{\mathit{x}}$〉: The longitudinal terms ${\mathit{J}}_{\mathit{z}}$ tend to enhance the sublattice magnetization, while the transverse terms ${\mathit{J}}_{\mathit{x}\mathit{y}}$ represent the quantum fluctuations that suppress the long-range order. We analyze the interplay between these two effects as a function of ${\mathit{K}}_{\mathit{z}}$ and ${\mathit{K}}_{\mathit{x}\mathit{y}}$ at the impurity link. The linearized spin-wave approximation breaks down for sufficiently large ${\mathit{K}}_{\mathit{z}}$ and ${\mathit{K}}_{\mathit{x}\mathit{y}}$ as a consequence of the frustration of the two plaquettes adjacent to the ferromagnetic bond.