Breakdown of the magnetization plateau of the S=1/2 ferromagnetic-ferromagnetic-antiferromagnetic trimerized spin chain with anisotropy

We study the breakdown of the magnetization plateau at the magnetization M=M_{S}/3 (M_{S} is the saturation magnetization) of the S=1/2 anisotropic spin chain with ferromagnetic-ferromagnetic-antiferromagnetic interactions. We consider the model with the isotropic ferromagnetic (trimer) coupling J_{F}, and anisotropic antiferromagnetic coupling (J_{x}=J_{y}=J_{AF} and J_{z}=\Delta J_{AF}). For the limit of large \gamma\equiv J_{F}/J_{AF}, the model is equivalent to the S=3/2 XXZ chain with the exchange anisotropy \Delta. There is a phase transition between the plateau (small-\gamma) and the no-plateau (large-\gamma) regions. This phase transition is of the Berezinskii-Kosterlitz-Thouless type, and we determine the phase boundary from the numerical diagonalization data. For \Delta=1, in particular, the phase transition between the plateau and the no-plateau regions occurs at the point \gamma_{c}=15.4.