2 A ug 1 99 9 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 = MS/3 (MS 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 JF , and the anisotropic antiferromagnetic coupling (Jx = Jy = JAF and Jz = ∆JAF ). For the large γ ≡ JF /JAF limit, the model is equivalent to the S = 3/2 XXZ chain with the exchange anisotropy ∆. There is a phase transition between the plateau (small γ) and the no-plateau (large γ) regions. This phase transition is of the Berezinskii-Kosterlitz-Thouless type, and we determine the phase boundary from the numerical diagonalization data. Especially for ∆ = 1, the phase transition point between the plateau and the no-plateau regions is γc = 15.4.