Quantum magnetization plateaux of an anisotropic ferrimagnetic spin chain

The magnetization curve of the $(S,s)=(1,1/2)$ ferrimagnetic alternating spin chain with the single-ion anisotropy $D$ is investigated with the numerical exact diagonalization of finite clusters and size-scaling analyses. The system has a plateau at 1/3 of the saturation moment, which corresponds to the spontaneous magnetization for D=0. Varying $D$ in the 1/3-magnetized ground state under the external field along the axis of $D$, a quantum phase transition is revealed to occur at the critical value $D/J=1.114 \pm 0.001$ where the plateau vanishes. Except for the critical point, the plateau is always opening, but the mechanism is different between $D<D_c$ and $D>D_c$. The change of mechanisms is an evidence to clarify that the plateau originates from the quantization of magnetization.