Coexistent quantum and classical aspects of magnetization plateaux in alternating-spin chains

Magnetization process of ferrimagnetic Heisenberg chains of alternating spins are theoretically studied. The size scaling analysis with the exact diagonalization of finite systems for ($S$,$s$)=(3/2,1) and (2,1) indicates a multi-plateau structure in the ground-state magnetization curve for $S$ and $s$ $>1/2$. The first plateau at the spontaneous magnetization can be explained by a classical origin, that is the Ising gap. In contrast, the second or higher one must be originated to the quantization of the magnetization. It is also found that all the $2s$ plateaux, including the classical and quantum ones, appear even in the isotropic case with no bond alternation.