Quantum magnetization plateaux in ferrimagnetic spin chains

Magnetization processes of ferrimagnetic Heisenberg chains of alternating spins S and s at T=0 are studied by the exact diagonalization and size scaling analyses. The results suggest that the spin-(S,s) system has 2s plateaux in the magnetization curve. It is also found that the second and higher plateaux are purely based on a quantum nature, in contrast to the first one that is not distinguished from classical one due to the Ising gap