  We discuss the critical behavior of the spin-(1,1/2) Heisenberg ferrimagnetic
chain in a magnetic field, whose magnetization curve exhibits a plateau at a
third of the full magnetization. A bond alternation stabilizes the massive
state, whereas an exchange anisotropy causes the breakdown of the plateau and
the onset of a gapless spin-fluid state, where the transition, lying in the XY
but ferromagnetic region, is of Kosterlitz-Thouless type. In order to elucidate
significant quantum effects, we investigate the model of classical version as
well.
