Critical behavior of anisotropic Heisenberg mixed-spin chains in a field

We numerically investigate the critical behavior of the spin-(1, 1/2) Heisenberg ferrimagnet with anisotropic exchange coupling in a magnetic field. A quantized magnetization plateau as a function of the field, appearing at a third of the saturated magnetization, is stable over:the whole antiferromagnetic coupling region. The plateau vanishes in the ferromagnetic coupling region via the Kostellitz-Tnouless transition. Comparing the quantum and classical magnetization curves, we elucidate what an essential quantum effects. [S0163-1829(99)00130-7].