One-Third Magnetization Plateau in an Anisotropic (S,S′)=(1,2) Spin-Alternating Chain

抄録

Employing numerical methods, we study the ground-state magnetization curve of a (S,S′)=(1,2) spin-alternating chain with uniaxial single-ion anisotropies, where S=1 and S′=2 spins are alternating and coupled antiferromagnetically. Special attention is paid to the magnetization plateau, the one-third plateau, which appears at one third of the saturation magnetization in the ground-state magnetization curve. We find that three kinds of one-third plateaux appear, one of which has a quantum nature, while the remaining two have a classical (Ising) nature. We determine the one-third plateau phase diagram on the D₂ versus D₁ plane, where D₁ (D₂) is the ratio of the single-ion anisotropy constant for the S=1 (S′=2) spin to the exchange interaction constant. This phase diagram consists of a phase boundary line between the quantum and classical one-third plateau states as well as three phase boundary lines between the one-third plateau and plateauless states. We emphasize that there is no phase transition between two classical one-third plateau states and both states are regarded as a single phase. Furthermore, the region of the one-third plateauless phase is divided into two parts, that is, the region in which the one-third magnetization state is not realized at any magnetic field as the ground state, and the region in which it is realized but no one-third plateau appears in the ground-state magnetization curve.