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First‐order phase transition between one‐third magnetization plateau states in an anisotropic (S, S′) = (1, 2) spin‐alternating chain

Takashi Tonegawa, Kiyomi Okamoto, Tôru Sakai

Published Feb 15, 2010• 1 citation
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Abstract

Abstract In recent studies we find that three kinds of magnetization plateaux appear at one‐third of the saturation magnetization in the ground‐state magnetization curve of an anisotropic (S, S′) = (1, 2) spin‐alternating chain, in which S = 1 and S′ = 2 spins with on‐site anisotropies are alternatingly arranged and coupled antiferromagnetically. One of these plateaux is a quantum plateau, while the other two are classical plateaux. We here show that the first‐order phase transition between the two classical one‐third plateau states occurs when the on‐site anisotropy for the S = 1 spin is of easy‐axis type and its magnitude is sufficiently large. We numerically determine the phase transition line for this first‐order transition and briefly discuss the origin of its occurrence.

DOI
10.1002/pssb.200983007

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Abstract In recent studies we find that three kinds of magnetization plateaux appear at one‐third of the saturation magnetization in the ground‐state magnetization curve of an anisotropic (S, S′) = (1, 2) spin‐alternating chain, in which S = 1 and S′ = 2 spins with on‐site anisotropies are alternatingly arranged and coupled antiferromagnetically. One of these plateaux is a quantum plateau, while the other two are classical plateaux. We here show that the first‐order phase transition between the two classical one‐third plateau states occurs when the on‐site anisotropy for the S = 1 spin is of easy‐axis type and its magnitude is sufficiently large. We numerically determine the phase transition line for this first‐order transition and briefly discuss the origin of its occurrence.
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Using mainly numerical methods, we investigate the ground-state magnetization curve of a mixed spin 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 1/3-plateau, which appears at one third of the saturation magnetization in the magnetization curve. It is found that there appear three kinds of 1/3-plateaux, one of which has a quantum nature, while the remaining two have a classical (Ising) nature.
The magnetization curve of the (S,s)=(1,1/2)(S,s)=(1,1/2)(S,s)=(1,1/2) ferrimagnetic alternating spin chain with single-ion anisotropy D is investigated with the numerical exact diagonalization of finite clusters and size-scaling analyses. The system has a plateau at (1/3) of the saturation moment, which corresponds to the spontaneous magnetization for D=0.D=0.D=0. Varying D in the 1/3-magnetized ground state under the external field along the axis of D, a quantum phase transition is revealed to occur at the critical value $D/J=1.114\ifmmode\pm\else\textpm\fi{}0.001$ where the plateau vanishes. Except for the critical point, the plateau is always opening, but the mechanism is different between D<DcD<{D}_{c}D<Dc​ and D>Dc.D>{D}_{c}.D>Dc​. The change of mechanisms is an evidence to clarify that the plateau originates from the quantization of magnetization.
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We study the breakdown of the magnetization plateau at the magnetization M = M S /3 ( M S is the saturation magnetization) of the S = 1/2 anisotropic spin chain with ferromagnetic-ferromagnetic-antiferromagnetic interactions. We consider the model with the isotropic ferromagnetic (trimer) coupling J F , and anisotropic antiferromagnetic coupling ( J x = J y = J AF and J z = J AF ). For the limit of large J F / J AF , the model is equivalent to the S = 3/2 XXZ chain with the exchange anisotropy . There is a phase transition between the plateau (small- ) and the no-plateau (large- ) regions. This phase transition is of the Berezinskii-Kosterlitz-Thouless type, and we determine the phase boundary from the numerical diagonalization data. For = 1, in particular, the phase transition between the plateau and the no-plateau regions occurs at the point c = 15.4.
The magnetization process of the S=3/2S=3/2S=3/2 quantum spin chain with the XXZXXZXXZ anisotropy and the single-ion anisotropy DDD is investigated using the numerical diagonalization of finite-size clusters and the level spectroscopy analysis. We obtain the phase diagrams at 1/3 and 2/3 of the saturation magnetization to find that the translational-symmetry-broken magnetization plateau appears for the first time. The similarity and the difference between the phase diagrams of the present model and the related models are discussed by use of the discrete parameters of the models. In addition several typical magnetization curves are presented.