坂井 徹

The magnetization process of the S = 1/2 two-dimensional magnet where ferromagnetic dimers connected by the antiferromagnetic bond on the Shastry-Sutherland lattice is investigated using the numerical diagonalization of a finite-size cluster. We introduce the easy-axis anisotropy at the intradimer ferromagnetic interaction and consider the external magnetic field along the easy axis. It is found that the two-magnon bound state appears at higher magnetization regions for sufficiently large anisotropy. Base on the previous studies of several one-dimensional systems and the present calculation of the spin correlation function, the spin nematic order perpendicular to the external field is expected to occur in this two-magnon bound phase.

The magnetization process of the S = 1 spin ladder with two anisotropies competing each other is investigated using the numerical diagonaization of finite-size clusters and the level spectroscopy analysis. As a result, it is found that the translational symmetry broken magnetization plateaux appear at 1/4 and 3/4 of the saturation magnetization. The phase diagrams with respect to the anisotropies at the 1/4, 1/2 and 3/4 magnetizations are obtained. The magnetization curves for several typical parameters are also presented.

The magnetization process of the S = 1/2 Kondo necklace spin chain with the anisotropic ferromagnetic interaction is investigated using the numerical diagonalization of finite-size systems. As a result, it is found that the two-magnon bound state appears in the field-induced Tomonaga-Luttinger liquid phase for sufficiently large anisotropy. The critical exponent analysis indicates that the spin nematic correlation is dominant at higher magnetization region in the two-magnon bound phase. The phase diagram with respect to the anisotropy and the magnetization is presented.

Abstract The magnetization process of the S = 1/2 distorted diamond spin chain with ferromagnetic interactions is investigated using the numerical diagonalization of finite-size clusters. The level spectroscopy analysis applied for the model with the spin anisotropy indicates that two different magnetization plateau phases appear at 1/3 of the saturation magnetization. The phase diagrams for some typical interaction parameters are presented. In addition the magnetization curves for several typical parameters are obtained.