| Using the numerical diagonalization based on the Lanczos algorithm, we investigate the magnetization process of several spin ladder systems with the next-nearest-neighbor interaction, or the ring exchange interaction. The finite size scaling analysis based on the conformal field theory and the recently developped level spectroscopy method reveal that a magnetization plateau would appear at half the saturation magnetization, if the next-nearest-neighbor or ring exchange interaction is sufficiently large. In addition the mean-field approximation for the interladder interaction suggests that the quasi-1D system possibly exihibit a field-induced incommensurate order in some regions close to the magnetization plateau. Several phase diagrams in the ground state of the 1D system are also presented. |
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