基于横场 Ising 模型，利用费米型格林函数的通常退耦合近似( MFA) 和高阶退耦合近似，研究了自旋为 1 /2 的正方晶格纳米岛的相变性质.极化强度计算表明: 在初始自旋取向随机时，可能产生多解及伪解，从而展现出非常规结果，即可重入现象.而利用高阶退耦合近似不但可以解释更为丰富的相变现象，同时绝大部分情况下高阶退耦合近似方程组仅有唯一解，可以很好地避免伪解的产生.另外 JS / J 是造成极化曲线出现阶梯形状的主要原因，但高阶退耦合近似加大了各参量之间的联系.

Based on the transverse field Ising model，the phase transition properties of

square lattice nanois- lands with a spin of 1 /2 are investigated using the usual decoupling

approximation and higher order decoupling ap- proximation of the Fermi green function. The

polarization diagram shows that when the initial spin orientation is ran- dom，multiple and pseudo

solutions may be produced，thus show unconventional results: reentry phenomenon. The

use of high-order decoupling approximation can not only explain the richer phase transition

phenomenon，but also in most cases，the higher-order decoupling approximate equations have only a unique solution，which

can avoid the generation of pseudo solutions. JS / J is the main reason for the stepped shape of

the polarization curve，but the high order decoupling approximately increases the relationship between the parameters.