S=2的单轴各向异性横向Ising模型的基态磁性质  

The Ground-State Magnetic Properties of the Single-Ion Anisotropy Ising Model in Transverse Field

作　　者：赵杰[1] 许星光[1] 魏国柱[2] 

机构地区：[1]辽宁石油化工大学理学院,辽宁抚顺113001 [2]东北大学理学院,辽宁沈阳110006

出　　处：《石油化工高等学校学报》2006年第1期89-92,共4页Journal of Petrochemical Universities

基　　金：教育部博士点基金项目(20040145019)

摘　　要：对处于外界横向磁场中,单自旋S=2的具有单轴各向异性场的Ising铁磁体模型,采用平均场理论(MFA),基于Bogoliubov不等式,以Gibbs自由能为判据,推导出系统基态磁矩及自由能的理论公式,重点考察了单轴各向异性场D和横场Ω对自旋系统基态磁化曲线及自由能的影响,并且从磁化曲线可观察系统一阶、二阶相变的特征。研究发现随着单轴各向异性场和横场的变化,磁化曲线可分成3种:系统的磁矩由2跃变到1,发生一阶有序-有序相变;系统的磁矩由2或1跃变到0,发生一阶有序-无序相变;系统的磁矩由2或1平滑地变化到0,发生二阶有序-无序相变。在m-D/(zJ)基态磁化曲线中随着Ω的增加,系统的磁矩由2到1,1到0的一阶相变现象相继消失;同样在m-Ω/(zJ)基态磁化曲线中随着D的增加,也相继消失。通过对基态自由能曲线的分析还发现当Ω增加或D增加时,自由能总是呈下降的趋势。The spin 2 Ising model with single- ion anisotropies was studied in the transverse magnetic field. Using the mean - field theory, based on Bogoliubov inequality and used the Gibbs free energy as standard, the theoretic expressions of ground - state magnetic moment and free energy of system were deduced. The influences of single- ion anisotropy D and transverse field Ω on the magnetization and free energy curves of spin system at ground state were mainly explored and the characteristic of first order and second order phase transition was observed from magnetization curves. It is found that there are three kinds of magnetization curves with the change of single - ion anisotropy and the tmns - verse magnetic field： first, the magnetic moment of the Ising system suddenly decreases from 2 to 1, which exhibits first order- order phase transition; .second, the magnetic moment suddenly changcs from 2 or 1 to 0, which exhibits first order- disorder phase transition; third, the magnetic moment smoothly changes from 2 or 1 to 0, which exhibits second order- disorder phase transition. The results show that first order- order （2 - 1 ）, first order- disorder （ 1 -0） phase transitions disappear one after another with the increasing of transverse field Ω in the m - D/（zJ） ground state magnetization curves. At the same time first order- order （2 - 1）, first order- disorder （1 - 0） phase transitions also disappear in the same way with the increase of the single - ion anisotropy D in the rn - D/（zJ） ground state magnetization curves. It is also found that the free energy always decreases when Ω or D increases through analysis of free energy curves at ground state.

关 键 词：平均场理论 基态 磁化曲线 Gibbs自由能 一阶相变 

分 类 号：O482.52[理学—固体物理]