磁性薄膜热动力学性质的变分累积展开研究

Thermodynamic properties of ferromagnetic films in variational cumulant expansion

采用变分累积展开方法 ,研究了立方格点上磁性薄膜的热动力学性质 .计算自发磁化强度、内能和热容到了三级累积展开 ,并对每一级给出了这些物理量对薄膜原子层数的依赖关系 .虽然变分累积展开的收敛性还没有严格证明 ,但计算结果显示这些物理量的变分累积展开收敛很快 .三级计算结果足已表明 :对低于某一临界厚度的薄膜 ,自发磁化强度随原子层数的减少而减小 ;不论在临界温度以下 ,还是在临界温度以上 ,每单位格点的内能都随原子层数的减少而增大 ;每单位格点的热容在临界温度以下随原子层数的减少而增大 。

The thermodynamical properties of quantum Heisenberg films are studied in variational cumulant expansion. The spontaneous magnetization, internal energy and specific heat are calculated to the third order. For a thicknesses below a critical value, the magnetization decreases rapidly as the number of atomic layers is decreased. The internal energy increases with decreasing number of atomic layers at any given temperature, and the specific heat increases below the critical temperature and decreases beyond the critical temperature with decreasing number of atomic layers.