The partition function and the spontaneous magnetization of a two-dimensional (2D) Ising model with the next nearest interactions at zero magnetic field are derived analytically. At first, the transfer matrices are analyzed in three representations, i.e., Clifford algebraic representation, transfer tensor representation, and schematic representation, to inspect nontrivial topological structures in this system. The system is equivalent to a triangular Ising model plus an interaction along the $z$ axis, so that the approaches developed for the 3D Ising models are modified to be appropriable for analytically solving the solution of the 2D Ising model with the next nearest interactions. The comparison with the exact solutions of other Ising lattices reveals that either the increase of the number of interactions in a unit cell or the presence/increase of topological contributions enhances the critical point of the Ising lattices. The results obtained in this work are helpful for understanding the physical properties of the 2D magnetic materials.

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• Classical statistical mechanics
• Phase transitions