Klein, D. & Yang, W. J Stat Phys (1993) 70: 1391. doi:10.1007/BF01049441
We consider a spin system with nearest-neighbor antiferromagnetic pair interactions in a two-dimensional lattice. We prove that the free energy of this system is differentiable with respect to the uniform external fieldh, for all temperatures and allh. This implies the absence of a first-order phase transition in this system.
Phase transitionantiferromagnetGibbs statefree energypressureIsing model