The Nel temperature (T_{N}) of an antiferromagnet is defined as that temperature below which the sublattices (a)

The Néel temperature \(T_{N}\) of an antiferromagnet is defined as that temperature below which the sublattices \(a\) and \(b\) possess nonzero spontaneous magnetizations \(M_{a}\) and \(M_{b}\), respectively. Determine \(T_{N}\) for the model described in the preceding problem.