Atfinite temperatures, the system is disordered but the spin–
spin correlation order-parameter has a different behavior below
and above a critical temperatureTc. For 0oToTc, the correlation
has an algebraic decay while it falls exponentially forT4Tc.Itis
important to highlight that it is not a transition associated with a
broken symmetry as that considered by Mermin–Wagner theory
and so, it is more similar to the BKT transition. At low temperatures and in the classical limit, we have