Onsager Reaction Field Theory of the Heisenberg Model

We discuss a simple Onsager reaction field treatment of the Heisenberg model. For d = 2 in particular, this predicts correctly the absence of long-range order for any T > 0, produces an exponentially divergent correlation length as T → 0, and gives good quantitative agreement over a wide temperature range with both Monte Carlo and (where appropriate) high-temperature series expansions.