Classical Heisenberg ferromagnet in two dimensions

Modified spin-wave theory is applied to the classical Heisenberg ferromagnet on a square lattice: $H_c$=-$J_0$Σ. The energy, the correlation function, and the magnetic susceptibility are calculated at low temperatures. These agree excellently with the Monte Carlo results for a 64×64 system. For an infinite system, the reduced susceptibility behaves as $T^2$exp(4π$J_0$/T) and the correlation length behaves as exp(2π$J_0$/T). The preexponential factors differ from those of renormalization-group Monte Carlo calculations.