We demonstrate that the finite temperature effective potential methods used in φ^4 field theory to study the symmetry behavior can be successfully applied to magnon fields. The Heisenberg Hamiltonian is converted into field theoretical form by the use of the Holstein-Primakoff transformations and continuum limit methods. Diagrammatic procedures are utilized to calculate the effective potential to the two-loop approximation. The critical temperature is determined by locating the point where the symmetry is restored. In constrast with φ^4 theory, the gauge symmetry is broken above the critical point and is restored below the critical point. Both high and low temperature approximations are utilized.