  The $O(3)$ nonlinear $\sigma$ model is studied in the disordered phase, using
the techniques of the effective action and finite temperature field theory. The
nonlinear constraint is implemented through a Lagrange multiplier. The finite
temperature effective potential for this multiplier is calculated at one loop.
The existence of a nontrivial minimum for this potential is the signal of a
disordered phase in which the lowest excited state is a massive triplet. The
mass gap is easily calculated as a function of temperature in dimensions 1, 2
and 3. In dimension 1, this gap is known as the Haldane gap, and its
temperature dependence is compared with experimental results.
