1/z Expansion for the Heisenberg Antiferromagnet at Low Temperatures

It is shown that the expansion in density of spin deviations for the Heisenberg antiferromagnet at low temperatures leads to a 1/z expansion for the free energy. In contrast to the 1/S expansion, the kinematic properties of spin operators are realized within each order of the expansion. The spin-wave energy at long wavelength is also calculated.