Spin-wave stiffness of the Heisenberg antiferromagnet at zero temperature

The spin-wave stiffness ${\mathrm{\rho }}_{\mathit{s}}$ of the Heisenberg antiferromagnet at zero temperature is calculated in two ways, by spin-wave theory and by series expansion about the Ising limit, extending previous results. The two methods give values of ${\mathrm{\rho }}_{\mathit{s}}$ for the isotropic model which are in good agreement, and which satisfy the hydrodynamic relation.