  We use neutron scattering to study the spin and lattice structures of single
crystal and powder samples of Na$_{1-\delta}$FeAs ($T_c = 23$ K). On cooling
from room temperature, the system goes through a series of phase transitions:
first changing the crystal symmetry from tetragonal to orthorhombic at 49 K,
then ordering antiferromagnetically with a spin structure similar to that of
LaFeAsO and a small moment (0.09$\pm$0.04 $\mu_B$), and finally becoming
superconducting below about 23 K. These results confirm that antiferromagnetic
order is ubiquitous for the parent compounds of the iron arsenide
superconductors, and suggest that the separated structural and magnetic phase
transition temperatures are due to the reduction in the c-axis exchange
coupling of the system.

Superconductivity