Diffractive scattering of metastable 2 3S He (He*) has been used for the first time to determine the long-range spin ordering on the surface of an antiferromagnetic insulator. Although the long-lived He* has a high probability of deexcitation on impact with a surface, its survival probability can be enhanced when an appropriately located electron energy gap and localized spins are present on the surface. A periodic arrangement of the orientation of the local moments on the surface then leads to a periodic modulation of the beam attenuation that is reflected in the corresponding diffraction pattern. We have measured the electron spin ordering on a NiO(100) surface and found a structure quite different from the bulk antiferromagnetic arrangement. We have developed a model of the scattering process that is based on a modified version of the eikonal approximation and uses a dissipative hard corrugated wall to include the effects of a spatially modulated attenuation of the He beam. Calculations performed with this model are in good qualitative agreement with our experimental data.