SPIN ORDERING ON A NIO(100) SURFACE MEASURED BY METASTABLE HE-2 S-3 SCATTERING

Diffractive scattering of metastable 2 3S He (He) has been used for t he 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 surviva l probability can be enhanced when an appropriately located electron e nergy gap and localized spins are present on the surface. A periodic a rrangement of the orientation of the local moments on the surface then leads to a periodic modulation of the beam attenuation that is reflec ted in the corresponding diffraction pattern. We have measured the ele ctron spin ordering on a NiO(100) surface and found a structure quite different from the bulk antiferromagnetic arrangement. We have develop ed a model of the scattering process that is based on a modified versi on of the eikonal approximation and uses a dissipative hard corrugated wall to include the effects of a spatially modulated attenuation of t he He beam. Calculations performed with this model are in good qualita tive agreement with our experimental data.