Spin Ordering on the Niobium OXIDE(100) Surface Measured by Elastic Metastable Helium Scattering.

Magnetic insulating solids show a rich variety of electron spin ordering configurations due to the existence of several competing magnetic coupling mechanisms. Calculations indicate that spin ordering on the 2-dimensional surface of a solid may differ from the 3-dimensional bulk spin arrangement. However, the lack of suitable experimental probes has greatly limited the study of spin ordering on solid surfaces. In this thesis, I present a novel experimental technique that is capable of measuring surface long-range spin ordering using elastic scattering of metastable He 2^3S atoms (ESMA). I also present results from the first application of this technique, the determination of the spin ordering on a NiO(100) surface. ESMA is based on the spin dependent de-excitation probability of the He 2^3S atoms (He*) at a magnetic surface. The electronic metastable 2^3S state of He is very long lived in vacuum, but on impact with a surface a large fraction of the metastable atoms de-excite to the electronic ground state. However, on an insulating surface with localized spins, the local nature of the atom-surface interaction together with Auger selection rules will prevent the de -excitation of a He* atom with a spin orientation parallel to the local surface electron spin. Thus, a periodic arrangement of the local surface moments leads to a periodic modulation of the elastically scattered He* beam which is manifested in the corresponding diffraction pattern. Application of this technique to the NiO(100) surface reveals a spin structure that is quite different from the bulk anti-ferromagnetic arrangement. The data suggests the presence of a frozen spin wave arrangement on the surface, where the surface spin ordering has acquired a longer super-periodicity in a high symmetry direction, due to a spatial precession of the spins around the spin directions defined by the bulk ordering. In order to better understand these results, I have developed a model of the scattering process that includes the effect of a spatially modulated attenuation of the metastable He* beam due to the spin dependent de-excitation probability of the He* atoms. The results show good qualitative agreement with the NiO(100) data, and demonstrate that a small spatial modulation of the de-excitation probability over the surface will give rise to corresponding diffraction peaks.