Metastable helium atom (He*) scattering studies of the antiferromagnetic cobalt oxide(001) surface.

摘要

This thesis presents results obtained from elastic scattering of coherent, metastable helium (He*) atomic beams from the antiferromagnetic (AFM) CoO(001) surface. This surface exhibits an anomalous behavior in the temperature dependence of its AFM surface spin-ordering. This ordering was manifest in the appearance of half-order magnetic diffraction peaks which present a clear evidence of an AFM surface spin structure with 2 x 1 periodicity. Contrary to conventional behavior, the diffraction peak intensity exhibits a maximum over the temperature range 250–300 K, after which it decreases to zero at 320 K. This range straddles the bulk Néel temperature $TbN$ = 290 K at which point the intensity enhancement, is interrupted and a very narrow minimum appears.; The technique of metastable 23S He* scattering has the advantage of being highly sensitive to the surface AFM ordering. It is based on the fact that the post scattering survival probability of a He* atom depends on the relative orientation of the local surface electron spin and the electron spin polarization of the He* atoms. A periodic modulation of the surface electron spin orientation, such as, for example, on an AFM surface, will then result in a diffraction pattern of the scattered He* atoms that manifests this periodicity. Previously developed theoretical formalisms have established a mathematical relation between the intensity of the magnetic diffraction peak and the AFM sublattice magnetization. Based on this, I interpret the intensity enhancement as reflecting an increase in the sublattice magnetization and consequently, an increase in the surface AFM spin-ordering.; Electronic structure calculations; performed for small Co-O clusters, have revealed the presence of spin-excited states on the surface, separated from the ground state by a small energy gap of 29 meV. Incorporating these states into a phenomenological mean-field model, as well as computer simulations performed for a slab geometry of interacting spins, clearly reproduces the anomalous enhancement. Moreover, the suppression in the diffraction intensity at $TbN$ is attributed to significant magnetoelastic interactions, characteristic of CoO, and associated with a lattice instability known as the Jahn-Teller effect. This interpretation is supported by the results of both mean-field theory and computer simulations when magneto elastic interactions are included.