Cobalt oxide is an important material for magneto-electronic devices. With the latter’s ever decreasing size the structural modifications due to nanoscaling become important, in particular as there is an intimate correlation between structure and magnetism.

In this light the structure of ultrathin cobalt-oxide films has been investigated whereby low-energy electron diffraction (LEED) together with scanning tunnelling microscopy (STM) was applied. Films of different thickness were grown on an Ir(100) surface. For oxygen-rich preparation conditions spinel-type Co₃O₄ is observed which transforms to rocksalt-type CoO by loss of oxygen during thermal annealing. Surprisingly in view of the quadratic substrate the films are in the polar (111) orientation. Also as a surprise comes the structure of CoO films whose rocksalt-type layer stacking switches at the surface, so that the latter forms a slab with wurtzite structure. Additionally, the surface is reconstructed and so exhibits some intrinsic surface roughness on the nanoscale. With further decreasing oxygen content other film structures develop which were investigated, too.
In the talk the power of quantitative LEED in combination with STM is demonstrated. The crystallography resolved for the oxide films is discussed, in particular with respect to their possible use in devices and in the light of how they avoid the polarity problem.

Im Auftrag der Hochschullehrer des Fachbereichs Physik

Prof. Dr. H. Zacharias