STRUCTURAL REARRANGEMENT AND SURFACE MAGNETISM ON OXIDE SURFACES - A TEMPERATURE-DEPENDENT LOW-ENERGY-ELECTRON DIFFRACTION-ELECTRON ENERGY-LOSS SPECTROSCOPY STUDY OF CR2O3(111)/CR(110)

Structural rearrangements of the (111) surface in the system Cr2O3/Cr(110) as a function of temperature were investigated by means of low-energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS). At room temperature, one observes a simple (1 x 1) LEED pattern of the clean (111) surface. If the temperature was lowered to 150 K a (root 3 x root 3)R30 degrees superstructure was observed. The structure reached its maximum intensity at about 150 K substrate temperature. Below 150 K the superstructure vanished again and the simple (1 x 1) LEED pattern of the (111) surface was recovered at 90-100 K. Parallel to this, a considerable change in the electron energy loss spectra with varying temperature was observed. With the help of quantum-chemical cluster calculations the low-energy excitations in the range between 0.8 and 2.5 eV were assigned to local d-d excitations of Cr3+ ions at the Cr2O3(111) surface. Some of these peaks were quenched upon adsorption of gases such as CO, NO or CO2. We propose a model of two successive phase transitions the first of which is a disorder-to-order transition above 150 K whereas the second is an order-to-order transition below 150 K. The transitions may be driven by antiferromagnetic coupling of the surface chromium ions to those in the second layer.