Influence of finite size and image charge screening on the antiferromagnetic ordering of CoO ultrathin films

Abstract The surface antiferromagnetic (AFM) ordering of ultrathin CoO(0 0 1) films grown on Ag(0 0 1) has been investigated using low energy electron diffraction (LEED) and valence band photoemission spectroscopy. We have observed coherent exchange scattered half-order spots in LEED with a periodicity of the magnetic unit cell of the CoO(0 0 1) surface. These spots show very low intensities (2–3% of the integer-order spots) and are visible only below a critical temperature for low electron beam energies ( T N ) of CoO(0 0 1) for different film coverages has been observed and it is exceeding the bulk T N even in the ultrathin limit. The enhancement of surface T N has been explained by the combined effects of finite size, surface strain, and polarizability of the underlying substrate. The critical behaviour near T N shows a coverage dependent evolution as the magnetic critical exponent ( β ) varies from 0.16 to 0.48 upon increasing film coverage from 0.5 ML to 10 ML, suggesting crossovers in the critical behaviour. Furthermore, upon transition from paramagnetic (PM) to AFM phase, minor modifications of the valence band electronic structure have been observed such as enhancement of spectral weight in the Co 3d band as well as widening of an energy gap relative to E F by ~ 150 meV, in agreement with the theoretical predictions.