Critical adsorption and dimensional crossover in epitaxial FeCo films

The critical behavior of thin FeCo films grown on MgO has been studied usin g phase-sensitive synchrotron x-ray diffractian. These studies unravel seve ral novel features of criticality in thin films, as the simultaneous appear ance of the 3D-2D crossover and the truncation of the correlation length no rmal to the film at approximately 1/3 of the film thickness. Above the crit ical temperature of the film we observe a pronounced pinning of the order p arameter at the MgO-FeCo interface, which indicates a novel critical adsorp tion behavior.