Thickness-dependent coercivity of ultrathin Co films grown on Cu(111)

By using the magneto-optic Kerr effect (MOKE) we have investigated the coercivity of ultrathin epitaxial Co films grown by molecular beam epitaxy with the aid of Pb as the surfactant on Cu(lll). We find two different regimes: below similar to 6 ML (ML = monolayer), the coercive field H-c increases continuously, whereas for thicker layers it falls at a rate that is inversely proportional to the film thickness. While this latter behaviour is typical for bulk systems, we show that the initial one is an effect of reduced dimensionality, reflecting the variation of the Curie temperature of the magnetic films in the same thickness range. This phenomenon could have important implications for atomic-scale engineering of magnetic materials.