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

By using the magneto-optic Kerr effect (MOKE) we have investigated the coer civity 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 c ontinuously, whereas for thicker layers it falls at a rate that is inversel y proportional to the film thickness. While this latter behaviour is typica l for bulk systems, we show that the initial one is an effect of reduced di mensionality, reflecting the variation of the Curie temperature of the magn etic films in the same thickness range. This phenomenon could have importan t implications for atomic-scale engineering of magnetic materials.