Magnetic anisotropy of Co/Cu(111) ultrathin films

The magnetic properties of ultrathin Co/Cu(lll) films (2-10 ML) were investigated by ferromagnetic resonance in situ in ultrahigh vacuum. The surface anisotropy K-2(S)= -0.16 meV per atom and volume anisotropy K-2(V) =98 mu eV per atom were determined at constant, reduced temperature T/T-c(d) = 0.2 for 2 < d < 6 ML. For thicker films reversed signs of the anisotropy contributions are measured with a nearly vanishing bulk-like anisotropy which is related to a structural transformation occurring near 6 ML. Previous experiments on Cu-capped Co films which had been performed at constant temperature (300 K) yielded different signs of K-2(S) and did not identify the change near 6 ML. It is shown that only by analysing the magnetic anisotropy at constant, reduced temperature - that is taking into account the thickness dependence of the Curie temperature T-c(d) - is the correct anisotropy behavior determined. These results are quantitatively correlated with theoretical calculations for the fee and hcp structure. (C) 1999 Elsevier Science B.V. All rights reserved.