Oscillatory interlayer exchange coupling in Co/Ru multilayers and bilayers

The interlayer exchange coupling in several Co/Ru multilayers and bilayers prepared by sputtering has been investigated from magnetization and ferromagnetic resonance experiments. The hysteresis loops have been discussed in detail, particularly regarding their dependence on the number of layers and the effect of an easy axis direction perpendicular to the film plane. Ferromagnetic as well as antiferromagnetic coupling strengths have been determined in an extended range of Ru thicknesses up to 44 Å. The analysis yields an oscillatory magnetic coupling as a function of the Ru thickness having a period of approximately 12 Å. The peak-to-peak-distance has been found to be smaller at thinner Ru layers indicating a preasymptotic or multiperiodic behavior in the coupling. The envelope function of the coupling was found to decrease, initially as ${\mathit{t}}_{\mathrm{Ru}}^{\mathrm{-}2}$ but considerably faster above a Ru thickness of 20 Å. The latter is discussed in terms of mean-free-path effects and of Fermi surface smearing effects both of which are active as a result of structural defects and of the finite measuring temperatures.