Effect of transition-metal overlayers on the perpendicular magnetism of MBE-grown, ultra-thin Co films

Abstract

We have used in situ polar Kerr ellipticity measurements to study the perpendicular magnetic behavior of MBE-grown Pd/Co/TM sandwich structures, where TM is the nonmagnetic transition metal overlayer Pd, Cu, or Ag. These structures are epitaxially deposited on thick Pd (111) buffer layers grown on Co-seeded GaAs (110) substrates. Hysteresis curves were measured in situ for systematically varied Co and TM layer thicknesses 2 Å≤tCo≤10 Å and 0 Å≤tTM≤200 Å. We observed perpendicular loops with a coercive field of Hc≤200 Oe for the uncovered Co films for tCo≤6 Å, becoming in-plane above this thickness. However, subsequent deposition of just one atomic layer (≊2 Å) of any of the TM over the Co resulted in strongly perpendicular, square hysteresis curves with Hc≥700 Oe for all films in the Co thickness range studied. Deposition of TM overlayers causes nonmonotonic behavior in Hc as a function of coverage. We find a peak in Hc at a TM coverage of tTM∼1.5 Å for all materials, with a subsequent monotonic increase above tTM≊4 Å to a limiting value by tTM≊100 Å. The peak is most pronounced for Cu, where Hc changes by more than 200%.