EFFECT OF TRANSITION-METAL OVERLAYERS ON THE PERPENDICULAR MAGNETISM OF MBE-GROWN, ULTRA-THIN CO FILMS

We have used in situ polar Kerr ellipticity measurements to study the perpendicular magnetic behavior of MBE-grown Pd/Co/TM sandwich structu res, where TM is the nonmagnetic transition metal overlayer Pd, Cu, or Ag. These structures are epitaxially deposited on thick Pd (111) buff er layers grown on Co-seeded GaAs (110) substrates. Hysteresis curves were measured in situ for systematically varied Co and TM layer thickn esses 2 angstrom less-than-or-equal-to t(Co) less-than-or-equal-to 10 angstrom and 0 angstrom less-than-or-equal-to t(TM) less-than-or-equal -to 200 angstrom. We observed perpendicular loops with a coercive fiel d of H(c) less-than-or-equal-to 200 Oe for the uncovered Co films for t(Co) less-than-or-equal-to 6 angstrom, becoming in-plane above this t hickness. However, subsequent deposition of just one atomic layer (alm ost-equal-to 2 angstrom) of any of the TM over the Co resulted in stro ngly perpendicular, square hysteresis curves with H(c) greater-than-or -equal-to 700 Oe for all films in the Co thickness range studied. Depo sition of TM overlayers causes nonmonotonic behavior in H(c) as a func tion of coverage. We find a peak in H(c) at a TM coverage of t(TM) alm ost-equal-to 1.5 angstrom for all materials, with a subsequent monoton ic increase above t(TM) almost-equal-to 4 angstrom to a limiting value by t(TM) almost-equal-to 100 angstrom. The peak is most pronounced fo r Cu, where H(c) changes by more than 200%.