  We investigate in this paper the origin of perpendicular anisotropy in Co
(1.6 nm)/Pt (3.0 nm) bilayers grown on alumina and annealed up to
650$^{\circ}$C. Above 350$^{\circ}$C, all layers exhibit perpendicular
anisotropy. Then coercive fields increase linearly with annealing temperature
following two different rates: 0.05 T/100$^{\circ}$C below 550$^{\circ}$C and
0.8 T/100$^{\circ}$C above. By making careful structural characterizations
using x-ray diffraction and transmission electron microscopy, we demonstrate
the presence of short range correlation of L1$_{1}$ type below 550$^{\circ}$C
whereas above 550$^{\circ}$C, L1$_{0}$ chemical ordering is observed. We
conclude that perpendicular anisotropy observed in Co/Pt bilayers grown on
alumina and annealed may not only be due to interface anisotropy as usually
invoked but also to CoPt alloying and chemical ordering that take place during
post-growth annealing.
