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.
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