Origin of perpendicular anisotropy in thin Co/Pt bilayers grown on alumina

Abstract

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.

submitted to Phys. Rev. B