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

TL;DR

This study explores the origin of perpendicular magnetic anisotropy in Co/Pt bilayers on alumina, revealing that chemical ordering and alloying during annealing significantly influence magnetic properties.

Contribution

It demonstrates that perpendicular anisotropy arises not only from interface effects but also from chemical ordering and alloying in annealed Co/Pt bilayers.

Findings

Perpendicular anisotropy appears above 350°C.

Different coercive field increase rates below and above 550°C.

L1₁ and L1₀ chemical orderings are observed at different annealing temperatures.

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.