L1o-CoPt/Co bilayer ferromagnetic films: interdiffusion, structure and microstructure

Detailed understanding of the connections between nanostructure and magnetic properties is key to the realization of optimal exchange-spring magnets. To this end, x-ray and electron diffraction and transmission electron microscopy were used to study the evolution of phases, microstructure and texture of model exchange-spring bilayers of L10 - CoPt/Co upon annealing at temperatures ranging from 300-550 deg. C. The work was motivated by previously--detailed changes in the magnetic exchange coupling of the system induced by the annealing treatments . Unannealed CoPt/Co bilayers comprised a <111> fiber-textured L10 CoPt layer and a <0001> fiber-textured Co layer. The Co layer predominantly consisted of the hcp (A3) form, but contained a minority fcc (A1) phase. Annealing the bilayers at low temperatures (300-450 deg. C) or for short times resulted in a strengthening of the hcp Co <0001> fiber texture and disappearance of the minor fcc phase. By contrast, annealing at higher temperatures (450-550 deg. C) or for longer times resulted in the interdiffusion of the Co and CoPt layers and the formation of new Co-Pt solid solutions. One of these solid solutions was found to be hcp in structure, while the other was fcc. The amount of the fcc solid solution increased at the expense of the L10 CoPt, hcp Co and hcp Co-Pt solid solution phases as annealing time and temperature were increased