Tantalum clustering in the A1 to L1o iron-platinum-tantalum chemical ordering phase transformation

A series of Fe52Pt48, Fe52.3Pt 46.3Ta1.4 and Fe52Pt40.7Ta7.3 thin films were sputter-deposited and subsequently annealed at 550°C and 750°C for 30 min. The as-deposited films adopted the A1 phase. The addition of Ta resulted in the change in the {111} to {200} fiber texture for these films. This has been explained in terms of the competition between surface energy and strain energy. The Fe52Pt48 and Fe 52.3Pt46.3Ta1.4 films ordered into L1 o at 550°C but the Fe52Pt40.7Ta7.3 did not. Upon annealing at 750°C all three films phase transformed to the L1o phase. As the Ta concentration increased, the grain size was refined and hindered grain growth. Atom probe tomography revealed nano-scale clustering upon annealing for films that contained Ta. For the lower 1.4 at. % Ta concentration film, these clusters were predominately within the grains. For the 7.3 at. % Ta film, the clusters formed predominately in the grain boundaries. The formation of clusters appears to be a necessary initial step to allow the short range A1 to L1o ordering processes in FePt to occur.