Correlations between morphology, crystal structure and magnetization of epitaxial cobalt-platinum films grown with pulsed laser ablation

Abstract

The effects of growth rate (Gr), deposition temperature (Td), film thickness (tF), and substrate induced strain (epsilon) on morphological, crystallographic and magnetic characteristics of equiatomic CoPt epitaxial films synthesized with PLD are investigated. The (001) substrates of MgO, STO and LAO provide different degree of epitaxial strain for growth of the disordered face centered cubic (fcc) and ordered face centered tetragonal (L10) phases of CoPt. The films deposited at Td~600 0C on all three substrates are fcc with in-plane magnetization and a narrow hysteresis loop of width~200 Oe. The L10 phase, stabilized only at Td~700 0C becomes predominantly c-axis oriented as Td is increased to 800 0C. While the crystallographic structure of the films depends solely on the Td, their microstructure and magnetization characteristics are decided by the growth rate. At the higher Gr (~1A/sec) the L10 films have a maze-like structure which converts to a continuous film as the tF is increased from 20 to 50 nm. The Hc of these films increases as the L10 phase fraction grows with Td and its orientation becomes out of the film plane. The evolution of microstructure with Td is remarkably different at lower growth rate (~0.4A /sec). Here the structure changes from a self-similar fractal pattern to an assembly of nano-dots as the Td is raised from 700 to 800 0C, and is understood in terms of the imbalance between strain and interfacial energies. MFM of such films reveals no distinct domain walls within the nano-islands while a clear contrast is seen between the islands of reversed magnetization. The simple picture of coherent rotation of moment appears incompatible with the time dependence of the remanent magnetization in these films.