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

Abstract

The effects of growth rate (G_r), deposition temperature (T_d), film thickness (t_F), and substrate-induced strain (ε ) on morphological, crystallographic, and magnetic characteristics of equiatomic CoPt epitaxial films synthesized with pulsed laser deposition are investigated. The (001)-oriented single-crystal substrates of MgO, SrTiO₃, and LaAlO₃ provide different degrees 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 T_d ≈ 600° C 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 T_d = 700°C, becomes predominantly c-axis oriented as T_d is increased to 800°C. While the crystallographic structure of the films depends solely on the T_d, their microstructure and magnetization characteristics are decided by the growth rate. At the higher G_r( ≈ 1 Å /s) the L1₀ films have a maze-like structure which converts to a continuous film as the t_F is increased from 20 to 50 nm. The magnetic coercivity of these films increases as the L1₀ phase fraction grows with T_d and its orientation becomes out of the film plane. The evolution of microstructure with Td is remarkably different at a lower growth rate ( ≈ 0.4 Å /s). Here, the structure changes from a self-similar fractal pattern to a disordered assembly of nanodots as the T_d is raised from 700 to 800°C, and is understood in terms of the imbalance between strain and interfacial energies. Magnetic force microscopy of such films reveals no distinct domain walls within the nanoislands, while a clear contrast is seen between the islands of reversed magnetization. Magnetic relaxation measurements on these assemblies of single-domain islands show a negligible decay of magnetization unless a reverse field close to the coercive field (H_c ≈30 kOe) is applied. The simple picture of coherent rotation of moment appears incompatible with the time dependence of the remanent magnetization in these films.