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

TL;DR

This study investigates how growth conditions like temperature, rate, and strain influence the structure and magnetic properties of epitaxial CoPt films grown by pulsed laser deposition, revealing phase, microstructure, and magnetization dependencies.

Contribution

It provides new insights into the relationship between growth parameters, microstructure, and magnetic behavior of CoPt films, highlighting the effects of strain and growth rate on phase stabilization and microstructural evolution.

Findings

L1_0 phase stabilized at higher T_d (~700-800°C) with c-axis orientation.

Microstructure varies from maze-like to nano-dots depending on growth rate and T_d.

Magnetic coercivity increases with L1_0 phase fraction and out-of-plane orientation.

Abstract

The effects of growth rate (G_r), deposition temperature (T_d), film thickness (t_F), 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 (L1_0) phases of CoPt. The films deposited at T_d~600 ^0C on all three substrates are fcc with in-plane magnetization and a narrow hysteresis loop of width~200 Oe. The L1_0 phase, stabilized only at T_d~700 ^0C becomes predominantly c-axis oriented as T_d is increased to 800 ^0C. 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 (~1A/sec) the L1_0 films have a maze-like structure which converts to a continuous film as the t_F is increased from 20 to 50 nm. The H_c of these films increases as the L1_0 phase fraction grows with T_d and its orientation becomes out of the film plane. The evolution of microstructure with T_d 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 T_d 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.