High L10-Ordering and Flat Surface Introducing a Two-Step Heating Process with Frank–Van der Merwe Initial Growth Mode at Low Temperature

Abstract

FePd films were epitaxially grown on SrTiO₃ (001) substrates by rf magnetron sputtering. The preferred crystal orientation of FePd was (111)_fcc when the substrate temperature (T_s) was room temperature during film deposition. Above T_s = 150 °C, the main preferred orientation of FePd changed from (111) to (001). Above T_s = 400 °C, the degree of L1₀ ordering (S_L10) of FePd increased monotonically and reached 0.88 at T_s = 600 °C. Below T_s = 350 °C, a flat surface was formed, which might be considered Frank–Van der Merwe’s (FM) mode growth. Above T_s = 400 °C, the surface roughness (R_a) increased and reached R_a of 3.9 nm at T_s = 600 °C. We call this as a ‘one-step heating process’. The high S_L10 and flat surface were not simultaneously obtained by the one-step heating process. We introduced the ’two-step heating method’ to obtain high S_L10 and flat surface simultaneously: the films were grown at low T_s, followed by postannealing at T_a = 600 °C. This two-step heating method realized a high S_L10 of 0.98 (considering the Pd excess composition) and a flat surface (R_a = 0.3 nm) by T_s = 150 °C followed by T_a = 600 °C. By the two-step heating, the S_L10 increased from 0.88 to 0.98, and the flatness improved from 3.9 to 0.3 nm. The flat surface is thought to be due to FM growth mode because FePd film was deposited at low T_s such as 150 °C, and once a flat surface was formed at low T_s, the flat surface was maintained even after T_a = 600 °C. For the reason for the high degree of L1₀ ordering at a flat surface, we consider that the flat surface reducing the atoms on the slope caused by coarse grains result in promoting L1₀ ordering. K_u was 1.04 MJ/m³ at 300 K (1.20 MJ/m³ at 10 K), and the high perpendicular magnetic anisotropy (PMA) resulted in a small temperature dependence of K_u and M_s. It is expected that the PMA can be increased to a theoretical value (2 MJ/m³) by removing the twin-like dislocations observed in the cross-sectional transmission electron microscope images.