Comparison of high temperature growth versus post-deposition in situ annealing in attaining very low Gilbert damping in sputtered Co2FeAl Heusler alloy films

Abstract

In this work, a detail study on the effectiveness of high substrate temperature (TS) during deposition versus in situ annealing of the films at high temperature (TA) after sputtering at room-temperature is compared with an objective to obtain Heusler alloy thin films of Co2FeAl (CFA) having optimally low αint (with an upper bound of 1.75 × 10-3) and 12–17% smaller MS by employing ion beam sputtering technique and Si(100) as substrates for current induced magnetization switching application. In each of the two series of CFA films, prepared with different choice of TS and TA (lying in 300–773 K range) were explored for optimally tailoring their structural, static and dynamic magnetization properties. Structural study revealed that although all the CFA films were polycrystalline but the post-deposition annealed films were structurally optimal with higher density (6.36 ± 0.09 g/cm3) and lower interface roughness (0.48 ± 0.03 nm) compared to the CFA films grown at high TS (6.23 ± 0.06 g/cm3, 0.61 ± 0.01 nm). In plane field-angle dependent longitudinal magneto optical Kerr effect (L-MOKE) study revealed the existence of a coupling between the weak uniaxial and biaxial magnetic anisotropies present in these films, attributed to the employed deposition geometry. Ferromagnetic resonance (FMR) spectroscopy measurements, performed in both the geometries - in-plane as well as out of plane, revealed a lowest value of 1.19 (±0.08) × 10-3 of αint in the film post-annealed at 773 K. In addition, the observed non-linear relation between the αint and the dynamical ‘g’-factor suggests that the contribution of spin orbit interaction to the damping is far less compared to the damping contribution from the DOS present near the Fermi level. It is remarkable to note that the as-grown CFA films sputtered at room temperature exhibited a record lowest value of 1.73 (±0.09) × 10-3 of αint. Attainment of such small value of Gilbert damping and having moderate magnetization in high spin polarized Co2FeAl Heusler films sputtered on the Si (100) substrate opens up their great application potential in future spintronics devices.