Influence of ferromagnetic layer thickness on the Gilbert damping and magnetocrystalline anisotropy in PLD grown epitaxial Co2FeSi Heusler alloy thin films

Abstract

In the present work, full Heusler alloy (FHA) Co2FeSi (CFS) epitaxial thin films with thickness (tCFS) ranging from 5–25 nm were grown by Pulse laser deposition (PLD) on MgO (001) substrates at an optimized deposition temperature of 600°C. Dynamic magnetization response of PLD-grown CFS thin films was systematically investigated as a function of tCFS. Thickness-dependent structural and magnetic correlation exhibit a non-monotonic variation in Gilbert damping parameter, α. The ferromagnetic resonance (FMR) investigation revealed that the presence of lattice strain and a lower degree of atomic ordering in the thinner FHA CFS films, results in additional enhancement of α. The lowest α value of 4×10−3 is obtained from the theoretical fit of experimental data in a 20 nm thick film. This low value of α is suitable in magnonic devices, such as spin-transfer torque magnetic random-access memories (STT-MRAMs) where an efficient spin injecting source from a FHA ferromagnet to a non-magnetic interface is required. Further, in-plane angle-dependent (φ-variation) FMR measurements of the sample yield an intrinsic four-fold magnetocrystalline anisotropy (MCA) along with the presence of uniaxial anisotropy that has non-monotonicity with tCFS. A significant value of cubic anisotropy constant of 5.16×105erg/cm3 has been obtained in the film with a thickness of around 15nm. Further investigation suggests that uniaxial anisotropy in the CFS/MgO film is a combination of strain-induced magneto-elastic and interface-related contribution that is intrinsic in nature. The experimental observations have been verified by using micromagnetic simulations run on the epitaxial crystal structure. FMR mode position and linewidth of varied thickness CFS films are generated by micromagnetic simulations utilizing the Landau–Lifshitz–Gilbert formalism.