Uniaxial anisotropy, intrinsic and extrinsic damping in Co2FeSi Heusler alloy thin films

Ferromagnetic resonance (FMR) technique has been used to study the magnetization relaxation processes and magnetic anisotropy in two different series of the Co2FeSi (CFS) Hensler alloy thin films, deposited on the Si (1 1 1) substrate by ultra high vacuum magnetron sputtering. While the CFS films of fixed (50nm) thickness, deposited at different substrate temperatures (T-S) ranging from room temperature (RT) to 600 degrees C, constitute the series-I, the CFS films with thickness t varying from 12nm to 100nm and deposited at 550 degrees C make up the series-II. In series-I, the CFS films deposited at Ts = RT and 200 degrees C are completely amorphous, the one at T-S = 300 degrees C is partially crystalline, and those at T-S = 450 degrees C, 550 degrees C and 600 degrees C are completely crystalline with B2 order. By contrast, all the CFS films in series-II are in the fully-developed B2 crystalline state. Irrespective of the strength of disorder and film thickness, angular variation of the resonance field in the film plane unambiguously establishes the presence of global 'in-plane' uniaxial anisotropy. The uniaxial anisotropy field decreases as the crystalline order in the films increases and goes through a minimum at t = 50nm as a function of film thickness. Landau-Lifshitz-Gilbert damping and two-magnon scattering dominantly contribute to the line-broadening in both 'in-plane' and 'out-of-plane' configurations. The two-magnon scattering has larger magnitude in the amorphous films than in the crystalline ones. Angular variation of the linewidth in the film plane reveals that, in the CFS thin films of varying thickness, a crossover from the 'in-plane' local four-fold symmetry (cubic anisotropy) to local two-fold symmetry (uniaxial anisotropy) occurs as t exceeds 50nm. Gilbert damping constant alpha decreases monotonously from 0.047 to 0.0078 with decreasing disorder strength (increasing T-S) and jumps from 0.008 for the CFS film with t = 50nm to 0.024 for the film with t = 75 nm. Such variations of alpha with T-S and t are understood in terms of the changes in the total (spin-up and spin-down) density of states at the Fermi level caused by the disorder and film thickness. Spin pumping across the Co2FeSi film/Ta cap-layer interface makes negligible contribution to alpha. We propose that disorder and/or the film thickness can be used as control parameters to tune alpha, whose value is decisive in choosing a ferromagnetic film for a given spintronics application.