Thickness-dependent enhancement of damping in Co2FeAl/β-Ta thin films

In the present work Co2FeAl (CFA) thin films were deposited by ion beam sputtering on Si (100) substrates at the optimized deposition temperature of 300 degrees C. A series of CFA films with different thicknesses (t(CFA)), 8, 10, 12, 14, 16, 18, and 20 nm, were prepared and all samples were capped with a 5-nm-thick beta-Ta layer. The thickness-dependent static and dynamic properties of the films were studied by SQUID magnetometry, in-plane as well as out-of-plane broadband vector network analyzer-ferromagnetic resonance (FMR) measurements, and angle-dependent cavity FMR measurements. The saturation magnetization and the coercive field were found to be weakly thickness dependent and lie in the range 900-950 kA/mand 0.53-0.87 kA/m, respectively. The effective damping parameter (alpha(eff)) extracted from in-plane and out-of-plane FMR results reveals a 1 tCFA dependence, the values for the in-plane aeff being larger due to two-magnon scattering (TMS). The origin of the aeff thickness dependence is spin pumping into the nonmagnetic beta-Ta layer and in the case of the in-plane aeff, also a thickness-dependent TMS contribution. From the out-of-plane FMR results, it was possible to disentangle the different contributions to aeff and to the extract values for the intrinsic Gilbert damping (alpha(G)) and the effective spin-mixing conductance (g(eff)) of the CFA/beta-Ta interface, yielding alpha(G) = (1.1 +/- 0.2) x 10(-3) and g(eff) = (2.90 +/- 0.10) x 1019 m(-2).