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Gilbert damping is of crucial importance for spintronic devices due to its practical effect on the response time and energy consumption. Lower damping would enable more energy-efficient excitations and, thus, less current is needed. The Gilbert damping constant of the Co2FeAl film, a half-metal material important for spintronics with its 100% spin polarization at the Fermi level, has shown an abnormal increase at the thin film limit due to the inter-diffusion of Al atoms. Here, we report that the Gilbert damping of ultra-thin Co2 FeAl 1 + x films of nanometer thicknesses can be effectively tuned by delicately controlling the stoichiometric ratio during the growth. Gilbert damping has been found to be the lowest of 0.065 in Co2 FeAl 1 + 0.1 , which is deduced by ∼ 50 % compared to that in Co2FeAl. We have further found that the damping constant of the ultra-thin Co2 FeAl 1 + 0.1 film is restored to the value of 0.062 of the nominal stoichiometric Co2FeAl by compensating the Al composition deficiency, as supported by 10.37% of composition change from scanning transmission electron microscopy with energy-dispersive-spectroscopy. This work offers a unique path to manipulate the Gilbert damping constant in ultra-thin Co2FeAl films by Al concentration control. [ABSTRACT FROM AUTHOR]