We investigated an effect of sputtering gas species (Ar, Kr, and Xe) for deposition of a W insertion layer in the CoFeBW/ CoFeB free layer on magnetic properties of the free layer and tunnel magnetoresistance (TMR) ratio of magnetic tunnel junctions (MTJs) stacks using the free layer annealed at 400 degrees C for 1 h. As the W insertion layer thickness tW increased, we found the degradation of perpendicular anisotropy and larger reduction of saturation magnetic moment per unit area mS in the free layer using Ar compared to those using Kr and Xe. We also found a smaller TMR ratio for the MTJ stack using Ar compared to those using Kr and Xe. Energy-dispersive X-ray spectrometry line analysis revealed more significant interdiffusion between W and CoFeB layers in the free layer using Ar than those using Kr and Xe, that could result in the smaller mS and perpendicular anisotropy in the free layer and smaller TMR ratio for the MTJ stack using Ar than those using Kr and Xe. We also investigated concentration of Ar, Kr, and Xe in W layers deposited using Ar, Kr, and Xe, respectively, by high-resolution Rutherford backscattering spectrometry, revealing that 0.2 at% Ar was detected in theW layer using Ar, while Kr and Xe were not detected inW layers using Kr and Xe. Such a difference in concentration of inert gas atoms in the W layer could be one possible reason for the difference about degree of interdiffusion between W and CoFeB layers.