Critical Role of W Insertion Layer Sputtering Condition for Reference Layer on Magnetic and Transport Properties of Perpendicular-Anisotropy Magnetic Tunnel Junction

We investigated an effect of sputtering gas species Ar, Kr, and Xe for deposition of a W insertion layer W(Ar, Kr, and Xe) in the synthetic ferrimagnetic reference layer consisting of [Co/Pt] multilayer/Ru/[Co/Pt] multilayer/W/CoFeB on magnetic properties of the reference layer and tunnel magnetoresistance ratio (TMR ratio) of magnetic tunnel junction (MTJ) stacks with the reference layer. In all the cases, the TMR ratio increased with the increase of the W insertion layer thickness t_W and showed a maximum at certain t_W. Although the maximum TMR ratio is almost the same for all the cases, t_W range in which we observed the maximum TMR ratio is dependent on the gas species; t_W = 0.2 nm for Ar, t_W = 0.2-0.4 nm for Kr, and t_W = 0.2-0.5 nm for Xe, indicating that t_W margin giving high-TMR ratio becomes wider with increasing atomic number of the gas species. The lower TMR ratio for W (Ar) at larger t_W region is due to the degradation of ferromagnetic coupling between the CoFeB and Co/Pt multilayer sandwiching W, resulting in canting of magnetization in the CoFeB layer. We found that an intermixing of Pt atoms in the Co/Pt multilayer occurred in the MTJ stack with W (Ar), whereas it did not intermix with W (Kr), indicating that the Pt interdiffusion caused the degradation of ferromagnetic coupling through W layer.