Perpendicular magnetization of Ta/Ru/Ta/Co/Fe/MgO multilayer

Magnetic tunnel junctions (MTJs) using perpendicular magnetic anisotropy (PMA) in CoFe(B)/ MgO interface have attracted much attention for high density magnetoresistive random access memory (MRAM) with advantages of scale-down in driving current using for spin transfer torque (STT). In perspective of reliability, further development of the magnetic anisotropy energy (K_u) is required to improve the thermal stability. In this regard, it was theoretically reported that Fe/MgO has a high interfacial anisotropy energy (IAE) for achieving a high K_u [1]; however, in-plane magnetic anisotropy (IMA) is experimentally dominated. Especially with Ta seed layer, PMA has not been achieved because magnetocrystalline anisotropy energy of bulk anisotropy (K_b) overwhelms IAE [2]. In this work, we introduce Co film between Ta and Fe/MgO stacks to reduce the effective value of K_b for Co/Fe bilayer. Perpendicular magnetization is successfully obtained in Ta/Co/Fe/ MgO structure with ultrathin films of Co and Fe. With multilayered structure, Ta (3 nm)/Ru (3 nm)/ Ta (1 nm)/Co(x: 0.36-0.6 nm)/Fe(y: 0.39-0.91 nm)/MgO (2 nm)/Ta (6 nm), PMA is formed with a proper combination of x and y, and the largest value of K_u is estimated as 5.28 MJ/m³ with Co (0.48 nm)/Fe (0.65 nm) which is larger than 2.4 MJ/m³ of CoFe alloy [2].