Interfacial giant tunnel magnetoresistance and bulk-induced large perpendicular magnetic anisotropy in (111)-oriented junctions with fcc ferromagnetic alloys: A first-principles study

We study the tunnel magnetoresistance (TMR) effect and magnetocrystalline anisotropy in a series of magnetic tunnel junctions (MTJs) with L11-ordered fcc ferromagnetic alloys and MgO barrier along the [111] direction. Considering the (111)-oriented MTJs with different L11alloys, we calculate their TMR ratios and magnetocrystalline anisotropies on the basis of the first-principles calculations. The analysis shows that the MTJs with Co-based alloys (CoNi, CoPt, and CoPd) have high TMR ratios over 2000%. These MTJs have energetically favored Co-O interfaces where interfacial antibonding between Co d and O p states is formed around the Fermi level. We find that the resonant tunneling of the antibonding states, called the interface resonant tunneling, is the origin of the obtained high TMR ratios. Such a mechanism is similar to that found in our recent work on the simple Co/MgO/Co(111) MTJ [K. Masuda et al., Phys. Rev. B 101, 144404 (2020)]. In contrast, different systems have different spin channels where the interface resonant tunneling occurs; for example, the tunneling mainly occurs in the majority-spin channel in the CoNi-based MTJ while it occurs in the minority-spin channel in the CoPt-based MTJ. This means that even though the mechanism is similar, different spin channels contribute dominantly to the high TMR ratio in different systems. Such a difference is attributed to the different exchange splittings in the particular Co d states contributing to the tunneling though the antibonding with O p states. Our calculation of the magnetocrystalline anisotropy shows that many L11 alloys have large perpendicular magnetic anisotropy (PMA). In particular, CoPt has the largest value of anisotropy energy Ku≈10 MJ/m3. We further conduct a perturbation analysis of the PMA with respect to the spin-orbit interaction and reveal that the large PMA in CoPt and CoNi mainly originates from spin-conserving perturbation processes around the Fermi level.