Prediction of High Tunnel Magnetoresistance Ratios in (111)-Oriented Junctions with a SrTiO3 Barrier

We theoretically study the tunnel magnetoresistance (TMR) effect in novel (111)-oriented magnetic tunnel junctions (MTJs) with SrTiO
3
barriers X/SrTiO
3
/X(111) (X = Co, Ni). Our ballistic transport calculation combining the density-functional theory and the Landauder formula obtains high TMR ratios of ~ 500% for the Co-based MTJ and ~300% for the Ni-based MTJ. By analyzing band structures in each region of the MTJs, we find that the high TMR ratios originate from the coherent tunneling of the half-metallic Λ
1
state in Co (Ni), where the Λ
1
state is the rotationally symmetric electronic state along the Λ line corresponding to the [111] direction. Since the in-plane lattice periodicity of SrTiO
3
is twice as long as that of fcc Co (Ni), the original band structures of Co (Ni) are folded in the k
x
-k
y
plane, leading to the half-metallicity in the Λ
1
state. The band-folding-driven high TMR ratios found in this study is a novel mechanism of high TMR ratios in (111)-oriented MTJs, in sharp contrast to the interfacial-resonance mechanism reported in our previous works.