We theoretically study the tunnel magnetoresistance (TMR) effect in (111)-oriented magnetic tunnel junctions (MTJs) with SrTiO3 barriers, Co/SrTiO3/Co(111) and Ni/SrTiO3/Ni(111). Our analysis combining the first-principles calculation and the Landauer formula shows that the Co-based MTJ has a high TMR ratio over 500%, while the Ni-based MTJ has a smaller value (290%). Since the in-plane lattice periodicity of SrTiO3 is about twice that of the primitive cell of fcc Co (Ni), the original bands of Co (Ni) are folded in the kx-ky plane corresponding to the ab plane of the MTJ supercell. We find that this band folding gives a half-metallic band structure in the Λ1 state of Co (Ni) and the coherent tunneling of such a half-metallic Λ1 state yields a high TMR ratio. We also reveal that the difference in the TMR ratio between the Co- and Ni-based MTJs can be understood by different s-orbital weights in the Λ1 band at the Fermi level.
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