First-principles study of tunneling magnetoresistance in Fe/MgAl2O4/Fe(001) magnetic tunnel junctions

We investigated the spin-dependent transport properties of Fe/MgAl₂O₄/Fe(001) magnetic tunneling junctions (MTJs) on the basis of first-principles calculations of the electronic structures and the ballistic conductance. The calculated tunneling magnetoresistance (TMR) ratio of a Fe/MgAl₂O₄/Fe(001) MTJ was about 160%, which was much smaller than that of a Fe/MgO/Fe(001) MTJ (1600%) for the same barrier thickness. However, there was an evanescent state with Δ₁ symmetry in the energy gap around the Fermi level of normal spinel MgAl₂O₄, indicating the possibility of a large TMR in Fe/MgAl₂O₄/Fe(001) MTJs. The small TMR ratio of the Fe/MgAl₂O₄/Fe(001) MTJ was due to new conductive channels in the minority spin states resulting from a band-folding effect in the two-dimensional Brillouin zone of the in-plane wave vector (k_∥) of the Fe electrode. Since the in-plane cell size of MgAl₂O₄ is twice that of the primitive in-plane cell size of bcc Fe, the bands in the boundary edges are folded, and minority-spin states coupled with the Δ₁ evanescent state in the MgAl₂O₄ barrier appear at k_∥=0, which reduces the TMR ratio of the MTJs significantly.