Title: Bias voltage effects on tunneling magnetoresistance in Fe/MgAl${}_2$O${}_4$/Fe(001) junctions: Comparative study with Fe/MgO/Fe(001) junctions,
Abstract: We investigate bias voltage effects on the spin-dependent transport
properties of Fe/MgAl${}_2$O${}_4$/Fe(001) magnetic tunneling junctions (MTJs)
by comparing them with those of Fe/MgO/Fe(001) MTJs. By means of the
nonequilibrium Green's function method and the density functional theory, we
calculate bias voltage dependences of magnetoresistance (MR) ratios in both the
MTJs. We find that in both the MTJs, the MR ratio decreases as the bias voltage
increases and finally vanishes at a critical bias voltage $V_{\rm c}$. We also
find that the critical bias voltage $V_{\rm c}$ of the MgAl${}_2$O${}_4$-based
MTJ is clearly larger than that of the MgO-based MTJ. Since the in-plane
lattice constant of the Fe/MgAl${}_2$O${}_4$/Fe(001) supercell is twice that of
the Fe/MgO/Fe(001) one, the Fe electrodes in the MgAl${}_2$O${}_4$-based MTJs
have an identical band structure to that obtained by folding the Fe band
structure of the MgO-based MTJs in the Brillouin zone of the in-plane wave
vector. We show that such a difference in the Fe band structure is the origin
of the difference in the critical bias voltage $V_{\rm c}$ between the
MgAl${}_2$O${}_4$- and MgO-based MTJs.