Bias dependence and inversion of the tunneling magnetoresistance in ferromagnetic junctions

Abstract

This paper presents a microscopic theory for spin-polarized tunneling in the FM/I/FM junction under a finite applied voltage. A significant decrease of the tunneling magnetoresistance (TMR) with increasing bias is obtained from the theory. We note that the spin-dependent prefactor of the transmission coefficient plays an important role in the tunneling. In particular, the bias-dependent quantum-coherence factor ${\ensuremath{\kappa}}_{R}^{2}{(E}_{x},V)\ensuremath{-}{k}_{R\ensuremath{\uparrow}}{(E}_{x}{,V)k}_{R\ensuremath{\downarrow}}{(E}_{x},V)$ rather than the density of states controls the sign change of TMR at finite voltage. The effects of an asymmetric potential profile within the barrier region is calculated in comparison with the tunneling junctions of the composite barrier $({\mathrm{Al}}_{2}{\mathrm{O}}_{3}/{\mathrm{Ta}}_{2}{\mathrm{O}}_{5}).$ Numerical results are in qualitative agreement with experiments.