Barrier-height and bias-voltage-controlled spin-filter effect and tunneling magnetoresistance in full ferromagnetic junctions

Abstract

Within the framework of the single electron spintronic model, we systematically studied the barrier-height and bias-voltage-controlled spin-filter effect and tunneling magnetoresistance (TMR) in ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal (FM/FI/FM) tunnel junctions. We find that it is the extended quantum-coherence factor of Slonczewski, κL↑2−kL↑kL↓, that physically controls the sign of the zero-bias TMR. This factor is a linear function of the mean barrier height. The zero-bias TMR is positive when the mean barrier is high, and negative when the mean barrier gets low, which agrees well with the experimental results observed in GdOx-barrier junctions. As a cooperative result of the mean barrier and spin-filter effect, a positively or negatively large TMR can be maintained in a rather wide range near the zero bias if the mean barrier of the FI spacer is much higher or much lower. This property is believed to be of practical use in designing spintronic devices. Besides, the TMR can oscillate positively, or negatively, or alternately with the applied voltage within the high bias region, which is different from conventional FM/I/FM tunnel junctions.