Spin-flip associated sequential tunnelling through a magnetic double tunnel junction system

Abstract

Spin-flip (SF) associated sequential tunnelling through a semiconductor quantum dot (QD) sandwiched by two collinear ferromagnetic leads is theoretically analysed based on the Master equation method. The transport model considers two discrete energy levels in the dot, i.e. the lowest unoccupied and the highest occupied energy levels, with associated intra-level Coulomb correlation energies. The tunnelling current and the tunnelling magnetoresistance (TMR) are evaluated in the presence of SF effects both within the quantum dot (SF-QD) and during sequential tunnelling across a junction (SF-TJ). It was shown that the presence of both SF mechanisms only affects the tunnelling current when the leads are in the antiparallel configuration. The antiparallel current increases monotonically with increasing strength of SF-QD resulting in a suppression of the TMR. The increase occurs for both low and high bias regions corresponding to singly occupied and freely occupied QD states, respectively. For the SF-TJ effect, the increase in the antiparallel current and resulting suppression of TMR is more pronounced for high bias corresponding to the freely occupied state. The TMR suppression occurs for SF probability within the range of 0 ⩽ η ⩽ 0.5, and reaches a maximum at η = 0.5 where TMR vanishes completely. However, at higher SF probability of 0.5 ⩽ η ⩽ 1, the trend in the antiparallel current is reversed leading to an enhancement of the TMR. Overall, the η-dependence of the TMR is symmetric about η = 0.5 and roughly proportional to (1 − 2η)2. In the presence of both SF mechanisms, it was found that SF-QD has a stronger suppressive effect on the TMR compared with SF-TJ.