Abstract
In this paper, we present a spintronic tunnelling theory for ferromagnet/insulator/ferromagnet(FM/I/FM) junctions. With the use of Airy functions, it can analytically account for boththe low-bias and the high-bias tunnelling magnetoresistances (TMRs). We find that thesign-change behaviour of TMR can only occur in the low-bias region, due to the quantumcoherence in FM/I/FM junctions. In the high-bias region, the TMR will oscillate betweenpositive and negative with increasing bias voltage. Physically, this oscillation arisesfrom the interference between the incident and reflected electron waves in thebarrier region. The effects of the barrier height, the barrier width and the electroneffective mass in the barrier are studied systematically. The theoretical resultsobtained from the exact Airy functions agree well with TMR experiments onTa2O5- andAl2O3-barrier junctions, within the whole measurable range of bias voltage.
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