Spin-dependent tunnelling in magnetic tunnel junctions

Abstract

The phenomenon of electron tunnelling has been known since the advent ofquantum mechanics, but continues to enrich our understanding of many fields ofphysics, as well as creating sub-fields on its own. Spin-dependent tunnelling (SDT)in magnetic tunnel junctions (MTJs) has recently aroused enormous interestand has developed in a vigorous field of research. The large tunnellingmagnetoresistance (TMR) observed in MTJs garnered much attentiondue to possible applications in non-volatile random-access memories andnext-generation magnetic field sensors. This led to a number of fundamentalquestions regarding the phenomenon of SDT. In this review article wepresent an overview of this field of research. We discuss various factors thatcontrol the spin polarization and magnetoresistance in MTJs. Startingfrom early experiments on SDT and their interpretation, we considerthereafter recent experiments and models which highlight the role of theelectronic structure of the ferromagnets, the insulating layer, and theferromagnet/insulator interfaces. We also discuss the role of disorder in the barrierand in the ferromagnetic electrodes and their influence on TMR.