  In the past few years the phenomenon of spin dependent tunneling (SDT) in
magnetic tunnel junctions (MTJs) has aroused enormous interest and has
developed into a vigorous field of research. The large tunneling
magnetoresistance (TMR) observed in MTJs garnered much attention due to
possible application in random access memories and magnetic field sensors. This
led to a number of fundamental questions regarding the phenomenon of SDT. One
such question is the role of interfaces in MTJs and their effect on the spin
polarization of the tunneling current and TMR. In this paper we consider
different models which suggest that the spin polarization is primarily
determined by the electronic and atomic structure of the ferromagnet/insulator
interfaces rather than by their bulk properties. First, we consider a simple
tight-binding model which demonstrates that the existence of interface states
and their contribution to the tunneling current depend on the degree of
hybridization between the orbitals on metal and insulator atoms. The decisive
role of the interfaces is further supported by studies of spin-dependent
tunneling within realistic first-principles models of Co/vacuum/Al,
Co/Al2O3/Co, Fe/MgO/Fe, and Co/SrTiO3/Co MTJs. We find that variations in the
atomic potentials and bonding strength near the interfaces have a profound
effect resulting in the formation of interface resonant states, which
dramatically affect the spin polarization and TMR. The strong sensitivity of
the tunneling spin polarization and TMR to the interface atomic and electronic
structure dramatically expands the possibilities for engineering optimal MTJ
properties for device applications.
