Spin-Polarized Tunneling

Jagadeesh S. Moodera
Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology,
Cambridge, MA 02139, USA

Tunneling phenomenon has enriched our understanding of various branches of physics over the years. Spin-polarized tunneling phenomenon, discovered in 1970 by Tedrow and Meservey, showed that spin is conserved in the tunneling process and the electrons coming from a ferromagnet are spin-polarized. Using magnetic semiconductors tunnel barriers such as EuS or EuSe, one can obtain nearly 100% spin polarized tunneling electrons even with nonmagnetic electrodes. The recent success of spin dependent tunneling between two ferromagnetic films has led to excitement in this area. A ferromagnetic tunnel junction (FM/I/FM) such as Co/Al2O3/Ni80Fe20 has shown over 20% change in junction resistance (JMR) with the application of a small magnetic field which changes the relative orientation of the magnetization. Such results have led to intense activity in this topic and as a result, unique observations have been made.

The magnitude of JMR is basically related to the spin-polarization of tunnel current as first modeled by Julliere. However, experimentally significant dependence of JMR on temperature and dc bias has been observed, which is yet to be fully understood. Surface magnon excitation is believed to play a major role in the above dependencies. The role of tunnel barrier, with or without dopants in it, on the spin-polarization and tunneling is being explored both theoretically and experimentally. The convergence between these two is yet to happen. Magnetic tunnel junctions, with a nonmagnetic interface layer, have shown unique JMR dependence on the thickness of the interface layer as well as on the bias voltage, for example, with Au at the interface in Co/Al2O3/Ni80Fe20 junctions. This has been attributed to the development of quantum well states. Similar inverse JMR effect seen by others, with Ta2O5 or SrTiO3 tunnel barriers with no nonmagnetic layer has been interpreted as due to a change in the sign of spin-polarization of the FM at the interface.

Recent observation on the barrier dopant effects on JMR is rather intriguing. For example, with submonolayer / monolayer level doping of Al2O3 say with Si, Cu, Pd, Gd, Co or Ni the JMR reduces significantly even though some of them are nonmagnetic ions. In fact we observed that Au showed significantly higher decrease in JMR than even Co or Gd. On the other hand doping with Fe actually enhances the JMR by a factor of up to 1.25 for Fe less than a monolayer inside the Al2O3 barrier. This effect persists even at room temperature or in a dc bias as large as 0.6V. The origin of this enhancement or the spin transport through these tunnel barriers with magnetic or nonmagnetic ions in them is an open question. Such investigation allows one to study the behavior of ultra-thin magnetic layer inside an insulator, in addition to exploring the effects of spin exchange scattering on spin tunneling due to magnetic ions.