Spin polarized tunneling in ferromagnetic junctions

Spin polarized tunneling studies by Tedrow and Meservey in the early 1970s that showed the spin conservation in electron tunneling gave rise to the possibility of spin sensitive tunneling between two ferromagnetic (FM) films. Julliere put forward a quantitative model (1975) showing that tunneling in FM/I/FM junctions should lead to a large magnetoresistance (JMR). This conjecture was realized with repeatable results only in 1995, and since then JMR values >30% have been achieved at room temperature. This recent success has led to several fundamental questions regarding the phenomenon of spin tunneling besides showing tremendous potential for applications as nonvolatile magnetic memory elements, read head and picotesla field sensors. We briefly review the experimental results and the current theoretical understanding of FM-T-FM tunneling: its dependence on bias, temperature and barrier characteristics. The influence of inelastic tunneling processes, metal at the interface and material properties on the JMR is discussed. Early theories are reviewed and their relationship to the Linear response theory is presented. The future direction, both from the point of fundamental physics as well as applications, is also covered, (C) 1999 Elsevier Science B.V. All rights reserved.