Spin-tunneling in ferromagnetic junctions

Based on the spin conservation in electron tunneling across an insulator (I) and the spin polarization of conduction electrons in ferromagnets (FM) established, by Meservey and Tedrow, Julliere put forward a quantitative model (1975) showing that tunneling in FM-I-FM junctions should lead to a large junction 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 phenomenon has tremendous potential for applications as nonvolatile magnetic memory elements, read heads, and picotesla field sensors. We review the experimental results and the current theoretical understanding of FM-I-FM tunneling and its dependence on bias, temperature, and barrier characteristics. The influence of inelastic tunneling processes and material properties on the JMR is extensively discussed. Early theories are reviewed and their relationship to the linear response theory is presented. Future directions, both from the point of fundamental physics as well as applications, are also covered.