AVS 54th International Symposium
    Magnetic Interfaces and Nanostructures Friday Sessions
       Session MI-FrM

Invited Paper MI-FrM9
Beyond Fe-MgO-Fe: Alternative Barriers and Systems

Friday, October 19, 2007, 10:40 am, Room 619

Session: Spin Injection, Transfer, and Tunneling
Presenter: P. LeClair, University of Alabama
Correspondent: Click to Email

Magnetic tunnel junctions have been an intensely active area of research since the first reliable demonstrations of tunneling magnetoresistance (TMR). However, there are only a few systems to date that experimentally show a large TMR effect at room temperature. One of the most recent and effective are ordered Fe/MgO/Fe(001) trilayers (bcc FeCo-based alloys may also be substituted for pure Fe). This system was initially predicted theoretically by Butler et al. to exhibit large TMR, and later experimentally verified by Yuasa et al and Parkin et al. The nearly four-fold improvement in magnetoresistance over earlier polycrystalline/amorphous structures has been attributed to the complex energy band matching between Fe and MgO. This promotes the tunneling of electrons from specific ("delta-1") bands in Fe(001) which exist only for majority spin electrons. The MgO tunnel barrier thereby acts as a 'spin filter.' At the most basic level, the tunneling rates for specific metallic states are controlled by the symmetry of the insulating barrier, which gives a general mechanism for large TMR. In this talk, I will try to outline the theoretical and experimental criteria for large TMR effects based on this 'spin filtering' effect, and attempt to answer the questions "Why does the Fe-MgO system work so well?" and "Is Fe-MgO a unique system?" Both experimental and theoretical considerations are crucial for realizing large TMR effects in realistic structures, and both viewpoints are necessary to explain the (initially surprising) large TMR effects in, e.g., CoFeB/ MgO/CoFeB. I will review our recent work on predicting and fabricating new TMR systems analogous to Fe-MgO-Fe, with a particular focus on alternative tunnel barriers, including organic systems. Finally, I will discuss spin-polarized tunneling characterization methods, in particular Meservey-Tedrow tunneling. This work is supported by the National Science Foundation.