Adjustable spin torque in magnetic tunnel junctions with two fixed   layers
  We have fabricated nanoscale magnetic tunnel junctions (MTJs) with an additional fixed magnetic layer added above the magnetic free layer of a standard MTJ structure. This acts as a second source of spin-polarized electrons that, depending on the relative alignment of the two fixed layers, either augments or diminishes the net spin-torque exerted on the free layer. The compound structure allows a quantitative comparison of spin-torque from tunneling electrons and from electrons passing through metallic spacer layers, as well as analysis of Joule selfheating effects. This has significance for current-switched magnetic random access memory (MRAM), where spin torque is exploited, and for magnetic sensing, where spin torque is detrimental. 
G. D. Fuchs, I. N. Krivorotov, P. M. Braganca, N. C. Emley, A. G. F.   Garcia, D. C. Ralph, and R. A. Buhrman