  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.
