Paul Haney and Mark Stiles from the NIST Center for Nanoscale Science and Technology (CNST) developed a new theory of current-induced torques that generalizes the relationship between spin transfer torques, total angular momentum current, and mechanical torques. This new theory is also applicable to more materials than previous theories.
The basic idea is that there are two types of current-induced torques: a mechanical torque acting on the lattice, and a spin transfer torque (STT) acting on the magnetization. STT is a known phenomenon that is the basic of several technologies such as STT-MRAM and nanoscale microwave oscillators.
The effect has been well understood in materials where total spin is conserved — the magnetization is equal to the total spin, and if there is a net spin current flux into or out of a volume, then the total spin, and therefore the magnetization, inside the volume must change. The researchers' theory extends this understanding to materials where total spin is not conserved, including materials with strong spin-orbit coupling, such as the magnetic semiconductor GaMnAs.