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Theorists Find Generalized Connection Between Spin Currents, Magnetic Torques, and Mechanical Torques
For Immediate Release: December 8, 2010
Paul Haney and Mark Stiles of the CNST have developed a theory of current-induced torques that generalizes the relationship between spin transfer torques, total angular momentum current, and mechanical torques, and is applicable to a much wider range of materials than previous theories. Their work, reported in Physical Review Letters,* describes two types of current-induced torques: a mechanical torque acting on the lattice, and a spin transfer torque acting on the magnetization. Spin transfer torque is a well established phenomenon in which the flow of spin current through a magnetic material can change its magnetization direction. This effect underlies a number of potential new technologies, including magnetic random access memory (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. The analysis of these systems requires the consideration of total angular momentum, which is always conserved. (Since magnetization is proportional to angular momentum, it contributes to the total angular momentum.) Conservation of total angular momentum arguments imply, for example, that a magnetic field-driven reversal of the magnetization is accompanied by a compensating change in the mechanical angular momentum of the sample, known as the Einstein-de Haas effect. This change in mechanical angular momentum is small, but measurable; in fact, Einstein himself performed the experiment demonstrating this effect. By accounting for the mechanical angular momentum of the material, Haney and Stiles found a general relationship between total angular momentum currents, magnetic torques, and mechanical torques. The theory is applicable to recent experiments which measure spin current-induced nanomechanical torques in nonmagnetic materials, and predicts that magnetic torques may become stronger in the presence of spin-orbit coupling, making them more effective for current-induced magnetic switching.