Spin-Transfer Torque in Helical Spin-Density Waves

The current driven magnetisation dynamics of a helical spin-density wave is investigated. Expressions for calculating the spin-transfer torque of real systems from first principles density functional theory are presented. These expressions are used for calculating the spin-transfer torque for the spin spirals of Er and fcc Fe at two different lattice volumes. It is shown that the calculated torque induces a rigid rotation of the order parameter with respect to the spin spiral axis. The torque is found to depend on the wave vector of the spin spiral and the spin-polarisation of the Fermi surface states. The resulting dynamics of the spin spiral is also discussed.