First-principles calculation of the nonadiabatic spin transfer torque in Ni and Fe

The magnetization dynamics of a ferromagnet subjected to an electrical current are described by an extension of the Landau-Lifshitz-Gilbert equation that contains two additional terms, the adiabatic and nonadiabatic spin-transfer torques. First-principles calculations of the nonadiabatic spin-transfer torque parameter beta for bcc iron and fcc nickel show that beta is related to and typically of the same order as alpha, the damping constant, but is distinct from it. Calculations as a function of the ratio of spin-dependent scattering rates show that (1) the minimum of the damping constant as a function of scattering rate does not change significantly, and (2) when the polarization of the current approaches zero, beta can become large but the implied domain-wall velocity does not.