Influence of current leads on critical current for spin precession in magnetic multilayers

Abstract

In magnetic multilayers, a dc current induces a spin precession above a certain critical current. Drive torques responsible for this can be calculated from the spin accumulation Δμ . Existing calculations of Δμ assume a uniform cross-section of conductors. But most multilayer samples are pillars with current leads flaring out immediately to a much wider cross-section area than that of the pillar itself. We write spin-diffusion equations of a form valid for variable cross-section, and solve the case of flat electrodes with radial current distribution perpendicular to the axis of the pillar. Because of the increased volume available for conduction–electron-spin relaxation in such leads, Δμ is reduced in the pillar by at least a factor of 2 below its value for uniform cross-section, for given current density in the pillar. Also, Δμ and the critical current density for spin precession become nearly independent of the thickness of the pinned magnetic layer, and more dependent on the thickness of the spacer, in better agreement with measurements by Albert et al. (Phys. Rev. Lett. 89 (2002) 226802).