Non-adiabatic spin-torques in narrow magnetic domain walls

Abstract

There is considerable debate over the size and direction of the non-adiabatic component of the spin-torque generated when a current flows across a domain wall in a ferromagnet. Measurements of this property in a wall just 1–10 nm wide suggest its value is small, arising from purely magnetic dissipation mechanisms. Torques appear between charge carrier spins and local moments in regions of ferromagnetic media where spatial magnetization gradients occur, such as a domain wall, owing to an exchange interaction. This phenomenon has been predicted by different theories1,2,3,4,5,6,7 and confirmed in a number of experiments on metallic and semiconductor ferromagnets8,9,10,11,12,13,14,15,16,17,18,19. Understanding the magnitude and orientation of such spin-torques is an important problem for spin-dependent transport and current-driven magnetization dynamics, as domain-wall motion underlies a number of emerging spintronic technologies20,21. One outstanding issue concerns the non-adiabatic spin-torque component β, which has an important role in wall dynamics, but no clear consensus has yet emerged over its origin or magnitude. Here, we report an experimental measurement of β in perpendicularly magnetized films with narrow domain walls (1–10 nm). By studying thermally activated wall depinning, we deduce β from the variation of the Arrhenius transition rate with applied currents. Surprisingly, we find β to be small and relatively insensitive to the wall width, which stands in contrast to predictions from transport theories2,5,6,7. In addition, we find β to be close to the Gilbert damping constant α, which, in light of similar results on planar anisotropy systems15, suggests a universal origin for the non-adiabatic torque.