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 theories 1-7 and confirmed in a number of experiments on metallic and semiconductor ferromagnets 8-19. Understanding the magnitude and orientation of such spin-torques is an important problem for spin-dependent transport and currentdriven magnetization dynamics, as domain-wall motion underlies a number of emerging spintronic technologies 20,21. 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 theories 2,5-7. In addition, we find β to be close to the Gilbert damping constant α, which, in light of similar results on planar anisotropy systems 15 , suggests a universal origin for the non-adiabatic torque. The adiabatic torque, which accounts for transport processes in which the conduction spin follows the local spatial magnetization variation by remaining in either the majority or minority state, is well understood and has been reproduced by a number of different transport theories. In contrast, the non-adiabatic contribution, characterized by a dimensionless parameter β (ref. 22), remains the subject of much debate. Various mechanisms have been put forward to explain its origin, such as momentum transfer 2,7 , spinmistracking 4,6 or spin-flip scattering 3. It is predicted that large nonadiabatic effects should appear in narrow domain walls because of large magnetization gradients 2,5,6 , whereby the wall width becomes comparable to important transport scales such as the spin-diffusion length 2 or the Larmor precession length 6 , which are of the order of a few nanometres in ferromagnetic transition metals. The presence of a non-adiabatic term is of fundamental importance, because its existence implies that current-driven wall motion is possible for any finite current in a perfect system, even in the absence of an applied magnetic field. Difficulty in characterizing β experimentally therefore stems in part from being able to distinguish between extrinsic sources of wall pinning, due to structural defects, for example, from the intrinsic finite threshold current predicted 2 for β = 0. We have studied current-driven domain wall dynamics in two different pseudo spin-valve systems based either on CoNi
