Domain wall ͑DW͒ depinning and motion in the viscous regime induced by magnetic fields, are investigated in planar permalloy nanowires in which the Gilbert damping ␣ is tuned in the range 0.008-0.26 by doping with Ho. Real time, spatially resolved magneto-optic Kerr effect measurements yield depinning field distributions and DW mobilities. Depinning occurs at discrete values of the field which are correlated with different metastable DW states and changed by the doping. For ␣ Ͻ 0.033, the DW mobilities are smaller than expected while for ␣ Ն 0.033, there is agreement between the measured DW mobilities and those predicted by the standard one-dimensional model of field-induced DW motion. Micromagnetic simulations indicate that this is because as ␣ increases, the DW spin structure becomes increasingly rigid. Only when the damping is large can the DW be approximated as a pointlike quasiparticle that exhibits the simple translational motion predicted in the viscous regime. When the damping is small, the DW spin structure undergoes periodic distortions that lead to a velocity reduction. We therefore show that Ho doping of permalloy nanowires enables engineering of the DW depinning and mobility, as well as the extent of the viscous regime.

We study the relationship between the damping ͑␣͒ and the nonadiabaticity of the spin transport ͑␤͒ in permalloy nanowires. ␣ is engineered by Ho doping, and from the characteristics of the current-induced domain-wall velocity, determined by high-resolution x-ray magnetic circular-dichroism photoemission electron microscopy, ␤ due to spin relaxation is measured. We find that ␤ scales with ␣ and conclude that the spin relaxation that leads to nonadiabatic spin torque originates from the same underlying mechanism as the angular momentum dissipation that causes viscous damping. DOI: 10.1103/PhysRevB.80.132403 PACS number͑s͒: 75.75.ϩa, 72.25.Ba, 75.60.Ch Magnetic domain-wall ͑DW͒ propagation by spinpolarized current 1 has attracted increasing attention in the last few years due to fundamental interest in the interaction between current and magnetization, 2,3 and the possibility of applications in spintronics. 4 However, despite a number of experimental [5][6][7][8][9][10][11] and theoretical investigations, 12-14 the mechanism of current-induced DW motion in magnetic nanowires is not fully understood. In a phenomenological approach, two spin-torque terms were added to the LandauLifshitz and Gilbert equation of magnetization dynamics to describe the influence of a current:Here the first and second terms describe the precession and damping of a magnetic moment m in a magnetic field H with ␥ as the gyromagnetic ratio and ␣ as the Gilbert damping constant. The third and fourth terms, denoted the adiabatic and nonadiabatic spin torque, respectively, 12 account for the two possible directions of the spin torque acting on m with u an effective DW velocity equal to ͑Pg B / 2eM S ͒j, where P is the spin polarization, g is the Landé factor, B is the Bohr magneton, e is the electron charge, M S is the saturation magnetization, and j is the current density. The adiabatic spin torque arises when the conduction electron spins follow the spatially varying magnetization within the DW as they travel through it. Conservation of angular momentum then dictates that the electron spins exert a torque on the magnetization, leading to DW motion. The nonadiabatic spin torque ͑"␤ term"͒ was studied theoretically in detail by Tatara et al.,14 who split it into two components: ͑i͒ a deviation of the electron spin from perfect adiabaticity as a result of spin relaxation and ͑ii͒ a nonadiabaticity arising from the rapidly varying magnetization direction ͑which can be neglected for the wide domain walls investigated here 15 ͒. Following, 14 we associate the parameter ␤ only with the nonadiabaticity due to spin relaxation. The ␤ term and its relation to the damping ␣ is key to understanding current-induced DW motion. The ratio ␤ / ␣ is predicted to control the nature of the DW motion 12,13 and is the subject of much debate. 3,[16][17][18] The discussion about ␤ / ␣ is connected to the question of whether Landau-Lifshitz or Gilbert damping provides the more natural description of dissipative magnetization dynamics. 3,16 This is because, for the...

Details are presented of a single shot focused magneto-optic Kerr effect (MOKE) magnetometer which is used to capture the movement of single domain walls (DWs) in permalloy (Ni 80 Fe 20 ) nanowires ( 400 nm width and 20 nm thickness) in real time. By probing the DW motion within the 1 µm diameter laser spot of the instrument, DW velocity and pinning field distributions were obtained. An external field was ramped up linearly, and depinning of a DW from the same start position was observed at three different fields, indicating the stochastic nature of the DW motion.

By direct x-ray photoemission electron microscopy imaging, we probe current-induced domain wall motion in 20nm thick CoFeB wires. We observe transverse walls for all wire widths up to 1500nm as a consequence of the small saturation magnetization of the material. High critical current densities above 1×1012A∕m2 for wall displacement due to the spin transfer torque effect are found. The critical current densities jc increase further with decreasing wire width indicating that jc is governed by extrinsic pinning due to edge defects. In addition to wall displacements, we observe wall transformations to energetically favorable wall types due to heating. Owing to the high Curie temperature though, the sample temperature stays below the Curie temperature even for the highest current densities where structural damage sets in.

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