Magnetic-field-induced domain-wall motion in permalloy nanowires with modified Gilbert damping

Abstract

Domain wall (DW) depinning and motion in the viscous regime induced by magnetic fields, are investigated in planar permalloy nanowires in which the Gilbert damping $\ensuremath{\alpha}$ 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 $\ensuremath{\alpha}<0.033$, the DW mobilities are smaller than expected while for $\ensuremath{\alpha}\ensuremath{\ge}0.033$, 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 $\ensuremath{\alpha}$ 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.