Current-induced domain wall motion in nanoscale ferromagnetic elements

The manipulation of a magnetic domain wall (DW) by a spin polarized current in ferromagnetic nanowires has attracted tremendous interest during the last years due to fundamental questions in the fields of spin dependent transport phenomena and magnetization dynamics but also due to promising applications, such as DW based magnetic memory concepts and logic devices. We comprehensively review recent developments in the field of geometrically confined domain walls and in particular current induced DW dynamics. We focus on the influence of the magnetic and electronic transport properties of the materials on the spin transfer effect in DWs. After considering the different DW structures in ferromagnetic nanowires, the theory of magnetization dynamics induced by a spin polarized current is presented. We first discuss the different current induced torques and their origin in the light of recent theories based on a simple s-d exchange model and beyond. This leads to a modified Landau-Lifshitz-Gilbert equation of motion where the different spin transfer torques are included and we discuss their influence on the DW dynamics on the basis of simple 1D models and recent micromagnetic simulations studies. Experimental results illustrating the effects of spin transfer in different ferromagnetic materials and geometries constitute the body of the review. The case of soft in-plane magnetized nanowires is described first, as it is the most widely studied class of ferromagnetic materials in this field. By direct imaging we show how confined domain walls in nanowires can be displaced using currents in in-plane soft magnetic materials and that using short pulses, fast velocities can be attained. While a spin polarized current can trigger DW depinning or displacement, it can also lead to a modification of the DW structure, which is described in detail as it allows one to deduce information about the underlying spin torque terms. High perpendicular anisotropy materials characterized by narrow domain walls have also raised considerable interest. These materials with only a few nanometer wide DWs combined several key advantages over soft magnetic materials such as higher non-adiabatic effects leading to lower critical current densities and high domain wall velocities. We review recent experimental results obtained in this class of materials and discuss the important implications they entail for the nature of the spin torque effect acting on DWs. (C) 2011 Elsevier B.V. All rights reserved.