“Observation of the intrinsic pinning of a magnetic domain wall in a ferromagnetic nano-wire”

Published in ” Nature Materials” , 20 February 2011

Displacing the domain wall (DW) is one of the most promising methods to switch the local magnetization. Using the reaction of spin torque is a promising method for electrically doing so. The adiabatic spin torque, which is exerted by transferring the change of the spin angular momentum of carrier electrons to local spins, is the most ubiquitous scenario. This torque moves a DW by changing its structure periodically between a Bloch wall and Néel wall. The energy barrier to be overcome to change the domain wall structure is called intrinsic pinning, which determines the threshold current density, J_th, for the DW displacement (see Figure). This scenario, however, is not applicable in the most intensively investigated NiFe nano-wires with in-plane magnetization, because J_th in this system is two orders smaller than theoretical expectation as well as depends on the extrinsic pinning strength brought by inevitable edge/surface roughnesses, defects, or intentional notches in artificial wires. In contrast, for the adiabatic spin torque, which is theoretically well established, no experimental evidence that the intrinsic pinning dominates J_th has been reported, although comparisons between the theories and experiments have been attempted.

We demonstrate by using Co/Ni nano-wires with perpendicular magnetization that J_th is tunable by varying the wire width, thus by controlling the strength of the intrinsic pinning. J_th shows a minimum value at the critical width where the DW energies of Bloch wall and Néel walls are equivalent. Insensitivity of J_th to the external magnetic field is also shown. These results do not only prove that J_th in Co/Ni nano-wires is dominated by the intrinsic pinning but also shed light on the pivotal physics of the spin torque and its advantage in reliable device application.