Dynamics of three-dimensional helical domain wall in cylindrical NiFe nanowires

Abstract

We report on a micromagnetic study on the dynamics of current-driven helical domain wall (DW) in cylindrical NiFe nanowires. The helical DW is a three-dimensional transition region between magnetizations with clockwise and anticlockwise vortex orientations. A minimum current density is needed to overcome an intrinsic pinning to drive the helical DW, and the propagation along the nanowire is accompanied by a rotational motion. As the driving current strength is increased, the rotation ceases while the DW propagates at an increased velocity. However, a velocity barrier is experienced which results in the decrease of the DW mobility. Throughout its motion, the propagated helical DW maintains a stable profile without showing any sign of structural breakdown even at relatively high driving current.

Cherenkov-type three-dimensional breakdown behavior of the Bloch-point domain wall motion in the cylindrical nanowire

“…As depicted by the green isosurface in the snapshots, the BP-DW conical structure begins to deform from one side and completely collapses at t ¼ 0.78 ns. As t ¼ 0.78 ns, no BP is observed, and an unstable 3D transverse-DWlike helical spin structure is formed 53,54 (see the green isosurface in this snapshot). In the process of forming the helical spin structure, it is found that the DW stops moving forward, and the SW formed in front of the DW disappears as well [see the dotted box area in Fig.…”

Current-induced three-dimensional domain wall propagation in cylindrical NiFe nanowires

“…14 The material parameters used in the simulations correspond to Permalloy (Ni 80 Fe 20 ), i.e., saturation magnetization M s ¼ 860  10 3 A/m, exchange stiffness constant A ex ¼ 1.3  10 À11 J/m, magnetocrystalline anisotropy k ¼ 0, Gilbert damping constant a ¼ 0.005, and non-adiabatic constant b ¼ 0.04-0.15. 8,[15][16][17][18][19] The helical DW dynamics does not show any significant difference for all b values considered. A 350-nm-diameter cylindrical nanowire with a length of 2 lm was used, and a unit cell size of 5 nm  5 nm  5 nm was chosen for all simulations.…”

“…As the applied magnetic field is reduced, a clockwise and an anticlockwise vortex are nucleated at the ends of the nanowire, which start to move toward the center of the nanowire, leading to a gradual reduction of the parallel magnetization component that causes a drop in the magnetoresistance (II). At 0 Oe (III), a helical DW 8,9 is found to be nucleated when the two vortices meet at the center of the nanowire. As the magnetic field direction is reversed, the helical DW is annihilated at H % -10 Oe.…”

“…A 350-nm-diameter cylindrical nanowire with a length of 2 lm was used, and a unit cell size of 5 nm  5 nm  5 nm was chosen for all simulations. 8,9…”

“…7 At a diameter of 350 nm, a three-dimensional helical DW can be obtained. [8][9][10] The helical DW is a three-dimensional magnetization configuration that connects clockwise and anticlockwise vortices at the center of the nanowire. Although two-dimensional DWs in planar nanowires have been intensively studied, there are only few experimental studies on three-dimensional DWs in cylindrical nanowires.…”