Metastable magnetic domain walls in cylindrical nanowires

C.A Ferguson,D.A Maclaren,S Mcvitie

doi:10.1016/j.jmmm.2015.01.027

C.A Ferguson, D.A Maclaren + Show 1 more

Open Access

https://doi.org/10.1016/j.jmmm.2015.01.027

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Abstract

The stability of the asymmetric domain wall (ATDW) in soft magnetic cylindrical nanowires and nanotubes is investigated using micromagnetic simulations. Our calculated phase diagram shows that for cylindrical permalloy nanowires, the transverse domain wall (TDW) is the ground state for radii below 20 nm whilst the Bloch point wall (BPW) is favoured in thicker wires. The ATDW stabilises only as a metastable state but with energy close to that of the BPW. Characterisation of the DW spin structures reveals that the ATDW has a vortex-like surface spin state, in contrast to the divergent surface spins of the TDW. This results in lowering of surface charge above the critical radius. For both cylindrical nanotubes and nanowires we find that ATDWs only appear to exist as metastable static states and are particularly suppressed in nanotubes due to an increase in magnetostatic energy.

Highlights

Much research into domain walls (DWs) in magnetic nanowires has focused on flat, planar DW guides that have potential spintronic applications including racetrack memory [1], shift registers [2] and domain wall logic devices [3]
In contrast to the 2-D transverse DW, in which the transverse region has two degenerate orientations, the transverse component of the (3-D) transverse domain wall (TDW) has no fixed direction and can point at any azimuthal angle. This rotational degeneracy is evident in simulations that show a corkscrew motion of the DW about the wire axis as it propagates under the torque of applied field or spin-polarised current [9,13]
The external spin structure of the Bloch point wall (BPW) is similar to that of the vortex DW (VDW), which is observed in cylindrical nanotubes (CNTs)

Summary

Introduction

Much research into domain walls (DWs) in magnetic nanowires has focused on flat, planar DW guides that have potential spintronic applications including racetrack memory [1], shift registers [2] and domain wall logic devices [3]. There is a minimum vortex diameter set by exchange energy so that reduction of the wire width below this value leads to a firstorder [5] phase change and the production of a transverse DW This second DW type has a triangular region of magnetisation lying perpendicular to the wire axis and its formation can be mediated by the third DW type, an asymmetric transverse DW, which has both a triangular form and retains some of the curling magnetisation of the vortex state but lacks a vortex core. A full understanding of both ground state and metastable (or transient) DW structures is essential for device design

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