Magnetization pinning in modulated nanowires: from topological protection to the "corkscrew" mechanism

TL;DR

This paper investigates magnetization pinning mechanisms in diameter-modulated nanowires, revealing complex behaviors including topologically protected walls and a novel 'corkscrew' pinning mode involving skyrmion tubes, with implications for magnetic nanostructure design.

Contribution

It introduces a detailed analysis of magnetization response in modulated nanowires, identifying new pinning phenomena and the role of topological structures in magnetic behavior.

Findings

Small diameter difference increases coercive field due to topologically protected walls.

Large diameter difference leads to the 'corkscrew' pinning mode with skyrmion tubes.

Complex dependence on disorder and topological magnetization structures.

Abstract

Diameter-modulated nanowires offer an important paradigm to design the magnetization response of 3D magnetic nanostructures by engineering the domain wall pinning. With the aim to understand its nature and to control the process, we analyze the magnetization response in FeCo modulated polycrystalline two-segment nanowires varying the minor diameter. Our modelling indicates a very complex behavior with a strong dependence on the disorder distribution and an important role of topologically non-trivial magnetization structures. We demonstrate that modulated nanowires with a small diameter difference are characterized by an increased coercive field in comparison to the straight ones which is explained by a formation of topologically protected walls formed by two 3D skyrmions with opposite chiralities. For a large diameter difference we report the occurrence of a novel pinning type called here the "corkscrew": the magnetization of the large diameter segment forms a skyrmion tube with a core position in a helical modulation along the nanowire. This structure is pinned at the constriction and in order to penetrate the narrow segments the vortex/skyrmion core size should be reduced.