Magnetic field-tuned size and dual annihilation pathways of chiral magnetic bobbers

TL;DR

This paper develops an analytical model and simulations to understand the size, annihilation mechanisms, and transformation pathways of chiral magnetic bobbers, advancing their potential use in spintronic devices.

Contribution

It introduces a predictive model for CB size based on magnetic parameters and identifies two distinct annihilation pathways, including a novel fragmentation process.

Findings

Validated the size model with micromagnetic simulations.

Identified droplet-like instability and Bloch point depinning as annihilation mechanisms.

Discovered a fragmentation pathway transforming CBs into skyrmion tubes and ferromagnetic states.

Abstract

Magnetic chiral bobbers (CBs) are three-dimensional (3D) topological spin textures that consist of a tapered skyrmion tube terminating in a Bloch point, promising applications in high-density spintronics. However, the mechanisms controlling their size and the dynamics of their annihilation are still not fully understood. In this study, we present an analytical model that predicts the radius $R$ of the CB as a function of the external magnetic field, the Dzyaloshinskii-Moriya interaction (DMI), the magnetic anisotropy, and the exchange interaction. The micromagnetic simulations validate this model across a broad range of parameters. We also identify two mechanisms of annihilation of CBs: (i) a droplet-like instability that occurs under rapid changes in the magnetic field, which we describe using a proposed magnetic Weber number $We$ and its critical field step scaling; and (ii) Bloch point depinning mechanism at interfaces, for which we determine the threshold magnetic field $B_{\text{th}}$ for annihilation. Importantly, we uncover a novel fragmentation pathway in which CBs transform into skyrmion tubes, then into half-CBs, and finally into ferromagnetic states. These findings lay the groundwork for understanding and manipulating 3D CBs as next-generation devices.