Micromagnetic simulations of helimagnetic nanostructures

Abstract

Complex magnetic materials hosting topologically non-trivial, particle-like objects such as skyrmions or Bloch points are under intensive research and could fundamentally change the way we store and process data. One important class of materials in which such chiral magnetisation configurations can be found are helimagnetic materials with Dzyaloshinskii-Moriya interaction. Recently, it was demonstrated that nanodisks consisting of two layers with opposite material chirality can host a single stable Bloch point—a point singularity in the magnetisation configuration—of two different types at the layer interface.

In this work, we use finite-difference micromagnetic simulations to study static and dynamic properties of Bloch points in the two-layer system. To conduct the simulations, we use a Python package called Ubermag, which defines a domain-specific language to express micromagnetic problems and provides a high-level interface to existing micromagnetic calculation packages such as OOMMF. Its functionality and capabilities have been significantly enhanced and extended as part of this work.

In the first part of the work, we focus on the static properties of Bloch points in rectangular two-layer FeGe nanostrips. We find that multiple Bloch points in any possible combination of the two different types can coexist—a crucial prerequisite for potential applications. The number of Bloch points in stable configurations depends on the strip geometry. We can predict suitable geometries for a given Bloch-point number, which we demonstrate for an 80-Bloch-point configuration.

In the second part of the work, we study the dynamics of the stable Bloch points when an electric current is applied, which couples to the magnetisation via the spin-transfer torque. We demonstrate that the Bloch points move in the current direction without deflection. Constrictions in a nanostrip can be used to control the motion: at lower current densities, Bloch points are trapped at the constrictions; at higher current densities, they can move past them. This can be exploited to move arrays of Bloch points in a controlled manner. Finally, we demonstrate that Bloch points in more complex geometries with multiple possible paths, such as the T-shaped geometry studied in this work, can move along the different paths if current is applied in suitable directions.

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Identifiers

ORCID for Martin Lang: orcid.org/0000-0001-7104-7867

ORCID for Hans Fangohr: orcid.org/0000-0001-5494-7193

ORCID for Marijan Beg: orcid.org/0000-0002-6670-3994

ORCID for Ondrej Hovorka: orcid.org/0000-0002-6707-4325

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