X-ray holography of skyrmionic cocoons in aperiodic magnetic multilayers

TL;DR

This study uses high-resolution X-ray holography to investigate 3D skyrmionic cocoons in magnetic multilayers, revealing their size, vertical distribution, and interactions influenced by stray fields, advancing understanding of complex magnetic textures.

Contribution

The paper provides the first detailed 3D imaging and analysis of skyrmionic cocoons in multilayers, combining X-ray holography, magnetic force microscopy, and simulations.

Findings

Cocoons have an average diameter of ~100 nm at room temperature.

Vertical distribution of cocoons varies with field and temperature.

Stray fields influence the horizontal positioning and interactions of cocoons.

Abstract

The development and characterization of three-dimensional (3D) topological magnetic textures has become an important topic in modern magnetism both for fundamental and technological perspectives. Among the novel 3D spin textures, skyrmionic cocoons have been successfully stabilized in magnetic multilayers having a variable thickness of the ferromagnet in the vertical direction of the stack. These ellipsoidal 3D magnetic textures remain vertically confined in a fraction of the total thickness while coexisting with fully columnar skyrmions. Here, we use X-ray holography with about 15 nm lateral resolution to investigate how their properties depend on the field and temperature. We observe circular objects with different amplitude of contrast which evidences the presence of different 3D objects located in various vertical parts of the multilayer. Moreover, we witness during out-of-plane cycling an attractive interaction between cocoons located at various heights, mainly due to the stray field, which impacts their horizontal positioning. The X-ray holography measurements also allow to determine the size of the cocoons at remanence which, at room temperature, possess diameter close to 100 nm in average. Combining this transmission technique with magnetic force microscopy and micromagnetic simulations gives a precise insight into the 3D distribution of the magnetization which demonstrate the 3D nature of skyrmionic cocoons.