Observation of multi-skyrmion objects created by size and density control in Ta/CoFeB/MgO films

TL;DR

This paper demonstrates precise control over the size, density, and merging of magnetic skyrmions in Ta/CoFeB/MgO films, advancing their potential use in magnetic memory devices and exploring multi-skyrmion topological objects.

Contribution

It introduces a method to control skyrmion formation, size, and density via CoFeB thickness tuning and magnetic field initialization, and demonstrates creation of multi-skyrmion objects.

Findings

High degree of skyrmion size and density control.

Stable zero-field skyrmions achieved.

Formation of multi-skyrmion topological objects.

Abstract

Magnetic skyrmions are chiral spin textures with a nontrivial topology that offer a potential for future magnetic memory and storage devices. The controlled formation and adjustment of size and density of magnetic skyrmions in Ta/CoFeB/MgO trilayers is demonstrated. It is the ideal candidate for the use as a bottom electrode integration into CoFeB/MgO/CoFeB magnetic tunnel junctions. Varying the CoFeB thickness close to the out-of-plane to in-plane magnetic phase transition, we find that subtle energy contributions enable the skyrmion formation in a narrow thickness window, corresponding to only around 10 pm variation in CoFeB thickness. Using magneto-optical imaging with quantitative image processing, variations in skyrmion diameter and distribution below the Abbe limit can be analyzed. We demonstrate a high degree of diameter and density control. Zero-field stable skyrmions can be set with proper magnetic field initialization. This demonstrated tunability and degree of comprehension of skyrmion formation, paves the way for future skyrmion based magnetic memory. Moreover, we demonstrate a controlled merging of individual skyrmions to complex topological objects. We compare our results with the baby-Skyrme model, developed to describe the soliton nature, for any topological charge n, and demonstrate the ability to form multi-skyrmion objects. These objects will be interesting for fundamental mathematical studies of the topological behavior of solitons in the future.