Pinning and hysteresis in the field dependent diameter evolution of skyrmions in Pt/Co/Ir superlattice stacks

TL;DR

This study visualizes and analyzes the field-dependent size changes and hysteresis of Neel skyrmion bubbles in multilayer stacks, revealing complex behavior influenced by disorder and effective thickness variations.

Contribution

It provides the first direct imaging of skyrmion diameter evolution under magnetic fields and demonstrates the role of disorder in hysteretic behavior, challenging previous homogeneous models.

Findings

Skyrmion bubbles are stable at zero field with a diameter of 260 nm.

Applying opposing magnetic fields compress skyrmions to about 100 nm.

Hysteresis observed in skyrmion size changes, with abrupt expansion during removal of field.

Abstract

We have imaged N\'eel skyrmion bubbles in perpendicularly magnetised polycrystalline multilayers patterned into 1 \mu m diameter dots, using scanning transmission x-ray microscopy. The skyrmion bubbles can be nucleated by the application of an external magnetic field and are stable at zero field with a diameter of 260 nm. Applying an out of plane field that opposes the magnetisation of the skyrmion bubble core moment applies pressure to the bubble and gradually compresses it to a diameter of approximately 100 nm. On removing the field the skyrmion bubble returns to its original diameter via a hysteretic pathway where most of the expansion occurs in a single abrupt step. This contradicts analytical models of homogeneous materials in which the skyrmion compression and expansion are reversible. Micromagnetic simulations incorporating disorder can explain this behaviour using an effective thickness modulation between 10 nm grains.