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

K Zeissler,S A Nikitov,S Finizio,C H Marrows,G Burnell,A V Sadovnikov,S Mcvitie,J Raabe,P M Shepley,M Mruczkiewicz,K Fallon,S Mcfadzean,T A Moore

doi:10.1038/s41598-017-15262-3

Abstract

We have imaged Néel skyrmion bubbles in perpendicularly magnetised polycrystalline multilayers patterned into 1 µm 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.

Highlights

In the interfacial DMI systems the stabilised skyrmions are of the Néel type
A Néel skyrmion is characterised by the sense of rotation of the spins forming the boundary between the out of plane skyrmion core and the antiparallel surrounding spins
A drawback of these polycrystalline room temperature systems has increasingly become apparent in form of pinning which prevents smooth and reliable skyrmion dynamics[9,15]

Summary

Introduction

In the interfacial DMI systems the stabilised skyrmions are of the Néel type. A Néel skyrmion is characterised by the sense of rotation of the spins forming the boundary between the out of plane skyrmion core and the antiparallel surrounding spins. Recent experimental advances have showcased the potential of interfacial skyrmions at room temperature for applications[6,8,9,11,12,13]. These works pushed the greatly promising results of Fe on Ir(111)[3] and PdFe bilayer on Ir(111)[5,14], which exhibit nanoscale skyrmions at sub-liquid nitrogen temperatures, a huge step towards practical applications. The Magnus force is expected to help mitigate pinning effects This is expected to lead to good low current density performance of skyrmions[19]. Defects influence the motion, pinning leads to the occurrence of deformation which was observed experimental[15,22] and theoretical[16]

Methods

Results

Conclusion