Direct observation of topological charge impacting skyrmion bubble stability in Pt/Ni/Co asymmetric superlattices

Nisrit Pandey,Maxwell Li,Marc De Graef,Vincent Sokalski

doi:10.1063/5.0055971

Abstract

We characterize the magnetic properties and domain structure of Pt/Ni/Co asymmetric superlattices in comparison to the more established Pt/Co/Ni structure. This reversal in stacking sequence leads to a marked drop in interfacial magnetic anisotropy and the magnitude of the interfacial Dzyaloshinskii–Moriya interaction as inferred from the domain wall (DW) structure, which we speculate could be related to a degradation of the Pt/Co interface when Pt is deposited on top of the Co layer. Lorentz transmission electron microscopy exclusively reveals Néel-type DWs and, with a perpendicular field, Néel skyrmions in the Pt/Co/Ni films. Conversely, the Pt/Ni/Co samples show only achiral Bloch DWs, which leads to the formation of achiral Bloch and type II bubbles at an increased perpendicular field. Combined with the reduced anisotropy leading to greater bubble densities, the latter case makes for an excellent test bed to examine the benefits of topological charge on stability. Simultaneous observation of Bloch and type II bubbles shows a roughly 50 mT larger annihilation field for the former. An in-plane component to the magnetic field is shown to both impact the structure of the formed bubbles and separately suppress the topological benefit.

Highlights

∂ym)dxdy, where m is the unit magnetization vector.15 A strong interfacial Dzyaloshinskii–Moriya interaction (DMI) supports the formation of Néel skyrmions or domain walls (DWs) where their chirality is influenced by the sign of DMI
A strong interfacial DMI supports the formation of Néel skyrmions or domain walls (DWs) where their chirality is influenced by the sign of DMI
As noted later the result is a change in the wall structure to achiral Bloch, suggesting weaker DMI, which is coupled with decreased perpendicular magnetic anisotropy

Summary

Introduction

∂ym)dxdy, where m is the unit magnetization vector.15 A strong interfacial DMI supports the formation of Néel skyrmions or domain walls (DWs) where their chirality is influenced by the sign of DMI. Topologically trivial bubbles (Q = 0), known as type II bubbles, have been observed in the case of weak DMI alongside achiral Bloch bubbles (see Fig. 2).8 As noted later the result is a change in the wall structure to achiral Bloch (rather than a reversal of Néel chirality), suggesting weaker DMI, which is coupled with decreased perpendicular magnetic anisotropy.

Results

Conclusion