Bloch-to-N\'eel domain wall transition evinced through morphology of magnetic bubble expansion in Ta/CoFeB/MgO layers

TL;DR

This study demonstrates how He$^+$ ion irradiation can induce a transition from Bloch to Ne9el domain walls in Ta/CoFeB/MgO layers by altering magnetic properties, with bubble morphology serving as an effective indicator.

Contribution

It reveals the ion irradiation threshold for domain wall transition and introduces bubble shape analysis as a tool for sensing and engineering magnetic domain structures.

Findings

Ion irradiation increases DMI strength and reduces saturation magnetization.

Transition to Ne9el domain walls occurs between 12 and 16 d7 10^{18} He$^+$ /m$^2$.

Bubble morphology analysis effectively indicates internal domain wall structure.

Abstract

Ta/CoFeB/MgO trilayers with perpendicular magnetic anisotropy are often characterised by vanishing or modest values of interfacial Dzyaloshinskii-Moriya interaction (DMI), which results in purely Bloch or mixed Bloch-N\'eel domain walls (DWs). Here we investigate the creep evolution of the overall magnetic bubble morphology in these systems under the combined presence of in-plane and out-of-plane magnetic fields and we show that He$^+$ ion irradiation induces a transition of the internal DW structure towards a fully N\'eel spin texture. This transition can be correlated to a simultaneous increase in DMI strength and reduction in saturation magnetisation -- which are a direct consequence of the effects of ion irradiation on the bottom and top CoFeB interfaces, respectively. The threshold irradiation dose above which DWs acquire a pure N\'eel character is experimentally found to be between 12 $\times$ 10$^{18}$ He$^+$/m$^2$ and 16 $\times$ 10$^{18}$ He$^+$/m$^2$, matching estimations from the one dimensional DW model based on material parameters. Our results indicate that evaluating the global bubble shape during its expansion can be an effective tool to sense the internal bubble DW structure. Furthermore, we show that ion irradiation can be used to achieve post-growth engineering of a desired DW spin texture.