Depth-dependent magnetic crossover in a room-temperature skyrmion-hosting multilayer

TL;DR

This study investigates the depth-dependent magnetic properties of skyrmion-hosting multilayers using advanced scattering, simulation, and muon-spin techniques, revealing a transition from Ne9el to Bloch domain walls and a skyrmion crossover at high fields.

Contribution

It provides the first detailed experimental and simulation analysis of depth-dependent magnetism in multilayer skyrmion hosts, highlighting a magnetic crossover at room temperature.

Findings

Domain-wall helicity angle varies with depth, indicating a transition from Ne9el to Bloch-like structures.

Magnetic field reduces domain wall volume, leading to skyrmion dominance above 180 mT.

Distinct magnetic field distributions are observed at different depths within the multilayer.

Abstract

Skyrmion-hosting multilayer stacks are promising avenues for applications, although little is known about the depth dependence of the magnetism. We address this by reporting the results of circular dichroic resonant elastic x-ray scattering (CD-REXS), micromagnetic simulations, and low-energy muon-spin rotation (LE-$\mu^+$SR) measurements on a stack comprising [Ta/CoFeB/MgO]$_{16}$/Ta on a Si substrate. Energy-dependent CD-REXS shows a continuous, monotonic evolution of the domain-wall helicity angle with incident energy, consistent with a three-dimensional hybrid domain-wall-like structure that changes from N\'eel-like near the surface to Bloch-like deeper within the sample. LE-$\mu^+$SR reveals that the magnetic field distribution in the trilayers near the surface of the stack is distinct from that in trilayers deeper within the sample. Our micromagnetic simulations support a quantitative analysis of the $\mu^+$SR results. By increasing the applied magnetic field, we find a reduction in the volume occupied by domain walls at all depths, consistent with a crossover into a region dominated by skyrmions above approximately 180 mT.