  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.
