The recent observation of current-induced domain wall (DW) motion with large
velocity in ultrathin magnetic wires has opened new opportunities for
spintronic devices. However, there is still no consensus on the underlying
mechanisms of DW motion. Key to this debate is the DW structure, which can be
of Bloch or N\'eel type, and dramatically affects the efficiency of the
different proposed mechanisms. To date, most experiments aiming to address this
question have relied on deducing the DW structure and chirality from its motion
under additional in-plane applied fields, which is indirect and involves strong
assumptions on its dynamics. Here we introduce a general method enabling
direct, in situ, determination of the DW structure in ultrathin ferromagnets.
It relies on local measurements of the stray field distribution above the DW
using a scanning nanomagnetometer based on the Nitrogen-Vacancy defect in
diamond. We first apply the method to a Ta/Co40Fe40B20(1 nm)/MgO magnetic wire
and find clear signature of pure Bloch DWs. In contrast, we observe left-handed
N\'eel DWs in a Pt/Co(0.6 nm)/AlOx wire, providing direct evidence for the
presence of a sizable Dzyaloshinskii-Moriya interaction (DMI) at the Pt/Co
interface. This method offers a new path for exploring interfacial DMI in
ultrathin ferromagnets and elucidating the physics of DW motion under current.Comment: Main text and Supplementary Information, 33 pages and 12 figure