Quantifying chiral exchange interaction for Néel-type skyrmions via Lorentz transmission electron microscopy

Magnetic skyrmions are topological spin textures that have been observed in bulk magnets and magnetic multilayers. For bulk magnetic materials, their noncollinear spin profiles have often been studied by using Lorentz transmission electron microscopy (TEM). We experimentally utilized Lorentz TEM imaging to study an inversion asymmetric ${[\mathrm{Pt}(1.5\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Co}(1\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{W}(1\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{8}$ heterostructure that exhibits N\'eel-type skyrmions at zero field. By tracking the evolution of skyrmion diameters as a function of magnetic fields, we determined the strength of the interfacial Dzyaloshinskii-Moriya interaction (DMI). Our results suggest that in situ Lorentz TEM imaging combined with simulations can provide valuable quantitative information about the interfacial DMI strengths, which can be helpful for optimizing skyrmion materials. Furthermore, we show that in theory, Lorentz TEM can identify the spin chirality of N\'eel-type skyrmions, although an experimental verification is challenging due to the relatively low signal-to-noise ratio.