  We report on experimental verification of the rotational sense and type of
homogeneous chiral spin spiral order in a Mn monolayer on a W(110) substrate
using spin-polarized scanning tunneling microscopy. We found that the magnetic
contrast due to the spin spiral order almost vanishes with a magnetic tip
magnetized normal to the (001) plane, indicating that the spin spiral rotates
in the plane. From a shift in the most-contrasted sites by changing the tip
magnetization direction within the rotating plane, we reveal that the
rotational sense is left-handed, consistent with the previous results predicted
by first-principle calculations. By comparing the current system with a chiral
magnetic domain wall in Fe double layers on the same substrate, we found that
the polarity of the Dzyaloshinskii--Moriya interaction, the driving force of
those chiral magnets, is dominantly determined by the choice of the substrate
rather than the overlayer.
