  We observed a hump-like feature in Hall effects of SrRuO$_3$ ultrathin films,
and systematically investigated it with controlling thicknesses, temperatures
and magnetic fields. The hump-like feature is extremely stable, even surviving
as a magnetic field is tilted by as much as 85$^\circ$. Based on the
atomic-level structural analysis of a SrRuO$_3$ ultrathin film with a
theoretical calculation, we reveal that atomic rumplings at the thin-film
surface enhance Dzyaloshinskii-Moriya interaction, which can generate stable
chiral spin textures and a hump-like Hall effect. Moreover, temperature
dependent resonant X-ray measurements at Ru L-edge under a magnetic field
showed that the intensity modulation of unexpected peaks was correlated with
the hump region in the Hall effect. We verify that the two-dimensional property
of ultrathin films generates stable non-coplanar spin textures having a
magnetic order in a ferromagnetic oxide material.
