  We report the critical role of La doping in the topological Hall effect
observed in LaxEu1-xO thin films (~ 50 nm) grown by molecular beam epitaxy.
When the La doping exceeds 0.036, topological Hall effect emerges, which we
attribute to the formation of magnetic skyrmions. Besides, the La doping is
found to play a critical role in determining the phases, densities, and sizes
of the skyrmions in the LaxEu1-xO thin films. The maximum region of the
skyrmion phase diagram is observed on the La0.1Eu0.9O thin film. As the La
doping increases, the skyrmion density increases while the skyrmion size
decreases. Our findings demonstrate the important role of La doping for the
skyrmions in EuO films, which could be important for future studies of magnetic
skyrmions in Heisenberg ferromagnets.
