  We investigate skyrmion formation in both a single crystalline bulk and
epitaxial thin films of MnSi by measurements of planar Hall effect. A prominent
stepwise field profile of planar Hall effect is observed in the
well-established skyrmion phase region in the bulk sample, which is assigned to
anisotropic magnetoresistance effect with respect to the magnetic modulation
direction. We also detect the characteristic planar Hall anomalies in the thin
films under the in-plane magnetic field at low temperatures, which indicates
the formation of skyrmion strings lying in the film plane. Uniaxial magnetic
anisotropy plays an important role in stabilizing the in-plane skyrmions in the
MnSi thin film.
