  Topological spin textures have attracted much attention both for fundamental
physics and spintronics applications. Among them, antiskyrmions possess a
unique spin configuration with Bloch-type and N\'eel-type domain walls due to
anisotropic Dzyaloshinskii-Moriya interaction (DMI) in the noncentrosymmetric
crystal structure. However, antiskyrmions have thus far only been observed in a
few Heusler compounds with $D_{2\mathrm{d}}$ symmetry. Here, we report a new
material Fe$_{1.9}$Ni$_{0.9}$Pd$_{0.2}$P in a different symmetry class ($S_4$
symmetry), where antiskyrmions exist over a wide temperature region including
room temperature, and transform to skyrmions upon changing magnetic field and
lamella thickness. The periodicity of magnetic textures greatly depends on
crystal thickness, and domains with anisotropic sawtooth fractals are observed
at the surface of thick crystals, which are attributed to the interplay between
dipolar interaction and DMI as governed by crystal symmetry. Our findings
provide a new arena to study antiskyrmions, and should stimulate further
research on topological spin textures and their applications.
