Antiskyrmions, new topological spin textures with a topological number of the opposite sign to that of skyrmions, have attracted much attention in the research field of topological magnets. Here, we review our recent progress on a new antiskyrmion-host material with S₄ symmetry. We find that the magnetic anisotropy of schreibersite (Fe,Ni)₃P is significantly modified by chemical doping, and Lorentz transmission electron microscopy shows that stable antiskyrmions form above room temperature in thin plates of (Fe_0.63Ni_0.30 Pd 0.07)₃P. The size of the magnetic domain textures increases with increasing the crystal thickness, and sawtooth-shaped fractal magnetic domain patterns appear near the surface of thick crystals as revealed by magnetic force microscopy. Furthermore, recent small-angle neutron scattering measurements characterize anisotropic fractal nature of the magnetic domain walls in bulk crystals. These results demonstrate that uniaxial magnetic anisotropy and magnetic dipolar interaction in addition to Dzyaloshinskii–Moriya interaction play an important role in this material.