Unique Magnetic Phases in the Skyrmion Lattice and Fermi Surface Properties in Cubic Chiral Antiferromagnet EuPtSi

We measured the magnetization, ac susceptibility, Hall resistivity, and magnetoresistance for a cubic chiral antiferromagnet EuPtSi, and found the skyrmion lattice of the so-called "A-phase" in a wide field direction, except around H ∥ [110], together with a new skyrmion-related phase named the "B-phase" for H ∥ [001]. A large magnitude of a skyrmion-derived additional Hall resistivity is discussed on the basis of a small size of the skyrmion, 18 Å for H ∥ [111]. We also observed the de Haas–van Alphen oscillations and clarified the Fermi surface properties. Many nearly spherical Fermi surfaces are observed in EuPtSi, which are well explained by the results of full-potential linearized augmented plane wave band calculations for a non-4f reference compound SrPtSi. The splitting energy of main two split Fermi surfaces named α and α′ is determined as a large value of 1200 K, mainly reflecting the chiral (noncentrosymmetric) crystal structure and a large spin–orbit coupling of Pt-5d electrons.