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 anti-ferromagnet EuPtSi, and found the skyrmion lattice of the so-called "A-phase" in a wide field direction, except around H parallel to [110], together with a new skyrmion-related phase named the "B-phase" for H parallel to [001]. A large magnitude of a skyrmion-derived additional Hall resistivity is discussed on the basis of a small size of the skyrmion, 18 angstrom for H parallel to [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 alpha and alpha' 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.