Fermi Surface and Magnetic Properties in Ferromagnet CoS₂ and Paramagnet CoSe₂ with the Pyrite-type Cubic Structure

We succeeded in growing high-quality single crystals of pyrite-type cubic compounds CoSe₂ and CoS₂ using a transport agent of CoBr₂ and measured the electrical resistivity, specific heat, magnetic susceptibility, magnetization, and the de Haas-van Alphen (dHvA) effect. We confirmed that CoSe₂ is an exchange-enhanced paramagnet revealing a broad maximum at around 50 K in the temperature dependence of the magnetic susceptibility. The electronic specific heat coefficient is moderately large, γ = 18 mJ/(K²•mol). On the other hand, CoS₂ is a ferromagnet with a Curie temperature T _C = 122 K and an ordered moment μ_s = 0.93 μ_B/Co. The γ value of 21 mJ/(K²•mol) in CoS₂ is slightly larger than that of CoSe₂. A large ordered moment, together with a large γ value, is a characteristic feature in CoS₂ because CoS₂ is a half-metallic spin state in the ferromagnetic state. Correspondingly, we detected a main dHvA branch with a large cyclotron effective mass of 13m ₀ in the dHvA experiments. The detected dHvA branches in CoS₂ and CoSe₂ are discussed on the basis of the results of energy band calculations, revealing a broken four-fold-symmetry in the angular dependence of the dHvA frequency.