Large Cyclotron Mass and Large Ordered Moment in Ferromagnet CoS2 Compared with Paramagnet CoSe2

We succeeded in growing high-quality single crystals of the pyrite-type cubic compounds CoSe2 and CoS2 using the transport agent CoBr2 and measured the electrical resistivity, specific heat, magnetic susceptibility, magnetization, and 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, gamma = 18mJ/(K-2.mol). On the other hand, CoS2 is a ferromagnet with a Curie temperature T-C = 122K and an ordered moment mu(s) = 0.93 mu(B)/Co. The gamma of 21 mJ=(K-2.mol) of CoS2 is slightly larger than that of CoSe2. A large ordered moment, together with a large gamma, is characteristic of 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 fourfold symmetry in the angular dependence of the dHvA frequency.