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Magnetic properties of FeS2 and CoS2 having pyrites structure are studied. Magnetic susceptibility of a good natural pyrites (FeS2) crystal is almost temperature-independent down to 4.2°K. The constant susceptibility is explained by the Van Vleck paramagnetic term and diamagnetic one of the core. This suggests iron in pyrites is in divalent and low-spin state contrary to the assumption of L. R. Walker et al. that iron in pyrites is in a trivalent state from the Mössbauer isomer-shift data. CoS2 is a ferromagnet of a Curie temperature of 130°K. The spontaneous magnetization σ and the Curie constant Cmol are measured on a synthetic crystal of CoS2. We obtained the values, σ=44.4 emu/g and Cmol=0.37 emu·deg. The values are compatible when we assume Co is in divalent and low-spin state. This fact accords with the result of magnetic measurement of FeS2.

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