Magnetic Properties of Some Compounds with Pyrite Structure

The magnetic properties of the dichalcogenide compounds with pyrite structure, CoS2, CoSe2, NiS2, NiSe2, and Co(SxSe1−x)2 were studied by means of magnetic measurements, neutron diffraction, NMR, and electrical resistivity in order to discuss their physical properties. Results obtained are as follows: CoS2 is a ferromagnetic with TC=124°K, θp=220°K, peff=2.13 μB, n=0.84 μB (at 0°K), and the exchange interaction J was estimated to be 24 cm−1 from the analysis of the low-temperature thermomagnetic curve. The internal magnetic field was observed to be 52 kOe at 80°K by NMR measurement. CoSe2 is an antiferromagnetic with TN=93°K, θp=−160°K, peff=2.20 μB; MnSe2 type antiferromagnetic spin ordering was determined by neutron diffraction at 53°K. NiS2 has an antiferromagnetic susceptibility with θp≃ −740°K and peff≃3.0 μB, but no magnetic ordering was observed by neutron diffraction at 53°K. NiSe2 exhibits weak constant paramagnetism, χM≈2×10−4 emu/mole. Co(SxSe1−x)2 forms a complete solid solution (0≤x≤1) with cubic pyrite structure and without ordering of S and Se. The magnetic phase diagram was determined, and the ferromagnetic region was found to exist up to 12% of Se at 0°K. The magnetic parameters of this system were determined and they show the so-called ``low spin state'' of cobaltous ion with one Bohr magneton. Electrical resistivities for the above compounds were semiconductive except for NiSe2 (metallic). The theoretical and the experimental values of kTC/J for CoS2 (fcc, S=½) were compared. The experimental value was found to be 3.6. According to the result of the series-expansion method for the Heisenberg model, kTC/J was found to be 4.170 (kTC/J=6.00 for the Weiss approximation). It is concluded that the main origin of the discrepancy between theory and experiment is the influence of the more distant neighbor antiferromagnetic exchange interactions. The narrow ferromagnetic region in the Co(SxSe1−x)2 is explained by a statistical theory.