Transport and magnetic properties of highly densified CoS2 ceramic

Dense ceramics of the itinerant ferromagnet CoS2 have been prepared by the spark plasma sintering technique. The structural study confirms a cubic unit-cell for CoS2 with a = 5.539(6)Å (SG=Pa3¯). Scanning and transmission electron microscopy reveal the existence of sulfur deficient regions of composition “Co2S3” with a structure compatible to that of CoS2 where 0.5 S atom are missing in one out of two successive layers along the b direction. The volume of these regions estimated to be less than 1% is explained by possible S losses from the surface during the sintering. The physical properties measurements show a high TC (122 K), large saturated magnetization (0.88 μB/f.u.), with a ∼100% magnetoresistance at 5 K in 9 T. Also, the value of the residual resistivity ratio, ρ300 K/ρ5 K = 45, between those of crystal and conventional ceramics, attest for both good compacity and grain connectivity. Interestingly, the Seebeck coefficient exhibits negative values reaching S = −47.5 μV K−1 at 675 K and below TC, S can be fitted by a diffusion model with a magnon term related to the ferromagnetism. A large power factor (S2/ρ) of 1 mW m−1 K−2 is obtained over a broad T range but the too high thermal conductivity (κ675 K ∼ 8.7 W K−1 m−1) is a limiting physical parameter to use CoS2 ceramics in thermoelectric devices.