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)angstrom (SG = Pa-(3) over bar). 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 T-C (122 K), large saturated magnetization (0.88 mu(B)/f.u.), with a similar to 100% magnetoresistance at 5K in 9 T. Also, the value of the residual resistivity ratio, rho(300K)/rho(5K) = 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 mu V K-1 at 675K and below T-C, S can be fitted by a diffusion model with a magnon term related to the ferromagnetism. A large power factor (S-2/rho) of 1mW m(-1) K-2 is obtained over a broad T range but the too high thermal conductivity (kappa(675 K) similar to 8.7W K-1 m(-1)) is a limiting physical parameter to use CoS2 ceramics in thermoelectric devices. (C) 2013 AIP Publishing LLC.