${\mathrm{CoS}}_2$ has recently been predicted to be highly spin-polarized, but very little is known about its magnetotransport properties. We find that this system exhibits a large anomaly in the resistivity at the Curie temperature ${(T}_C),$ which shows a pronounced shift with applied field, resulting in negative magnetoresistance below $T_C$ and positive magnetoresistance above it. Magnetization measurements indicate an almost first-order character to the ferromagnetic transition, indicating that ${\mathrm{CoS}}_2$ is close to a tricritical point. Analyzing the field dependence of the magnetization and resistivity, we are able to prove that the resistivity near $T_C$ is controlled only by the magnetization, regardless of whether the ferromagnetic state is induced by cooling or by application of a field. We propose that the increase in resistivity on entering the ferromagnetic state is due to a spin-dependent band structure effect where exchange splitting results in a distinct decrease in the density of states at the Fermi level for the minority spins. Point-contact Andréev reflection measurements on sulfur-stoichiometric polycrystals give a spin polarization of 56%, consistent with indications of electron-magnon scattering in our transport measurements as well as our proposed spin-dependent band structure mechanism for the magnetotransport anomalies.

• Received 3 March 2003

DOI:https://doi.org/10.1103/PhysRevB.69.094412