Charge transport in cobalt-doped iron pyrite

The Hall effect and resistivity of the carrier-doped magnetic semiconductor ${\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x}{\text{S}}_{2}$ were measured for $0\ensuremath{\le}x\ensuremath{\le}0.16$, temperatures between 0.05 and 300 K, and fields of up to 9 T. Our Hall data indicate electron charge carriers with a density of only $10\char21{}30\text{ }\mathrm{%}$ of the Co density of our crystals. The charge-carrier transport is dominated by a Kondo-like anomaly below 20 K for $x$ less than that required to form a long-range magnetic state, ${x}_{c}$. For $xg{x}_{c}$, the resistivity and magnetoresistance resemble that of a spin glass with a reduction in the resistivity by as much as 35% in 5 T fields. Although the product of the Fermi wave vector and the mean-free path, ${k}_{F}\ensuremath{\ell}$, varies between 1.5 and 15 over the range of $x$ investigated, we observe no indication of quantum corrections to the resistivity, $\ensuremath{\rho}$, as $\ensuremath{\rho}$ is dominated by the Kondo and spin glasslike anomalies down to very low temperature. Despite the previous identification of magnetic Griffiths phase formation in the magnetic and thermodynamic properties of this system for the same range of $x$, we measure a saturating resistivity below 0.5 K indicating Fermi liquidlike transport. We also observe an enhancement of the residual resistivity ratio by almost a factor of 2 for samples with $x\ensuremath{\sim}{x}_{c}$ indicating temperature-dependent scattering mechanisms beyond simple carrier-phonon scattering. We speculate that this enhancement is due to charge carrier scattering from magnetic fluctuations which contribute to the resistivity over a wide temperature range.