The Hall effect and resistivity of the carrier doped magnetic semiconductor Fe$_{1-x}$Co$_x$S$_2$ were measured for $0le x le 0.16$, 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 to 30% of the Co density of our crystals. 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 temperature independent resistivity below 0.5 K indicating Fermi liquid-like transport. We also observe no indication of quantum corrections to the conductivity despite the small values of the product of the Fermi wave vector and the mean-free-path, $1.5 le k_Fell le 15$, over the range of $x$ investigated. This implies a large inelastic scattering rate such that the necessary condition for the observation of quantum contributions to the carrier transport, quantum coherence over times much longer than the elastic scattering time, is not met in our samples. Above 0.5 K we observe a temperature and magnetic field dependent resistivity that closely resembles a Kondo anomaly for $x$ less than that required to form a long range magnetic state, $x_c$. For $x>x_c$, the resistivity and magnetoresistance resemble that of a spin glass with a reduction of the resistivity by as much as 35% in 5 T fields. We also observe an enhancement of the residual resistivity ratio by almost a factor of 2 for samples with $xsim x_c$ 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.
Doping of the band insulator FeS$_2$ with Co on the Fe site introduces a small density of itinerant carriers and magnetic moments. The lattice constant, AC and DC magnetic susceptibility, magnetization, and specific heat have been measured over the $
0le xle 0.085$ range of Co concentration. The variation of the AC susceptibility with hydrostatic pressure has also been measured in a small number of our samples. All of these quantities show systematic variation with $x$ including a paramagnetic to disordered ferromagnetic transition at $x=0.007pm 0.002$. A detailed analysis of the changes with temperature and magnetic field reveal small power law dependencies at low temperatures for samples near the critical concentration for magnetism, and just above the Curie temperature at higher $x$. In addition, the magnetic susceptibility and specific heat are non-analytic around H=0 displaying an extraordinarily sharp field dependence in this same temperature range. We interpret this behavior as due to the formation of Griffiths phases that result from the quenched disorder inherent in a doped semiconductor.
Critical points that can be suppressed to zero temperature are interesting because quantum fluctuations have been shown to dramatically alter electron gas properties. Here, the metal formed by Co doping the paramagnetic insulator FeS$_2$, Fe$_{1-x}$C
o$_x$S$_2$, is demonstrated to order ferromagnetically at $x>x_c=0.01pm0.005$ where we observe unusual transport, magnetic, and thermodynamic properties. We show that this magnetic semiconductor undergoes a percolative magnetic transition with distinct similarities to the Griffiths phase, including singular behavior at $x_c$ and zero temperature.
