Paramagnetic Semiconductor to itinerant Ferromagnet in Fe$_{1-x}$Co$_x$S$_2$

Carrier doping of ``fool's gold'', the paramagnetic insulator FeS$_2$ by partial substitution of Co for Fe, results in an insulator-to-metal transition at $x\le 0.001$. Magnetization and susceptibility measurements for samples with Co substitution beyond $x~0.01$ reveal the emergence of a highly itinerant ferromagnet with no discontinuous changes with $x$, field, or temperature ($T$) suggesting either a continuous crossover or quantum phase transition to ferromagnetism. For $x\le 0.01$ and low-$T$, the conductivity decreases with smaller $T$ and displays a positive magnetoconductance. Samples which order magnetically show similar magnetoconductance, but with a minimum in conductivity near $T_c$. We conclude that either Kondo or magnetic polaron effects are responsible for these low-$T$ anomalies. Specific heat measurements display extraordinarily large carrier masses that diverge logarithmically or with a small power law at low-$T$ close to the ferromagnetic critical concentration, similar to the behavior near quantum critical points.