The strongly paramagnetic-to-spin polarized ferromagnetic transition in Fe$_{1-x}$Co$_x$S$_2$

Carrier doping of “fool’s gold”, the paramagnetic insulator FeS$_2$, by way of Co substitution for Fe, results in a insulator-to-metal transition at $x\le 0.001$. Further Co substitution beyond $x~0.035$ produces an itinerant and fully spin polarized ferromagnet by way of either a crossover or quantum phase transition. In order to explore the thermodynamics of this magnetic semiconductor near the paramagnetic-to- ferromagnetic phase transition the specific heat of Fe$_{1-x} $Co$_x$S$_2$ (x=0.045,0.03,0.005) was measured for temperatures down to 0.1 K. Our $x=0.045$ sample displayed a logarithmic like divergence of $C/T =\gamma$ in zero field which saturates to a Fermi liquid like constant below 0.5 K. For $x$=0.03 at zero field, the enhancement of $\gamma$ is extended down to 0.3K while for x=0.005 the logarithmic-like divergence continues down to the lowest temperature measured. For this last sample, $\gamma$ can be enhanced by the application of small magnetic fields ($H<0.15$ T) while higher fields tend to suppress $\gamma$ in all of our samples.