Discovery of the Griffiths Phase in the Itinerant Magnetic SemiconductorFe1−xCoxS2

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 ${\mathrm{FeS}}_2$, ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{S}}_2$ is demonstrated to order ferromagnetically at $x>x_c=0.01\pm 0.005$, 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.