  The conductivity and magnetization of Fe1-xCoxS2 were measured to investigate
quantum critical behavior in disordered itinerant magnets. Small x (<0.001) is
required to convert insulating iron pyrite into a metal, followed by a
paramagnetic-to-ferromagnetic metal transition at x = 0.032+/-0.004. Singular
contributions are discovered that are distinct from those at either
metal-insulator or magnetic transitions. Our data reveal that disorder and low
carrier density associated with proximity to a metal-insulator transition
fundamentally modifies the critical behavior of the magnetic transition.
