The Effect of Disorder on a Quantum Phase Transition

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.