Evolution of magnetic fluctuations through the Fe-induced paramagnetic to ferromagnetic transition in Cr sub(2) B

In itinerant ferromagnets, the quenched disorder is predicted to dramatically affect the ferromagnetic to paramagnetic quantum phase transition driven by external control parameters at zero temperature. Here we report a study on Fe-doped Cr sub(2) B, which, starting from the paramagnetic parent, orders ferromagnetically for Fe-doping concentrations x larger than x sub(c)= 2.5%. In parent Cr sub(2) B, super(11) B nuclear magnetic resonance data reveal the presence of both ferromagnetic and antiferromagnetic fluctuations. The latter are suppressed with Fe doping, before the ferromagnetic ones finally prevail for x> x sub(c) Indications for non-Fermi-liquid behavior, usually associated with the proximity of a quantum critical point, were found for all samples, including undoped Cr sub(2) B. The sharpness of the ferromagneticlike transition changes on moving away from x sub(c) indicating significant changes in the nature of the magnetic transitions in the vicinity of the quantum critical point. Our data provide some constraints for understanding quantum phase transitions in itinerant ferromagnets in the limit of weak quenched disorder.