Quantum Griffiths Phase in theWeak Itinerant Ferromagnetic Alloy Ni1-xVx

We present magnetization (M) data of the d-metal alloy Ni1-xVx at vanadium concentrations close to x(c) approximate to 11.4% where the onset of long-range ferromagnetic (FM) order is suppressed to zero temperature. Above x(c), the temperature (T) and magnetic field (H) dependencies of the magnetization are best described by simple nonuniversal power laws. The exponents of M/H similar to T-gamma and M similar to H-alpha are related by 1 - gamma = alpha for wide temperature (10 < T <= 300 K) and field (H <= 5 T) ranges. gamma is strongly x dependent, decreasing from 1 at x approximate to x(c) to gamma < 0.1 for x = 15%. This behavior is not compatible with either classical or quantum critical behavior in a clean 3D FM. Instead it closely follows the predictions for a quantum Griffiths phase associated with a quantum phase transition in a disordered metal. Deviations at the lowest temperatures hint at a freezing of large clusters and the onset of a cluster glass phase.